More correctly known as an algal bloom, a so-called Red tide is a natural event in which estuarine, marine or freshwater algae accumulate rapidly in the water column and can convert entire areas of an ocean or beach into a blood-red color. This phenomenon is caused by high levels of phytoplankton accumulating to form dense, visible clouds near the surface of the water. While some of these can be relatively harmless, others can be harbingers of deadly toxins that cause the deaths of fish, birds, and marine mammals. In some cases, even humans have been harmed by red tides though no human exposure is known to have been fatal. While they can be fatal, the constituent phytoplankton in ride tides is not harmful in small numbers.
