1.0 INTRODUCTION

2.0 INVENTORY AND STATUS OF PPH WETLANDS

3.0 FUNCTIONS AND VALUES

• 3.1 Maintenance of Runoff Volume

• 3.2 Maintenance of Runoff Timing

• 3.3 Groundwater Recharge

• 3.4 Sediment Retention

• 3.5 Phosphorus Retention

• 3.6 Nitrogen Removal

• 3.7 Detoxification

• 3.8 Vascular Plant Production and Carbon Cycling

• 3.9 Invertebrate Production

• 3.10 Fish Production

• 3.11 Waterfowl Production

• 3.12 Winter Wildlife Shelter

• 3.13 Furbearer Production

• 3.14 Biodiversity

4.0 FUNCTIONAL LOSS

• 4.1 Losses Due to Conversion by Filling or Leveling

• 4.2 Losses Due to Artificial Drainage

• 4.3 Losses Due to Groundwater Pumping

• 4.4 Losses Due to Dugout and Impoundment Construction

• 4.5 Losses Due to Grazing and Mowing

• 4.6 Losses Due to Tillage

• 4.7 Losses Due to Sedimentation

• 4.8 Losses Due to Pesticide Use

• 4.9 Losses Due to Excessive Nutrient Inputs

• 4.10 Losses Due to Excessive Human Visitation

5.0 REPLACEMENT POTENTIAL

• 5.1 Status of Replacement Efforts

• 5.2 Potential for Replacing Specific Wetland Functions

• 5.3 Indicators of Replacement Potential

6.0 LANDSCAPE STUDIES AND INDICATORS

• 6.1 Previous Landscape Studies in the Region

• 6.2 Sources of Data on Indicators

• 6.3 Existing GIS Capabilities

7.0 LITERATURE CITED

This report focuses on the basin-type wetlands of the Prairie Pothole Region (PPR), specifically, those wetlands which lack surface-water outlets during most years, are smaller than about 20 acres, and intermittently become dry. This report contains both information collected from an extensive literature review, and inferences made from that information. Information in the report was compiled to establish a common information source for all members of a panel charged with qualitatively assessing risks of loss of prairie pothole (PPH) wetlands. The process used in that risk assessment is described in the Summary report, and its application is documented in another report (Adamus 1992a). As explained in the Summary report, the risk assessment of which this literature review was a part has treated the entire PPR as a single entity. Yet clearly, considerable variability exists within the region. Thus, attempts to describe "overall" regional conditions have resulted in considerable simplification of actual relationships.

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2.0 Inventory and Status of PPH Wetlands

The U.S. portion of the PPR, which is the focus of this report, comprises 36% of the total PPR. Although the PPR contains only about 5% of the wetlands in the conterminous U.S., the density of wetlands in the PPR is greater than in most regions of the United States. Accurate, current figures on wetland acreage for the entire region or individual states are not available. The best potential source—the National Wetlands Inventory of the U.S. Fish and Wildlife Service (USFWS)—is not yet complete (see sections 4.3 and 6.0). A secondary source—the county soil surveys of the Soil Conservation Service (SCS)—also does not contain complete regional coverage that would allow quantification of acreage of hydric soils. Estimates of statewide wetland acreage, and percent of land as wetland, have been made by the USFWS (1990a) as follows:

	Total Land Acreage in PPR	Statewide Total Wetland Acreage	Wetland Acreage in PPR
IA:	7,680,000	421,900 ( 1.2%)	35,000*
MN:	13,440,000	8,700,000 (16.2%)	1,635,000*
MT:	5,760,000	1,882,176 ( 2.0%)*	no data
ND:	24,960,000	2,490,000 ( 5.5%)	1,500,000*
SD:	17,280,000	1,780,000 ( 3.6%)	1,546,000
* Data on wetland acreage located specifically in the PPR is from Iowa Dept. Natural Resources (1990), Minnesota Pollution Control Agency (1990), and North Dakota Dept. Health and Consolidated Laboratories (1990b). PPR boundaries used by the states may differ from those used by this report, shown in Figure 1 of the summary report, thus affecting acreage estimates. The figure for total wetland acreage in Montana is from Montana Dept. Health and Environmental Sciences (1990).

 

Most of these figures do not include cropped wetlands (those that were tilled at the time of the survey). In the PPR, the proportion of wetlands has seldom been compiled on a watershed basis. Data compiled by U.S. Army Corps of Engineers St. Paul District (1989) for watersheds of the Red River of the North (Minnesota and North Dakota) show that "storage" exceeded 10% of watershed area in 9 of 44 watersheds, the largest figures being 51% and 35% of watershed area. Other portions of the PPR would be expected to have a generally higher incidence of storage and wetlands. "Storage" in the U.S. Army Corps of Engineers (1989) study was defined as the area of lakes, ponds, and swamps colored blue on standard U.S. Geological Survey (USGS) topographic maps; the author reports much inconsistency in this among adjoining maps, and wetland acreage is surely underestimated. A sample of 17 North Dakota PPH basins had watershed:basin ratios ranging from 2.5 to 15.3 (Borthwick 1988).

Wetland depressions (hereinafter called "basins," see Cowardin 1982) of the PPR have most often been classified according to the system of Stewart and Kantrud (1971). This classification will be used in this report, and includes the following classes:

Permanent: The center of the wetland basin contains surface water during all years.

Semipermanent: The center of the wetland basin contains surface water during most years.

Seasonal: The center of the wetland basin contains surface water through mid-summer during most years of normal precipitation.

Temporary: The center of the wetland basin contains surface water for less than about two weeks during most years of normal precipitation, although in some years, heavy summer precipitation can reflood these wetlands briefly.

"Saturated" wetlands (e.g., bogs, fens) also occur to a very limited extent in the region, but along with streams and rivers. Although they may be of considerable importance in the PPR for support of biodiversity, they are mostly non-depressional and are not emphasized in this report.

Wetland basins vary greatly in their salinity and in their annual change. During an average year, perhaps 26% of the temporary, 51% of the seasonal, and 72% of the semipermanent basins contain surface water (Cowardin et al. 1988). Over an idealized wet-dry cycle, the vegetation in semipermanent wetlands goes through four stages: dry marsh, regenerating marsh, degenerating marsh, and lake marsh, with associated consequences for functions and values (Weller 1981).

Although comprehensive data on the percentages of the basic basin types are lacking, some reasonable estimates have been made from statistical samples (e.g., Cowardin et al. 1988). For the PPR overall, temporary and seasonal basins are believed to be most numerous, whereas seasonal and semipermanent basins probably comprise the largest acreage (Stewart and Kantrud 1973). The generalized distribution of various types can be inferred from state-level geologic maps and topographic relief maps. Semipermanent basins tend to occur in areas of dead-ice and terminal (stagnation) moraines, and in areas of moderately to steeply rolling relief with few streams (Krapu and Duebbert 1989). Geographically, such areas occur largely in eastern South Dakota (in the Missouri-Prairie Coteau subregion) and along the western edge of the region in the Dakotas (Kantrud et al. 1989)(see Figure 1 in the summary report). Semipermanent basins also prevail in Iowa, because the greater intensity of agricultural activity there has resulted in tile-drainage of nearly all temporary and seasonal wetlands (pers. comm., A. van der Valk, Iowa St. Univ., Ames). Temporary and seasonal basins tend to occur in areas with ground moraines and lake plains, or are areas of gentle relief with scattered, poorly-developed channels (Krapu and Duebbert 1989). Geographically, such areas increase as a proportion of all basins in a westward direction. The annual variability in extent of surface water tends to increase in a southerly direction within the central and western parts of the PPR.

Sample-based estimates of wetland size distribution, by type, are available, but comprehensive data are lacking. In North Dakota, 92% of the basins in the PPR are reported to be <10 acres (Stewart and Kantrud 1973), and in South Dakota portions of the PPR, "most" of 2342 sampled wetlands were <1 acre in size (Cowardin et al. 1981). Semipermanent basins are typically the larger wetland basins. Large basins also occur in situations of collapsed glacial outwash (Bluemle 1977). Some of the widest, shallowest wetland basins occur in the lake plains subregions of north-central North Dakota and northeastern South Dakota. PPH basins within the PPR commonly occur at densities exceeding 20 per square mile and may exceed 100 per square mile in some undrained areas (Smith et al. 1964). Similar types of data are summarized by Hubbard and Linder (1986) and Hubbard (1988).

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3.0 Functions and Values

The following pages of the report discuss each potential* wetland function, along with its potentially* associated values, and potential* indicators of both the function and its values. This reflects the major components of wetland risk assessment, as discussed in section 1.3 of the Summary Report. Each function's discussion of indicators begins with a discussion of indicators appropriate at landscape and regional scales, and then addresses indicators that are best assessed and interpreted site-specifically.

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3.1 Maintenance of Runoff Volume

DESCRIPTION: The volume of surface water runoff (i.e., landscape input) is diminished when water evaporates or is transferred to long-term or permanent storage in aquifers. Some PPH wetlands reduce runoff partly by efficiently evaporating (and transpiring) water, or in some cases transferring it to underground storage. The detention function of wetlands is discussed separately, in section 3.2.

DOCUMENTATION OF FUNCTION OCCURRENCE: The role of PPH wetlands in regulating the volume of runoff, as opposed to its timing, is uncertain.

On one hand, some evidence suggests PPH wetlands dissipate water and thus reduce the volume of runoff. Relatively high rates of water loss from evapotranspiration and groundwater recharge have been documented during the mid-growing season, especially in basins smaller than about 5 acres (e.g., Allred et al. 1971, Millar 1971, Shjeflo 1968). Although evapotranspiration and deep recharge are slow processes, measurable in days, weeks, or even years (vs. runoff from storm events, which is measurable in hours), these processes can prime a landscape's soil absorptive capacity before storm events, so that runoff volume is reduced or prolonged following a storm. The long hydraulic detention times in PPH basins can result in large total losses of runoff to infiltration, recharge, and evapotranspiration, even though the rates of these processes are usually small relative to rates occurring in uplands.

On the other hand, other evidence suggests PPH wetlands conserve water and thus maintain the volume of runoff. Vegetation of PPH wetlands appears to reduce evaporation of open water at either end of the growing season, probably through reduction in wind velocity and shading (Eisenlohr 1966, Shjeflo 1968). If wetlands do, indeed, conserve a measurable amount of runoff in this manner, they might help maintain local water table levels. This might benefit crop production and wildlife production, because during the nongrowing season period, subsurface storage of water becomes a crucial determinant of crop yields the following growing season (Schroeder and Bauer 1984). "Conserved soil moisture" also is reported to be a better annual index of waterfowl numbers than are indices based on the extent of ponded water (Boyd 1981). Artificial drainage of wetlands can eliminate the ability of subsurface soil moisture to move upward in winter to replenish moisture in frozen soil above it (e.g., Malo 1975). In certain types of flat landscapes not characterized by recharge basins, this moisture might contribute measurably to sustaining crops and wildlife habitat the following growing season (Hubbard and Linder 1986). However, this increase in antecedent springtime soil moisture also could theoretically aggravate regional flooding. Regardless of whether PPH wetlands are dissipators or conservers of runoff volume, it still remains uncertain whether they dissipate or conserve enough volume of runoff (at least during the growing season) to cumulatively exert a detectable and socially important effect some distance downslope.

ASSOCIATED POTENTIAL VALUES: Changing the volume of runoff can potentially have implications for several values. If wetlands predominantly dissipate runoff, via evapotranspiration or deep recharge (i.e., recharge that is not readily released to surface waters downslope), then landowners with property in low areas surrounding each basin, as well as in downstream areas, could theoretically benefit from lower water levels during severe runoff events. On the other hand, this reduction in runoff could potentially be detrimental to a goal of maintaining adequate water supplies for livestock and crops during summer months. If, as discussed above, PPH wetlands have the opposite effect (i.e., they conserve water on the landscape), then the resultant increase in runoff volume could aggravate flood losses but could also help sustain crops, livestock, and wildlife.

DOCUMENTATION OF VALUE: Losses of property as a result of flooding are of great concern in the PPR. There are over 200,000 households located within the 6209 square miles of 100-year floodplains in the PPR (FEMA data files). The largest concentration of floodable residences is in North Dakota, and flooding there damages up to $100 million per year worth of crops, roads, and property (ND Dept. Health and Consolidated Laboratories 1990b). In the Devils Lake area, Leitch and Scott (1984) assumed that for each acre of wetland restored, annual flood damages to transportation facilities would be reduced by $3.17, and flooding of crops would be reduced on 1.11 acres. Also, they estimated the benefits of making an acre of agricultural land flood-free would be $14.83. Including recreation values, total income generated by an undrained wetland for its flood control functions was estimated to be about $29/acre.

It is apparent that many PPR citizens (e.g., 79% in a survey by Grosz and Leitch 1990) value wetlands for their ability to reduce downstream flood losses. No studies have established a causal link between loss of runoff volume as a result of wetland evapotranspiration or recharge, and actual decreases in economic losses due to flooding.

TEMPORAL EFFECTS: Hydrologic inputs to PPR wetlands vary greatly among years. Throughout most of the PPR, landscape inputs (i.e., runoff volume) are greatest during early spring. Comparing two years (or locations) with equal amounts of total annual precipitation, the volume of spring runoff is likely to be much greater for the year (or location) in which, during the preceding autumn, a major rainstorm was followed by freezing, which then was followed by a snow cover that persisted through the winter. Conversely, less spring moisture is available during years (or locations) where winters are mild and lack continuous insulating snow cover. Springtime weather conditions also affect runoff. At years or locations where frozen soils persist late into the spring, wetland capacity for detaining runoff inputs and permitting their infiltration may be reduced (a point which, at least for clay hydric soils, is debatable; pers. comm., J.L. Richardson, North Dakota St. Univ., Fargo). Any of the temporal effects just described can overwhelm the effects of spatial characteristics described in the following sections.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs to wetlands can be represented by indicators of runoff (such as watershed size, springtime rainfall, snow depth, and total annual precipitation) and indicators of groundwater discharge (such as subregion within the PPR and geology). Runoff inputs may be represented more accurately if the actual water yield to wetlands is used to represent landscape input. Water yield is precipitation, minus the evapotranspirative and other losses that occur in uplands before runoff from precipitation can reach wetlands. Evapotranspirative losses in uplands are measurably influenced in some situations (e.g., Deibert et al. 1986, Sophocleous and McAllister 1987) but not all (e.g., Khakural 1988) by crop type and tillage practices, as well as by grazing regimes (e.g., Hofmann et al. 1983).

Runoff inputs to wetlands are also affected by geomorphic characteristics of the watershed area upslope of the wetlands. Runoff entering wetlands is likely to be greater, and storm hydrographs sharper, where upslope watersheds are elongate in shape, relatively steep, comprised of clayey or similar less-permeable soils, and are channelized or artificially drained. Conversely, as a watershed becomes more rounded in shape and dominated by generally flat permeable soils that are not artificially drained, runoff inputs to downslope wetlands diminish, having the same effect as a decrease in watershed area. However, if wetlands are mainly of a type whose water budget is dominated by groundwater (e.g., semipermanent basins), then conversion of upland grasslands to tillage can cause water tables to drop and wetlands to dry up (pers. comm., J.L. Richardson, North Dakota St. Univ., Fargo).

In the PPR, available data suggest that the potential hydrologic input to wetlands (i.e., precipitation surplus plus groundwater) is greatest in the southern and eastern part of the region. The actual input, as noted above, depends on factors such as crop type, soil type, slope, soil permeability, watershed shape, and extent of upstream channelization/drainage. Within the PPR, soil permeability (as predicted by soil type) probably shows few geographic patterns, as the ratio of till to outwash is about the same in Iowa as in North Dakota (pers. comm., J.L. Richardson, North Dakota St. Univ., Fargo). Also, the form of precipitation (snow vs. rain) measurably affects the degree to which it is stored or evaporated by the landscape. Citing evidence from a study in northern Ontario, Winter and Woo (1990) state that wetlands delay streamflow response to rainfall but not to snowmelt; it is not apparent that this is the case in the PPR (pers. comm., J.L. Richardson, North Dakota St. Univ., Fargo). In the northwestern part of the PPR, over 25% of the annual precipitation occurs as snow, whereas less than 12% occurs as snow in southeastern South Dakota (Winter and Woo 1990).

Capacity for losing water via evaporation in wetlands depends partly on landscape factors. As is apparent in the conceptual model presented in a related report (Adamus 1992b), the effects of wind velocity and air temperature—which measurably influence evapotranspiration—can be indicated by landscape/regional characteristics such as latitude, longitude, elevation, and general topographic relief. Regional geologic patterns can also determine characteristics that operate within wetlands and are conducive to evaporative loss of runoff, as described in (B) below. The relative extent of runoff lost to recharge can also be estimated from geologic and topographic patterns; in watersheds having a large proportion of wetland basins located near regional drainage divides, these basins are often groundwater recharge areas (Sloan 1972). Similarly, the contagion characteristics of the wetland spatial distribution (e.g., distribution of acreage in wetlands is dispersed vs. clumped) are likely to affect potential for evaporative loss. As expressed by Hubbard (1988):

Complexes should provide for better groundwater recharge and better water retention than similar acreages of wetlands in a large, single basin...small [temporary, seasonal] wetlands will regenerate their storage capacity and be ready to store the next runoff event much more quickly than large wetlands.

In addition, climatic patterns influence and indicate relative losses from evapotranspiration. Subregions within the PPR having a larger portion of their hydrologic inputs occurring during summertime (vs. spring) may lose considerable water via transpiration, assuming water tables do not drop below the root zone for extended periods. Wetlands located in subregions with a relatively greater portion of runoff occurring in summertime probably are more capable of reducing runoff volume. However, the differences among PPH wetlands in their capacity to reduce runoff volume may be attributable more to differences in recharge capacity than to differences in evapotranspiration (Sloan 1972). Indicators of recharge are discussed in section 3.3 (page B-15).

B. Site-level (Within-wetland) Indicators of Function

Wetlands of all types may reduce runoff volume, but the magnitude of this function is partly indicated by wetland water regime (i.e., as used in this report, meaning the basin hydroperiod or permanence type—temporary, seasonal, semipermanent, or permanent). Water regime in turn can be indicated generally by plant species, soil profile, landscape geology, relief, and geographic position. Drier basins, such as temporary and seasonal types, are often densely vegetated. Until the water table drops below the root zone in early summer, the vegetation in these wetlands can efficiently reduce runoff volume by evapotranspirating water and directing some runoff into underlying groundwater storage (i.e., recharge). However, wetland basins that are relatively deep (e.g., semipermanent) tend to lose a smaller portion of their water to evaporation, because a smaller portion of the water volume is directly exposed to wind and sunlight. Moreover, the peripheral vegetation of semipermanent basins, although it transpires water throughout the growing season, shields much of the water surface from wind and sun and thus reduces losses due to evaporation (Eisenlohr 1966, Shjeflo 1968).

Another site-specific indicator of wetland capacity for reducing runoff volume is basin size and shape. Wetland basins that are small, with convoluted and/or gently sloping shorelines, are likely to have a larger proportion of their area as shallows, i.e., smaller depth-to-volume ratio (Millar 1971). Such conditions lead to warmer water temperatures and a larger percent cover of vegetation, which in turn can lead to greater rates of water loss via summertime evapotranspiration (Crow and Ree 1964), at least in temporary and seasonal basins. Small, convoluted basins may be optimal for loss of surface water via recharge.

A third site-specific indicator is the ratio of emergent or woody vegetation to open water. Because during the growing season an equal or greater volume of runoff can be lost via vegetative transpiration than by unvegetated open water, densely vegetated wetlands, if temporary or seasonal, may have a greater capacity for reducing runoff volume. However, during months when vegetation is dormant, evaporative water losses may be less where conditions allow a continuous ground cover or dense litter layer to protect wet sediments from exposure to wind and sun (e.g., Rickerl and Smolik 1990).

Fourth, the vegetation type probably influences evapotranspirative losses. Wetland basins dominated by submersed, shallow-rooted, or fine-leaved plants (as indicated partly by the successional status of the basin) generally experience less evapotranspirative loss of water than those dominated by broad-leaved, deep-rooted species.

Finally, water chemistry both influences and indicates potential for reduction in runoff volume. Loss of water can be less in permanently saline basins than in freshwater basins, due to reduced evaporation (e.g., Whiting 1984), and reduced transpiration due to impedance of osmotic tension in plants. Moreover, saline wetlands typically represent groundwater discharge situations, whereas loss of runoff volume is enhanced in situations of groundwater recharge. In the PPR, saline basins mostly occur in topographically low positions on glacial outwash in western and northern areas.

Each of the above five site-specific indicators can be manifested on a regional level as well. That is, because these indicators correlate roughly with regional geologic and climate patterns, in a general sense they may show spatial trends within a region.

POSSIBLE INDICATORS OF VALUES:

A. Regional and Landscape-level Indicators of Value

Economic value of the runoff volume reduction function can be indicated partly by number of floodplain properties, the market value of these properties, and their position and proximity relative to those wetland complexes that cumulatively have the greatest capacity for reducing runoff volume. Also, the season of flooding may partly determine the value of runoff control. Spring floods can delay planting, summer floods tending to damage maturing crops as well as dwellings, and autumn floods can delay harvest. Ecological values of maintaining runoff volume also depend on the season of flooding and the position and proximity of wetlands to areas of greatest intrinsic ecological importance.

B. Site-level (Within-wetland) Indicators of Value

Local economic values of this function for water supply are indicated partly by the density of livestock, the extent of cropped wetlands, and the drought vulnerability of the most widely grown crops. The value of wet areas to livestock and crops increases in proportion to the severity of drought occurring during a particular year.

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3.2 Maintenance of Runoff Timing

DESCRIPTION: Surface water runoff (i.e., landscape input) is delayed in its down gradient journey when water is detained (i.e., increased functional capacity) in wetland basins. PPH wetlands facilitate detention of runoff because they mostly lack well-defined surface water outlets, and interbasin subsurface flows are very slow (e.g., 0.05 meters/day, Tipton et al. 1972). When runoff is detained in a regionally dispersed manner by PPH basins, pulses of water that eventually enter downstream areas in most cases are staggered (desynchronized). This broadens the storm hydrograph and reduces streamflow peaks.

