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 microphyto benthos was studied in the turbid estuary. Because of the high turbidi ty and high nutrient concentrations primary production by phytoplankto n is light-limited. In the inner and central parts of the estuary maxi mum rates of primary production were therefore measured during the sum mer, whereas in the more marine part spring and autumn bloom were obse rved. Organic loading is high, causing near anaerobic conditions upstr eam in the river Schelde. Because of this there were no important phyt oplankton grazers in this part of the estuary and hence the grazing pr essure on phytoplankton was minimal. As this reduced losses, biomass i s maximal in the river Schelde, despite the very low growth rates. On a number of occasions, primary production by benthic micro-algae on in tertidal flats was studied. Comparison of their rates of primary produ ction to phytoplankton production in the same period led to the conclu sion that the contribution to total primary production by benthic alga e was small. The main reason for this is that the photosynthetic activ ity declines rapidly after the flats emerged from the water. It is arg ued that CO2-limitation could only be partially responsible for the no ticed decrease in activity.