Temporal variability of micro- and nanoplankton in the German Bight in relation to hydrographic structure and nutrient changes

Sources of variability include not only succession of plankton populations within the seasonal cycle but also changes in the hydrographic structure of the nearby stratified water masses of the outer German Bight. The expected eVect of increasing inorganic nutrient concentrations (eutrophication) on long-term changes of phytoplankton stocks could not be determined definitively, because of the overriding eVect of hydrographic changes. Indications for eutrophication eVects ‐ frequent plankton blooms and oxygen depletion in the bottom water due to accumulated organic particles ‐ are restricted to the outer German Bight. Diatoms and flagellates exhibit diVerent seasonal cycles, with particularly large inter-annual fluctuations in the summer dinoflagellate stocks. Nanoplankton populations appear to have increased sharply at the end of the 1970s. As other plankton and abiotic factors have changed in the southern North Sea at the same time, possible larger-scale events must be considered as well as regional causes. ? 1998 International Council for the Exploration of the Sea