DOCUMENTATION OF FUNCTION OCCURRENCE: Prairie pothole wetlands can reduce peak flows occurring in channels downslope, because they have considerable capacity for detaining runoff (e.g., Hubbard and Linder 1986, Ludden et al. 1983). Detention of precipitation occurs largely because most PPH basins lack surface water outlets, and also because robust vegetation that occurs in some wetlands is capable of effectively intercepting and storing drifting snow (Sloan 1972), allowing water to infiltrate in depressions rather than moving downslope during melt periods.

Several empirical landscape studies and landscape simulation studies in the PPR have attempted to demonstrate diminuation of downstream peak flows as a result of the presence of many wetlands, or aggravation of peak flows due to artificial drainage of wetlands (Brun et al. 1981, Moore and Larson 1979, U.S. Army Corps of Engineers St. Paul District 1989). These studies have not differentiated whether any peak flow diminuation is due to volume reductions, as just discussed in section 3.1, or to runoff timing alterations (i.e., desynchronization) associated with wetlands, as discussed below. Artificial drainage of wetlands was implicated by Sidle (1983) in the increase of economically damaging floods of the James River in North Dakota. In flat landscapes of Florida, watersheds where most of the wetlands have been drained or channelized have detention times of only 2.2 days, vs. 4.5 days for undrained, unchannelized watersheds (Bedient et al. 1976). However, as discussed in section 4.2, wetland drainage does not inevitably cause increased flood stages downstream. In some cases, alteration of main-channel wetlands may do more than artificial drainage of isolated wetlands to aggravate flooding (Moore and Larson 1979, Ogawa and Male 1983), and in some instances artificial drainage could theoretically ameliorate peak flows by decreasing antecedent moisture conditions prior to storm events (Hill 1976).

Nonetheless, PPH basins have considerable potential for storing runoff. In the large wetland complexes of Salyer National Wildlife Refuge of North Dakota, undrained, mostly unconnected wetlands were reported to be storing 58% of the inflow, plus all local runoff (Malcolm 1979). In the Devils Lake Basin of North Dakota, wetland basins store between 41% of the runoff from severe (100-year) storm events, and up to 72% of the runoff from smaller events (Ludden et al. 1983). In the Pembina River Basin of North Dakota, each undrained wetland can store up to one acre-foot of runoff (Kloet 1971), a figure also supported by the data of Hubbard and Linder (1986) from 213 wetlands in northeastern South Dakota. Moreover, that study noted that most wetlands were not filled to capacity at the time of measurement.

ASSOCIATED POTENTIAL VALUES: At a landscape level, detaining runoff in wetland basins has implications for several values. By flattening the storm hydrograph, some wetlands help reduce flooding in downslope areas. This can potentially reduce economic damage to property, as quantified in section 3.1. The effects on ecological resources of releasing runoff more gradually can be either adverse (e.g., reduced frequency of scouring flows needed to maintain habitat of some species) or positive (e.g., increased annual minimum flows, improved purification of runoff due to lengthened processing time).

On a site-specific level, by detaining water later into the growing season, wetlands can provide water for livestock, soil moisture for surrounding croplands, and habitat for aquatic wildlife, particularly in areas of glacial till (Hubbard 1988). However, this renders some temporary wetlands unsuitable for cultivation.

DOCUMENTATION OF VALUE: On a landscape level, damages to property as a result of flooding are of great concern in the PPR. On a local level, water supplies for livestock and crops are also a great concern, particularly in the western part of the region. Maintaining soil moisture in that area is a major public concern. It is apparent that many PPR citizens (e.g., 79% in survey by Grosz and Leitch 1990) think that wetlands are important for their ability to reduce downstream flood losses. Although some regional studies have supported a link between wetlands and peak flow reduction, no studies have established causal links between shortened detention times, increased flow synchronization, increased peak flows, and actual economic losses due to flooding.

TEMPORAL EFFECTS: Hydrologic inputs to PPR wetlands vary greatly among years. Throughout most of the PPR, landscape inputs (i.e., runoff volume) are greatest during early spring. Comparing two years (or locations) with equal amounts of total annual precipitation, the volume of spring runoff is likely to be much greater for the year (or location) in which, during the preceding autumn, a major rainstorm was followed by freezing, which then was followed by a snow cover that persisted through the winter. Conversely, less spring moisture is available during years (or locations) where winters are mild and lack continuous insulating snow cover. Springtime weather conditions also affect runoff. At years or locations where frozen soils persist late into the spring, wetland capacity for detaining runoff inputs and permitting their infiltration may be reduced (a point which, at least for clay hydric soils, is debatable; pers. comm., J.L. Richardson, North Dakota St. Univ., Fargo). Any of the temporal effects just described can overwhelm the effects of spatial characteristics described in the following sections.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Landscape inputs to wetlands can be represented by indicators of precipitation and water yield or their surrogates (e.g., watershed shape, soil type, slope, and channelization extent) as described on page B-7. Also, differences in rainfall or snowmelt intensity exist within the PPR; subregions with shorter, more intense rainfall or snowmelt may yield proportionally more runoff to wetlands.

Capacity for detaining runoff in wetlands can be indicated by within-wetland factors, but the important consequences of detention—i.e., whether detention will result in synchronization or desynchronization of downstream flows—depend on between-wetland landscape factors, specifically, the spatial arrangement of wetlands relative to receiving channels, and the diversity of their sizes (storage volumes). Science has not advanced to the point where it is possible to specify all combinations of wetland spatial position, size, and type that are optimal for desynchronizing flows.

B. Site-level (Within-wetland) Indicators of Function

Review of the conceptual model suggests several site-specific indicators that determine or correlate with hydrologic detention. Perhaps the most important indicator of the ability of a wetland to detain flow is the frequency and magnitude of connection to other basins. This is indicated partly by the height of the rim separating basins in a complex, as well as the subsurface flow patterns and the extent of artificial drainage connections. Basins that remain hydrologically isolated, regardless of the size of the runoff event, are essentially "noncontributing areas." These basins can have the largest influence on runoff timing because all runoff that reaches them is retained. The ratio of basin size to watershed (catchment) size is also important. Provided their water budgets are not dominated by groundwater discharge, basins that are large relative to the watershed (catchment) area are likely to be effective in detaining runoff. A third indicator of the magnitude of runoff detention in wetland basins is wetland water regime. Water regime can be indicated generally by plant species, soil profile, landscape geology, relief, and geographic position. Drier wetland basins, such as temporary and seasonal types, probably have a larger proportion of their basin available for storage and infiltration of spring runoff. Infiltration occurs because (a) the topographic position of most temporary and seasonal basins enhances their ability to recharge groundwater, (b) thawing of sediments occurs earlier in the season than in semipermanent and permanent basins, thus making interpore space available in wetland soils for water storage, and (c) when frozen, the clayey soils that typify many of these basins contain macropores which facilitate loss of runoff to ground water recharge. In contrast, semipermanent and permanent basins are often dominated by the more sustained inflows from groundwater discharge, leaving little space available for subsurface storage of runoff. Wetland soil type may also partially indicate wetland capacity to desynchronize inputs. Hydric soils which during years of snow cover do not freeze deeply, or which thaw earlier in the spring (e.g., blackish mineral soils), allow runoff to infiltrate during the weeks when most runoff occurs. Finally, wetland capacity for detaining runoff is suggested by wetland size and shape. As noted earlier, smaller wetlands and wetland basins with convoluted shorelines are likely to have more gently sloping shorelines and a larger proportion of their area as shallows. Such zones are more likely than deepwater to support recharge and infiltration, which in turn makes space available for storing runoff.

POSSIBLE INDICATORS OF VALUES:

A. Regional and Landscape-level Indicators of Value:

Economic value of the hydrologic detention function can be indicated partly by the season of flooding (summer floods tending to damage crops as well as dwellings), number of floodplain properties, the market value of these properties, and their position and proximity relative to wetlands that cumulatively have the greatest capacity for reducing runoff volume. Ecological values of maintaining runoff timing also depend on the season of flooding and the position and proximity of wetlands to areas of greatest intrinsic ecological importance.

B. Site-level (Within-wetland) Indicators of Value:

The values of this function are expressed primarily at the landscape scale. See the discussion above.

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3.3 Groundwater Recharge

DESCRIPTION: Surface water runoff (i.e., landscape input), when delayed in storage areas during its downgradient journey, can move downward into underlying aquifers, recharging the groundwater.

DOCUMENTATION OF FUNCTION OCCURRENCE: Recharge has been documented in several PPH basins, particularly those with temporary or seasonal water regimes in the western portion of the PPR (Sloan 1972, Eisenlohr et al. 1972). Infiltration rates of up to 0.5 foot per day have been reported (Sloan 1972). In a Minnesota part of the PPR, wetlands recharge aquifers probably by applying a relatively constant hydraulic head that forces water into underlying unweathered till, Wall et al. (1989). It is not apparent that the presence of wetland vegetation or soils enhances recharge; rather, such wetland basins just happen to occur in topographic situations that are intrinsically supportive of recharge.

ASSOCIATED POTENTIAL VALUES: Primarily at a landscape level, recharge from wetlands can play a role in sustaining groundwater used domestically and for pastured livestock. Because loss of recharge functions can result in greater well-drilling and pumping costs, and possibly greater agricultural expenses, loss of wetlands might aggravate such expenses, if indeed the role of wetlands is large relative to that of other landscape components that contribute to groundwater (e.g., Hubbard and Linder 1986). Recharge from wetlands also sustains groundwater that discharges to other wetlands (e.g., semipermanent and permanent basins), thus potentially supporting their ecological, water purification, and economic values (Lissey 1971; Richardson and Arndt 1989). Seasonally fluctuating water levels in wetlands, as controlled largely by natural rates of recharge, are important to maintaining wetland productivity. On the other hand, loss of recharge wetlands can cause remaining wetlands and ditches to become groundwater discharge areas which increase the salinity of soils, rendering them unsuitable for cultivation (Arndt and Richardson 1986, Hendry and Buckland 1990).

DOCUMENTATION OF VALUE: Groundwater is a major source of consumed water in the PPR, contributing from 39% (in North Dakota) to 83% (in Iowa) of the water used in 1980. Use specifically for domestic human consumption varies from 34% (in Iowa and Montana) to 52% statewide in Minnesota. Use for livestock and irrigation varies from 30% in Iowa and Minnesota to 86% in Montana (Moody et al. 1986). The U.S. Geological Survey considers future groundwater development in North Dakota to be limited partly by "insufficient aquifer recharge" (Paulson 1983). Where PPH basins recharge groundwater, it helps ensure adequate water supplies for livestock and crops. This is a particularly great concern in the western part of the region.

However, the role of wetlands specifically, as opposed to other landscape elements, in supporting these values is debated. The geographic extent to which recharge via PPH wetlands enters groundwater tapped by wells, as opposed to recharging zones higher or lower than those used domestically, is unknown. Although some PPH basins are known to recharge groundwater, there appear to be no data that link recharge rates specifically from wetlands to actual use rates and economic values of the users. However, if indeed recharge in the western and flatter parts of the PPR occurs only rarely in uplands (Freeze and Banner 1970, Lissey 1971, Malo 1975), the value of wetlands in that subregion can be assumed directly.

TEMPORAL EFFECTS: Hydrologic inputs to PPR wetlands vary greatly among years. Throughout most of the PPR, landscape inputs (i.e., runoff volume) are greatest during early spring. Comparing two years (or locations) with equal amounts of total annual precipitation, the volume of runoff available for springtime recharge of groundwater is likely to be much greater for the year (or location) in which, during the preceding autumn, a major rainstorm was followed by freezing, which then was followed by a snow cover that persisted through the winter. Conversely, less spring moisture is available during years (or locations) where winters are mild and lack continuous insulating snow cover. Springtime weather conditions also affect water available for recharge. At years or locations where frozen soils persist late into the spring, wetland capacity for detaining runoff and permitting its infiltration as recharge may be reduced (a point which, at least for clay hydric soils, is debatable; pers. comm., J.L. Richardson, North Dakota St. Univ., Fargo). Any of the temporal effects just described can overwhelm the effects of spatial characteristics described in the following sections.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs to wetlands that support groundwater recharge can be represented by indicators of precipitation and water yield, including watershed shape, slope, crop type, soil type, and artificial drainage, as described on page B-7.

Capacity for recharging groundwater is indicated primarily by regional and landscape factors. In particular, climate, as represented by regional position within the PPR, is a primary indicator of recharge. Undrained basins in the western and northern parts of the PPR (including the Lake Agassiz plain) contain predominantly recharge wetlands, whereas basins in Iowa, southern Minnesota, and parts of eastern South Dakota mostly contain discharge or flow-through wetlands (pers. comm., J.L. Richardson, North Dakota St. Univ., Fargo).

Topographic position is another landscape indicator of recharge. At least in the eastern portion of the PPR, wetland complexes located near major regional divides often recharge groundwater (Swanson et al. 1988). Thus, when local terrain is homogeneously flat or slopes sharply away from a wetland (i.e., great local relief and large regional slope, cf. Winter 1977), the water table often slopes away as well, resulting in a hydraulic gradient favorable for movement of water into the groundwater system. In contrast, wetlands located at the bottom of a relatively steep slope are often areas of groundwater discharge, because the water table crops out at the surface near the deflection point of the slope.

Also, the contagion characteristics of the wetland spatial distribution (e.g., distribution of acreage in wetlands is dispersed vs. clumped) are likely to affect potential for recharge. Wetland acreage occurring as complexes rather than as single large wetlands should provide for better groundwater recharge (Hubbard 1988) because such landscape are more likely to indicate diversified potentiometric gradients, which are more conducive to recharge.

B. Site-level (Within-wetland) Indicators of Function

Wetland water regime is a prominent indicator of groundwater recharge. Water regime can be indicated generally by plant species, soil profile, landscape geology, relief, and geographic position. Drier wetland basins, such as temporary and some seasonal basins, have been documented as being recharge areas in much of the northwestern PPR (Lissey 1971, Loken 1991), particularly where they occur as part of a wetland complex. In contrast, the larger semipermanent and permanent basins are often dominated by groundwater discharge or are flow-through systems. However, in the western PPR (Richardson et al. 1991) or in landscapes with water tables recently lowered by extensive artificial drainage, semipermanent basins probably assume increased importance for recharging groundwater. Another site-specific indicator (but not determinant) of the ability of a wetland to recharge groundwater is the ratio of wetland size to watershed size. Large basins that have very small watersheds, or whose watersheds contain many other basins, are likely to be groundwater discharge areas. Conversely, small basins (at least in the western PPR) tend to be groundwater recharge areas (Loken 1991).

Water and soil chemistry can indicate, but not determine, the direction of groundwater exchange where the magnitude of exchange is great. Basins that have higher specific conductivity, pH, alkalinity, hardness, magnesium, sulfates, and total dissolved solids; and lower concentrations of calcium and total bicarbonates, are likely to be dominated by groundwater discharge, not recharge (Sloan 1972, Arndt and Richardson 1986). In the PPR, such basins occur mostly in western and northern areas. In soil profiles, recharge is suggested by presence of only small quantities of gypsum (CaSO4 . 2H2O) and absence of calcite (CaCO3). Recharge basin soils are generally nonsaline, noncalcareous, with deep sola (depth of soil development). Also, they usually have well-developed eluvial (highly leached) and argillic (clay enriched) horizons (Miller et al. 1985, Hubbard 1988). They would generally be classified as Argiaquols.

The texture of soil and subsurface materials is often a major determinant of groundwater exchange, although not of recharge or discharge specifically. In theory, groundwater should move most rapidly through coarse sands or gravels and successively slower through fibric peats, deep sapric peats, and clays. In some cases, this ranking is easily overridden by topographic factors and the presence of rooted wetland plants. Macropores created by plants enhance infiltration into underlying aquifers through shallow clay layers or compacted bottom sediments (Eisenlohr 1966). On the other hand, the surface layer of organic matter formed by these plants progressively accumulates (unless removed by waves, currents, wind, animals, fire, sulfate reduction, or decomposition). In doing so, it might progressively isolate or seal a wetland from groundwater systems.

The above site-specific indicators can be manifested on a regional level as well, because they are partly a reflection of regional geologic patterns.

POSSIBLE INDICATORS OF VALUES: Economic values of this function are partly indicated by the density of livestock or humans dependent on wells, the depth of wells relative to wetland inputs to the groundwater, and the drought vulnerability of the subregion. Ecological values of recharging groundwater also depend on the local drought vulnerability and the proximity of recharging wetlands to areas of greatest intrinsic ecological importance.

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3.4 Sediment Retention

DESCRIPTION: Sediment retention is the process by which sediment borne by overland runoff (e.g., sheet flow) and incoming surface waters is deposited (sedimentation) and retained (stabilization). PPH wetlands retain sediments by (a) trapping them in basins closed to surface outflow, (b) anchoring sediments with plant roots, and (c) intercepting and reducing erosional energies (e.g., wind, waves). To be stabilized over long periods of time, sediment entering wetlands must either be deposited in deep permanent waters, or be stabilized by encrusting precipitates or roots of wetland vegetation. High winds typical of the region can remove a small portion of the sediment from large (especially saline) basins during drought periods.

DOCUMENTATION OF FUNCTION OCCURRENCE: PPH wetlands retain virtually all sediment which enters them because they lack surface water outlets. Sediment retention, perhaps because it seems so obviously present in PPH basins, has seldom been documented in the PPR. One study of a series of South Dakota potholes reported accretion rates of between 430 and 800 g/m2/year (Martin and Hartman 1987a). An open lake system in South Dakota removed 43, 33, and 100% of the suspended solids from tributary input during three successive years (SDRCWP 1990), and two open wetland complexes in North Dakota removed 13 and 28% of the tributary suspended sediment, vs. a 2000% increase in sediment exported from a ditch-drained wetland complex (Malcolm 1979). Sedimentation rates might also be inferred from data collected by Callender 1969, Frickel 1972, Churchill et al. 1975, and Okland 1978.

ASSOCIATED POTENTIAL VALUES: Excess sediment deposition impairs fish production, biodiversity, and flood storage and conveyance. For example, little of the sediment entering the James River in South Dakota is transported downriver (Benson 1988). At a landscape level, wetlands are among the most effective landscape features for mitigating sedimentation problems. They do so by intercepting and retaining eroded sediment before it reaches larger, more permanent waterbodies. Thus, protection and enhancement of this function in wetlands could help maintain and restore public uses of downslope lakes and rivers, such as fish production, biodiversity, and flood conveyance. However, on a site-specific level, retaining sediments in PPH wetlands can adversely impact the ecological and hydrologic values of the wetlands themselves. Thus, the potential landscape-level values of some wetlands to effectively treat nonpoint nutrient runoff must be balanced against the likelihood that site-specific (and eventually, landscape) ecological values may compromised.

DOCUMENTATION OF VALUE: All of the states of the PPR, in their annual 305b water quality report, declare sedimentation the most important threat to public uses of surface waters. In South Dakota, 26% of the assessed river miles statewide have water quality that is so degraded it does not support any of their designated uses, and 44% show partial support; about 12% are not fishable due to water quality problems. Some 20% of the lakes do not meet their designated uses (SD Dept. Water and Natural Resources 1990). In North Dakota, 25% of the assessed river miles, and 36% of the lake acres, have water quality that is so degraded it does not fully support all of their designated uses (ND Dept. Health and Consolidated Laboratories 1990b).

Also, public opinion surveys indicate that many PPR citizens (e.g., 90% in survey by Grosz and Leitch 1990) think that wetlands are important for their ability to purify water. Despite reductions of erosion in some counties as a result of implementing the federal Conservation Reserve Program (CRP), sedimentation problems may be increasing, as a trend of converting grasslands (range, hay, pasture) to row crops continues (Kantrud et al. 1989). Although PPH basins are known to retain sediment, no attempts have been made to link quantitative estimates of sedimentation rates specifically from wetlands to actual use rates and economic values of the users of downslope water that is benefitted.

TEMPORAL EFFECTS: Throughout most of the PPR, runoff inputs to wetland basins are greatest during early spring. Often, this early spring runoff bears the largest portion of sediment entering wetlands. Much of the sediment originates from lands tilled the previous autumn. Comparing two years (or locations) with equal amounts of total annual precipitation, the transport of sediment is likely to be much greater for the year (or location) in which, during the preceding autumn, a major rainstorm was followed by freezing, which then was followed by a snow cover that persisted through the winter and was followed by a warm early spring. Under such conditions, the large spring thaw is likely to mobilize considerable sediment. Conversely, springtime sediment runoff may be less during years (or locations) where winters are mild and lack continuous insulating snow cover. In other instances, springtime conditions affect sediment inputs to a greater degree than conditions during the preceding autumn or winter. Although PPH basins retain nearly all sediment that reaches them, their cumulative removal efficiency may be greatest during years when the runoff occurs later in the season, or during years when summer (vs. early spring) storms contribute a larger portion of the annual sediment input to wetlands. That is because as the season progresses, wetland soils thaw and vegetation develops more fully, thus increasing trapping efficiency. Any of the temporal effects just described can overwhelm the effects of spatial characteristics described in the following sections.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs of sediment to wetlands can be represented by indicators of the amount, intensity (Young and Wiersma 1973), and timing of precipitation and water yield, as described on page B-7. Other factors indicate potential sediment inputs to wetlands resulting both from erosion of upland areas, and from the lack of interception of runoff-borne sediment before it reaches wetlands. These include proximity to wetlands of land covers, soil types, and slope conditions that are considered highly erodible or poorly interceptive (i.e., weak buffers). In watersheds with erosion-resistant soils, wetlands surrounded by wide buffer strips of dense vegetation usually receive the least inputs of sediment, unless tile(vs. ditch) drainage is prevalent. Also, the type of grazing regime (Hofmann et al. 1983) and crop management practices (e.g., conservation tillage, contour plowing) determine both the magnitude of sediment runoff (Young and Wiersma 1973) and lack of interception before reaching wetlands. The Soil Conservation Service office in each county of the PPR has integrated erosion factors to identify and list soil mapping units considered to be Highly Erodible Land (HEL). Where these HEL units are based on vulnerability to non-wind related erosion (as predicted mainly by slope), these data can be used to assess relative sedimentation risks of various wetland complexes and other water bodies. Artificial drainage of individual wetlands normally causes initially large exports of sediment to downslope areas. In localized situations, drainage might occasionally reduce long-term sediment export to downslope areas, in two ways. First, at least in watersheds where drainage is accomplished by use of tile rather than ditches, artificial drainage can cause a large portion of the precipitation to infiltrate rather than move downslope as overland runoff and carry soil with it. Second, wetland drainage could theoretically reduce landscape-level sediment inputs if drainage results in farm operators planting crops in drained lands rather than in highly erodible uplands, which under the CRP are frequently returned to a permanent cover such as hay (Danielson and Leitch 1986).

