LIVING IN THE TWILIGHT - ESTIMATING NET PHYTOPLANKTON GROWTH IN THE WESTERSCHELDE ESTUARY (THE NETHERLANDS) BY MEANS OF AN ECOSYSTEM MODEL (MOSES)

Net phytoplankton productivity in the Westerschelde, a relatively deep , highly turbid and eutrophic estuary in the Southwest Netherlands, wa s examined by means of a 13-compartment dynamic simulation model. The description of the light-limited primary production was based on the m odel of Eilers and Peeters (1988, Ecol. Model., 42, 185-198). This lig ht limitation was parameterized to the different model compartments by integrating the Eilers-Peeters model over a day and over depth, takin g into account the morphology of the Westerschelde compartments, the d aily fluctuating light regime and characteristics of the photosynthesi s-light curves of the phytoplankton community. The availability of lig ht to the phytoplankters limited primary production to on average simi lar to 2% of maximal production. Basin morphology, the turbidity gradi ent and light emission were important in the light-limitation function . Nutrient limitation was almost non-existent, except for diatoms in t he marine part in summer, where production decreased to similar to 85% due to silicate limitation. A large fraction of algal biomass was res pired by the algae in the model. From late fall to early spring, the e ntire estuary was a respiratory sink for modeled phytoplankton biomass . Viewed on a yearly scale, phytoplankton respiration exceeded gross p roduction in three model compartments where turbidity and basin morpho logy were most limiting. Due to the high turbidity of the area and the relatively large mixing depth, net pelagic primary production in the model was relatively low: the global estuarine average was 41 g C m(-2 ) year(-1). It fluctuated from a negative production of -74 g C m(-2) year(-1) in the turbidity zone to a maximum net production of 60 g C m (-2) year(-1) near the sea. Salinity stress mortality was the main fac tor causing freshwater algal stocks to decline, while marine and brack ish phytoplankton biomass was mainly controlled by grazing.