What is plankton?

   Many living things in the sea belong to a group of plants and ani­mals called plankton. This name refers to the countless marine plants and animals that live in surface water and are carried along by currents. Those organisms, especially animals, that actively swim at the surface of the water are called nekton.
   There are a few fairly large types of plankton, such as the sargassum plants and the jellyfish. However, most of the organ­isms classed as plankton are very small. These range from the copepods, which are barely visible, to the microscopic forms called diatoms and bacteria.
   The animal representatives of plankton are one-celled protozoa and the larvae of such forms as oysters, snails, fish, and worms. These forms of plankton are a very necessary food supply to the fish of the sea. Small fish, as well as some very large whales, feed entirely on plankton. Larger fish feed upon the small fish. Eventually man uses some of the larger fish as a source of his food supply.
   The terms limnoplankton and haliplankton are sometimes applied to fresh-water and marine organisms, respectively. There are also other subdivisions.