Capacity for retaining sediment is related, on a landscape scale, to the spatial arrangement of wetland complexes relative to inputting land uses. Science has not advanced to the point where it is possible to specify all combinations of wetland spatial position, size, and type that are optimal for retaining sediment at a landscape scale. Nonetheless, the contagion characteristics of the wetland spatial distribution (e.g., proportion of subregional wetland acreage that is dispersed vs. clumped) are likely to affect both the potential extent of sediment input to wetlands and wetland capacity to retain it. Wetland acreage occurring as scattered complexes rather than as single large wetlands should be more likely to be located near and downslope from sediment sources, partly because complexes tend to occur in hummocky terrain. Although in hummocky terrain the predominant land cover (rangeland) is usually less supportive of erosion than is cropland, where soil tillage does occur, erosion may be great. Moreover, the larger shoreline-to-area (or smaller depth-to-volume) ratio associated with a more dispersed wetland acreage implies that a larger portion of sediment-bearing runoff will enter wetlands via shoreline zones of dense vegetation. Although such vegetation has minimal effect on long-term sedimentation of a basin, it may reduce the amount of sediment in suspension and thus benefit some other wetland functions (e.g.) Dieter 1990).

B. Site-level (Within-wetland) Indicators of Function

Most of the site-specific indicators described for the function, Maintenance of Runoff Timing (section 3.2), are suitable as indicators of sediment retention, because retention is closely linked to hydraulic retention or settling time. Again, the frequency and magnitude of connection to other basins and the ratio of wetland size to watershed size are important indicators of sediment retention capacity. Also, wetland water regime influences sediment retention. Water regime can be indicated generally by plant species, soil profile, landscape geology, relief, and geographic position. The drier wetland basins, such as temporary and seasonal types, probably have a larger proportion of their basin available for storage and infiltration of spring runoff, with concommitant deposition of suspended sediment. However, large temporary and seasonal basins that are saline, burned, or otherwise sparsely vegetated may become minor sources rather than sinks for sediments during windy or overflow conditions, whereas sediments deposited in semipermanent and permanent basins are usually protected from wind erosion by overlying waters.

Where concern focuses on turbidity within the open water portions of an individual wetland or lake basin, other indicators are appropriate. Turbidity may be caused by either inorganic (e.g., clay) or organic (e.g., plankton) in the open water area. Turbidity problems are likely to be aggravated if the basin is tilled or re-ditched during seasonal dryout periods or drought years; has steep banks of erodible soil; is excessively enriched; contains animals that disturb shallow bottom sediments (e.g., carp, livestock, humans in boats); has recently lost protective shoreline vegetation as a result of overgrazing, burning, or salination; or is shallow and exposed to strong winds (Carper and Bachmann 1984). On the other hand, indicators of diminished problems with open water turbidity include dense vegetative cover (both submerged aquatics and emergents) close to the path of incoming sediment during the season of greatest sediment runoff (e.g., Dieter 1990); high densities of filter-feeding zooplankton; and high salinity or specific conductance (approximately greater than 500 mS/cm, Akhurst and Breen 1988). Specific conductance is generally greater in basins in glacial outwash areas than in glacial till (Swanson et al. 1988). Conductance fluctuates greatly from year to year within PPH wetland basins; in-basin sediment deposition rates might be expected to fluctuate accordingly.

POSSIBLE INDICATORS OF VALUES: The values of retaining sediment in wetlands can be indicated by the ecological, commercial, and recreational importance of the receiving waters relative to those of the retaining wetlands (Ribaudo 1986), and the vulnerability as of receiving waters as judged by factors described above under the discussion of indicators of inputs. All of the PPR states, in their 305b water quality reports, have listed water bodies impacted by nonpoint sediment runoff.

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3.5 Phosphorus Retention

DESCRIPTION: Phosphorus retention is the process by which phosphorus borne by overland runoff (e.g., sheet flow), incoming surface waters, and perhaps groundwater and precipitation, is held for long periods within the sediments, water column, or biota of a wetland basin. While phosphorus is being retained within a basin, it can be converted from one form to another, e.g., from organic to inorganic form, or from oxidized to reduced form.

DOCUMENTATION OF FUNCTION OCCURRENCE: On a landscape level, PPH wetlands would seem to retain virtually all phosphorus which enters them. This is because most phosphorus is retained by adsorption to surface sediments within wetland basins, or by seasonal incorporation into plant material. In the PPR generally, phosphorus is unlikely to migrate downward into groundwater systems, partly because of adsorptive calcareous soils and alkaline pH values (Hubbard 1988). However, in the eastern PPR, groundwater originating from recharge sites mainly in upland cultivated areas with non-sulfatic soils may contain elevated concentrations of phosphorus (e.g., SDRCWP 1990, Wall et al. 1989). Isolated PPH wetlands become sources rather than sinks for phosphorus only when (a) biological materials from within a basin are dispersed outside the basin by mobile vertebrates, emerging insects, or severe runoff events that connect basins, or (b) phosphorus-bearing sediments and plants are blown from large saline basins during seasonal dryout or drought years.

On a site-specific level (i.e., within wetland basins) phosphorus may be repeatedly reduced and oxidized as influenced by seasonal and annual wetland water levels. The highly productive and diverse biological communities may also cycle the phosphorus rapidly between particulate and dissolved form; between sediments, biota, and water column; and between open water and marsh zones. During severe winters when basins freeze completely to their bottoms, phosphorus in the water column may be forced into the sediments (pers. comm., J. Kadlec, Utah State Univ., Logan). Phosphorus retention has been studied in few PPH basins. An open lake system in South Dakota removed 70, 80, and 100% of the tributary phosphorus input during three successive years (SDRCWP 1990). Two open wetland complexes in North Dakota removed 52 and 85% of the tributary phosphorus, vs. a 200-to-600% increase from two drained wetland complexes (Malcolm 1979).

ASSOCIATED POTENTIAL VALUES: Excess phosphorus causes blooms of algae which potentially impair drinking water quality, biodiversity, and opportunities for swimming and fishing. Fishing is popular throughout the region, yet fish populations in many rivers and lakes are subject to die-offs as a result of oxygen depletions caused by excessive algal growth, which in turn had been stimulated by abnormal enrichment by phosphorus-laden runoff. If wetlands play a measurable role in retaining phosphorus, and if phosphorus is a primary factor limiting the algae which diminish ecological and recreational values of waters used or valued by the public, then wetland loss could further diminish these values and increase the cost to taxpayers of waste treatment plant construction.

On a landscape level, much of the phosphorus in runoff is associated with suspended sediment. Because wetlands (as discussed above) are among the most effective landscape features for mitigating sedimentation problems, they may also intercept much of the associated phosphorus before it reaches larger, more permanent waterbodies. Thus, protection and enhancement of this function in wetlands could help maintain and restore important public uses of downslope lakes and rivers. However, on a site-specific level, retaining phosphorus in wetlands will have both positive and adverse effects on the ecological values of the wetlands themselves. In particular, the competition between algae and vascular wetland plants for phosphorus has complex implications for all trophic levels, and ultimately, overall wetland production and biodiversity. Also, some experienced observers have speculated that apparent declines in emergent plant species richness (and simultaneous increases in dominant stands of cattail) in some PPH wetlands might be the result of increased inputs of nutrients, including phosphorus (see section 4.9). Thus, the potential landscape-level values of some wetlands to effectively treat nonpoint nutrient runoff must be balanced against the likelihood that site-specific (and eventually, landscape) ecological values may compromised. For protecting most surface waters from these problems, the state of North Dakota specifies a criterion of 0.100 mg/l phosphate (Minnesota's is a similar 0.090 mg/l), but recommends a more stringent target value of <0.025 mg/l for lake improvement projects. Minnesota considers waters having <0.040 mg/l phosphorus to be fully supporting of all designated uses.

DOCUMENTATION OF VALUE: Virtually all of the lakes in the PPR are considered hypereutrophic or eutrophic, according to annual 305b water quality reports. While this condition to some degree preceded human settlement, sediment cores indicate it has been aggravated by increasing agricultural and urban runoff (e.g., Allan et al. 1980). In South Dakota, 26% of the assessed river miles statewide have water quality that is so degraded it does not support any of their designated uses, and 44% show partial support; about 12% are not fishable due to water quality problems. Some 20% of the lakes do not meet their designated uses (SD Dept. Water and Natural Resources 1990). In North Dakota, 25% of the assessed river miles, and 36% of the lake acres, have water quality that is so degraded it does not fully support all of their designated uses (ND Dept. Health and Consolidated Laboratories 1990b).

Public opinion surveys indicate that many PPR citizens (e.g., 90% in survey by Grosz and Leitch 1990) think that wetlands are important for their ability to purify water. Although PPH basins are known to retain phosphorus, no attempts have been made to link quantitative estimates of phosphorus retention rates specifically from wetlands to ecological resources or to actual economic values associated with downslope use.

TEMPORAL EFFECTS: Throughout most of the PPR, runoff inputs to wetland basins are greatest during early spring. Often, this early spring runoff bears the largest portion of phosphorus-bearing sediment that enters wetlands. Much of the sediment phosphorus originates from lands tilled the previous autumn. Comparing two years (or locations) with equal amounts of total annual precipitation, the transport of phosphorus in sediment is likely to be much greater for the year (or location) in which, during the preceding autumn, a major rainstorm was followed by freezing, which then was followed by a snow cover that persisted through the winter and was followed by a warm early spring. Under such conditions, the large spring thaw is likely to mobilize considerable sediment-borne phosphorus. Conversely, springtime sediment runoff may be less during years (or locations) where winters are mild and lack continuous insulating snow cover. In other instances, springtime conditions affect phosphorus inputs to a greater degree than conditions during the preceding autumn or winter. Wetlands may retain a larger portion of the phosphorus borne by spring runoff during years when the runoff occurs later in the season, or during years when summer (vs. early spring) storms contribute a larger portion of the annual phosphorus input to wetlands. That is because as the season progresses, wetland soils thaw and vegetation develops more fully, thus increasing trapping efficiency. Any of the temporal effects just described can overwhelm the effects of spatial characteristics described in the following sections.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs of phosphorus depend partly on characteristic methods, rates, frequencies, and seasonal timings of fertilizer application. As shown in the detailed map by Omernik (1976), instream concentrations of phosphorus are greatest in Iowa, and decline in a generally westerly direction within the PPR. At least in Montana, phosphorus is the predominant plant nutrient applied as fertilizer (Bauder et al. 1991). Phosphorus inputs to wetlands from fertilizer can be inferred directly from fertilizer type and indirectly by soil and crop type (i.e., assuming particular fertilizers are associated with particular soils and crops). Fertilizer input to wetlands also depends on (a) type of cultivation and associated specific practices (e.g., conservation tillage, contour plowing, irrigation), (b) soil erodibility, as indicated by soil type and slope; (c) proximity of treated soils to wetlands, (d) transport mechanisms, as indicated by precipitation and water yield volume (White 1983) or their surrogates (e.g., rainfall/snowmelt intensity, watershed shape, soil type, and slope), and (e) width and type of buffer strips (vegetated, unfertilized sediment-filtering zones) surrounding wetlands. Use of upstream artificial drainage as an indicator of increased phosphorus input may depend on whether drainage is accomplished by ditches or subsurface tiles. Ditches draining PPR wetlands have been reported to have elevated levels of phosphorus (e.g., Malcolm 1979). In contrast, reports of water quality from watersheds drained by subsurface tile suggest lower phosphorus concentrations, at least after several years have passed following installation. This is probably because tile drains increase infiltration and thus reduce transport in overland flow. Landscape inputs of phosphorus may be indicated as well as by indicators of soil erosion, as described on page. Additional phosphorus enters wetlands from dry deposition (e.g., windborne sediments); groundwater (e.g., SDRCWP 1990); pesticide and road salt runoff; upslope wetlands (especially those with organic substrates) whose water levels have recently been drawn down following a period of stagnated, anoxic conditions in the sediment; and animals (e.g., livestock, waterfowl, sewage).

Capacity for retaining phosphorus is related, on a landscape scale, to the spatial arrangement of wetland complexes relative to contributing land uses. Science has not advanced to the point where it is possible to specify all combinations of wetland spatial position, size, and type that are optimal for retaining phosphorus at a landscape scale, but it may be reasonable to assume that this would be roughly comparable to the pattern that would be optimal for retaining sediments. Again, the contagion characteristics of the wetland spatial distribution (e.g., proportion of subregional wetland acreage that is dispersed vs. clumped) are likely to affect both the potential extent of phosphorus input to wetlands and wetland capacity to retain it. Wetland acreage occurring as scattered complexes rather than as single large wetlands should, by chance alone, be more likely to be located near and downslope from phosphorus sources, providing more opportunities for input. At the same time, the larger shoreline-to-area (or smaller depth-to-volume) ratio associated with a more dispersed wetland acreage implies that a larger portion of phosphorus-bearing runoff will enter wetlands via aerobic shoreline zones of dense vegetation and weathered sediments, which have a high capacity for taking up and transforming phosphorus.

The basin water regime probably influences phosphorus retention. Water regime can be indicated generally by plant species, soil profile, landscape geology, relief, and geographic position. The drier wetland basins, such as temporary and seasonal types, probably have a larger proportion of their basin available for storage and infiltration of spring runoff, with concomitant deposition of suspended sediment and retention of phosphorus. These wetland types are also likely to have more weathered surface horizons, with consequently greater potential for phosphorus adsorption. However, the seasonally fluctuating water levels in temporary and seasonal wetlands may cause a large portion of the phosphorus pool to be biologically available. However, the phosphorus retention capacity in semipermanent and permanent basins is not negligible, because the higher salinity and alkalinity that typify these basins is conducive to precipitating phosphorus from the water column.

B. Site-level (Within-wetland) Indicators of Function

Most of the site-specific indicators described for the functions, Maintenance of Runoff Timing (section 3.2), and Sediment Retention (3.4) are suitable as indicators of phosphorus retention. Again, the ratio of wetland size to watershed size, and the frequency and magnitude of connection to other basins are important indicators. Where concern focuses on phosphorus transformations within the open water portions of an individual wetland or lake basin, other indicators are appropriate. Phosphorus availability may increase if the basin is tilled or re-ditched during seasonal dryout periods or drought years, is shallow and exposed to strong winds (e.g., Kenney 1985), has steep banks of erodible soil, or contains animals that disturb shallow bottom sediments (e.g., carp, humans in boats). On the other hand, indicators of diminished capacity for dispersing phosphorus into open water (i.e., longer phosphorus turnover times) include alkaline soils with large calcium content; dense vegetative cover (both submerged aquatics and emergents) close to the path of incoming sediment; and high salinity. In the PPR, saline basins occur mostly in topographically low positions on glacial outwash in western and northern areas, whereas basins with calcitic water quality occur on glacial till in other areas.

POSSIBLE INDICATORS OF VALUES: The values of retaining phosphorus in wetlands can be indicated by the ecological, commercial, and recreational importance of the receiving waters relative to those of the retaining wetland, and their vulnerability as judged by factors described above under the discussion of indicators of inputs. All of the PPR states, in their 305b water quality reports, have listed water bodies impacted by nonpoint fertilizer runoff.

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3.6 Nitrogen Removal

DESCRIPTION: Nitrogen removal is the process by which dissolved nitrogen (a) disappears from the immediate landscape as a result of being converted to gaseous forms, or (b) is retained for long periods within the sediments, water column, or biota of a wetland basin. While nitrogen is being retained within a basin, it can be converted from one form to another, e.g., from organic to inorganic.

DOCUMENTATION OF FUNCTION OCCURRENCE: On landscape and regional levels, PPH wetlands would seem to retain or remove much of the nitrogen which enters them. As with phosphorus, this is due mainly to the physically closed nature of individual pothole basins with regard to surface water flow. Unlike phosphorus, which to some degree accumulates over time in such basins, nitrogen can be removed permanently from a basin. This happens mainly as a result of denitrification, a biologically-mediated process which converts nitrate to nitrogen gases. To a perhaps lesser extent, nitrogen may be lost from PPH wetlands by a process known as ammonia volatilization (e.g., Murphy and Brownlee 1981).

In one study in South Dakota, between one-third and one-half of the total nitrogen applied as fertilizer was removed from the landscape by denitrification (SDRCWP 1990). In semipermanent basin wetlands specifically, Davis and van der Valk (1978) in Iowa reported 86% removal of the inputted nitrate and 78% of the ammonia. An open lake system in South Dakota removed 44, 46, and 100% of the tributary nitrogen input during three successive years; this was less than the proportion of phosphorus removed (SDRCWP 1990). Two open wetland complexes in North Dakota removed 13 and 58% of the tributary nitrate, vs. a >10-fold increase from a drained wetland complex (Malcolm 1979). An unvegetated wet basin in South Dakota that was loaded with municipal wastewater removed 1765 kg N/ha (White and Dornbush 1988). On a regional basis, Jones et al. (1976) in northwestern Iowa found that of 34 watersheds, those with a larger percentage of land as wetlands had less nitrate in streamflow. Nitrogen can be removed temporarily from surface waters by being buried below the root zone in rapidly accreting sediments or by adsorption of ammonium nitrogen to clays (e.g., Martin and Hartman 1987a).

Isolated PPH wetlands become sources rather than sinks for nitrate only when (a) biological materials from within a basin are dispersed outside the basin by mobile vertebrates, emerging insects, or severe runoff events that connect basins, (b) nitrogen-bearing sediments and plants are blown out of a wetland basin during seasonal dryout or drought years, or (c) denitrification rates are so low that nitrate accumulates and leaches into groundwater systems or laterally into subsurface flow, which may carry it into nearby wetlands.

On a site-specific level (within wetland basins), nitrogen may be repeatedly reduced and oxidized from ammonium to nitrate forms, with consequences for aquatic community structure. The potential for this to happen can be indicated by fluctuations in seasonal and annual wetland water levels, which in turn may be evidenced by soils with strong argillic horizons (Hubbard 1988).

ASSOCIATED POTENTIAL VALUES: On landscape and regional levels, because wetlands are among the most effective landscape features for denitrification (Groffman and Tiedje 1989) and are often located so as to intercept most runoff and groundwater, they also intercept much of the associated nitrate before it reaches larger, more permanent waterbodies. Thus, protection and enhancement of nitrogen removal functions of wetlands could help maintain and restore important public uses of downslope lakes, rivers, and aquifers. Fishing is very popular throughout the region, yet fish populations in many rivers and lakes are subject to die-offs as a result of oxygen depletions caused by excessive algal growth, which in turn had been stimulated by abnormal enrichment by nutrient-laden runoff. In surface waters of less than 3000 mmhos conductance (Barica 1978), algal blooms occur where there is excess nitrate (e.g., Moore and Haertel 1975). Perhaps of even greater public concern, nitrate in glacial outwash areas can readily infiltrate into groundwater and impair the quality of groundwater withdrawn for drinking. If wetlands play a measurable role in removing nitrate from ground or surface waters used or valued by the public, then wetland loss could increase the cost to taxpayers of measures to remediate contaminated groundwater or treat contaminated surface waters.

On a site-specific level, retaining nitrogen could have both adverse and positive effects on the ecological values of the wetlands themselves. In particular, the competition between algae and vascular wetland plants for nitrogen, while perhaps less important than competition for phosphorus, nonetheless has complex implications for all trophic levels, and ultimately, overall wetland production and biodiversity. Selective removal of nitrate by wetlands might trigger increased growths of blue-green algae that may be less useful to typical PPH food webs (cf. Barica et al. 1980). Also, some experienced observers have speculated that apparent declines in emergent plant species richness (and simultaneous increases in dominant stands of cattail) in some PPH wetlands might be the result of increased inputs of nutrients, including nitrogen (see section 4.9). Thus, the potential landscape-level values of some wetlands to effectively treat nonpoint nutrient runoff must be balanced against the likelihood that site-specific (and eventually, landscape) ecological values may compromised.

DOCUMENTATION OF VALUE: Public opinion surveys indicate that many PPR citizens (e.g., 90% in survey by Grosz and Leitch 1990) think that wetlands are important for their ability to purify water. If wetlands are indeed effective for removing nitrate, then the currently degraded water quality in parts of the PPR where wetland losses have been greatest (e.g., Iowa) can be blamed at least partly on these past losses. In South Dakota, 26% of the assessed river miles statewide have water quality that is so degraded it does not support any of their designated uses, and 44% show partial support; about 12% are not fishable due to water quality problems. Some 20% of the lakes do not meet their designated uses (SD Dept. Water and Natural Resources 1990). In North Dakota, 25% of the assessed river miles, and 36% of the lake acres, have water quality that is so degraded it does not fully support all of their designated uses.

Groundwater contamination by nitrate has also been widely documented. Wells having severe (>10 mg/l) levels of nitrate contamination comprise 17% of those in Montana PPR counties, 6% of those in Iowa PPR counties, and 2% of those in South Dakota counties. In one area of South Dakota (Oakwood-Poinsett watershed), 37% of the domestic wells exceeded 10 mg/l (SDRCWP 1990), a level hazardous to humans. In one area of the Minnesota PPR, 10 of 15 sampled wells exceeded the level (Wall et al. 1989). Groundwater with nitrate levels great enough to potentially produce adverse ecological effects in wetlands occurred in 38% of the South Dakota PPR counties (compiled from DeMartino and Jarrett 1991).

