What causes such discoloration? Red tides are generally caused by several species of microscopic, single-celled algae or by protozoans called dinoflagellates. These tiny organisms have hairlike projections called flagella—whiplike extensions, which they use to propel themselves in water. There are about 2,000 varieties of dinoflagellates, 30 of which carry poisonous substances. These minute organisms usually stay in warm waters with high salinity.

A red tide occurs when there is a sudden and rapid increase in, or bloom of, these microscopic dinoflagellates. The concentration of these organisms may swell to 50,000,000 per quart of water! Although scientists do not fully understand why this happens, it is known that dinoflagellates accumulate when certain conditions simultaneously affect the water. These include abnormal weather, optimum temperatures, an oversupply of nutrients in the water, a generous amount of sunlight, and favorable water currents. When a heavy rainfall occurs, minerals and other nutrients are sometimes washed from the land into coastal waters. These nutrients can contribute to the proliferation of dinoflagellates. The result? Red tides!

Certain species of dinoflagellates release toxic substances into the water that paralyze and kill fish and other marine life.

Red-Tide Poisoning

One of the toxins released by some dinoflagellates is known as saxitoxin. This is a water-soluble salt that attacks the human nervous system. Thus, it is classified as a neurotoxin. The New Encyclopædia Britannica reports that “toxins released into the water are irritating to the human respiratory system.” Beach resorts have had to close when red-tide toxins have been released into the air as a result of wave action.