Learn About Freshwater and Estuarine Harmful Algal Blooms

What are cyanobacteria and freshwater harmful algal blooms (FHABs)?

Cyanobacteria, also known as blue-green algae or toxic algae, occur in freshwater and estuarine waterbodies, have been around for billions of years, and created oxygen on earth. Cyanobacteria look and act similar to true or non-harmful green algae but are phylogenetically distinct. Cyanobacteria and algae are natural components of ecosystems and perform many roles that are vital to our aquatic communities, by being a food source and producing oxygen. However, when certain conditions are favorable for these organisms, algae and cyanobacteria can rapidly grow to cause “blooms.”

Algae and cyanobacteria can produce harmful compounds, such as toxins and taste and odor compounds, that cause health risks to humans and animals. When blooms pose a risk to humans, animals, and the environment, they are referred to as freshwater harmful algal blooms (FHABs). Floating or planktonic blooms in lakes or bays are the more familiar type of FHABs, however, other types can grow attached to the bottom (benthic) of waterbodies and form toxic algal mats. For more information, see FAQs on FHABs and toxic algal mats.

How do I know if there is a FHAB in the water?

Sometimes the bloom is easily visible, forming a “scum” or discoloration on the water surface (planktonic bloom). Other times, it is less visible, growing beneath the surface on the bottom of a water body (benthic bloom). Blooms can appear green, blue, yellow, red, or brown. Cyanotoxins produced by cyanobacteria, are small colorless molecules that cannot be visually identified in water or tissues.  Several guidance documents are available to aid identification of algae and cyanobacteria (Fact Sheet & Visual Guide), and the California Freshwater FHAB Field Guide is available to assist in monitoring. 

What causes FHABs?

Increased inputs of nutrients like nitrogen and phosphorus (from fertilizers and human or animal wastes) promote cyanobacterial growth and can lead to increased occurrences of FHABs. Low flows, stagnant water, increased intensity and duration of sunlight, and sustained high temperatures create the ideal conditions for FHABs. Current research suggests that the rising temperatures and changing precipitation patterns caused by climate change will favor FHAB formation.

What are the possible health concerns of FHABs?

Cyanotoxins and algal toxins pose risks to the health and safety of people and pets that recreate in or drink water  sourced from areas that are affected by FHABs. They can also accumulate in fish and shellfish at levels that are unsafe for consumption. 

For planktonic FHABs, pets such as dogs, are especially susceptible to FHABs because they swallow more water while swimming and playing in the water. Dogs are also curious and less deterred by green, smelly water that may contain cyanobacteria and/or their toxins. 

For benthic FHABs, most of the cyanotoxin load is in the mat material so the primary exposure to cyanotoxins is the ingestion of mat material, rather than just the surrounding water. Dogs can be attracted to benthic mats when they detach and float to the surface or often play on shorelines where stranded mats accumulate, making them especially susceptible to these FHABs as well. 

Learn more about Human Health Impacts, Domestic Animal Impacts and Fish and Wildlife Impacts.

What guidelines does California use for FHABs?

Currently, there are no federal or state regulatory standards for cyanotoxins in recreational waters or drinking water. Participating state agencies have developed California voluntary guidance for responding to FHABs in recreational waters. The Department of Public Health, local health departments, and water body managers are encouraged to use this guidance for posting water bodies when FHABs pose a health threat. Guidance is also available for addressing cyanotoxins in drinking water, however, this page focuses on recreational impacts.

How do I Report a FHAB?

To report a bloom please visit the State FHAB Report Form. Once this form is submitted the information will appear on the State Incident Report Map.

Want to learn more?

Visit the California State FHAB portal