HAB (Harmful Algal Bloom)

A harmful algal bloom (HAB) is a rapid proliferation of phytoplankton that harms marine life or human health through biotoxins, oxygen depletion, or sheer biomass. Toxin-producing genera such as Alexandrium, Karenia, Dinophysis, and Pseudo-nitzschia cause paralytic, neurotoxic, diarrhetic, and amnesic shellfish poisoning, closing fisheries and shellfish beds. Non-toxic high-biomass blooms strip dissolved oxygen as they decay, driving hypoxia and fish kills. HABs are promoted by nutrient loading from agricultural and sewage runoff, warm stratified water, and ballast-water or current transport of resting cysts. The IOC-UNESCO HAB programme and national monitoring support early warning, shellfish-harvest closures, and toxin testing.