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Red Tide

Marine toxins and their effects

Red tides are a marine phenomenon in which water is stained a red, brown, or yellowish color because of the temporary abundance of a particular species of pigmented dinoflagellates (these events are known as "blooms"). Also called phytoplankton, or planktonic algae, these single-celled organisms of the class Dinophyceae move using a tail-like structure called a flagellum. They also photosynthesize and it is their photosynthetic pigments that can tint the water during blooms. Dinoflagellates are common and widespread. Under appropriate environmental conditions, various species can grow very rapidly, causing red tides. Red tides occur in all marine regions with a temperate or warmer climate.

The environmental conditions that cause red tides to develop are not yet understood. However, they are likely related to some combination of nutrient availability, nutrient ratios, and water temperature. Red tides are ancient phenomena and were, for example, recorded in the Bible. However, it is suspected that human activities that affect nutrient concentrations in seawater may be having an important influence on the increasingly more frequent occurrences of red tides in some areas. In particular, the levels of nitrogen, phosphorous, and other nutrients in coastal waters are increasing due to runoff from fertilizers and animal waste. Complex global changes in climate also may be affecting red tides. Water used as ballast in ocean-going ships may be introducing dinoflagellates to new waters.

Sometimes the dinoflagellates involved with red tides synthesize toxic chemicals. Genera that are commonly associated with poisonous red tides are Alexandrium, Dinophysis, and Ptychodiscus. The algal poisons can accumulate in marine organisms that feed by filtering large volumes of water, for example, shellfish such as clams, oysters, and mussels. If these shellfish are collected while they are significantly contaminated by red-tide toxins, they can poison the human beings who eat them. Marine toxins can also affect local ecosystems by poisoning animals. Some toxins, such as that from Ptychodiscus brevis, the organism that causes Florida red tides, are airborne and can cause throat and nose irritations.

Red tides can cause ecological damage when the algal bloom collapses. Under some conditions, so much oxygen is consumed to support the decomposition of dead algal biomass that anoxic conditions develop. This can cause severe stress or mortality in a wide range of organisms that are intolerant of low-oxygen conditions. Some red-tide algae can also clog or irritate the gills of fish and can cause stress or mortality by this physical effect.

Saxitoxin is a natural but potent neurotoxin that is synthesized by certain species of marine dinoflagellates. Saxitoxin causes paralytic shellfish poisoning, a toxic syndrome that affects humans who consume contaminated shellfish. Other biochemicals synthesized by dinoflagellates are responsible for diarrhetic shellfish poisoning, another toxic syndrome. Some red tide dinoflagellates produce reactive forms of oxygen—superoxide, hydrogen peroxide, and hydroxyl radical—which may be responsible for toxic effects. A few other types of marine algae also produce toxic chemicals. Diatoms in the genus Nitzchia synthesize domoic acid, a chemical responsible for amnesic shellfish poisoning in humans.

Paralytic, diarrhetic, and amnesic shellfish poisoning all have the capability of making large numbers of people ill and can cause death in cases of extreme exposure or sensitivity. Because of the risks of poisoning associated with eating marine shellfish, many countries routinely monitor the toxicity of these foods using various sorts of assays. One commonly used bioassay involves the injection of laboratory mice with an extract of shellfish. If the mice develop diagnostic symptoms of poisoning, this is an indication of contamination of the shellfish by a marine toxin. However, the mouse bioassay is increasingly being replaced by more accurate methods of determining the presence and concentration of marine toxins using analytical biochemistry. The analytical methods are generally more reliable and are much kinder to mice.

Marine animals can also be poisoned by toxic chemicals synthesized during blooms. For example, in 1991 a bloom in Monterey Bay, California, of the diatom Nitzchia occidentalis resulted in the accumulation of domoic acid in filter-feeding zooplankton. These small animals were eaten by small fish, which also accumulated the toxic chemical and then poisoned fish-eating cormorants and pelicans that died in large numbers. In addition, some humans who ate shellfish contaminated by domoic acid were made ill.

In another case, a 1988 bloom of the planktonic alga Chrysochromulina polylepis in the Baltic Sea caused extensive mortalities of various species of seaweeds, invertebrates, and fish. A bloom in 1991 of a closely related species of alga in Norwegian waters killed large numbers of salmon that were kept in aquaculture cages. In 1996, a red tide killed 149 endangered manatees (Trichechus manatus latirostris) in the coastal waters of Florida.

Even large whales can be poisoned by algal toxins. In 1985, 14 humpback whales (Megaptera novaeangliae) died in Cape Cod Bay, Massachusetts, during a five-week period. This unusual mortality was caused by the whales eating mackerel (Scomber scombrus) that were contaminated by saxitoxin synthesized during a dinoflagellate bloom. In one observed death, a whale was seen to be behaving in an apparently normal fashion, but only 90 minutes later it had died. The symptoms of the whale deaths were typical of the mammalian neurotoxicity that is associated with saxitoxin, and fish collected in the area had large concentrations of this very poisonous chemical in their bodies.



Freedman, B. Environmental Ecology. 2nd ed. New York: Academic Press, 1994.

Okaichi, T., D. M. Anderson, and T. Nemoto, eds. Red Tides: Biology, Environmental Science, and Toxicology. New York: Elsevier, 1989.

Bill Freedman


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—An event of great abundance of phytoplankton, to the degree that the water is distinctly colored by the algal pigments.

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