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Drivers of harmful algal blooms (HABs) include nutrient pollution, warmer temperatures, and stagnant waters. HABs often discolor water. When they block sunlight needed by oxygen-producing algae or aquatic macrophytes, or run out of a nutrient, die and are broken down by oxygen-consuming organisms, hypoxic dead zones are produced. Also, directly toxigenic HABs kill fish, fish-eating birds, marine and terrestrial mammals, and people. Although irritation results from skin, eye, or respiratory exposures, the most serious toxic effects typically follow ingestion of water, contaminated shellfish or fish. Microcystins, nodularins, and cylindrospermopsins cause lethal liver and sometimes kidney damage, and repeated exposures promote liver cancers. Paralysis of respiratory muscles can follow ingestion of anatoxin-A, which acts like nicotine, anatoxin-A(s), which inhibits acetylcholinesterase, or saxitoxins, which block sodium channels. Brevetoxins and ciguatoxins cause cell death, neurotoxicity, and respiratory irritation. Manatees, dolphins, and fish-eating birds have died from brevetoxin poisoning. In humans, ciguatera poisoning is common after eating contaminated large predatory fish from coral reefs. Okadaic acid and dinophysistoxins in shellfish cause severe diarrhea. Domoic acid contaminates shellfish and fish, and persistent neurotoxicity affects people, fish-eating birds and marine mammals. Preventing nutrient pollution and climate change, and restoring watershed biodiversity can prevent many harmful algal blooms. |
