Continuous plankton records: Changes in the plankton of the North Sea and its eutrophic southern bight from 1948 to 1975

Abstract

Patterns of long-term variability in the plankton of the North Sea were remarkably uniform over large areas; indeed, major changes during the last decades were similar in all the North Sea and eastern North Atlantic regions considered. Clearly, the plankton part of the ecosystem responded in a standard, predictable way to large-scale changes in the environment not immediately associated with human activities. Patterns of annual variation in the herbivorous zooplankton were not related to those in the phytoplankton. In the southern North Sea the decrease in copepod numbers between 1960 and 1966 did not correspond with a change in the number of diatoms registered by the Continuous Plankton Recorder: the decline in this latter group did not start before the mid-sixties. Phytoplankton groups that contributed to the coloration of the silks but were not recorded in the analysis of C.P.R. samples ( e.g. microflagellates, small or fragile diatoms) came to profusion in spring and early summer all over the North Sea during the late sixties and early seventies; and during the last 5 years dinoflagellates were much more abundant than usual all over the southern North Sea. The gradual decrease in abundance of the colonial μ-flagellate Phaeocystis poucheti between 1948 and 1970 and the delay in the spring production of Temora and Acartia since the early fifties may have been related to the decline of sea temperature on the European shelf reported in the literature; but in the southern North Sea the annual fluctuations in the phytoplankton did not correspond to temperature variations in any season. The increasing green coloration of the Recorder silks showed some resemblance to a trend of increasing solar radiation between 1956 and 1975. Trends in the annual plankton fluctuations in the survey region most under the influence of the eutrophic Rhine water were similar to those in adjacent areas. However, the decrease in diatoms and in copepods was less dramatic than elsewhere, while the increase in microflagellates and other unidentified species between 1966 and the early seventies was greater than in any other region. It is possible that the increased fertilization of this area has stimulated phytoplankton and zooplankton production in the eastern part of the Southern Bight. However, the natural long-term variability can still be recognized clearly. de Jonge & Postma's (1974) estimate of a twofold increase in the amount of suspended particulate organic matter in Dutch coastal waters between 1950 and 1970 comes close to the increase in silk colour intensity (a phytoplankton index) of the Recorder samples taken in the eastern Southern Bight. In less eutrophic southern North Sea waters and in the oligotrophic Channel the silk colour value registered during the early seventies was also double that of the early fifties.