In August 2014, 500,000 people in Toledo, Ohio were told their tap water was toxic and could not be drunk or even boiled for safety, all because of a harmful algal bloom. Scientists are phasing out the term red tide because blooms can be green, brown, or yellow, can be invisible, and have nothing to do with tides. The main culprits are freshwater cyanobacteria, which are bacteria rather than algae, and marine diatoms and dinoflagellates. Many rest as armored cysts in bottom mud, then migrate to the surface when temperature, salinity, and nutrients line up. Blooms are ancient, and Cabeza de Vaca described them in the Gulf of Mexico in the 1500s, but fertilizer runoff, untreated sewage, thermal pollution, and warming have made them more frequent and severe since the 1980s.

The toxins are specific: diatoms make domoic acid that attacks the nervous system, cyanobacteria make liver damaging microcystins, and Karenia brevis makes brevetoxins that crashing waves turn into an airborne mist on Florida beaches. Shellfish concentrate these toxins and pass them up to otters, ducks, dolphins, and people. When a bloom dies, bacteria feeding on it strip oxygen from the water, creating dead zones, and the CDC’s One Health system now links human, animal, and water data.

• The Pacific Blob of 2015 closed the West Coast shellfish fishery, costing Washington $40 million.
• A 2016 bloom in Chile killed 23 million farmed salmon.
• In spring 2004, 107 bottlenose dolphins died on the Florida Panhandle after eating contaminated fish.
• The United States has more than 166 coastal dead zones, which may also release methane.
• Fossil whales clustered at Chile’s Cerro Ballena are linked to an ancient toxic bloom.