From August to October 1996, a station in the freshwater tidal reaches of the Schelde estuary was sampled every two days to study short-term changes in the phytoplankton community. The monitoring study started after a long period of low and constant discharge and our data suggest that at that time, phytoplankton productivity was bottom-up limited by silica and/or top-down controlled by grazers. Rainfall in the middle of August resulted in a flushing of almost half of the estuarine volume upstream of our monitoring station. This fresher ended the period of limitation by removing grazers as well as supplying new silica, while having no direct effect on total phytoplankton biomass. It was followed by a gradual increase in phytoplankton biomass. An extremely high runoff event caused by a storm at the end of August resulted in a flushing of the entire freshwater tidal reach. Phytoplankton biomass dropped and the estuarine phytoplankton community was replaced with one of riverine origin which was not adapted to the growth conditions prevailing within the estuary. Afterwards, autochthonous phytoplankton populations were not able to recover due to the presence of grazers and a general decrease in light levels and temperature. Our results illustrate how freshets may influence phytoplankton dynamics directly through longitudinal transport of phytoplankton populations and indirectly by influencing nutrient levels and grazer populations.
OMES: Monitoring van het fytoplankton in de Zeeschelde, meer
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