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...