More correctly known as an algal bloom, the so-called Red tide is a natural event in which estuarine, marine, or fresh water algae accumulate rapidly in the water column and can convert entire areas of an ocean or beach into a blood red color. This phenomena 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 are known to have been fatal. While they can be fatal, the constituent phytoplankton in ride tides are not harmful in small numbers.