Light absorption in the waters of the Ems-Dollard estuary and its consequences for the growth of phytoplankton and microphytobenthos

Light absorption and concentrations of suspended matter were measured in the Ems-Dollard estuary (The Netherlands) from 1976 to 1980. The mean coefficients of quantum irradiance attenuation ranged from 1.6 in the outer Ems estuary to 6.7 m−1 in the inner part of the estuary, whilst the mean concentrations of suspended matter ranged from 30 in the outer part to 193 mg · m−1 in the inner part. Both factors closely correlate.

The consequences of this gradient on the growth of phytoplankton and microphytobenthos are discussed. The mean column irradiance levels in the channels and above the submersed tidal flats were calculated. It is demonstrated that the total irradiance available for phytoplankton depends to a small extent on the irradiance received during the submersion of the tidal flats. This contribution may accelerate spring growth of phytoplankton by 2 weeks. Both the onset and the period of phytoplankton growth calculated for the outer part of the estuary corresponded with actual observations on the phytoplankton biomass in the estuary. The column irradiance for phytoplankton growth constrast with that of microphytobenthos growth which reaches a saturation level throughout most of the year.