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Great Lakes Benthos Monitoring

Benthic macroinvertebrates or benthos, are small aquatic animals and the aquatic larval stages of insects that dwell on the bottom of the lake. They include dragonfly and stonefly larvae, snails, worms, and beetles. 


EPA added benthos sampling to the biological monitoring program in 1997. The disappearance of the mayfly Hexagenia from the benthos in the western part of Lake Erie provided some of the first strong evidence of water quality deterioration in the Great Lakes. 


  • EPA's research vessel The Lake Guardian is used to gather samples every August from between ten and sixteen benthos sites in each lake.
  • Four of these sites are sampled every April to monitor Hexagenia populations.


  • A piece of equipment called a Ponar grabs a bucket of mud from a known area of a soft lake bottom to sample benthos
  • After sifting the finer materials out of the samples, scientists begin the painstaking process of picking the organisms out of the mud.
  • A special dye that turns the benthos pink called rose bengal makes them easier to see.
  • The benthos are weighed to estimate the bio-mass which provides a better idea of changes in their populations

Trends and Changes

Trends Indicate Problems with the Lakes

  • Diporeia, a rich food source to many important fishes in the Great Lakes food-web, are declining.
  • This decline has allowed the populations of invasive zebra and quagga mussels to grow.
  • These invasive mussels have an extreme effect on both the water and lake bottoms 
  • They feed on algae by filtering water, which make the water clear
  • Increased visibility of water allows more light to get to the bottom, which enables aquatic plants to grow in deeper water
  • The good news is the plants increase habitat for young fish and shallow water benthos