Seawater always contains tiny plant-like organisms called algae. Usually algae can only be seen with a microscope. Under certain conditions, algae can begin to reproduce rapidly. This causes the algae population to increase rapidly or “bloom." During an algal bloom, a single gallon of seawater can contain millions of algae.
Not all algal blooms are harmful. Some algae, however, produce harmful substances called toxins. For example, the Florida Gulf Coast often experiences harmful algal blooms of a type of algae called Karenia brevis, which produces toxins called brevetoxins.
Blooms of Karenia brevis can discolor the water, turning it yellow, orange, pink, brown, or red. This discoloration is probably what led some people to use the term red tide. Scientists don't like the term red tide, however, for several reasons.
Harmful algal blooms don't always discolor the water and, when they do, it doesn't always appear red. Moreover, harmful algal blooms have nothing to do with tides. This is why scientists prefer the more accurate term harmful algal bloom instead of red tide.
Harmful algal blooms are called harmful because the toxins produced by the blooming algae can kill millions of fish and other marine organisms. One tell-tale sign of a harmful algal bloom is a beach covered with dead fish.
Harmful algal blooms can also be harmful to humans. People have been known to get sick from eating shellfish that feed on the toxic algae. Some experts also believe there may be negative effects associated with breathing the air near or swimming in water during a harmful algal bloom.
Because of the effects on marine and human health, as well as shoreline quality and the fishing industry, scientists are trying to learn more about the causes of harmful algal blooms. Some believe they're simply natural occurrences caused by the seasonal shifting of ocean currents. Others believe human factors, such as pollution, may also play a role.
Scientists have learned that even harmful algal blooms may have benefits, too. For example, even though some fish are killed, scientists have noted that other marine populations seem to increase. This has led some scientists to theorize that harmful algal blooms may be one of nature's ways of balancing the populations of various marine species.