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Nutrient Pollution

Harmful Algal Blooms

Harmful algal blooms are a major environmental problem in all 50 states. Red tides, blue-green algae, and cyanobacteria are examples of harmful algal blooms that can have severe impacts on human health, aquatic ecosystems, and the economy.

Algal blooms can be toxic. Keep people and pets away from water that is green, scummy or smells bad. 

On this page:

Learn about harmful algal blooms

A green algal bloom along a rocky shoreline
Harmful algal blooms can be green, blue, red or brown. They can be scummy or look like paint on the surface of the water.

What are harmful algal blooms?

Harmful algal blooms are overgrowths of algae in water. Some produce dangerous toxins in fresh or marine water but even nontoxic blooms hurt the environment and local economies.

What are the effects of harmful algal blooms?

Harmful algal blooms can:
  • Produce extremely dangerous toxins that can sicken or kill people and animals
  • Create dead zones in the water
  • Raise treatment costs for drinking water
  • Hurt industries that depend on clean water

Learn more about these and other effects.

Did you know?

Climate change might lead to stronger and more frequent algal blooms.
Find out how.

What causes harmful algal blooms?

Harmful algal blooms need:
  • Sunlight
  • Slow-moving water
  • Nutrients (nitrogen and phosphorus)

Nutrient pollution from human activities makes the problem worse, leading to more severe blooms that occur more often.

Learn where nutrient pollution comes from.

What you can do to help

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Volunteer to monitor waterbodies for algal blooms

The following organizations help identify toxic algae outbreaks:

Report suspected algal blooms to your state

State departments of health or environment are the best sources for local information about harmful algal blooms.

Find your state's contact information.

Help prevent nutrient pollution

Simple actions around your home and yard can make a big difference

Learn how you can help.



Keeping pets- and people- safe from toxic algae.
Two years ago, our family planned to take our Yellow Labrador puppy Fiona to Lake Needwood near our home in Rockville, Maryland for a swim. Read more...

Click to read other related blogs.


Partner resources

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National resources

State resources

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