The Annual Cycle of Kiel Bight Plankton: A Long-Term Analysis

Abstract

Over the past decade, the annual cycle of the major pelagic processes in relation to environmental factors and species composition of the plankton has been studied intensively at a fixed station in Kiel Bight. A series of sequential phases, differentiated according to characteristic properties, succeed each other in a recurring pattern each year. The following phases have been differentiated: the spring diatom bloom, the late spring copepod maximum, the summer stratification, the fall blooms and the winter dormancy. Each phase represents a particular pattern of biogenous element cycling, both within the pelagic system and between the pelagic and benthic systems. Each phase is also characterized by a spectrum of dominant species, many of which do not recur each year. Greatest variation is found amongst bloom diatoms, whereas large, slow-growing species such as the Ceratia and most metazooplankton are highly recurrent. Variation in species composition is not related to long-term trends since the past century, in spite of the considerable increase in anthropogenic nutrient input to the Bight. Short-term events appear to determine occurrence of fast-growing species, many of which have benthic resting stages in their life histories. It is concluded that more attention should be paid to life history strategies of species if the mechanisms of seasonal succession are to be elucidated. Long-term observations on appearance or absence of the various species in relation to environmental properties can provide clues as to the nature of these life history strategies.