Wetlands remaining in the most contaminated areas, as well as in areas where groundwater is particularly vulnerable to contamination due to hydrogeologic conditions, are especially valuable to society because whatever nitrate they can remove reduces the severity of the local problem and its potential threat. Similarly, wetland restoration efforts whose primary objectives are nonpoint source management and drinking water protection might target such areas. All of the PPR states have mapped areas of groundwater vulnerability and/or groundwater contamination from nitrate over at least a portion of the state.

Although PPH basins are known to remove nitrogen, no attempts have been made to link quantitative estimates of denitrification rates specifically in wetlands to economic values of maintaining fishing opportunities and groundwater quality in surrounding areas.

TEMPORAL EFFECTS: Throughout most of the PPR, landscape inputs to wetland basins of nitrate (e.g., from fertilizer and livestock) are often greatest during early spring. Although wetland plants take up and store considerable nitrate at this time, they store most of it only temporarily. In contrast, denitrification results in permanent losses of nitrogen, and denitrification rates may be equal or greater at the beginning and end of the growing season than during mid-summer (Christensen 1985, Myrold 1988, SDRCWP 1990, Zak and Grigal 1991). Thus, denitrification functions in wetlands may be of greatest value in removing nitrate during years when runoff inputs occur early or late in the growing season. However, if runoff resulting from the spring melting of snow surrounding wetlands occurs prior to ice-out in wetlands, it flows under the ice, purging basins of anoxic, ammonia-rich water which can subsequently be released into receiving waters (if seasonal connections exist) without being substantially denitrified, thus causing water quality problems (ND Dept. Health and Consolidated Laboratories 990b). This adverse impact may be more likely to occur in landscapes dominated by semipermanent and permanent basins, because they tend to remain frozen longer into the spring.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

PPH wetlands are focal points for nitrate accumulation. Regional and landscape inputs of nitrogen depend partly on characteristic amounts, methods, rates, frequencies, and seasonal timings of fertilizer application. This can be inferred directly from fertilizer type and indirectly by soil and crop type (i.e., assuming particular fertilizers are associated with particular soils and crops). Fertilizer input to wetlands also depends on (a) soil leaching potential, as indicated by soil type, crop type, and crop management practices (e.g., summer fallowing, irrigation, conservation tillage, contour plowing); (b) transport mechanisms, as indicated by precipitation and water yield or their surrogates (e.g., rainfall/snowmelt intensity, watershed shape, soil type, and slope), (c) proximity of treated soils to wetlands and connecting subsurface flow zones, and (d) width and type of buffer strips (vegetated, unfertilized, filtering zones) surrounding wetlands. As shown in the detailed map by Omernik (1977), instream concentrations of nitrate are greatest in Iowa, and decline in a generally westerly direction within the PPR. Most streams east of the Missouri Coteau exceed 1 mg/l nitrate on a mean annual basis. With regard to groundwater, the incidence of both actual and potential nitrate contamination is greatest in eastern South Dakota, Iowa, and southwestern Minnesota (Nielsen and Lee 1987).

The extent of artificial drainage upstream may also be a useful indicator of increased transport and nitrogen input. In other regions with flat terrain drained by agricultural ditches, watersheds which had less than 7% remaining wetland cover (Chescheir et al. 1987), were appreciably less effective for removing nitrate than were watersheds with more extensive wetlands (Bedient et al. 1976). Nitrate also enters wetlands via groundwater discharging from contaminated aquifers (e.g., many semipermanent and permanent basins in the eastern and southern PPR); from atmospheric deposition (e.g., White 1983); from nitrogen fixation processes of the cyanobacteria that dominate many PPH basins (e.g., Barica et al. 1980, Brownlee and Murphy 1983); and from animals (e.g., livestock, waterfowl, sewage). In fact, livestock feedlots (and perhaps also spills from fertilizer storage tanks) may be a more frequent cause of severe (>10 mg/l nitrate) groundwater contamination than is routine fertilizer application (Kross et al. 1990, SDRCWP 1990, Meyer 1985).

Capacity for removing nitrate is partly related to regional-scale factors. Denitrification is probably hindered somewhat in the eastern PPR because soils of most wetlands there are relatively sulfur-poor; this deficit exacerbates competition among microbes for available carbon, thus reducing denitrification rates (pers. comm., J. Richardson, North Dakota St. Univ., Fargo). Except where irrigated and underlain by sandy soils, wetlands in the western part of the PPR would be expected to play a larger role in preventing groundwater contamination because they are both good sites for denitrification and tend to be areas of groundwater recharge.

Capacity for removing nitrate is also related to landscape-scale factors, particularly the spatial arrangement of wetland complexes relative to contributing land uses and associated contaminated aquifers. Science has not advanced to the point where it is possible to specify all combinations of wetland spatial position, size, and type that are optimal for removing nitrate at a landscape scale. Nonetheless, the contagion characteristics of the wetland spatial distribution (e.g., proportion of subregional wetland acreage that is dispersed vs. clumped) are likely to affect both the potential extent of nitrate input to wetlands and wetland capacity to remove it. Wetland acreage occurring as scattered complexes rather than as single large wetlands should, by chance alone, be more likely to be located near and downslope from nitrate sources. Finally, the larger shoreline-to-area (or smaller depth-to-volume) ratio associated with a more dispersed wetland acreage implies that a larger portion of nitrate-bearing runoff will enter wetlands via shoreline zones which support high rates of denitrification due to their characteristic soils, fluctuating water levels, and high density of plants.

B. Site-level (Within-wetland) Indicators of Function

Most of the site-specific indicators described for the functions, Maintenance of Runoff Timing (section 3.2), and Sediment Retention (section 3.4) are suitable as indicators of nitrate removal. Thus, the frequency and magnitude of connection to other basins and the ratio of wetland size to watershed (or groundwater source) size, are important indicators.

Volumetric soil moisture, as inferred from wetland water regime also is perhaps the most important indicator of denitrification capacity (Groffman and Tiedje 1989). Measurements of denitrification in a South Dakota wetland soil indicated that conditions of less than 22% volumetric soil moisture completely inhibit denitrification (Lemme 1988). A wetland does not have to be exposed to runoff for very long to reach these moisture levels and remove nitrate. In fact, studies of an emergent wetland by Lindau et al. (in press) showed that under loading rates typical of the PPR, denitrification begins within a day of when nitrogen enters a wetland, and reaches a peak at 7 days. Yet some citizens may not consider areas saturated for so brief a time to really be wetlands.

Other factors associated with wetland water regime can be used to infer relative capacity for denitrification. First, water table levels fluctuate the most in temporary and seasonal wetland basins, partly as a result of both diurnal and seasonal evapotranspiration. Such fluctuations, from saturated to drained condition, are associated with switches in sediments between anoxic (anaerobic, or reduced) and oxic (aerobic, or oxidized) conditions. Fluctuating water levels might be expected to enhance denitrification so long as (a) anaerobic conditions still occur, (b) moisture levels in the upper soil layers are not too severely depleted (i.e., pore space is 30-60% water-filled; Linn and Doran 1984, Lemme 1988), (c) carbon supplies also are not limiting, and (d) salinity conditions are not extreme. Second, soil temperature might be expected to be warmer in temporary and seasonal wetlands during much of the year, due to their shallow depths. Third, denitrification can be hindered in wetlands lacking sulfatic soils (e.g., western parts of the PPR), as sulfate reduction processes compete for available carbon. This happens to a much lesser degree in temporary and seasonal wetlands than in semipermanent wetlands (pers. comm., J. Richardson, North Dakota St. Univ., Fargo).

Thus, on the basis of seasonal hydrologic fluctuations, warmer temperature, and diminished sulfate reduction processes, temporary and seasonal wetlands might be more capable of removing nitrate from surface runoff than are semipermanent and permanent basins. As noted by Kantrud et al. (1989), "It would seem that temporary and seasonally flooded wetlands would be especially efficient in removal of excess nitrogen."

However, other logic suggests that semipermanent and permanent wetlands might be more effective than temporary and seasonal wetlands for removing nitrate. Because semipermanent and permanent wetlands are usually groundwater discharge or flow-through systems, they are less susceptible to drought, and by definition, remain saturated and thus favorable to denitrification for longer periods. Prolonged drought in temporary wetlands not only results in moisture deficits inhospitible to denitrifying microbes, but also can result in diminuation (via mineralization) of organic matter essential for sustaining denitrifiers. Organic matter content of soils in semipermanent and permanent wetlands generally seems to be greater than in temporary and seasonal wetlands (however, Loken (1991) reported less organic matter in soils of semipermanent groundwater discharge wetlands; he attributed this to the inhibiting of production by the high salinity of these basins). Also, once nitrate has contaminated groundwaters, the permanent and semipermanent wetlands—which, often being groundwater discharge or flow-through systems, are in direct contact with the contamination for a longer portion of the year—may be expected to play a relatively larger role in removing nitrate.

In summary, it is probable (but generally undocumented) that in areas of sulfatic soils of the eastern PPR, semipermanent wetlands have a greater capacity to remove nitrogen than do temporary/seasonal wetlands, whereas in the western PPR or in areas with sandy soils or rapid infiltration, all wetland water regimes are about equally capable of removing nitrogen (pers. comm., J. Richardson, North Dakota St. Univ., Fargo). Overall, wetlands in the western part of the PPR would be expected to play a larger role than eastern PPR wetlands in preventing groundwater contamination, because they tend to be areas of groundwater recharge.

Soil fertility is also an indicator of denitrification potential. Nitrogen removal by wetlands is typically greatest where soils are especially fertile (moderately alkaline clays with adequate organic matter) and have large water-holding capacity (i.e., saturated for long duration). Microbial biomass in North Dakota soils can be greater in areas underlain by siltstone than in areas underlain by sandstone or shale parent material (Schimel et al. 1985). Tillage and fertilization of soils over time also might increase the suitability of remaining soil carbon as an energy source for denitrifying microbes (Groffman et al. 1991). As a result, denitrification rates may be greater in wetlands that have been exposed to nutrient runoff than in relatively pristine wetlands (pers. comm., J. Kadlec, Utah State Univ., Logan).

The percent cover or root biomass of rooted plants (as possibly inferred from plant species or community type) is another possible indicator of denitrification capacity. Although some rooted plants are capable of "pumping" nitrates from the sediment into aboveground tissues and eventually into the water column, wetland plants also enhance denitrification by (a) enhancing the availability of carbon (e.g., from litterfall), (b) speeding the diffusion of oxygen (via roots) into otherwise anaerobic subsurface zones, especially during mid-growing season, and (c) increasing diurnal and seasonal fluctuations in the water table, and consequently the oxidation status, as a result of evapotranspiration. Denitrification may be greatest where soil organic matter reaches a maximum just below the soil surface, but above the depth limit of the root zone (Parkin and Meisinger 1989). In this zone, impeded lateral flow increases the time available for nitrate loads to interact with prolific microbial populations present in the surrounding root masses. A minor amount of denitrification can occur within shallow aquifers, depending on the amount of oxidizable organic matter that has infiltrated (Hiscock et al. 1991); this could in turn be greatly enhanced by recharge from carbon-producing temporary wetlands (e.g., Trudell et al. 1986). Although the effects of different native wetland communities on denitrification have not been determined in the PPR, limited studies elsewhere have found greater denitrification in grassy buffer strips than in forested wetlands (Groffman et al. 1991) and more denitrification in soils cropped with oat/clover cover than in those with corn (Fraser et al. 1988).

Indicators such as basin hydrologic type and soil organic matter can be manifested on a regional level as well. That is, because these indicators correlate roughly with geologic and climate patterns within the region, in a general sense within-region spatial trends in denitrification might exist. For example, the occurrence of anaerobic conditions and fluctuating water levels might be greater in subregions where the wetland resource, due partly to geologic conditions, is comprised of proportionately more small, temporary, densely-vegetated wetlands with anoxic, under-ice conditions in winter.

POSSIBLE INDICATORS OF VALUES: The values of removing nitrate from the landscape can be indicated by the ecological, domestic, and recreational importance of the aquifers and receiving waters relative to those of the wetland, and their vulnerability as judged by factors described above under the discussion of indicators of inputs.

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3.7 Detoxification

DESCRIPTION: For purposes of this report, detoxification is the process by which xenobiotic contaminants, including synthetic hydrocarbons and atypical concentrations of heavy metals, are converted from forms toxic to plants or animals to forms that are relatively harmless.

DOCUMENTATION OF FUNCTION OCCURRENCE: On a landscape level, PPH wetlands would seem to retain much of the contaminant load that enters them, due largely to the physically closed nature of individual pothole basins with regard to surface water flow. However, studies of detoxification functions of PPH wetlands are lacking. Circumstantially, occurrence of natural detoxification processes might be inferred from the data of Martin and Hartman (1985). Their analyses of sediment and/or fish in five PPH basins found no evidence of toxic concentrations of PCBs or organochlorine pesticides, despite expected exposure. Isolated PPH wetlands become sources rather than sinks for contaminants only when (a) contaminants are highly soluble and infiltrate into underlying groundwater systems before they can be naturally detoxified, (b) contaminants from within a basin are dispersed outside the basin by mobile vertebrates or emerging insects, or (c) sediment-adsorbed contaminants are blown out of a wetland basin during seasonal dryout or drought years.

On a site-specific level (i.e., within wetland basins) contaminants may be transformed to less harmful substances by physicochemical processes or microbial communities. The former generally results only in deactivation of pesticides, while the latter results in actual degradation (Rao et al. 1983). Among various ecosystems, wetlands frequently have the largest year-round microbial densities (e.g., Henebry et al. 1981). In being detoxified, contaminants may be repeatedly reduced and oxidized (depending on seasonal and annual wetland water levels). Contaminants also may be cycled rapidly between sediments, biota, and water column (as well as between open water and marsh zones) by the highly productive and diverse biological communities. Detoxification mechanisms have generally not been studied in PPH basins. Although the theoretical occurrence of this function in PPH wetlands is inarguable, its mechanisms and extent are generally unknown in the PPR.

ASSOCIATED POTENTIAL VALUES: Contaminants impair potability of both groundwater and surface water, as well as having adverse ecological effects. Thus, contamination increases the cost to taxpayers of waste treatment plant construction and other remedial measures. On a landscape level, because wetlands are possibly among the most effective landscape features for retention and detoxification, and are generally located so as to intercept most runoff and contaminated groundwater, they may also intercept much of the contaminant load before it reaches larger, more permanent waterbodies. Thus, protection and enhancement of contaminant removal functions of wetlands could help maintain and restore important public uses of downslope lakes and rivers. However, on a site-specific level, retaining contaminants could impair the ecological functions and values of the wetlands themselves. Long hydraulic detention times, complex food webs, presence of many mobile species, and intrinsically high production rates in PPH wetlands could lead to severe problems with direct toxicity (e.g., Grue et al. 1986) or bioaccumulation as has been documented in wetlands elsewhere. Thus, the potential landscape-level values of some wetlands to effectively treat contaminated runoff must be balanced against the likelihood that site-specific (and eventually, landscape) ecological values may compromised.

DOCUMENTATION OF VALUE: Pesticides, rather than heavy metals or chemicals related to mineral extraction or industrial processing, appear to be the primary contaminant within the PPR. Pesticides have not only entered PPH wetlands extensively, but have also contaminated groundwater in a few parts of the PPR (e.g., Kross et al. 1990, Moody et al. 1988). However, most parts of the region have yet to detect a problem (ND Dept. Health and Consolidated Laboratories 1990a). In some cases this may simply be because contamination plumes of more persistent contaminants have not yet had time to migrate downward into drinking water supplies, while in other cases such migration may never occur due to the nature of the pesticide and the detoxification capacity of the landscape (particularly its wetland component).

Certain trace metals appear to occur naturally within the PPR at levels potentially toxic to some aquatic life, and a few instances have been recorded of PPH wetlands being contaminated by metals associated with inputs specifically from drainage waters. At the Milk River Reclamation Project near Bowdoin National Wildlife Refuge, Montana, potholes were contaminated by arsenic, boron, selenium, and zinc (Willard et al. 1988). Also, sediment sampling in just five PPH basins by Martin and Hartman (1984) found that levels of arsenic, cadmium, lead, and selenium were higher than in riverine wetlands nearby, but were within normal or background ranges. Acidic deposition, which has been documented to occur at least in North Dakota (Smith 1988), could compound such problems in some types of PPH wetlands, as well as posing a direct threat to their productivity. Its extent of occurrence in the PPR is unknown. Temporary, recharge wetlands are likely to be most vulnerable. In western parts of the PPR, their water budget is dominated by direct snowmelt whose acidity has had less opportunity to be diminished by movement across or within upland soils.

Wetlands remaining in the most contaminated areas, as well as in areas where groundwater is particularly vulnerable to contamination due to hydrogeologic conditions, are especially likely to be valued by society. That is because their ability to remove any amount of contamination reduces the severity of the local problem and its potential threat to downslope areas. Similarly, wetland restoration efforts whose primary objectives are drinking water protection may target such areas. All of the PPR states have mapped areas of groundwater vulnerability over at least a portion of the state.

Although PPH basins can be assumed to detoxify some of the contaminant load, no attempts have been made to link quantitative estimates of detoxification rates specifically in wetlands to economic values of maintaining groundwater quality and ecological values in surrounding areas. Public opinion surveys indicate that many PPR citizens (e.g., 90% in survey by Grosz and Leitch 1990) think that wetlands are important for wetlands for their ability to purify water.

TEMPORAL EFFECTS: Rainfall volume (Haith 1986) or irrigation volume (Kolberg et al. 1990) and timing relative to timing of pesticide application (Isensee et al. 1990) are critically important to leaching and runoff of pesticides. Throughout most of the PPR, landscape inputs to wetland basins of pesticides are greatest during early spring and summer (Grue et al. 1988). This is often when microbial activity and associated detoxification processes are greatest (Myrold 1988). Thus, detoxification processes in wetlands may be slightly more effective during years when runoff input or pesticide spraying occurs later in the season, or during years when summer (vs. early spring) storms contribute a larger portion of the pesticide input to wetlands.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs of pesticides (the major PPR contaminant) to wetlands depend on characteristic methods, rates, frequencies, and seasonal timings of pesticide application, as well as the pesticide's physicochemical mobility (e.g., transported by sediment vs. rapidly infiltrates vs. remains airborne), persistence, and bioaccumulation potentials. These can be inferred directly from pesticide type and indirectly by crop type (i.e., assuming particular pesticides are associated with particular crops). Pesticide input to wetlands and groundwater also depends (a) on position and proximity of wetlands relative to pesticide sources; (b) on soil leaching potential, as indicated by soil type (e.g., particle size and organic content), crop type, and crop management practices (e.g., irrigation, conservation tillage, contour plowing); depth and type of subsurface geologic materials and formations (e.g., their permeability and hydraulic conductivity, with glacial outwash areas often having more well contamination than glacial till areas; see SDRCWP 1990), (c) on width and type of buffer strips (vegetated, unsprayed, filtering zones) surrounding wetlands, and (d) on transport mechanisms, as indicated by timing and amount of precipitation and water yield or their surrogates (e.g., rainfall/snowmelt intensity, watershed shape, soil type, and slope). Transport of insecticides is also indicated by wind direction and velocity relative the spatial positions of crop acreage and wetlands at the time of spraying. The extent of artificial drainage upstream may not be a useful indicator of increased pesticide input. Some studies suggest that subsurface tile drains, at least, reduce downslope pesticide concentrations by increasing infiltration and thus reducing transport in overland flow (Southwick et al. 1990, VanScoyoc and Kladivko 1989). Within the PPR, the incidence of both actual and potential pesticide contamination of groundwater is greatest in eastern South Dakota, Iowa, and southwestern Minnesota (Nielsen and Lee 1987).

Capacity for removing contaminants is related, on a landscape scale, to the spatial arrangement of wetland complexes relative to contributing land uses and associated contaminated aquifers. Science has not advanced to the point where it is possible to specify all combinations of wetland spatial position, size, and type that are optimal for detoxifying various types of contaminants at a landscape scale. Nonetheless, the contagion characteristics of the wetland spatial distribution (e.g., proportion of subregional wetland acreage that is dispersed vs. clumped) are likely to affect both the potential extent of contaminant input to wetlands and wetland capacity to detoxify it. Wetland acreage occurring as scattered complexes rather than as single large wetlands should, by chance alone, be more likely to be located near and downslope from contaminant sources. Moreover, the larger shoreline-to-area (or smaller depth-to-volume) ratio associated with a more dispersed wetland acreage implies that a larger portion of contaminant-loaded runoff will enter wetlands via shoreline zones of dense vegetation, where much of the plant uptake, fluctuating sediment oxygen conditions, and microbial activity associated with detoxification processes is likely to occur.

B. Site-level (Within-wetland) Indicators of Function

Most of the site-specific indicators described for the functions, Maintenance of Runoff Timing (section 3.2), and Sediment Retention (section 3.4) are suitable as indicators of detoxification capacity. Thus, the ratio of wetland size to watershed (or groundwater source) size, and the frequency and magnitude of connection to other basins are important determinants. Wetland soil type is also a possible indicator of detoxification potential. Microbial density, and thus detoxification capacity, is typically greatest in wetlands with sediments that are fertile (or enriched by manure fertilizers) and highly organic (however, in the case of mercury contamination, this characteristic may be detrimental, as it can increase the bioavailability of mercury; Jackson 1986). Also, sediments with a high cation exchange capacity are often capable of immobilizing many contaminants. Similarly, basins whose water quality is highly alkaline or saline might be more capable of chemically adsorbing, degrading, and/or immobilizing many contaminants (e.g., Cairns and Dickson 1977, Wayland and Boag 1990). However, microbial degradation processes may be less effective at extreme salinities. In the PPR, saline basins occur mostly in topographically low positions on glacial outwash in western and northern areas.

