A wide variety of species of algae grow in the oceans. Some few occasionally grow very fast or "bloom" and accumulate into dense, visible patches near the surface of the water. Blooms are sometimes of nuisance value only, or they may produce ecological impacts. A small number of algal species produce potent neurotoxins that can be transferred through the food web where they affect and even kill the higher forms of life such as zooplankton, shellfish, fish, birds, marine mammals, and even humans that feed either directly or indirectly on them.
Algal blooms usually occur in summer and are increasingly occurring in unprecedented places, for reasons not well understood. Some may be so thick they block sunlight reaching lower levels and other species. The density of the organisms may be so great that dissolved oxygen in the water is depleted when they die and decompose. Some species give off toxic wastes killing fish and other marine life; some, when consumed by shellfish can kill people. Algal blooms contribute to acid rain by producing various sulphur compounds. They effect the weather by absorbing carbon dioxide from the atmosphere and reflect sunlight. They destroy tourist beaches by making the sea unattractive and dangerous to swim in. The increase in algae blooms is, in some places, caused by pollution coming from cities, industries and farms.
A common name for nuisance algal blooms is "red tide". This is where certain phytoplankton species contain reddish pigments and "bloom" such that the water appears to be coloured red. The term "red tide" is a misnomer because the phenomenon is not associated with tides. Species involved in "red tides" are usually not harmful. It is also the case that species that are harmful may never reach the densities required to discolour the water.
Red tides have been reported off the east coast of America, Tasmania, Taiwan, Guatemala, Korea, Hong Kong, Venezuela, Iceland, the UK, Papua New Guinea, Sabah, Brunei, the Philippines, Scandinavia, Germany and Italy.