Harmful Algal Bloom

Algae bloom in Lake Winnipeg in the Canadian province of Manitoba, with Reindeer Island visible in the lower-right part of the image. Image: contains modified Copernicus Sentinel data (2017), processed by ESA, CC BY-SA 3.0 IGO.


Harmful algal blooms (HABs) are proliferations of certain noxious and/or toxic micro- and macroalgae and cyanobacteria, regardless of their concentration, with negative impacts on aquatic ecosystems, and human health and wellbeing. HABs are naturally occurring phenomena that are also facilitated by anthropogenic pressures (including eutrophication, habitat modification and introduction of exogenous HAB organisms). HABs constitute a complex global problem that might increase in severity and frequency, and be expanded in biogeographic range, in our changing planet (GlobalHAB).

Facts and figures

Harmful algal blooms occur when colonies of algae grow out of control and produce toxic or harmful effects on people, fish, shellfish, marine mammals and birds. HABs occur naturally, but human activities that disturb ecosystems seem to play a role in their more frequent occurrence and intensity. Increased nutrient loadings and pollution, food web alterations, introduced species, water flow modifications and climate change all play a role. Studies show that many algal species flourish when wind and water currents are favorable. In other cases, HABs may be linked to “overfeeding.” This occurs when nutrients (mainly phosphorus and nitrogen) from sources such as lawns and agriculture flow into bays, rivers, and the sea, and build up at a rate that “overfeeds” the algae that exist normally in the environment. Some HABs appear in the aftermath of natural phenomena like sluggish water circulation, unusually high water temperatures, and extreme weather events like hurricanes, floods, and drought (NOAA).

HABs are natural processes that occur in all aquatic systems and cause worldwide problems with significant economic, socio-cultural, and human health consequences. There is considerable concern that some HABs and/or their associated impacts may be increasing and expanding globally due to a combination of natural and human-driven forcing, including climate change. In the past two decades, improvements in scientific understanding of the complex processes involved in HAB dynamics have contributed to better management of the risks associated with some harmful events (GlobalHAB).

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