The percent cover or root biomass of fibrous-rooted plants (as possibly inferred by plant species, community type, and management history) is another possible indicator of detoxification capacity. Some scientists have speculated that some rooted plants facilitate contamination of groundwater by creating pore spaces in soils through which contaminants are rapidly carried by infiltrating runoff. Also, some evidence suggests that wetland plants can mobilize contaminants (particularly metals) from the soil by taking them up via roots and translocating ("pumping") them into aboveground tissues. On the other hand, wetland plants enhance microbial activity associated with detoxification, due to increasing the (a) availability of carbon (e.g., from litterfall), and (b) diffusion of oxygen (via roots) into otherwise anaerobic subsurface zones. In summary, it is likely that the capacity for detoxifying pesticides may be greatest where soil organic matter reaches a maximum just below the soil surface, but above the depth limit of the root zone. In situations where lateral flow in this zone is impeded, the time available for pesticide loads to interact with prolific microbial populations present in the surrounding root masses is increased, thus maximizing the detoxification process.

Wetland water regime also is likely to influence or at least indicate detoxification capacity. Water regime can be indicated generally by plant species, soil profile, landscape geology, relief, and geographic position. PPH basins, especially the temporary and seasonal ones, experience great diurnal and seasonal fluctuations in water level partly as a result of evapotranspiration. Such fluctuations, from saturated to drained condition, are associated with switches in sediments between anoxic (anaerobic, or reduced) and oxic (aerobic, or oxidized) conditions. Severe diurnal and seasonal changes occur in water column acidity (pH) as well, as a result of intense photosynthetic activity of algae. Such changes in pH and oxidation status can be expected to mobilize many contaminants (e.g., some heavy metals) and perhaps speed the degradation of others.

Thus, detoxification capacity might be greatest in the permanent and semipermanent wetlands. In temporary and seasonal wetlands, prolonged drought is more likely to cause mineralization of the organic matter essential for sustaining detoxifying microbes. Thus, soils in these basin types would seem to be generally lower in organic content, and higher in alkalinity, than soils in semipermanent and permanent basins. However, as noted earlier, Loken (1991) reported more organic matter in soils of temporary wetlands; he attributed this to the inhibiting of production in semipermanent wetlands by high salinity). Also, if contaminants have reached groundwater, the permanent and semipermanent wetlands—which, as groundwater discharge or flow-through systems, are in direct contact with the contamination for a longer portion of the year—might be expected to play a relatively larger role in removal processes.

POSSIBLE INDICATORS OF VALUES: The values of removing pesticides from the landscape can be indicated by the ecological, domestic, and recreational importance of the aquifers and receiving waters relative to those of the retaining wetland, and their vulnerability as judged by factors described above under the discussion of indicators of inputs. Many of the PPR states, in their 305b water quality reports or other reports, have documented the locations and extent of contamination problems in groundwater used for drinking, as has the USGS (e.g., Moody et al. 1988). The US Fish and Wildlife Service monitors contaminants at many National Wildlife Refuges and other areas. States also have listed water bodies impacted by nonpoint chemical runoff, and in some states, a subset of these water bodies has been assigned highest priority, based on a variety of social and technical factors.

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3.8 Vascular Plant Production and Carbon Cycling

DESCRIPTION: Wetland plants in the PPR produce large quantities of carbon as they grow. Carbon production per unit area is particularly great among emergent vascular plants, and to a lesser extent among woody and aquatic bed species. Rates of decomposition (decay) are also relatively great in most types of PPR wetlands.

DOCUMENTATION OF FUNCTION OCCURRENCE: Although sometimes less productive than fertilizer-subsidized crops and domesticated forages, the yields of plants in PPH basins are commonly at least twice those of upland native plants, and in some cases can provide up to four times the amount of forage or hay as adjacent uplands (Fulton et al. 1986).

On a landscape level, PPH wetlands would seem to retain much of the carbon that is produced within each basin, due largely to the physically closed nature of individual pothole basins with regard to surface water flow. Isolated PPH wetlands become sources rather than sinks for carbon only when (a) carbon is decomposed into its dissolved organic forms and infiltrates into underlying groundwater systems before it can be converted to particulate matter or gas, (b) carbon (e.g., ingested plant or invertebrate material) from within a basin is dispersed outside the basin by mobile vertebrates or emerging insects, (c) particulate carbon is blown out of a wetland basin during seasonal dryout or drought years, or (d) may be gassified (e.g., to methane or carbon dioxide). On a site-specific level (i.e., within wetland basins) microbial communities, invertebrates, and physicochemical processes cause routine shifts among the physical and chemical forms of carbon. Among various ecosystems, wetlands frequently have the largest year-round microbial communities (e.g., Henebry et al. 1981), and these help cycle carbon rapidly among particulate and dissolved forms.

ASSOCIATED POTENTIAL VALUES: The sustained high primary and secondary production in PPH wetlands is largely due to the capacity of PPH basins to physically retain carbon, especially under climatic conditions that facilitate its rapid cycling. On a landscape level, because PPH wetlands are highly productive, they can energetically subsidize wildlife populations and biodiversity over a wide region. On a site-specific level, vascular plant production in wetlands not only supports within-basin fish and wildlife recreational values, but also can support economically important cultivation, livestock grazing, and harvesting of hay and baitfish (e.g., Carlson and Berry 1990). Moreover, the production of vascular plants and associated carbon contributes to many other wetland functions, such as sediment retention (section 3.4), denitrification (section 3.6), and detoxification (3.7). However, in some basins, the decomposition of carbon reserves (e.g., vascular plants) under snow-covered winter ice can deplete dissolved oxygen, kill fish and some invertebrates, and ultimately affect fish and wildlife values both within the wetland basin and, in some instances, in waters farther downslope.

DOCUMENTATION OF VALUE: The wildlife values supported by vascular plant production are documented in section 3.8. Although the extent of agricultural activities in the PPR is known on a county basis, the percentage of these activities occurring specifically in wetlands cannot easily be determined. Only a few attempts have been made to link quantitative estimates of plant production in PPH wetlands to economic values of hay, baitfish, and other harvestable resources (e.g., Ogaard 1981).

TEMPORAL EFFECTS: Vascular plant production within individual basins will be greater during years that are relatively wet and have temperatures that support an extended growing season. However, an exceptionally wet year or an extended series of wet years can drown emergent plants and cause many basins to become dominated by submerged aquatic species, which generally are less productive than emergents. Total annual shoot production can change 18-fold as a PPH wetland passes through various successional stages (van der Valk and Davis 1981). Cycling of carbon (via decomposition and re-use) might be greater during years with mild winters and adequate soil moisture.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs of carbon to PPH basins are probably minor compared to carbon fixed within each basin by photosynthesis. Some carbon enters wetland basins from animals (e.g., livestock, waterfowl); groundwater (primarily dissolved organic matter); atmospheric deposition; and runoff, particularly where permanent or intermittently exposed wetlands located upslope have been drained. Vascular plant production is also sustained by landscape inputs in the form of seeds carried among wetlands by animals or wind. In the PPR, the capacity for supporting vascular plant production or cycling carbon has not been demonstrated to be related to processes occurring at a landscape scale.

B. Site-level (Within-wetland) Indicators of Function

The percent cover and diversity of rooted plants is a fairly direct indicator of production. Monotypic stands, despite their sometimes great percent cover, are often relatively stagnant with regard to annual production. In contrast, mixed-species stands interspersed with small patches of open water often are highly productive. The role of epiphytic algae in contributing to overall primary production also increases with increasing proportion of moderately deep (0.5-2 m) open water. Submerged aquatic plants, which also flourish in more open areas, especially when fish are removed from a basin (Hanson and Butler 1990), are often less productive than emergent species. Wetland water regime (or less direct surrogates of wetland water regime such as plant species, soil profile, landscape geology, relief, and geographic position) sometimes can indicate vascular plant production. In general, vascular plant production increases across a gradient of increasing moisture, from temporary to semipermanent wetlands (Fulton et al. 1986), at least until limited by salinity, buildup of organic matter, and associated hydrogen sulfide (van Mensvoort et al. 1985), or by excessive water depth (i.e., light penetration). In the 42 PPH wetlands examined by Loken (1991), less vegetation occurred in semipermanent (groundwater discharge) than in temporary or seasonal (groundwater recharge, or flowthrough) basins. Basins whose water levels remain relatively constant for more than a year or two may experience diminished production (Hubbard 1988b), as may wetlands exposed to years of sustained drought. Production depends not only on basin type, but the successional status of the plant community, e.g., number of years elapsed since severe drought (van der Valk and Davis 1991). Although most PPH basins are highly eutrophic, and thus not usually nutrient limited, wetland soil type may indicate somewhat the potential for vascular plant production. Elevated production capacity may be indicated by the presence of fertile soils that are moderately organic and not highly acidic. Nitrogen in particular limits the production of some key wetland plants in the PPR (Neill 1990).

POSSIBLE INDICATORS OF VALUES: The value of forage provided by PPH wetlands to livestock increases in proportion to the temporal and spatial severity of drought occurring during a particular year. The value of plant production also can be indicated by the ecological, commercial, and recreational importance of the wetland basins in which the production occurs, and their position and proximity relative to ecological, domestic, and recreational users. More specifically, the forage value of wetland plants can be indicated by the annual moisture condition (wetlands assuming increased importance to livestock during drought years, Ogaard 1981), and by the plant species (e.g., cellulose and lignin content), which in turn can be indicated by wetland water regime and time of year. Plants that typify seasonal and temporary wetlands are generally better-quality forage than those of semipermanent basins, especially during the early and mid-growing season (Hubbard 1988). Also, the diversity of productive plant communities may be a key to making their production valuable to waterbirds; higher waterbird production may be more likely to be sustained in wetland complexes where plant communities are not only productive, but diverse.

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3.9 Invertebrate Production

DESCRIPTION: PPH wetlands sustain a wide variety of aquatic and semi-aquatic insects and crustaceans. Individual taxa can be grouped as follows (after Jeffries 1989, McLachlan 1970, 1975, 1985, Wiggins et al. 1980):

• Overwintering Residents: disperse passively; include many snails, mollusks, amphipods, worms, leeches, crayfish.
• Overwintering Spring Recruits: reproduction depends on water availability; include most midges, mayflies, some beetles.
• Overwintering Summer Recruits: reproduce independent of surface water availability, requiring only saturated sediment; include phantom midges and some dragonflies, mosquitoes.
• Non-wintering Spring Migrants: mostly require surface water for overwintering, adults leave temporary water before it disappears in spring or summer; includes most water bugs, some water beetles.

DOCUMENTATION OF FUNCTION OCCURRENCE: Wetland invertebrate communities in the PPR occur seasonally at high densities and are highly diverse. On a landscape level, invertebrate production within PPH wetlands may subsidize other ecosystem types (e.g., upland passerines feeding on emerging insects) and wetlands in other regions (e.g., via transport in guts of migratory birds). However, most invertebrate production probably is utilized or recycled in the basins in which it originates. Thus, invertebrate production is primarily a site-specific function. High densities of invertebrates (which usually indicate, but are not synonymous with, high production) have been documented in several PPH basins (e.g., Schultz 1987, LaBaugh and Swanson 1988).

ASSOCIATED POTENTIAL VALUES: The capacity of PPH basins for supporting high densities of invertebrates during particular seasons is not only of intrinsic importance, but is essential for supporting other functions, particularly waterbird habitat functions. On a landscape level, because wetlands appear to host relatively diverse and abundant invertebrate communities the PPR, they can subsidize insectivorous wildlife populations and biodiversity over a wide region. On a site-specific level, invertebrate production in wetlands primarily supports within-basin fish and wildlife recreational and biodiversity values; certain species (leeches) that proliferate in fish-free basins also can help support local baitfish markets, and some wetlands are leased for this purpose (Hubbard 1988). Moreover, as demonstrated in the conceptual model (Adamus 1992b), invertebrates contribute to or help serve as catalysts of many other wetland functions, such as sediment retention (section 3.4), phosphorus retention (3.5), denitrification (3.6), detoxification (3.7), and carbon cycling aspects of vascular plant production (3.8).

DOCUMENTATION OF VALUE: The wildlife values supported in part by PPH invertebrate communities are documented in section 3.11. Some waterfowl seem to select habitat based on the total biomass of invertebrates, rather than the number (density) or mean size (dimension) of the invertebrates (Ball and Nudds 1989). Also, the type of invertebrate (e.g., mud-dwelling worm vs. epiphytic snail vs. swimming beetle) determines use by a particular waterfowl species (Swanson and Duebbert 1989), and this in turn may be influenced with organic inputs associated with interannual hydrologic conditions (Murkin and Kadlec 1986). Apparently no attempts have been made to link quantitative estimates of invertebrate production in PPH wetlands to economic values of the fish or wildlife resources that depend on invertebrates. Indeed, few attempts have been made to measure just invertebrate production (as opposed to density).

TEMPORAL EFFECTS: Invertebrate production within individual basins may be greater during years that are relatively wet and have temperatures that support an extended growing season. An extended series of wet years can cause many basins to become dominated by submerged aquatic species, which generally support greater densities of invertebrates than do emergents.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs of invertebrates to PPH basins occur as a result of dispersal of adults from surrounding basins. This dispersal may be active (i.e., flying adults) or passive (e.g., attached to vertebrates, see Swanson et al. 1984). In the PPR, the capacity for supporting invertebrate production (not diversity) has not been demonstrated to be related to processes occurring at a landscape scale.

B. Site-level (Within-wetland) Indicators of Function

The density of invertebrates is a fairly direct but imperfect indicator of invertebrate production. Its use at this stage of a regional risk assessment is impractical given the current lack of representative data from all portions of the PPR. The percent cover and diversity of submersed plants is also a fairly direct indicator of invertebrate production (e.g., Hanson and Butler 1990). Mixed-species stands of vegetation interspersed with small patches of open water often have the greatest invertebrate densities (Kaminski and Prince 1981; Murkin et al. 1982, Weller and Frederickson 1974, Broschart and Linder 1986). The actual size of the patch openings may be unimportant (Ball and Nudds 1989). Wetland water regime (or less direct surrogates of wetland water regime, such as plant species, soil profile, landscape geology, relief, and geographic position) may to some degree indicate invertebrate production capacity. In general, seasonal and especially semipermanent basins tend to support the greatest invertebrate densities (Swanson and Duebbert 1989, Nelson 1989). Basins whose water levels show naturally large annual fluctuation may be particularly productive. In contrast, those whose levels remain relatively constant over many years (Weller 1981), or which have very productive fish populations, may eventually experience diminished invertebrate production, as may temporary wetlands that are exposed to sustained drought. Basins not subject to drastic, artificial hydrologic alteration, such as sudden changes in water level, may also support greater invertebrate production. Wetlands which are subject to moderate levels of grazing (particularly by muskrat), burning, or especially mowing generally support 2 to 3 times the biomass of invertebrates of untreated wetlands, at least during the first year following treatment (Ball and Nudds 1989). Wetlands not subjected to artificial drainage and/or tillage have greater densities of invertebrates than those so altered (see sections 4.2 and 4.6). Although most wetland basins are highly eutrophic, and thus not usually nutrient limited, wetland soil type may indicate somewhat the potential for invertebrate production. Elevated production capacity may be indicated by the presence of fertile soils that are highly organic yet not highly acidic. Specifically, the most productive basins may be those with salinities in the 1000-3000 mmhos range (Barica 1978).

POSSIBLE INDICATORS OF VALUES: The values of invertebrate production can be indicated by the ecological and recreational importance of the wetland basins in which the production occurs, and the position and proximity of wetland basins relative to important ecological and recreational users. Also, the size class distribution or functional/life cycle diversity of productive invertebrate communities may be a key to making their biomass valuable to waterbirds; greater waterbird production might be sustained only in wetland complexes where invertebrate communities are not only productive, but diverse. Finally, the value of invertebrates to waterfowl varies by season and year, with invertebrate densities perhaps having a greater effect on waterfowl production during years of normal (vs. above-normal) precipitation (Murkin and Kadlec 1986b).

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3.10 Fish Production

DESCRIPTION: The more permanent PPH wetlands can support a productive fish community, comprised mainly of fathead minnows and brook sticklebacks.

DOCUMENTATION OF FUNCTION OCCURRENCE: Fish communities in the PPR can be very productive (e.g., Payer 1977). The function is mainly site-specific.

ASSOCIATED POTENTIAL VALUES: The capacity of PPH basins for supporting productive fish communities is important to some waterbirds (e.g., grebes, herons) and baitfish harvesting enterprises. Like the function, the associated values are mainly site-specific. In South Dakota, semipermanent PPH wetlands are often stocked and used seasonally as rearing ponds for northern pike and walleye (USFWS 1990a), and baitfish enterprises occur locally throughout the region. Fish production in wetlands also may contribute to or help serve as a catalyst of some other wetland functions, such as sediment retention (section 3.4), phosphorus retention (section 3.5), and carbon cycling aspects of vascular plant production (section 3.8). One recent study indicated that fish removal from certain PPH basins might improve water clarity and stimulate growth of macrophytes important to waterfowl (Hanson 1990).

DOCUMENTATION OF VALUE: The commercial and wildlife values supported in part by PPH fish communities have generally not been adequately documented, with only a few attempts (e.g., Carlson and Berry 1990) having been made to link quantitative estimates of fish production in PPH wetlands to local economies. Still, public opinion surveys indicate that many PPR citizens (e.g., 67% in survey by Grosz and Leitch 1990) think that wetlands are important for the fishing opportunities they provide.

TEMPORAL EFFECTS: Fish production within individual basins may be greater during years that are relatively wet, if flooding opens up dispersal corridors among basins and allows colonization of new basins. Years with relatively mild or snow-free winters, or cool and windy summers, may allow survival of more juvenile fish in permanent basins otherwise prone to fishkills from anoxia.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs of fish to PPH basins occur as a result of dispersal of fish from surrounding basins. This dispersal may be active (i.e., movements during high water conditions) or passive (e.g., stocked by humans or escaped from vertebrate predators). In the PPR, the capacity for supporting fish production (not diversity) at a regional scale would seem to be independent of interactions among basins. However, the rate of groundwater discharging into semipermanent basins may be important to overwinter survival of fish occurring in these basins. This discharge may be maintained locally by recharge focused in temporary and seasonal basins. If these basins are disrupted by drainage (i.e., local diversity of basin types is diminished), then fish production might decline (pers. comm., D. Hubbard, South Dakota St. Univ., Brookings). Also, fish production is influenced by climatic aspects of geographic position; in more northerly and easterly PPH basins, severe winter snow cover on ice-covered wetlands has the potential to reduce fish populations by aggravating anoxic conditions.

B. Site-level (Within-wetland) Indicators of Function

The density of fish or the "catch per unit effort" are fairly direct but imperfect indicators of fish production. Their use at this stage in a regional risk assessment is impractical given the current lack of representative data from all portions of the PPR.

Wetland water regime, (or less direct surrogates of wetland water regime, such as plant species, soil profile, landscape geology, relief, and geographic position) is probably the best indicator of fish production in PPH basins. Temporary, seasonal, and semipermanent basins cannot support sustained fish production because they periodically lack standing water. Fish occur only if (a) there is an intermittent connection to more permanent basins, or (b) fish are introduced intentionally, or accidentally by anglers or mobile vertebrates. Within semipermanent and permanent basins, overwinter survival may be greatest when oxygenated, natural spring seeps are present (Peterka 1989). Shallower basins are also less likely to have winterkill problems (e.g., Barica 1984). Basins not subject to drastic, artificial hydrologic alteration, such as sudden changes in water level, may also support greater fish production, as may basins whose shorelines are not subject to severe grazing, mowing, tillage, burning, drainage, frequent human visitation, or other factors discussed in section 4. Although most PPH basins are highly eutrophic, and thus not usually nutrient limited, wetland soil type may be a useful secondary indicator of fish production. Elevated production capacity may be indicated by the presence of fertile soils that are moderately organic and not highly acidic, because such soils support high densities of invertebrates fed upon by some fish. However, excessive production from fertile soils, followed by a winter with deep snow covering the ice, commonly causes fish to die from lack of oxygen or from ammonia toxicity (Baird et al. 1987, Barica and Mathias 1979). Salinity also limits use of some PPH basins by fish. Reproduction is sometime impaired at alkalinities of greater than 1000 mg/l (Peterka 1989). The particular anion that is dominant in a basin influences the precise tolerance threshold.

POSSIBLE INDICATORS OF VALUES: The values of fish production can be indicated partly by the ecological importance of the wetland basins in which the production occurs, the commercial importance of the resource (e.g., baitfish), and the position and proximity of wetland basins relative to important ecological and commercial users.

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3.11 Waterfowl Production

DESCRIPTION: This function consists of the capacity of wetlands to annually produce ducks and geese. Although the discussion below emphasizes the breeding season, PPH wetlands also provide crucial stopover sites for migrating waterfowl. Wetlands of the PPR are also important to many other birds; those values are discussed in the section on biodiversity (section 3.14).

DOCUMENTATION OF FUNCTION OCCURRENCE: Waterfowl depend on PPH wetlands extensively during courtship, nesting, rearing of young, shelter during feather molting periods, and staging prior to migration. The PPR, including the Canadian portion, produces 40-60% of the waterfowl of North America (Smith et al. 1964, Batt et al. 1989). The U.S. portion of the PPR, which comprises 36% of the continental PPR, supports 25-30% of the continental production (Mineau 1987). During some years, North or South Dakota alone annually supports nearly half of the waterfowl production occurring in the conterminous United States (USFWS 1990a). Waterfowl species for which over half the continental population uses the PPR (including Canadian portions), are, in order of dependency: gadwall (95% of population), blue-winged teal, ruddy duck, redhead, northern shoveler, mallard, canvasback, northern pintail, and American wigeon (Batt et al. 1989).

