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Causes of Harmful Algal Blooms and Toxicity

Environmental drivers regulating harmful algal bloom (HAB) growth and toxicity span a wide range of physical, chemical, and biological factors. They include a variety of complex interactions within and across these categories. Moreover, climate change is altering the way and the extent to which many of these factors influence HAB growth and toxin dynamics in natural settings. NCCOS is striving to identify the mechanisms involved in mediating changes and shifts in HAB abundance, distribution, and toxicity in order to develop predictive capabilities required to manage and mitigate the impacts of HABs and their toxins on public health and quality of life as well as impacts to coastal economies.

Causes of Harmful Algal Blooms

Oceanic, coastal, and Great Lakes waters of the United States are increasingly impacted by HABs with blooms reported in every state. Certain types of microscopic or larger plant-like cyanobacterial or algal species can cause HABs. Algae and cyanobacteria are considered to be the basis of most freshwater and marine food webs. While many of these cyanobacterial and algal species are present in healthy ecosystems, environmental factors can cause them to multiply in mass quantity (considered a bloom) and/or move onshore causing impact to wildlife and humans.

boat floating in thick, green water with a trail

Cyanobacteria

This blue-green algae causes harmful blooms in Lake Erie when warmer water (typically July to October) provides favorable bloom conditions.
Spongy orange substance (sargassum) lines a beach and is floating in the water.
Sargassum

This brown macroalgae is pushed ashore from winds, tides and currents, however scientists are working to understand what causes these larger blooms.
Brown plume in green water along a coastline with water front houses

Karenia brevis

Also known as “red tide”, this algal species persists in the bottom waters off the coast of Florida and in the Gulf of Mexico. Northerly winds create upwelling, causing the bloom to move onshore.

Factors Affecting Harmful Algal Bloom Toxicity

Identifying and characterizing the factors regulating toxin production are key to developing predictive models and strategies needed by managers to mitigate the impacts of toxic HABs. This effort is complicated by the fact that bloom toxicity is highly variable due to intrinsic (e.g., genetic) as well as environmental factors (i.e., physical, chemical, biological) and their interactions. Changes related to climatic shifts may also affect toxin production.

Graphic shows how instruments collect ocean data and transmit the information
Environmental Sample Processor Enhances Ocean Sensors to Inform Climate Change Predictions on HABs
uncrewed underwater vehicle on a boat before launch
A ‘Team’ of Uncrewed Vehicles Reveals the Extent and Toxicity of a Harmful Algal Bloom in Monterey Bay

Using the Latest Research and Technology

Our scientists are working with partners on cutting edge research and the latest technologies to help forecast, manage, and monitor harmful algal blooms and toxicity. Visit our additional HAB pages to discover how we are tackling this issue in each focus area.

Forecasting and Monitoring
Prevention and Control
Response and Readiness
Toxin Detection Technologies

Partnerships

HABs are one of the most scientifically complex issues facing our nation, and as such the problem requires scientists across a multitude of expertise. Therefore, to ensure the most accurate and reliable products are being developed, NCCOS scientists collaborate extensively with the HAB community, including other NOAA offices and federal agencies, state management agencies, academia, and the private sector, through professional connections and partnerships as well as through Harmful Algal Bloom and Hypoxia Research and Control Act (HABHRCA) Interagency Working Group. In addition, the NCCOS HAB Competitive Research Program helps develop technologies to understand the causes of HABs and toxicity.