MARINE PLANKTON FOOD CHAINS

Abstract

The view on the ecology of marine plankton has changed significantly within the last decade. In some respects the is sufficiently dramatic to make some authors speak about a change in paradigm (84). The new picture of plankton dynamics includes the recognition that phototrophic and heterotrophic microorganisms play a substantial and sometimes dominating role in the cycling of matter in the sea, that plankton food chains include a higher number of trophic levels than hitherto believed, and that a large fraction of the primary production is not consumed directly by herbivorous consumers but is channelled through a pool of dead organic matter before it becomes available-via bacterial production-to phagotrophic organisms. In this latter respect the structure of planktonic systems resembles benthic and terrestrial systems. This changing view is remarkable for more than one reason. Among different types of ecosystems, that of the open sea was long considered the simplest and best understood. Many ecology textbooks used marine plankton to introduce the concept of food chains. A graphical presentation might be in the form of a pyramid with its base representing primary producers; above them two hori-zontal layers represented zooplankton and fish, respectively; and at the top a fisherman (or a whale) represented the highest trophic level. While this essentially remains correct, it has certainly proven to be an incomplete picture of pelagic food chains. Modem biological oceanography began at the turn of the century. During the following decades it developed into an established discipline which successfully explained fundamental aspects of plankton dynamics. Organic