Regular phytoplankton research in the Marsdiep started in the early 1970s as a curiosity-driven project fitting in the International Biological Programme (IBP). Interest was primarily in seasonal variation in species composition, biomass and primary production. Initially our monitoring was not intended to extend beyond the IBP programme, which ended in 1971. However, the example and support of Jan Beukema and the interesting links with his work on secondary production in the Wadden Sea were decisive. in its continuation. As a result, we now have a time series of well over 30 years. We had the good luck to start in a period of relatively low phytoplankton values, whereas in the late 1970s biomass and primary production doubled in a short period. Duration of the Phaeocystis blooms, which regularly occur in spring and early summer in Dutch coastal waters, also increased. Light limitation plays a dominant role in turbid coastal waters. Our Secchi disc data up to 1985 did not indicate changes in turbidity, and therefore increases were seen as a eutroohication phenomenon. What would then be more natural than to expect phytoplankton to decrease again with the lowering of phosphate values in the 1980s and 1990s as a result of the successful cleaning of the Rhine? The data up to 1992 did not show any effect on phytoplankton of this deeutrophication. Annual primary production during the period 1992-2000 decreased from a peak value of >400 gC m(-2) in 1994 to ca. 250 now. Over the entire period 1974-2000, a slight increase in turbidity was observed. Phosphate concentrations have dropped a little further since 1992, but as could be expected, dissolved nitrogen (nitrate, nitrite and ammonia) and reactive silica did not show changes since the mid 1970s; inputs from freshwater and the atmosphere have not decreased. In accordance with the decrease of primary production, also chlorophyll-a and Phaeocystis cell numbers have dropped since 1994. These first signs of a decrease in phytoplankton parameters probably related to de-eutrophication make continuation of our Marsdiep monitoring highly interesting. No lengthening of the growing season of phytoplankton was observed comparable to that observed in terrestrial vegetation and related to global warming, but Phaeocystis growth seems to start earlier now. Our phytoplankton research covers only a small part of the period of man-induced changes of the Wadden Sea ecosystem, which started already in the Middle Ages. Certainly the recent overfishing of filter feeders such as cockles and mussels has affected phytoplankton, but also the cultivation of mussels and introduction of exotics such as Crassostrea gigas and Ensis directus must have influenced phytoplankton. The Marsdiep phytoplankton time series has proved to be useful in the continued ecosystem research in the western Wadden Sea. Continuation of this time series will also be helpful to study natural versus human-induced variations in this area. (C) 2002 Elsevier Science B.V All rights reserved.
