4.4.10 (HL)—Harmful algal blooms

A harmful algal bloom is identified by organism and toxin evidence, not by colour or density alone.

Freshwater cyanobacteria can release cyanotoxins through drinking, skin contact or aerosols; exposure may cause gastrointestinal, liver, neurological, skin or eye effects. At Salto Grande dam, Argentina, freshwater cyanobacteria exposure in 2007 was followed by severe illness and liver damage. In marine water, dinoflagellates such as Gonyaulax can form red tides and release neurotoxins that kill fish, close shellfish fisheries and biomagnify through food webs.

The Salto Grande case is a named freshwater HAB example; the dinoflagellate red tide in the Gulf of Mexico is a named marine example. In both, identify the organism and toxin evidence rather than relying on water colour.

Toxin concentration in shellfish/water plus exposure thresholds, not colour alone.

Not every bloom is toxic, and a clear-looking sample is not proof of safety.