Harmful Algal Blooms

Algal bloom in Lake Erie

Harmful algal blooms (HABs) are becoming an increasingly common occurrence across the U.S. and around the world. Blooms (large numbers or colonies) of the algae or related organisms produce one or more toxins that can be dangerous to fish, wildlife, pets and livestock. People can also be exposed to these toxins through outdoor recreation activities and consuming contaminated drinking water. HABs can occur in both freshwater and coastal marine waters.

What Causes Harmful Algal Blooms?

Algae include phytoplankton (or microscopic, free-floating plants), and are an essential part of any aquatic ecosystem, with green algae serving as the food base for microscopic animals (zooplankton) and some fish. Harmful algae types, including cyanobacteria or "blue-green algae", are a less desired food item for zooplankton and fish, and in addition, can produce toxic chemicals threatening other organisms in the water.  

Conditions that contribute to HABs:
  • Nutrients, particularly phosphorus and nitrogen
  • Warm temperatures
  • Adequate sunlight
  • Calmer waters

All types of algae need nutrients to survive and grow, and excessive nutrient levels can lead to blooms. Key sources of nutrient pollution in our waters are agricultural sources, like inorganic fertilizers and manure runoff, and point sources, particularly wastewater treatment plants. Studies have shown that reducing nutrient inputs to waters is important to reducing HAB occurrences.

Climate Change: HABs Threat Multiplier

According to Dr. Jay Martin of Ohio State University, "with climate change and warmer climates we see longer growing seasons. This is going to increase the opportunity that these organisms can grow. We also expect to see wetter and stormy winters and springs in the future, especially in the Midwest, which impacts the Great Lakes and Lake Erie. So because of this we expect to see an increase in nutrients coming into the lakes, which will increase the photosynthetic rate. Lastly, with higher CO2 emissions and higher CO2 concentrations in the atmosphere, this will increase the dissolved organic carbon or DOC, and this will also increase the photosynthetic rate." To get more information from Dr. Jay Martin, listen to his webinar: "Climate Change & Harmful Algal Blooms"

Heavy Precipitation
Image from

How Are Fish and Wildlife at Risk?

In fish, toxins (e.g. microcystins) are taken up by the liver, and at sufficient exposures, can affect the liver function and cause liver damage. In some cases, fish kills have been associated with HABs.

Wildlife can also be at risk from the toxins, both through direct ingestion or through consuming food containing the toxins. Though there have been few lab studies on impacts to birds, field studies have attributed deaths in songbirds, ducks, gulls, pheasants and hawks to HAB exposures, and research in the Chesapeake region associated great blue heron deaths with toxic algae. Pets and livestock are also at risk from exposure to HABs, with deaths documented at a number of locations around the country.

Where Have Harmful Algal Blooms Happened?

  • A major bloom of harmful algae in western Lake Erie in early August 2014 led to contamination of Toledo’s water by a microcystin toxin, and the cutoff of drinking water supplies to nearly 500,000 people for two days.

  • Numerous blooms in the Chesapeake Bay region, including nearly annual blooms in Maryland (with the first beach closure issued there in 2000).

  • Large blooms in the Lower St. John’s River in Florida (including in 2005 and 2013), leading to recreational water advisories.

  • A harmful algal bloom in Lake Pontchartrain, north of New Orleans, leading to a recreational use advisory, following diversion of Mississippi River water in 1997.

  • Blooms of Prymnesium parvum (or the “golden algae”) responsible for numerous fish kills in Texas waters over the past 15 years.

  • Blooms transported downstream to Monterey Bay, California, leading to sea otter deaths.

No national monitoring of harmful algal blooms is currently done in the U.S. However, NWF and Resource Media noted in a report that based on public reports, harmful algal blooms appear to be very common, with 21 states reporting blooms at 147 locations between May and September 2013. In a more recent report, NWF and Resource Media highlighted the need for more systematic monitoring of harmful algal blooms, with increased action at the federal, state, and local levels needed.

What Can Be Done?

There are ongoing federal programs that protect water quality and limit the sources of nutrient runoff that contribute to HABs. Programs like these need to be supported, including:

  • Waters of the U.S. Rule, which will ensure adequate protection of additional wetlands to help reduce nutrient runoff into our rivers and lakes.

  • U.S. Farm Bill, including conservation programs targeted at priority watersheds to reduce nutrient runoff from fields, and increased efforts to improve nutrient management at confined animal feeding operations.

  • Great Lakes Restoration Initiative, including ongoing support of projects to reduce nonpoint source pollution in targeted watersheds in the Great Lakes.

Reducing the problem:
  • We need increased actions to address nonpoint source pollution, in particular agricultural runoff. This includes increased targeting of Farm Bill and other programs to priority areas, and continuing research to identify key nutrient source areas.

  • We need increased wetland restoration efforts in priority areas, as well as wetland protection (including through enforcement of a strong Waters of the U.S. rule).

  • We need to address climate change and its potential to exacerbate blooms (through increasing nutrient runoff via more intense storms), including taking actions (such as adopting a strong power plant rule) to reduce greenhouse gas emissions.

Research and monitoring:
  • We need full implementation of (and adequate funding for) the recently reauthorized Harmful Algal Bloom and Hypoxia Control Research Amendments Act of 2014, including development of the integrated assessment for the Great Lakes, development of a strategic plan, and implementation by all relevant entities.

  • States should ramp up monitoring of their waters for harmful algal blooms, targeting monitoring as appropriate (e.g., based on likelihood of bloom presence or other relevant information)

Education and outreach:
  • States should ramp up education and outreach efforts to the public, including using online tools for notifications and provision of other information.

  • States should incorporate ways for citizens to report problems and observations.

  • Federal and state governments should increase collaboration (with other entities, including municipalities and nongovernmental organizations) on education and outreach efforts.


Related Resources

Get Our E-Newsletter 
Subscribe to National Wildlife Magazine!