Total duck production (i.e., ratio of broods per total ducks) has declined regionwide over the period 1955-85 (Batt et al. 1989). However, no consistent trend in nest success rates is apparent for most species (Klett et al. 1988). Only in Montana (of the U.S. portion of the PPR) is the decline in broods per total ducks statistically significant, and only in South Dakota are declines in average brood size statistically significant (Batt et al. 1989). Mallard and northern pintail are the only species appearing to have had a sustained population decline (Batt et al. 1989), and they consistently have had the lowest nest success rates (Klett et al. 1988). A nest success rate of 15-20% is needed to sustain continental waterfowl populations (Cowardin et al. 1985). Yet, within the PPR, the success rate in wetlands has consistently been less than this for all species (Klett et al. 1988). This rate has been attained for all waterfowl species only in idle grassland, and only in central South Dakota (all habitats combined).

ASSOCIATED POTENTIAL VALUES: The values of waterfowl production are mainly expressed at the landscape level. Migratory waterfowl produced by PPH wetlands support hunting in many states south and east of the PPR. Waterfowl are also enjoyed nonconsumptively by birders and other recreationists. Waterfowl production probably contributes to, or helps serve as a catalyst of, some other wetland functions, such as phosphorus retention (section 3.5), carbon cycling aspects of vascular plant production (section 3.8), and invertebrate production (section 3.9).

DOCUMENTATION OF VALUE: Public opinion surveys indicate that a large proportion of PPR citizens (e.g., 69% in survey by Grosz and Leitch 1990) think that wetlands are important for the wildlife they support. The North American Waterfowl Management Plan (USFWS and Canadian Wildlife Service 1986) recognizes the PPR as the highest priority region for protection. Waterfowl hunting, and the economy it supports, was valued at $21 million per year in North Dakota in 1986 (Baltezore et al. 1987). North Dakota has the greatest number of waterfowl hunters per capita of any state (Carney et al. 1983). In the Devils Lake Basin specifically, the values of wetlands for waterfowl hunting were theoretically valued at about $25 per wetland acre (Leitch and Scott 1984). In South Dakota, hunting in wetlands in 1985 generated $24 million per year (Johnson and Linder 1986). Other statistics documenting the value of hunting in the PPR are shown in Table B1. Some caution is required in interpretation because the data are not referenced to the source of the waterfowl production (e.g., an unknown proportion of the waterfowl may have been produced in Canada).

TEMPORAL EFFECTS: Waterfowl use of PPH wetlands varies according to annual water conditions and long-term weather cycles (Bellrose 1979, Hammond and Johnson 1984, Batt et al. 1989, Poiani and Carter 1991). Annual water conditions seem to have a greater effect on duck use of wetlands than on hatching success (average production); however, possible monitoring biases make this interpretation inconclusive (Batt et al. 1989). Comparing two years (or locations) with equal amounts of total annual precipitation, the number of basins with water is likely to be much greater for the year (or location) in which, during the preceding autumn, a major rainstorm was followed by freezing, which then was followed by a snow cover that persisted through the winter. Conversely, fewer basins may contain water during years (or locations) where winters are mild and lack continuous insulating snow cover. Although during most years potholes fill to less than 7 inches of standing water, during 100-year runoff events, many potholes will fill to a depth of at least 18 inches (Ludden et al. 1983), with corresponding increase in area. The type of basin water regime that waterfowl select also varies by year, with waterfowl becoming more dependent on permanent and semipermanent basins during drought years.

Table B1. Extent of Hunting in PPR States

State	# of Hunters and/or Anglers (x 1000)	Waterfowl Hunters (x 1000)	Waterfowl Hunting Days/Year (x 1000)
IA	746 (35%)a	52 (15%)	332
MN	1492 (48%)	181 (33%)	1578
MT	270 (45%)	23 (11%)	249
ND	220 (45%)	47 (40%)	419
SD	202 (40%)	35 (26%)	354
a Data are from 1985, as compiled mainly by USFWS (1988).

 

Species whose use of PPH wetlands appears most sensitive to regional water availability include northern pintail, northern shoveler, blue-winged teal, ruddy duck, and redhead (Stewart and Kantrud 1973, Ruwalt et al. 1979, Batt et al. 1989).

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs of waterfowl to PPH basins are indicated partly by basin proximity to continental flyways and other traditionally used areas. Subregional differences in waterfowl abundance are described by Brewster et al. (1976) and Batt et al. (1989). Also, landscape connectivity (or conversely, the degree of wetland isolation or wetland complex fragmentation) may be important for some waterfowl species in the PPR. Connectivity is defined somewhat differently than it would be in forested regions. Areas of the PPR with cumulatively greater waterfowl production may be those where wetlands are connected by ill-defined, semi-continuous corridors or networks of (a) lower-intensity land use (e.g., idle grassland, fewer roads, less frequent tillage, wetter soils) or perhaps (b) habitat with greater temporal predictability. Such conditions may represent reduced isolation among basins, provide greater nesting cover, and enhance landscape permeability for local movements, particularly of hens with broods.

The capacity of various subregions of the PPR for supporting waterfowl production is indicated somewhat by longitude--nesting success may be greater in the western portion of the PPR (USFWS 1990a). Capacity of landscapes to support waterfowl production is especially indicated by the diversity (number and proportions) of basin types within the subregions, as well as the spatial arrangement of wetland basins relative to the most essential upland habitats (idle grassland, planted cover). Within "complexes" comprised of diverse wetland types, waterfowl find conditions optimum for meeting many needs; indeed, individual females (depending on species and water conditions) may use up to 22 different wetland basins during a single summer (Dwyer et al. 1979). As expressed by Hubbard (1988):

Even if isolated, a semipermanent basin generally provides acceptable waterfowl breeding habitat; however, its status would be improved if it were part of a good complex. An isolated semipermanent basin may provide excellent breeding habitat if it contains large peripheral temporary and seasonal wetlands.

Temporary and seasonal wetlands are of greatest importance in landscapes where there is already at least one semipermanent basin per 4 square miles. Also, temporary and seasonal basins can provide the only suitable overwater nesting habitat during exceptionally wet years when peripheral nesting cover of semipermanent basins is completely inundated.

One expression of spatial arrangement of basins is the contagion (e.g., proportion of subregional wetland acreage that is dispersed vs. clumped). Wetland acreage occurring as scattered complexes rather than as single large wetlands should have a larger shoreline-to-area (or smaller depth-to-volume) ratio, and this is likely to be associated with greater secondary productivity. Although some preliminary evidence suggests that at least during wet years, clustered wetlands (mainly temporary basins) may have lower brood densities than scattered basins (Wishart et al. 1984), this may represent sampling bias rather than actual differences in production.

A crucial question is: How far from the rest of a "wetland complex" can a wetland basin be before it is considered functionally disjunct from the complex? This depends on type of functional use (breeding or migration stopover), species, surrounding habitat types, intrinsic productivity of the compared wetlands, and regional water conditions during a particular year. Essentially nothing is known regarding how far apart wetlands may be spaced before waterfowl movements between them are so energetically draining that (a) migration is severely delayed (with consequent increased mortality) or (b) individual birds, once arriving on breeding grounds, are too nutritionally depleted to successfully rear broods. During the breeding season, dabbling ducks have home ranges of between about 75 and 1200 acres. For breeding functions, temporary and seasonal basins are of greatest value if within 0.15 mile of another wetland (Sousa 1985), and temporary and seasonal wetlands located more than about 0.5-1 mile from another wetland might be assumed to receive little use (Hubbard 1988). Regionwide studies currently being initiated by the USFWS (researchers are Cowardin and Klaus) will examine waterfowl recruitment as a function of the degree of basin isolation and intervening land use types. Computer simulations using energetics models may also be used to estimate configurations of wetlands necessary to maintain brood success.

Based on a consensus of regional biologists, Sousa (1985) suggested that, for at least one waterfowl species (blue-winged teal), the greatest benefit of wetlands occurs where (a) wetlands suitable as nesting habitat are present at a density of not less than about 480 acres per square mile, (b) wetlands suitable for territorial pairing of waterfowl are at a density of not less than 160 acres per square mile, and (c) wetlands suitable as habitat for broods comprise not less than 50 acres per square mile, whichever is most limiting. For pairs, the 160 acres should be distributed such that there are ideally 150 individual wetlands per square mile. For broods, the 50 acres should be distributed in about 6 wetlands per square mile (Sousa 1985).

B. Site-level (Within-wetland) Indicators of Function

The density of waterfowl is a fairly direct but imperfect indicator of waterfowl production. Its use at this stage in a regional risk assessment is impractical given the current lack of representative data from all portions of the PPR.

Opportunities for inputs of colonizing waterfowl may be highly indicative of waterfowl production capacity. As can be inferred from the landscape discussion above, basins likely to support greater waterfowl production are those located (a) closest to patches of low-intensity land use or wetland, and perhaps (b) where intervening land cover between patches or basins is relatively permeable to brood movements. The land cover surrounding wetlands, as well as land cover intervening among wetlands, is important. Buffers of natural vegetation surrounding wetland basins can enhance waterfowl production within wetlands by providing additional habitat (structural) diversity; providing dense, protective cover that shelters broods from predators and extremes of weather; intercepting and immobilizing contaminants that would otherwise diminish diversity of wetland food chains; and reducing noise and visual intrusion by people and vehicles (Burger 1981, Pomerantz et al. 1988). In some cases, certain types of buffer zones of cropland surrounding wetlands can provide feeding opportunities for some waterfowl. With regard to feeding, Pederson et al. (1989) noted, "Waterfowl use of agricultural habitats is related to the proximity of refuges and staging areas and to the type and abundance of agricultural grain in the area." For nesting, however, most agricultural land surrounding PPH wetlands supports low waterfowl densities, compared to natural land covers.

Capacity for supporting waterfowl production within an individual basin is indicated partly by wetland water regime (or less direct surrogates of wetland water regime, plant species, soil profile, landscape geology, relief, and geographic position). Temporary and seasonal basins supply migrating and early-nesting birds with abundant foods, partly because they warm up sooner in the season (Hubbard 1988). Surveys that have compared waterfowl occurrence in various types of PPH wetlands during the breeding season show less than 10% of the waterfowl using temporary basins (Ruwalt et al. 1979, Stewart and Kantrud 1972), as opposed to use of seasonal basins by from 16% (Ruwalt et al. 1979) to 47% (Stewart and Kantrud 1973) of the nesting population, and use of semipermanent and permanent basins by from 16% (Stewart and Kantrud 1973) to 47% (Ruwalt et al. 1979) of the nesting population. However, these simple measures of occurrence, sometimes made at one or a few points in time, do not necessarily correlate with the relative importance and substitutability of various basin types for sustaining waterfowl (pers. comm., L. Flake, South Dakota St. Univ., Brookings). Temporary and seasonal basins are particularly important during wetter years (Duebbert and Frank 1984). Semipermanent and permanent basins are essential for late nesting, brood rearing, molting, and staging prior to fall migration. These functions are served by the sustained water habitat, large density of invertebrate foods, and protection from predators which semipermanent and permanent basins supply. However, the productive capacity of these more permanently inundated basins depends largely on years elapsed since last drawdown, with basins that have been more recently drawn down usually supporting greater production. Preference for particular basin water regimes varies by species, with diving duck species using semipermanent and permanent wetlands to a greater degree than dabbling duck species, due partly to presence of higher densities of their preferred non-insect invertebrate foods in these types of basins (Swanson 1988).

Basins not subject to drastic, artificial hydrologic alteration, such as sudden changes in water level, may support greater waterfowl production. Wetlands which are subject to moderate levels of grazing (particularly by muskrat), burning, or especially mowing generally support conditions favoring greater waterfowl production, at least during the first year following treatment (Ball and Nudds 1989). Wetlands not subjected to artificial drainage and/or tillage have greater densities of waterfowl than those so altered (see sections 4.2 and 4.6). Basins that lack fish populations may support greater densities of aquatic invertebrates preferred by nesting waterfowl. Although most PPH basins are highly eutrophic, and thus not usually nutrient-limited, wetland soil type may be a useful secondary indicator of the capacity of wetlands to support waterfowl. Elevated production may be indicated by fertile soils that are not highly acidic or saline. Although saline wetland basins provide some feeding habitat for adult waterfowl, their use by waterfowl broods is relatively limited. Recently hatched ducklings have difficulty surviving in basins where the specific conductance of waters exceeds about 20,000 mS/cm (Swanson et al. 1984).

Waterfowl production capacity is also indicated by habitat heterogeneity of a basin. Habitat patches can be defined according to various combinations of basin hydrologic permanence types, water depth, vegetation form and species, soil type, bank slope angle, natural disturbance frequencies, and other factors. The occurrence of vegetated islands suitable for nesting or roosting is one particularly good indicator of waterfowl production. Also, the presence of multiple vegetation forms, well-interspersed with a relatively equal portion of open water, strongly enhances waterfowl production. Under such conditions, ecotones between open water and vegetation provide birds with natural territorial boundaries (Weller and Spatcher 1965). For waterfowl, both the ecotone length and degree of interspersion with open water are important indicators. Waterfowl populations are more highly correlated with total length of wetland shoreline than total acreage of wetlands (e.g., Weller 1979). For ponds of equal area, higher brood densities have been observed on more irregularly shaped ponds (e.g., Mack and Flake 1980, Hudson 1983). Maximum waterfowl production is supported in basins where equal amounts of vegetation and open water are well-interspersed (Weller and Spatcher 1965, Kaminski and Prince 1981; Murkin et al. 1982, Weller and Frederickson 1974, Broschart and Linder 1986). At least for one species (mallard), open water patches in such a configuration should have a diameter of at least 150 feet (Ball and Nudds 1989). In an unmanaged marsh, the distribution and proportion of vegetation changes greatly from year to year. Over an idealized wet-dry cycle, the vegetation progresses from dry marsh, to regenerating marsh, to degenerating marsh, and finally to lake marsh before reversing (Weller 1981). Associated changes in interspersion and internal vegetative diversity profoundly affect waterbird community composition.

Wetland basins oriented so that they are sheltered from extreme exposure to wind may support greater waterfowl production. However, large open areas can be important for waterbird roosting (Hoy 1987).

POSSIBLE INDICATORS OF VALUES: The value of waterfowl production can be indicated by the productive capacity of the wetland complexes in which the production occurs, the proximity of these areas to nonconsumptive users, the annual harvest of locally-bred waterfowl, and the proximity of areas used by waterfowl during migration and wintering to both hunters and nonconsumptive users. In parts of the PPR where wetlands are leased for waterfowl hunting, a portion of the cost of hunting leases can be used to help estimate waterfowl value.

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3.12 Winter Wildlife Shelter

DESCRIPTION: This function consists of the capacity of wetlands to reduce thermal stress to non-aquatic birds and mammals during winter. Most of the species that are benefitted are year-round residents of the region, but not necessarily of wetlands.

DOCUMENTATION OF FUNCTION OCCURRENCE: In many landscapes within the PPR, a few species depend on PPH wetlands almost exclusively during winter months, at least during periods of severe weather. Examples include Ring-necked Pheasant and White-tailed Deer. Wetlands in winter provide shelter from strong winds and frequent human disturbance, as well as a limited supply of food for some species (Kramlich 1985, Sather-Blair and Linder 1980, Fritzell 1988). In fact, wind velocity within some PPH wetlands is 95% less than in deciduous-wooded shelterbelts (Schneider 1985). In one area of South Dakota, over 70% of the suitable wintering habitat for pheasants was wetland, even though wetlands comprised a relatively small proportion of the landscape (Sather-Blair and Linder 1980).

ASSOCIATED POTENTIAL VALUES: Many of the wintering species support opportunities for hunting and nonconsumptive recreation, mainly at a local scale. In fact, these species support higher local levels of recreation (more hunter use-days) than species using the wetland predominantly in summer (Johnson and Linder 1986). Some characteristics which make a wetland attractive as winter shelter also make it attractive for some waterfowl functions (e.g., roosting) while making it less suitable for other waterfowl functions (e.g., loafing areas).

DOCUMENTATION OF VALUE: Available estimates of economic values associated with hunting in the PPR are mentioned in section 3.11. The economic values of hunting generally, and (less often) of pheasant and deer hunting specifically, have been quantified in a few cases. However, such estimates are either very site-specific (i.e., cannot be reliably extrapolated to all PPH wetlands), or are not referenced according to what portion of the harvested population had depended specifically on wetlands for winter survival.

TEMPORAL EFFECTS: The magnitude of this function will be greater during winters with severe winds and cold. The function may be lessened during winters of heavy snow, as many wetlands tend to trap drifting snow, and this can reduce access of some species to sheltering vegetation.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

The ability of wetlands to provide winter shelter for wildlife is best assessed at regional and landscape levels, because most species that benefit from winter cover disperse into non-wetland ecosystems during other seasons. Landscape inputs of upland wildlife to PPH basins during the winter are mainly a function of basin proximity to populations of resident upland species, and the proximity to non-wetland habitat that is suitable for overwintering. Use of wetlands for overwintering probably is directly related to subregional climate (e.g., latitude), and to habitat suitability of proximate uplands. That, in turn, is indicated partly by crop type, tillage management practices, and extent of sheltering from upland vegetation (e.g., windbreaks). Natural factors, such as surficial geology and topography also play a role; for example, terminal moraines tend to have more woody cover than dead-ice moraines (Kantrud 1981). Capacity for supporting overwintering wildlife at a regional scale can be indicated by spatial distribution patterns of basin types most suitable for wintering wildlife. However, even basins that are not part of a complex can, if sufficiently large, provide essential cover to locally higher densities of resident species.

B. Site-level (Within-wetland) Indicators of Function

The density of wildlife wintering in wetlands is a fairly direct but imperfect indicator of overwintering capacity. Its use at this stage in a regional risk assessment is impractical given the current lack of representative data from all portions of the PPR.

The effective patch (vegetated wetland) size, percent cover, and height of robust perennial plants in a PPH basin largely determines its capacity for supporting overwintering wildlife. Pheasant use of one set of 15 South Dakota wetlands was greatest in wetlands larger than about 25 acres and within about one mile of other suitable wetlands (Sather-Blair and Linder 1980). To some degree, wetland size, percent cover, and vegetation height may be inferred from wetland water regime (or less direct surrogates of wetland water regime, such as plant species, soil profile, landscape geology, relief, and geographic position). Temporary and seasonal wetlands, if untilled, often have denser winter cover than semipermanent and permanent basins, although the latter may have larger patches of the more robust vegetation most valuable as winter cover. Basin capacity to support overwintering wildlife is also indicated by a relative lack of severe grazing, mowing, tillage, burning, drainage, frequent human visitation, or other factors discussed in Section 4.

POSSIBLE INDICATORS OF VALUES: The value of winter wildlife shelter can be indicated by the sheltering capacity of the wetlands, the proximity to hunters of autumn distributional ranges of wildlife that overwinter in these wetlands, and the proximity to nonconsumptive users of the distributional ranges (at any season) of the overwintering wildlife.

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3.13 Furbearer Production

DESCRIPTION: This function consists of the capacity of wetlands to support mammals that are trapped for fur.

DOCUMENTATION OF FUNCTION OCCURRENCE: A few furbearing species depend on PPH wetlands almost exclusively, and others may do so locally or during severe weather. Muskrat is the predominant and most dependent species. Others include raccoon, skunk, coyote, red fox, and mink. Wetlands provide abundant food and shelter to these species.

ASSOCIATED POTENTIAL VALUES: Furbearers support opportunities for trapping and nonconsumptive recreation, mainly at a local scale. The habitat value for furbearers is expressed mostly at a site-specific scale for muskrat, and at a landscape scale for most other furbearers because of their larger home ranges. Furbearers are also important to some other wetland functions, notably Vascular Plant Production, Invertebrate Production, and Waterfowl Production.

DOCUMENTATION OF VALUE: Available estimates of economic values associated with furbearer trapping in the PPR cannot be referenced according to what portion of the harvested population had depended specifically on wetlands. Only in the case of muskrat can value be attributed almost solely to wetlands. In North Dakota, statewide income from furbearer harvesting during 1985-86 averaged $103,000/yr for muskrat, $64,000/yr for mink, $255,000 for coyote, $202,000 and for raccoon (S.Allen, cited in Kantrud et al. 1989). In South Dakota, statewide income from furbearers during 1986-87 averaged $617,000/yr for muskrat, $203,000/yr for mink, $350,000/yr for coyote, $368,000/yr for fox, and $715,000/yr for raccoon (L. Fredrickson, cited in Kantrud et al. 1989).

TEMPORAL EFFECTS: Furbearers, particularly muskrats, undergo wide annual fluctuations in population levels in response to water conditions. The dependency of furbearers on wetlands will be greater during drought years and winters with severe cold.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators of Function

Regional and landscape inputs of furbearers to PPH basins are mainly a function of basin proximity to suitable habitat located in surrounding wetland basins or uplands (e.g., conditions suitable for den sites). Indicators of the suitability of wetland basins are described below; indicators of upland suitability include crop type, tillage management practices, extent of upland vegetation (e.g., windbreaks), and topography. Landscape inputs are also indicated by geographic location within the PPR; due to climate and other influences, the current range of some furbearers includes only part of the PPR. For example, muskrat abundance is greater in the southeastern PPR; the species' northern limit is determined by freezing to bottom of wetland basins, and its western limit is determined by increasing drought frequency (Kantrud et al. 1989). Coyotes tend to increase in a westerly direction in the PPR. Capacity for supporting furbearers at a regional scale can be indicated by the spatial distribution patterns of basin types (as described below) most suitable for furbearers. Even basins that are not part of a complex can, if sufficiently large and of the right type, support considerable furbearer production.

B. Site-level (Within-wetland) Indicators of Function

The density of furbearers, as estimated from counts of dwellings, trapping records, or tracks, is a fairly direct but imperfect indicator of wetland capacity for supporting furbearers. Its use at this stage in a regional risk assessment is impractical given the current lack of representative census data on furbearers from all portions of the PPR.

