Marine plankton

The word 'plankton' is taken from a Greek verb meaning to wander and is used to refer to those pelagic forms which are carried about by the movements of the water rather than by their own ability to swim. These organisms are called planktonts. The plants of the plankton are the phytoplankton, the animals the zooplankton. Early naturalists were enthralled by these tiny animals and plants and wrote some fascinating accounts of their observations (Fraser, 1962; Hardy, 1956 and 1967; Sverdrup et al., 1946). This huge subject can be only introduced in this chapter.

The phytoplankton is responsible for most of the primary production in the sea (see Chapter 7). There would be virtually no life in the ocean without the photosynthesis carried out by these microscopic plants. On land the energy-fixing plants dominate the landscape in the form of grasses, shrubs and trees. In contrast the phytoplankton is only visible to us as a cloudiness or discolouration of the water when reproduction is rapid and a 'bloom' occurs.

Some planktonts can only float passively, unable to swim at all. Others are quite active swimmers but are so small that swimming does not move them far compared to the distance they are carried by the water. The swimming movements serve chiefly to keep them afloat, alter their level, obtain food, avoid capture, find a mate or set up water currents for respiration. Although the majority of planktonts are small, mainly of microscopic size, a few are quite large. For example, the tentacles of Portugese Man O'War (Physalia) sometimes extend 15 m through the water, and there are jellyfish (Scyphomedusae) which grow to over 2 m in diameter.

Organisms, whose entire lifespan is planktonic, are termed holoplankton or permanent plankton. The numbers of planktonts are swelled by the addition of organisms passing through a pelagic phase which is only part of the total life-span; for example, planktonic spores, eggs or larvae of nektonic (free-swimming) or benthic (bottom-living) organisms. These organisms, which spend only part of their lifespan in the plankton, usually the early stages, are termed meroplankton or temporary plankton. For fixed, sessile species, the planktonic larval stages provide an essential means of dispersal.

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