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Algae Blooms in District Waters

Nuisance level filamentous algae bloom occurring on a river. Photo Credit: USGS


Algae and cyanobacteria, also known as blue-green algae, are found naturally in aquatic environments and are an essential part of the ecosystem. Algae are major sources of food for aquatic life and create oxygen that fish use. Algae blooms occur when large amounts of algal cells are present in a body of water. They can sometimes be observed as floating mats on the water surface or simply as a change in water color.

Although algae blooms are natural events, they can be accelerated by manmade factors. Some conditions that promote algal growth include slow-moving water, warm temperatures, and the availability of nutrients. Fertilizers, animal waste, sewage discharges, and stormwater runoff increase the availability of nutrients such as nitrogen and phosphorus and enhance conditions for algal growth.

While many forms of algae do not cause direct harm, some forms of algae and cyanobacteria can present health risks to wildlife, pets, or humans. An overabundance of algae or cyanobacteria can harm aquatic ecosystems by preventing light from reaching other plants below the surface of the water. Without sunlight, the submerged plants will not survive and will no longer provide food or habitat for fish. Large algae blooms can also deplete oxygen levels in the water as they die off. With low oxygen levels, fish cannot survive and the result may be a fish kill event. 

Algae Bloom Monitoring

DOEE staff conducts regular surveillance and sampling to track the water conditions of the District’s rivers and streams. DOEE analyzes continuous, real-time water quality measurements of Chlorophyll from the Potomac and Anacostia Rivers to monitor for algae and cyanobacteria blooms.

Algae Bloom Safety Cyanobacteria bloom causing thick, green, discolored water.  Photo Credit: Nara Souza, USGS

Blooms that have a negative impact on the environment are called Harmful Algal Blooms (HABs). Take caution to limit any exposure to a suspected HAB. Several species of cyanobacteria have the capacity to produce toxic chemicals that can harm human health and lead to illness or death for wildlife or pets. People and pets should avoid direct skin contact, inhalation, or ingestion of water affected by HABs.

Reporting Suspected Algae Blooms or Harmful Algal Blooms

To report suspected large algae blooms or HABs, please contact Matt English of DOEE’s Water Quality Division at (202) 481-3943 or [email protected]. Please include your name, phone number, the location of the algae bloom, and the date and time you observed it. If possible, please also include the approximate size of the bloom and a photo of the site.

For More Information

- US EPA Harmful Algal Blooms
- US EPA CyanoHABs