Plankton

Plankton, collective term for a variety of marine and freshwater organisms that drift on or near the surface of the water. Their movement depends largely on tides, currents, and winds, because they are too small or weak to swim against the currents. That component of the plankton comprising photosynthetic organisms is called the phytoplankton. Important algal groups in the phytoplankton include diatoms, golden algae, green algae, and cyanobacteria (formerly known as blue-green algae). The other component of the plankton, the zooplankton, comprises protozoa and small crustaceans, jellyfish, worms, and mollusks, together with the eggs and larvae of the many animal species inhabiting marine and fresh waters. Important protozoan groups in the zooplankton are dinoflagellates and foraminifera.

The density of plankton varies, depending on the availability of nutrients and the stability of the water. A liter of lake water may contain more than 500 million planktonic organisms. Marine plankton occasionally becomes so numerous that the organisms color the water; such sudden population increases are called tides. The so-called red tides are caused by billions of dinoflagellates of various species; such tides are sometimes dangerous, because they can poison both humans and fish. Red tides have occurred off the west coast of Florida and in the coastal waters of New England, southern California, Texas, Peru, eastern Australia, Chile, and Japan. In 1946 such a tide killed fish, turtles, oysters, and other marine organisms in the Gulf of Mexico near Fort Myers, Florida. The blue-green alga Trichodesmus sometimes, in dying off, imparts a reddish color to water; the Red Sea is so named because of this.

An estimated 90 percent of all photosynthesis and release of free oxygen takes place in the oceans. Marine phytoplankton is the first link in the vast aquatic food chain. The zooplankton, which feeds on the phytoplankton, is consumed in turn by larger animals such as fish and even by the largest mammal, the blue whale. The high protein content of plankton has stimulated research on it as a potential food source for humans. Phytoplankton is sensitive to ocean water temperature, growing faster in cooler waters. Researchers are concerned that increased ocean temperatures due to global warming could reduce the growth of phytoplankton and thereby impact the aquatic food chain. Because phytoplankton also absorbs atmospheric carbon dioxide, which is a greenhouse gas, reduced amounts of phytoplankton in the oceans could increase the amount of carbon dioxide in the atmosphere and thereby speed up global warming. See also Marine Life.