Nutrients,light and primary production by phytoplankton and microphytobenthos in the eutrophic,turbid Westerschelde estuary (The Netherlands)

Abiotic factors and primary production by phytoplankton and microphytobenthos was studied in the turbid Westeschelde estuary. Because of the high turbidity and high nutrient concentrations primary production by phytoplankton is light-limited. In the inner and central parts of the estuary maximum rates of primary production were therefore measured during the summer, whereas in the more marine part spring and autumn bloom were observed. Organic loading is high, causing near anaerobic conditions upstream in the river Schelde. Because of this there were no important phytoplankton grazers in this part of the estuary and hence the grazing pressure on phytoplankton was minimal. As this reduced losses, biomass is maximal in the river Schelde, despite the very low growth rates.On a number of occasions, primary production by benthic micro-algae on intertidal flats was studied. Comparison of their rates of primary production to phytoplankton production in the same period led to the conclusion that the contribution to total primary production by benthic algae was small. The main reason for this is that the photosynthetic activity declines rapidly after the flats emerged from the water. It is argued that CO₂-limitation could only be partially responsible for the noticed decrease in activity.