State of the Science Fact Sheet: Harmful Algal Blooms

Harmful algal blooms (HABs) occur when algae or cyanobacteria produce toxic or harmful effects on people, fish, shellfish, marine mammals, and birds. HABs can have adverse effects on human, animal, and ecosystem health, and cause significant economic impacts to coastal communities. Ranging from single-celled microalgae to large seaweeds, algae are photosynthetic organisms that form the base of aquatic food webs. Various types of algae can cause HABs. Only some of these algae produce toxins some of the time. HABs in marine and freshwater environments are different, but can overlap in estuaries (where rivers meet the sea).