Chapter 16: Plankton

Marine organisms are divided into three major ecological groups: (1) the benthos,which includes the attached, burrowing, creeping, or skipping organisms; (2) the nektonor strongly swimming pelagic animals, such as fish, certain crustaceans, cephalopods, and whales, capable of stemming most water currents; and (3) the plankton,which includes all the floating or feebly swimming animals and plants that are swept about by the prevailing currents. The plankton is divided into two natural groups: the phytoplanktonand the zooplankton.Their members are either oceanic—i.e.,living upon the high seas—or neritic,living in coastal waters. Each group will be discussed separately in so far as possible, and it is fitting to begin with the phytoplankton since it is basic to all life in the sea because of its overwhelming importance in organic production. By “organic production” is here meant the gross production or primary production. This may be defined as the amount of organic material synthesized by organisms from inorganic substances in unit time in a unit volume of water or in a column of water of unit-area cross section and extending from the surface to the bottom. The plant substance produced will be slightly less than gross production because of utilization by plants themselves in their living processes. ( SeeSverdrup, Johnson, and Fleming, 1946; Clarke, 1946, for additional definitions and units of production, and also Chapters 3, 17, 29). PHYTOPLANKTON The phytoplankton comprises all the freely floating photosynthetic forms. Individually they are microscopic in size and composed of...