Red tides are conditions when a dinoflagellate population increases to huge numbers. This "bloom" may be caused by nutrient and hydrographic conditions, although the environmental conditions which result in red tides are not completely understood. The water is discolored red or brown due to the presence of dinoflagellate cells numbering up to 20 million cells per liter. Red tides are composed primarily of one species of dinoflagellate which has been rapidly growing.
Some red tides are luminescent; most in southern California create dramatic nighttime displays of bioluminescence in the wakes breaking on the beach. A synopsis of the putative mechanisms responsible for these red tides is kindly provided by Prof. Wolfgang Burger, a geologist and former Interim Director of SIO:
"My understanding is this: if, after an upwelling or mixing event (storm?) there is plenty of sunshine, which warms the water and makes for stable stratification, conditions are right for a bloom. If, in addition, Gonyaulax cysts waiting around on the bottom have been stirred up into the water, and have by some means (change in temperature? light?) detected that the time is good for popping open, sufficient seeds are released to start the process. Rapid reproduction ensues (by cell division; these are unicellular organisms) and crowds out everything else, by taking away the light (the water was brown!) and perhaps also by chemical means. "
Some both not all red tides are toxic. In toxic red tides, the dinoflagellates produce a chemical which acts as a neurotoxin in other animals. When the dinoflagellates are ingested by shellfish, for example, the chemicals accumulate in the shellfish tissue in high enough levels to cause serious neurological affects in birds, animals, or people which ingest the shellfish.
There are several types of neurotoxins produced by dinoflagellates. These chemicals may affect nerve action by interfering with the movement of ions across cell membranes, thus affecting muscle activity. The toxin saxitoxin, produced by Gonyaulax off the west coast of North America, and Alexandrium off the northeast coast, accumulates in shellfish. Eating contaminated shellfish causes paralytic shellfish poisoning (PSP). The worst cases of PSP result in respiratory failure and death within 12 hours. Another toxin which accumulates in shellfish is brevetoxin, produced by the dinoflagellate Karenia brevis . Brevetoxin is unique in that it becomes aerosolized when the dinoflagellates end up in the surf zone and then blows onto the beach causing respiratory irritation in humans. If you are on a beach on the Gulf coast of Florida and notice asthma-like breathing symptoms, chances are you are experiencing toxicity from a Karenia bloom. A toxin produced by the dinoflagellate Dinophysis causes diarrhetic shellfish poisoning (DSP), which results in digestive upset but which is not fatal. Ciguatera is another form of dinoflagellate toxicity in tropical areas caused by eating contaminated fish.