The effective patch size, percent cover, and height of the dominant plant species in a PPH basin largely determines its capacity for supporting certain furbearers. Muskrat populations in particular are most productive in wetlands dominated by bulrush, common reed, or especially, cattail. Basins not subject to drastic, artificial hydrologic alteration, such as sudden changes in water level, may have greater capacity to support aquatic furbearers, as may basins not subject to severe grazing, mowing, tillage, burning, drainage, sustained and intensive furbearer harvest, or other factors discussed in section 4.

POSSIBLE INDICATORS OF VALUES: The value of PPH wetlands for furbearers may be indicated by the capacity of the wetlands and surrounding areas to support furbearers, and the proximity to trappers of the distributional ranges of furbearing species which depend on wetlands. A portion of the market price of fur can be used to estimate wetland value, and this depends on furbearer species, subregion, and year. Fur prices in the PPR can vary by at least an order of magnitude between years (Hubbard 1988).

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3.14 Biodiversity

DESCRIPTION: Biodiversity consists of the capacity of wetlands both individually and cumulatively to support a large variety of plants, invertebrates, and vertebrates. Biodiversity concerns the variety of genotypes, species, biotic communities, and trophic groups. For purposes of this report, biodiversity will be considered synonymous with species density (i.e., species richness per unit). It is recognized that wetland or landscape types capable of supporting a large number of species of one phylum (e.g., plants) are not always optimal for supporting maximum diversity of another phylum (e.g., aquatic insects). Also, it is recognized that genetic diversity is an intrinsic part of "biodiversity," and is not always associated with great species diversity or richness. Despite its potential importance, genetic diversity in this report is not considered because too little information is available on genetic diversity of PPH communities.

DOCUMENTATION OF FUNCTION OCCURRENCE: The number of PPR bird species is greater in landscapes with abundant wetlands than landscapes with fewer wetlands (Kantrud 1981). Many species depend on PPH wetlands almost exclusively, and others are dependent locally, seasonally (e.g., sandhill cranes and shorebirds, Krapu and Johnson 1990), or only during extreme weather conditions. Not all "wetland" species are equally dependent on wetlands; many also use non-wetland habitats, to an equal or greater degree.

Mammals that are particularly dependent on PPH wetlands are listed for part of the region by Fritzell (1988) and Kantrud et al. (1989); birds by Faanes (1982), and Kantrud and Stewart (1984); and plants by Reed (1988). Wetland plants considered by government or private groups to be threatened, endangered, or of special concern number 21 in South Dakota (SD Natural Heritage Program listing) and 62 in North Dakota (ND Chapter of the Wildlife Society 1986). A few native fish species, such as the finescale dace, are highly dependent on PPH basin wetlands (USFWS 1990a), as are at least three amphibians (Wheeler and Wheeler 1966, Kantrud et al. 1989).

ASSOCIATED POTENTIAL VALUES: Biodiversity may be considered both a function and a value. It is crucial to recognize its importance at multiple scales. One cannot assume that protecting wetlands or wetland types that have the most species results in protecting biodiversity, because some wetlands with few species can have the greatest incremental contribution to regional biodiversity, if those few species are present at no (or few) other wetlands. At all scales, biodiversity serves as one basis for nonconsumptive recreation (e.g., birding). Particularly at a landscape scale, biodiversity is important for contributions to maintaining regional pools of genetic material, as well as for maintaining communities that are resilient in the face of environmental change.

DOCUMENTATION OF VALUE: Public opinion surveys indicate that a large proportion of PPR citizens (e.g., 69% in survey by Grosz and Leitch 1990) think that wetlands are important for the wildlife they support. Estimates of economic values associated with biodiversity in the PPR are not available. Some general information exists on nonconsumptive values of wildlife, but cannot be attributed entirely to wetlands.

TEMPORAL EFFECTS: Temporal effects depend on scale, phylum, and wetland type. At one extreme, biodiversity of aquatic insects in temporary wetlands at a site-specific scale can fluctuate considerably among years. At the other extreme, biodiversity of bog-dwelling plants at a regional scale shows virtually no variation among years. Fluctuations of biodiversity are mostly related to severity of hydrologic conditions during particular sequences of years.

POSSIBLE INDICATORS OF FUNCTION

A. Regional and Landscape-level Indicators

Regional and landscape inputs of different species to various subregions of the PPR are partly indicated by geographic location within the PPR. A generally larger species pool may exist in the southeastern portion of the PPR, due to climate, proximity to flyways and continental biome boundaries, and other influences. Differences among subregions of the PPR may also be indicated by differences in landscape heterogeneity, defined as the number of distinct habitat patches per unit area, and the variety of their juxtapositioning combinations. Landscape connectivity (or conversely, the degree of wetland isolation or wetland complex fragmentation) may also be important for some PPR species, such as mammals, fish, and vascular plants, and is defined somewhat differently than in forested regions. Subregions of the PPR with cumulatively greater species density may be those where wetlands are connected by ill-defined, semi-continuous corridors or networks comprised of (a) lower-intensity land use (e.g., fewer roads, less frequent tillage, wetter soils) or (b) topographically low areas that during extreme wet years connect basins, or (c) habitat with greater temporal predictability. Such conditions may represent reduced isolation among basins and enhanced landscape permeability for species dispersal.

Capacity for supporting a high diversity of wetland species at a landscape or subregional scale is indicated partly by the presence of a wide range of conditions among wetlands of hydroperiod, water depth, soil type, vegetation form and species, water chemistry, natural disturbance frequency, and other factors. Some of the less common (but not necessarily rare) wetland types in the PPR, which may contribute the most to regional biodiversity, include:

saline basins

peat bogs (i.e., wetlands with hydroperiod classified as "saturated," and slightly acid conditions)

fens (special recognition by Iowa and Minnesota Departments of Natural Resources; somewhat alkaline conditions)

wild rice (Zizania) wetlands (special recognition by Minnesota DNR)

all palustrine emergent prairie potholes in 35 counties of Iowa (special recognition by Iowa DNR)

restored wetlands

forested basin wetlands

low-order riparian wetlands

riverine wetlands along Missouri and Mississippi Rivers (special recognition by Iowa DNR), especially oxbow and forested wetlands

basins of any type that have not recently been tilled, severely grazed, or otherwise directly altered by humans

basins of any type not surrounded by high-intensity agriculture or residential uses.

In some subregions of the PPR, certain of these types may be encountered more widely, but on a regionwide basis they are a relatively small proportion of the entire resource. Also, some types that are widespread on a regional basis, but are rare within a particular watershed or locality, may be particularly important for their contribution to biodiversity. In considering this, it is important to use wetland "type" in a broad sense to mean not only basins containing a locally uncommon water regime, but also those with a locally uncommon chemical/soil regime, size class, juxtaposition, and/or other classifier of biological importance.

B. Site-level (Within-wetland) Indicators of Function

Landscape inputs of species may be as indicative of site-level diversity as of landscape-level diversity. As can be inferred from the above, basins having the greatest potential for supporting a larger species density are those located (a) near biome boundaries or closer to the southeasterly parts of the PPR, (b) where intervening land cover between patches or basins is relatively permeable to species movements, and (c) closest to patches of low-intensity land use or wetland, particularly those of a different or locally rare type (e.g., wet meadow, ungrazed native prairie, prairie thicket, shelterbelt; see Faanes 1982). Land cover surrounding wetlands, as well as land cover intervening among wetlands, is important. Buffers of natural vegetation surrounding wetland basins can enhance animal diversity within wetlands by providing additional habitat (structural) diversity; providing dense, protective cover that shelters young from predators and extremes of weather; intercepting and immobilizing contaminants that would otherwise diminish diversity of wetland food chains; and reducing noise and visual intrusion by people and vehicles (Vaske et al. 1983, Ream 1980, Dickman 1987, Simpson and Kelsall 1979, Ryder et al. 1980). Buffer zones of cropland surrounding wetlands can provide feeding opportunities for some wetland wildlife in regions of low human population density and infrequent disturbance. However, agricultural land surrounding most PPH wetlands normally contributes little to biodiversity, compared to natural land covers. The importance of isolation to biodiversity (species density) in individual basins is documented by Brown and Dinsmore 1986.

Capacity for supporting great species densities within an individual basin is indicated partly by habitat heterogeneity within the basin. As with the landscape scale, this addresses the number of distinct habitat patches per unit area, the variety of their juxtapositioning combinations, and their spatial arrangement within a basin. Habitat patches can be defined according to various combinations of hydroperiod, water depth, vegetation form and species, soil type, bank slope angle, natural disturbance frequencies, and other factors. Islands and sand/gravel bars are particularly effective in increasing spatial heterogeneity. They are disproportionately used as resting and feeding sites by migratory shorebirds, and for nesting by many species, because of the protection they provide from small mammalian predators. Also, the presence of multiple vegetation forms, well-interspersed with a relatively equal portion of open water, contributes strongly to avian diversity within wetlands. Under such conditions, ecotones between open water and vegetation provide birds with natural territorial boundaries, and support habitat elements important both for open water species and species characteristically requiring more vegetated environments (Weller and Spatcher 1965). In addition, transition zones are inhabited by species which seem adapted specifically to the edge environment (e.g., yellow-headed blackbirds, gallinules, American coots, and least bitterns). Maximum wetland bird richness usually is favored by equal proportions of open water and emergent vegetation, well interspersed (Weller and Spatcher 1965, Kaminski and Prince 1981; Murkin et al. 1982, Weller and Frederickson 1974; Weller 1979; Broschart and Linder 1986). However, at some point, especially for animals with larger territories or requirements for isolation, too much interspersion of open water with vegetation might lead to within-basin fragmention of requisite shelter and territorial markers. Structurally heterogeneous wetlands also can support a greater diversity of macroinvertebrates (e.g., Dvorak and Best 1982). Invertebrate richness tends to be greatest where aquatic bed and emergent classes are interspersed with each other or with open water (Voights 1975).

Within-basin biodiversity is also correlated with wetland water regime (or less direct surrogates of wetland water regime, such as plant species, soil profile, landscape geology, relief, and geographic position). Although the greatest within-basin avian nesting density is supported by semipermanent basins, either semipermanent (Kantrud and Stewart 1984) or permanent basins (Faanes 1982) can support the largest mean number of species. The greatest within-basin floristic diversity is supported by untilled basins whose shorelines are not abrupt (Kantrud and Stewart 1984). Temporary wetlands may become less diverse after being exposed to sustained drought. Basins not subject to drastic, artificial hydrologic alteration, such as sudden changes in water level, may also be richer in species, as may basins not subject to severe grazing, mowing, tillage, burning, drainage, frequent human visitation, or other factors discussed in Section 4.

Basin size also is an indicator, but perhaps not a strong determinant, of species density. It may be expressed as acreage of basin, acreage of wetland (excluding open water), linear feet of shoreline (i.e., ecotone between open water and vegetation), or some combination of these. Plant diversity sometimes is correlated with size of nonpermanent wetlands (e.g., Ebert and Balko 1987), as is invertebrate diversity, at least of mollusks (e.g., Lassen 1975, Aho 1978), midges (e.g., Driver 1977), and crustaceans (e.g., Fryer 1985). Effects are difficult to separate from effects of increased permanency associated with larger basins, and some studies (Driver 1977, Ebert and Balko 1987) have suggested that the latter effect is more determining for invertebrates. One study (Brown and Dinsmore 1986) found greater aquatic bird species richness in larger PPH basins, and over a range of 0.5-450 acres, 10 of 25 species did not use wetlands smaller than about 2 acres. Large open basins can be important for shorebird feeding, waterbird roosting, and molting (e.g., Hoy 1987). However, because large basins in the PPR are often more spatially heterogeneous, it is also difficult to separate the effects of size from those of habitat heterogeneity (Patterson 1976). Also, if water levels in large isolated basins remain relatively constant over many years, diminished oxygen and reduced nutrient availability may restrict production and diversity.

Although most PPH basins are highly eutrophic, and thus not usually nutrient-limited, wetland soil type may be a useful secondary indicator of within-basin biodiversity. Elevated production and perhaps elevated biodiversity (e.g., Kantrud 1981) may be indicated by the presence of fertile soils that are not highly acidic or saline. Saline wetland basins generally have the lowest within-basin biodiversity (Ungar 1974), but the relatively few species they contain, e.g., piping plover, may be regionally rare or highly restricted (Faanes 1982). Thus, as noted above, the biodiversity value of such wetlands is primarily at a regional scale.

POSSIBLE INDICATORS OF VALUES: On a landscape scale, the biodiversity value of an individual wetland (or wetland complex) can be indicated by its proximity to nonconsumptive users, and the proportion of its wetland-dependent flora and fauna that is comprised of species that are highly restricted in their distribution within wetlands across the region. Indicators of wetlands likely to have the largest proportion of highly restricted species are described above. Many procedures exist for summarizing information on species restrictedness (or ubiquity) into indices useful for planning (e.g., Usher 1986, Cable et al 1989). On a site-specific scale, the biodiversity value of an individual wetland (or wetland complex) can be indicated again by its proximity to nonconsumptive users, and on its species density. Indicators of wetlands likely to have great species density are described above.

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6.0 Landscape Studies and Indicators

6.1 Previous Landscape Studies in the Region

Relatively few research studies in the PPR have taken a regionwide, landscape-level, comparative approach. Perhaps the most common studies of this type have been hydrologic statistical analyses involving regression of watershed characteristics against streamflow in multiple watersheds. For example:

In North Dakota: Crosby 1974, 1975, U.S. Army Corps of Engineers St. Paul District 1989

In South Dakota: Becker 1974

In Minnesota: Guezkow 1977, Moore and Larson 1979, Jacques and Lorenz 1988, U.S. Army Corps of Engineers St. Paul District 1989

In Iowa: Melvin et al. 1971, Lara 1973, 1974

In Montana: Dodge 1972, Johnson and Omang 1976, Johnson et al. 1976, Cunningham and Peterson 1983

The streamflow regression studies contain abundant data on geomorphic characteristics (e.g., sizes, soil types, slopes) of a large number of watersheds, and in a few cases include estimated acreages of wetlands, drained areas, or surface storage areas specifically (e.g., Moore and Larson 1979, Jacques and Lorenz 1988). Other data sources (e.g., Benson et al. 1987) provide watershed boundaries but do not include geomorphic information. Issues surrounding use of regression and simulation modeling approaches, as applied specifically to the PPR, are summarized by Moore and Larson 1979, Miller and Frink 1984, and U.S. Army Corps St. Paul District 1988, 1989. Simulation approaches have been used in several instances in the PPR to understand groundwater and runoff processes at a landscape level (e.g., DeBoer and Johnson 1971, Larson 1975, Crowe and Schwartz 1981, Emmons 1988, Winter and Carr 1980), but few have included an explicit wetland component.

Few broadscale regression studies involving water purification functions of wetlands have been published in the PPR. In northwestern Iowa, Jones et al. (1976) regressed nitrate and phosphorus against land cover variables from 34 watersheds. In eastern Montana, Lambing (1984) regressed sediment yield against land cover and soils data from 121 areas. Landscape-level simulations of nutrient or sediment runoff also have been conducted for several PPR watersheds (e.g., Felderman and Eno 1976). Some have used the AGNPS model, but have not focused specifically on the role of wetlands.

Several regression studies relating waterfowl use to landscape characteristics have been published. These include studies by Ducks Unlimited (see Koeln et al. 1988), Brewster et al. 1976, Brown and Dinsmore 1986, Evans and Kerbs 1977, Flake et al. 1977, Heitmeyer and Vohs 1984, Hudson 1983, Klett et al. 1988, Mack and Flake 1980, Kantrud and Stewart 1977. Only one landscape-level simulation (Cowardin et al. 1988) of waterfowl dynamics has been published, and this did not explicitly examine consequences to production of different wetland configurations and type combinations.

Few time-series regression studies have been done in PPR landscapes, in which changes in drainage, crop management practices, or other potential loss factors have been regressed against changes in streamflow, waterfowl production, or water quality. Most of these few studies have involved trends analyses of streamflow (e.g., Brun et al. 1981).

Landscape data are also becoming available in a few localities for use in runoff simulation models. Specifically, such data are being digitized and compiled for use in applications of the AGNPS model in the state of South Dakota's Oakwood Lakes-Poinsett Research Project (Brookings, Hamlin, and Kingsbury Counties). That 10-year, watershed-scale project is evaluating the effects of crop management practices, and fertilizer and pesticide applications, on groundwater and surface water quality (SDRCWP 1990). In South Dakota, land cover and soils are being digitized for AGNPS applications to the following watersheds: Canyon, Punished Woman (Minnesota River Basin), Redfield (James River Basin), Richmond (James River Basin), State (James River Basin). Similar information is being compiled in the Big Sioux Basin for Lakes Norden and Campbell. About 60 other lake watersheds in South Dakota are being delineated and digitized as part of Section 314 planning studies, but there are no current plans to compile their attribute data. Non-digitized land cover and road mileage maps also are available for all watersheds designated as priority watershed for nonpoint management by the state of North Dakota.

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6.2 Sources of Data on Indicators

Future multi-function research and planning efforts in the PPR may be limited by the high cost of acquiring new data at a landscape scale. Despite the paucity of existing sets of comparable, digitized, landscape-level data, such data are perhaps the most practical to use in future analyses. At least for planning-level, relative categorizations, such data can be used to quantify and map the indicators described in previous chapters. In Table B3 (p. B-96), indicators presented in section 3.0 are summarized by function. Then, datasets that might be used to estimate these indicators are indexed to the specific indicators in Table B4 (p. B-98). In some cases, indicators considered to be dependent variables in some analyses may be considered to be independent variables in other analyses. For example, wetland acreage could be used as a dependent variable in a regression against "miles of drainage ditch", but could be used as an independent variable (indicator) in a regression against peak flow. Major characteristics of each data set are briefly summarized at the end of Table B4 (page B-98), with emphasis on current status of regional coverage; a more thorough analysis of each of their strengths and limitations is presented by Adamus (1992).

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6.3 Existing GIS Capabilities

In general, there appear to be few central repositories of diverse, digital resource data in the region. In Montana, a Geographic Information System (GIS) and database called "MAPS" is capable of summarizing data on about 20 resource themes by 8 square-mile cell for anywhere in the state (Nielsen et al. 1990). Minnesota also has a raster-based statewide GIS (MLMIS) with information on land use and soil landscape unit, with a minimum resolution of 40 acres. They anticipate completing the vector digitization of NWI maps covering the entire state by June 1992, and are proceeding with an update of the land cover data. Iowa is in the process of developing statewide GIS resource databases, with initial focus on detailed mapping of soils (pers. comm., K. Kane, Iowa Dept. Natural Resources, Des Moines). Of the Dakotas, only South Dakota appears to be moving ahead with GIS capabilities. Their developing GIS currently includes digital versions of 32 NWI quad maps. They expected to complete the 1:100,000 digital soils coverage (STATSGO, State Soil Geographic Database) for the entire state in 1992; more detailed (SSURGO) digitization has nearly been completed for 6 PPR counties ("GIS News in South Dakota" newsletter, SD State Univ.). They hope (in work done jointly with the East Dakota Water Development District) to complete digital coverage for six eastern South Dakota counties in FY92. This coverage will include 1:24,000 scale wetlands, soils, geology, aquifers, floodplains, land cover, and elevation. North Dakota produced a statewide land cover map at 1:500,000 scale in 1978 (Mower 1978), and digital data on land cover and irrigation suitability are available for the Oakes and Lincoln Valley areas from the Bureau of Reclamation.

Table B3. Summary of Indicators, by Wetland Function and Measurement Scale
C=indicator of capacity, I=Indicator of landscape input

Functions		1	2	3	4	5	6	7	8	9	10	11	12	13	14
REGIONAL-SCALE INDICATORS
Position relative to flyway												1			1
Position relative to biome edge															1
Precip-evapotranspir. ratio	4	IC	I	I	I	I	I	I
Rainfall/snowmelt intensity	4	I	I	I	I	I	I	I
Growing season length	4	C	C	C	C	C	C	C	C	C	C	C	C		C
LANDSCAPE-SCALE INDICATORS
Wetland-watershed acre ratio (position in watershed)	3	IC	IC	C	IC	IC	IC	IC							1
Distance to another wetland (contagion)	3	C		C	IC	IC	IC	IC				C	I	I	I
Local basin type diversity	2			C	C	C	C	C	C	C			C	C	C
SITE-SPECIFIC INDICATORS
Basin type (hydrologic regime)	4	C	C	C	C	C	C	C	C	C	C	C	C	C	C
Frequency of basin connections	2		C		IC	IC	IC	IC			I				I
Years of constant wet or dry (succesional status)	2	C	C	C	C	C	C	C	C	C	C	C	C	C	C
Water chemistry
salinity	3	C			C	C	C	C	C	C	C	C	C		C
other				C
Sediment type
organic content	2						C	C	C
permeability
general fertility	3	C	C	C	C	C	C	C
Basin depth/volume ratio	3						C	C	C	C	C	C	C	C	C
Open water % (% veg.cover)	2	C			C	C	C	C	C	C	C	C			C
Open water interspersion (o.w. edge complexity)	2					C	C	C	C	C	C	C	C		C
Vegetation form richness	3								C	C		C		C	C
Islands & upland inclusions	1											C			C
Vegetation types
evapotranspiration rates	2	C		I
root mass density	2						C	C		C					C
primary productivity	1								C	C			C
typical stand density	2				C	C	C	C	C	C		C	C	C	C
submerged aquatic %	2	C							C	C	C	C			C
eaten by wildlife	1								C			C	C	C	C
In-basin stressor exposure
pesticides	4							I	C	C	C	C	C	C	C
fertilizer/sewage/manure	1					I	I		C	C	C	C	C		C
artificial drainage	4	C	C	C	C	C	C	C	C	C	C	C	C	C	C
tilled often	3	C	C	C	C	C	C	C	C	C		C	C	C	C
burned often	2	C			C	C	C	C	C	C		C	C	C	C
mowed often	1	C			C	C	C	C	C	C	C	C	C	C	C
visited often	2										C	C		C	C
water level changes often	1		C	C	I	I	I	I	C	C	C	C	C		C
Upslope watershed or buffer zones
land slope	4	I	I	I	I	I	I	I
storage (% lakes, wetlands)	3	I	I	I	I	I	I	I
Soil types
erodibility	4	I	I	I	I	I	I	I
permeability	4	I	I	I	I	I	I	I
Vegetation/crop types
evapotranspiration rate	2	I	I	I	I	I	I	I
root mass density	1	I	I	I	I	I	I	I
general cover density	1	I	I	I	I	I	I	I				C	C	C	C
eaten by wildlife	1											C	C	C	C
tillage practices	3	I	I	I	I	I	I	I				C	C	C	C
Abbreviations
GEO =	Is this indicator likely to exhibit regionwide spatial trends? 1=no; distribution is entirely random and unpredictable, or is even. 3=distribution is somewhat predictable from geography, despite moderate local spatial variation. 5=yes; distribution is geographically distinct; very little local spatial variablity.
	Some examples: The geographic distribution of basin types generally follows definable regional geologic patterns (=4), and vegetation density is somewhat correlated with basin type, so vegetation type is rated "3." The regional patterns in landform/slope are generally known, and wetland density (distance to another wetland) and frequency of wetland connections have a greater probability of occurring in flatter subregions.

