© 1998 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Temporal variability of micro- and nanoplankton in the German Bight in relation to hydrographic structure and nutrient changes
Biologische Anstalt Helgoland Notkestrasse 31, 22607, Hamburg, Germany. Tel. +49 40 89693 203, fax: +49 40 89693 115
Variability in micro- and nanoplankton biomass in the German Bight is described based on 34 annual cycles measured five times week–1 near the island of Helgoland. 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 effect of increasing inorganic nutrient concentrations (eutrophication) on long-term changes of phytoplankton stocks could not be determined definitively, because of the overriding effect of hydrographic changes. Indications for eutrophication effects – 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 different 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.
Keywords: Eutrophication effects, German Bight, nanoplankton, North Sea, phytoplankton