Red tide

Red tide is a common name for a phenomenon known as an algal bloom (large concentrations of aquatic microorganisms) when it is caused by a few species of dinoflagellates and the bloom takes on a red or brown color. Red tides are events in which estuarine, marine, or fresh water algae accumulate rapidly in the water column, resulting in coloration of the surface water. It is usually found in coastal areas. It kills many manatees every year.

These algae, a form of phytoplankton, are single-celled protists, plant-like organisms that can form dense, visible patches near the water's surface.

Certain species of phytoplankton, dinoflagellates, contain photosynthetic pigments that vary in color from green to brown to red. When the algae are present in high concentrations, the water appears to be discolored or murky, varying in color from purple to almost pink, normally being red or green. Not all algal blooms are dense enough to cause water discoloration, and not all discolored waters associated with algal blooms are red. Additionally, red tides are not typically associated with tidal movement of water, hence the preference among scientists to use the term algal bloom.

Some red tides are associated with the production of natural toxins, depletion of dissolved oxygen or other harmful effects, and are generally described as harmful algal blooms. The most conspicuous effects of these kinds of red tides are the associated wildlife mortalities of marine and coastal species of fish, birds, marine mammals, and other organisms.

Red tides in the Gulf of Mexico are a result of high concentrations of Karenia brevis, a microscopic marine algae that occurs naturally but normally in lower concentrations. In high concentrations, its toxin paralyzes the central nervous system of fish so they cannot breathe. Dead fish have been observed to wash up on gulf beaches of Mexico and Texas. Dense concentrations appear as discolored water, often reddish in color. It is a natural phenomenon, but the exact cause or combination of factors that result in a red tide outbreak are unknown. Red tide causes economic harm and for this reason red tide outbreaks are carefully monitored. For example, the Florida Fish and Wildlife Conservation Commission provides an up-to-date status report on the red tide in Florida. Texas also provides a current status report.

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