 

Table B4. Data Sets of Possible Use for Quantifying Indicators.
Numbers are keyed to specific data sources footnoted in Table B5.
Indicators are rated as D=direct, P=partial, I=indirect or inferred estimate of indicator (this does not necessarily imply good accuracy or high spatial resolution)

Potential Data Source	1	2	3	4	5	6	7	8	9	10	11
I. QUANTIFICATION OF FUNCTION
Wetland effects on streamflow	I			I		I
Wetland effects on water quality				I		I		D
Wetland effects on habitat			D	D				I
II. INDICATORS OF FUNCTION
Regional Scale Indicators
Position relative to biome edge			D
Precip.-evapotranspiration ratio	I					D
Rainfalls/snowmelt intensity	I			I		D
Growing season length						D
Landscape-Scale Indicators
Wetland-watershed acre ratio (position in watershed)				D
Distance to another wetland (contagion)				D
Local basin type diversity				D
Site-Specific Indicators
Basin type (hydrologic regime)				D
Frequency of basin connections				I
Years of constant wet or dry (successional status)	I	D
Water chemistry (salinity/other)				D
Sediment type
organic content				I		D
permeability				I		D
general fertility				I		D
Open water% (% veg. cover)				D
Open water interspersion (o.w. edge complexity)				D
Vegetation form richness				D
Islands & upland inclusions				D
Vegetation types
evapotranspiration rates				I		I
root mass density				I
primary productivity				I
typical stand density				D
submerged aquatic %				D
eaten by wildlife			I
Upslope watershed or buffer zones
land slope				I	I	D
storage (% lakes, wetlands)				D			I
soil types
erodibility				I		D		I
permeability				I		I
vegetation/crop types
evapotranspiration rate				I	I	I
root mass density				I
general cover density				D	I
eaten by wildlife			I
tillage practices				D	I			I
III. INDICATORS OF VALUE
A. HYDROLOGIC VALUES
Number of floodplain residences				D					D
Value of floodplain residences									D
Proximity of floodprone developments to upslope wetlands				D					D
Number of groundwater users	D									D
Proximity of wells to wetlands										I
Usual season of flooding	I							8
Livestock density					I
Cropped wetland acreage				D	D			I
Drought vulnerability of crops				I	I
Average depth of wells	I
(see also Water Quality Values below)
(see also Habitat Values below)
B. WATER QUALITY VALUES
Number of groundwater users	D
Relative importance of the receiving waters
Proximity of important receiving waters or aquifers to upslope wetlands				D
(see also Habitat Values below)
C. HABITAT VALUES
Annual moisture condition	I	D
Fish/wildlife licenses/leases											D
Fish/wildlife harvest											D
Person-days of use											D
Threatened/endangered status of wetland species locally present			D								D
Proximity of wetlands to urban centers				D	D	I		I
INDICATORS OF FUNCTIONAL LOSS
Population growth								D
Wetland conversion trends		D						I
Filling and leveling activity		I						I
Proximity and frequency of:
artificial drainage		I	I	I	I	D	I
groundwater pumping	D	I						I
dugout/impoundment construction				D	D
grazing/mowing		I			D			D
dugout/impoundment construction				D	D
grazing/mowing		I			D			D
tillage		I		I	I	I		D
sedimentation		I				I
pesticide exposure								D
fertilizer/sewage/manure		I			I			I
human visitation				I	I			I

 

Table B5. Potential Sources of Existing Regionwide Indicator Data in the PPR

Data Source Number from Table B3.	Type of Data
1=	Long-term changes in watershed hydrology and water quality: (a) Data for streams, rivers and wetlands. Much data, some of it long-term, is available from gauging stations supported by the U.S. Geological Survey and/or state agencies. However, in many cases, gauges are located below impoundments or channelized reaches that would be expected to confound most wetland influence. Seasonal flow duration tables have been prepared for 81 South Dakota rivers by Dornbush (1985). Many of the USGS stations also have simultaneously-collected water quality data. Additional stream water quality data have been collected by state and other federal agencies, as have limited data on contaminants in wetland sediments and fauna. (b) Data for groundwater. USGS water quality data, collected at a three-year interval from about 40 North Dakota wells, might be analyzed to determing if groundwater levels and water quality have changed in concert with wetland drainage. However, few monitoring wells in South Dakota are located where the effects of wetland loss would be detectable (pers. comm., D. Hubbard, South Dakota St. Univ., Brookings). There also is a series of county-level summaries of water resource information (data on use, recharge and runoff rates and their variability) for North Dakota (e.g., Randich and Kuzniar 1986) and South Dakota (e.g., Koch and McGarvie 1988). Data on groundwater quality are available from some state water resource agencies (e.g., DeMartino and Jarrett 1991).
2=	Long-term changes in wetland and wildlife distribution and/or abundance. (a) USFWS annual aerial counts of "ponds." These wetland counts have been conducted along standard transects by federal and state wildlife agencies since the mid-1960's, to be used to index annual climate changes. Waterfowl brood densities have been estimated along these same transects, and USFWS has computed 10-year and long-term means. Data from both the pond counts and the waterfowl brood surveys have been aggregated such that it is not possible to reference the data to exact spatial locations within the transect. Other regionwide surveys of waterbirds have also been sponsored in a few instances by the USFWS Northern Prairie Wildlife Research Center and state wildlife agencies. (b) Breeding Bird Survey routes; Christmas Bird Counts. These datasets cover decades, but spatial and annual coverage is spotty. Specific areas can be located within a few miles; accuracy and representativeness is quite variable. (c) USFWS National Wetlands Trends Surveys (Tiner 1984, Dahl et al. 1991). This periodic survey does not include a sufficient number of plots in the PPR to reliably estimate trends in wetland acreage within the PPR, either mid-50s to mid-70s, or mid-70s to mid-80s. (d) USFWS Northern Prairie Waterfowl Research Center database. The NPWRC has interpreted 1980-84 land cover and wetland types from airphotos of a stratified random sample of 422 plots, each about 10 square kilometers in size and covering all but the Montana part of the PPR (Cowardin et al. 1983). Recent wetland loss rates have been estimated from this (e.g., Klett et al. 1988). (e) National Resource Inventory (NRI) database. This database is a successor to the "Conservation Needs Inventories" conducted in the 1960's and 1970's. It can be used to compute changes in the 1982-1987 acreage of nonfederal wetlands in a county, major river basin, or major land resource area. However, results are coarse, sample-based estimates of wetland acreage (as of about 1986). Data are difficult to interpret because of uncertainty in how wetlands were defined.
3=	Fish and wildlife distribution, abundance, and species characteristics. (a) State Fish and Wildlife Departments. Agencies in various PPR states are developing computerized fish and wildlife information systems (CFWIS) that are databases in which species are referenced to habitat needs, geographic distribution, life history characteristics, and other features. (b) State Natural Heritage Programs/The Nature Conservancy. Computerized data are available for some PPH states on distribution of wetland species by county, and (as part of the Vertebrate Characterization Abstracts) on life history characteristics.
4=	Wetland maps and/or regionwide data on wetland distribution: (a) National Wetlands Inventory maps. These are the most reliable, precise, and widely used source of information on wetland distribution in the PPR. However, map coverage is not yet complete for the region, and only a small portion of the maps have been digitized. For general information, contact Ronald Erickson, USFWS, Federal Building, Fort Snelling (AS/BSP), Twin Cities, MN 55111. To order maps, contact Earth Science Information Center, U.S. Geological Survey, 507 National Center, Reston, VA (phone 1-800-USA-MAPS). (b) Thematic Mapper (TM) data. Ducks Unlimited, Inc., has assembled remotely sensed Landsat TM scenes for the entire PPR (Koeln et al. 1988). These have not been converted to maps or compiled with regard to their spatial data characteristics, but could be. The TM scenes were both from 1986 (a wet year) and 1987 (a dry year). For general information, contact Dr. Gregory Koeln, Ducks Unlimited Inc., One Waterfowl Way, Long Grove, IL 60047 (phone 312-438-4300). (c) "Swampbuster" wetland maps. USDA Soil Conservation Service staff have hand-drawn wetlands, especially those subject to periodic cultivation, on airphotos at scales of about 1:12,000 or 1:20,000. The basis for these approximate delineations was presence of surface water visible in airphotos, and/or presence of hydric soils based on available county soil survey maps. Only a single copy of these delineations exists, usually at SCS county offices; accuracy of the delineations is unknown. (d) STATSGO soil distribution + SOILS5 soil attribute data. These digital maps of soil series, prepared by the USDA Soil Conservation Service, could be used to indicate wetlands by showing hydric soils, depth-to-water table, or drainage condition as a percent of polygon area. They can be produced at a 1:250,000 scale (about 100 acres minimum resolution). Although the digital soils maps have been completed in all PPR states except North Dakota, the SCS timeframe for completing STATSGO's linkages with attribute data (that would allow crude portrayal and summarization of wetland distribution) is uncertain. Much of the PPH wetland resource may not be included because at the coarse scale used, wetland basins may have been categorized not as wetland, but as water or (more often) lumped with adjoining upland soil types. For general information, contact state office of the USDA Soil Conservation Service. (e) Groundwater or water table maps. Some state water agencies and the USGS state offices have such maps, or data that could produce such maps (e.g., NDWCA 1991, Hoyer and Hallberg 1991). These might be tested for their ability to infer presence and function of wetlands. (f) Federal Emergency Management Agency (FEMA) maps. These maps potentially depict river-associated wetlands. Map scale varies depending on locality, and ranges from 1"=2000 feet to 1"=200 feet. A small subset of the maps (some of the first ones prepared) were also printed on USGS quadrangles, at 1:24,000 or 1:62,500 scale; printed copies of these are no longer available for purchase, but they can be obtained on microfiche or reviewed in government map libraries. FEMA has budgeted for the eventual digitization of all its floodplain maps; digital versions for about 40 densely-populated counties will be available beginning in mid-1992. For maps, contact Flood Map Distribution Center, Federal Emergency Management Agency (FEMA), 6930 San Tomas Rd., Baltimore, MD 21227-6227 (phone 1-800-333-1363). For general information, contact state office of FEMA. (g) James River wetland maps. As part of assessments for the Garrison Diversion Project, the Bureau of Reclamation digitized emergent and aquatic bed vegetation (at scales of 1:12,000 and 1:24,000) within wetlands of four national wildlife refuges only along the James River. (h) National Resource Inventory (NRI) database. This is not a map source, but rather a database from which the acreage of nonfederal wetlands in a county, major river basin, or major land resource area may be compiled. It is a successor to the "Conservation Needs Inventories" conducted in the 1960's and 1970's. These are coarse, sample-based estimates of wetland acreage (as of about 1986). Accuracy is unknown, but in 1982 the NRI estimated the PPH wetland acreage at 4,213,000 acres, somewhat lower than most other estimates (Heimlich and Langner 1986). Contact the Resources Inventory Division, USDA Soil Conservation Service, P.O. Box 2890, Washington, D.C. 20013.
5=	Other land cover/land use maps or regionwide data on land cover distribution: (a) USGS Land Use/Land Cover (LUDA) maps. Digital map data are at 1:250,000 scale (about 40 acres minimum resolution), and show land cover only as of the 1970's or early 1980's. For obtaining maps, contact Earth Science Information Center, U.S. Geological Survey, 507 National Center, Reston, VA (phone 1-800-USA-MAPS). (b) Thematic Mapper (TM) data. See 1(b) above for information. (c) USDA National Resource Inventory (NRI) database. Data on land cover and management practices are compiled by county, river basin (USGS accounting unit), or major land resource unit, and are for nonfederal lands during 1982 and 1987. Contact the Resources Inventory Division, USDA Soil Conservation Service, P.O. Box 2890, Washington, D.C. 20013.
6=	Soil, landform, or climate characteristics (data and/or maps showing actual/potential erosion, salinization, productivity): (a) STATSGO soil distribution + SOILS5 soil attribute data. When fully developed (in about one year), this source could be linked to soil attribute information in SCS's SOILS5 computerized database, allowing estimation of the proportion of soils within a polygon (at 1:100,000 scale) that have steep slopes, severe erosion potential, high organic matter, moderate water retaining capacity, and/or other characteristics potentially useful as indicators of wetland function. For general information, contact state office of the USDA Soil Conservation Service. (b) USDA National Resource Inventory data. Data on erosion, landform, and slope are compiled by county, river basin (USGS accounting unit), or major land resource unit, and are for nonfederal lands during 1982 and 1987. Contact the Resources Inventory Division, USDA Soil Conservation Service, P.O. Box 2890, Washington, D.C. 20013. (c) USGS digital elevation model (DEM) data. Altitude/elevation data are available in digital format by contacting the state USGS office or the USGS-EROS Data Center, Sioux Falls, South Dakota. (d) U.S. Geological Survey's National Atlas. This contains coarse-scale maps of annual runoff, evapotranspiration, and water yield. (e) Soil fertility data. A series of reports for South Dakota (e.g., Malo et al. 1990) describes general fertility of soils, including some hydric series.
7=	Artificial drainage of wetlands. (a) State water agency data. Some state agencies compile data on artificial drainage by county. For example, in Minnesota, the extent of artificial drainage was measured in a few watersheds included in the Willey (1988) regression study, as well as in reports prepared for Nicollet County (Dunsmore and Quade 1979a), Blue Earth County (Dunsmore and Quade 1979b), and Brown County (Dunsmore et al. 1979). (b) Data on center-pivot irrigation. In northwestern parts of the PPR, extent of new wetland drainage might be crudely estimated from data on increases in center-pivot irrigation in areas of high wetland density, because much wetland loss results from new irrigators draining wetlands that pose obstacles to rotating equipment (J. Leitch, North Dakota St. Univ., pers. comm).
8=	Other existing impairment/disturbance of wetland/landscape functions: (a) State section 305(b) water quality reports. Each state's semi-annual report rates the water quality of each assessed lake and river, and identifies possible causes of water quality degradation. (b) U.S. Census Bureau data. Data on population density by county or municipality might be used to infer degradation risks to wetlands and use of some wetland functions. (c) USDA National Agriculture Survey (NAS) database. For annual data reports (data compiled only by county), contact the state agricultural statistics office: IA: phone (515) 284-4340 MT: phone (406) 449-5303 ND: phone (701) 239-5306 SD: phone (605) 330-4235 (d) Resources for the Future database. To arrange for county-level estimates and mapping of pesticide use, phone (202) 328-5036.
9=	Beneficiaries of flood storage functions: (a) Federal Emergency Management Agency (FEMA) data. These computerized data, which are easily converted to maps, describe number and value of residences located in floodplains (current and projected for the year 2002), and past economic losses due to flooding. Data are indexed to community and county. For general information, contact state office of FEMA. (b) Data on distribution of ecological resources that potentially benefit from wetland hydrologic functions is available partly from databases listed in (3) above.
10=	Beneficiaries of water quality functions: (a) USGS databases. Some state offices of the USGS have computer files that quantify users of surface and groundwater in specific counties and river basins. (b) State water resource agencies. Some state agency efforts to define risks to groundwater have quantified users of potentially vulnerable or contaminated aquifers (e.g., DeMartino and Jarrett 1991). (b) Data on distribution of ecological resources that potentially benefit from wetland water quality functions is available partly from databases listed in (3) above.
11=	Beneficiaries of habitat functions: (a) State Fish and Wildlife Department data. These agencies may have current statistics on hunting, fishing, trapping (licenses and harvest), and nonconsumptive uses. As indicated partly in (3) above, these agencies as well as state heritage programs have information that might be used to indicate where wildlife would be most expected to benefit from wetland functions.

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7.0 Literature Cited

Adamus, P.R. 1992. Data sources and evaluation methods for addressing wetland issues. p. 171-224 In: Statewide Wetlands Strategies. World Wildlife Fund, Washington, D.C. and Island Press, Washington, D.C.

Adamus, P.R. and K. Brandt. 1990. Impacts on Quality of Inland Wetlands of the United States: A Survey of Indicators, Techniques, and Applications of Community Level Biomonitoring Data. EPA/600/3-90/073. U.S. EPA Environmental Research Laboratory, Corvallis, Oregon. 406 p.

Aho, J.M. 1978. Freshwater snail populations and the equilibrium theory of island biogeography. II. Relative importance of chemical and spatial variables. Annales Zool. Fenn. 15:155-164.

Akhurst, E.G.J., and C.M. Breen. 1988. Ionic content as a factor influencing turbidity in two floodplain lakes after a flood. Hydrobiologia 160:19-31.

Allan, R.J., J.D. Williams, S.R. Joshi, and W.F. Warwick. 1980. Historical changes and relationship to internal loading of sediment phosphorus forms in hypereutrophic prairie lakes. J. Envir. Qual. 9:199-206.

Allred, E.R., P.W. Manson, G.M. Schwartz, P. Golany, and J.W. Reinke. 1971. Continuation of Studies on the Hydrology of Ponds and Small Lakes. Tech. Bull. 274. Agric. Exp. Stn., Univ. Minnesota, Minneapolis.

Andersson, L. and A. Sivertun. 1991. A GIS-supported method for detecting the hydrological mosaic and the role of man as a hydrological factor. Landscape Ecol. 5:107-124.

Arndt, J.L. and J.L. richardson. 1986. The effects of ground water hydrology on salinity in a recharge -flowthrough-discharge wetland system in North Dakota. p. 269-278 In: G. Van der Kamp and M. Madunicky (eds.) Proc. 3rd Geohydrol. Conf., Saskatchewan Res. Council, Saskatoon.

Baird, D.J., T.E. Gates, and R.W. Davies. 1987. Oxygen conditions in two prairie pothole lakes during winter ice cover. Can. J. Fish. Aquat. Sci. 44:1092-1095.

Baker, J.L. 1987. Hydrologic effects of conservation tillage and their importance relative to water quality. In: T. H. Logan et al. (ed.). Effects of Conservation Tillage on Groundwater Quality. Lewis Publ., Chelsea, Michigan.

Ball, I.J. and R.L. Eng. 1989. A thriving duck population: implications for management. Research Info. Bull. No. 89-111. U.S. Fish & Wildl. Serv., Lakewood, Colorado. 13 p.

Ball, J.P. and T.D. Nudds. 1989. Mallard habitat selection: An experiment and implications for management. p. 659-671 In: R.R. Sharitz and J.W. Gibbons (eds.). Freshwater Wetlands and Wildlife. CONF-8603101, Symposium Series No. 61. U.S. Dept. Energy, Oak Ridge, Tennessee.

Ball, I.J., R.L. Eng, and S.K. Ball. 1988. Density and productivity of ducks on certain western rangelands: an assessment in north central Montana and evaluation of methodology. Report to Central Flyway Council, U.S. Fish & Wildl. Serv., Lakewood, Colorado. 13 p.

Baltezore, J.F., J.A. Leitch, and W.C. Nelson. 1987. Economic analysis of draining wetlands in Kidder County, North Dakota. Agric. Exp. Stn., North Dakota St. Univ., Fargo. 58 p.

Baltezore, J.F., J.A. Leitch, T. Golz, and A.K. Harmoning. 1987. Resident hunter and angler expenditures and characteristics in North Dakota in 1986. Rep.AE 87008, Dept. Agric. Econ., Agric. Exp. Stn., North Dakota St. Univ., Fargo.

Baltezore, J.F., J.A. Leitch, S.F. Beekie, P.F. Schutt, and K.L. Grosz. 1991. Status of Wetlands in North Dakota in 1990. Agric. Econ. Rep. No. 269. Dept. Agric. Econ., North Dakota St. Univ., Fargo.

Barari, A., T. Cowman, and D. Iles. 1990. Groundwater recharge through till. North Dakota Acad. Sci. Proc. 44:41.

Barica, J. 1978. Variability in ionic composition and phytoplankton biomass of saline eutrophic prairie lakes within a small geographic area. Arch. Hydrobiol. 81:304-326.

Barica, J. 1984. Empirical models for prediction of algae blooms and collapses, winter oxygen depletion, and a freeze-out effect in lakes: Summary and verification. Int. Ver. Theor. Angew. Limnol. 22:309-319.

Barica, J. and J.A. Mathias. 1979. Oxygen depletion and winterkill risk in small prairie lakes under extended ice cover. J. Fish. Res. Bd. Can. 36:980-986.

Barica, J., H. Kling, and J. Gibson. 1980. Experimental manipulation of algal bloom composition by nitrogen addition. Can. J. Fish. Aq. Sci. 37:1175-1183.

Barker, W.T., K.K. Sedivec, T.A. Messmer, K.F. Higgins, and D.R. Hertel. 1990. Effects of specialized grazing systems on waterfowl production in southcentral North Dakota. Trans. N. Am. Wildl. Nat. Res. Conf. 55:462-474.

Batt, B.D.J., M.G. Anderson, C.D. Anderson, and F.D. Caswell. 1989. The use of prairie potholes by North American ducks. p. 204-227 In: A.G. van der Valk (ed.). Northern Prairie Wetlands. Iowa St. Univ. Press, Ames.

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