STRUCTURAL CHARACTERISTICS OF PHYTOPLANKTON ASSEMBLAGES IN TIDAL AND NON-TIDAL FRESH-WATER SYSTEMS - A CASE-STUDY FROM THE SCHELDE BASIN, BELGIUM

1. The Schelde estuary, its side basins and their tributaries were sam pled in August 1995 and April 1996 for phytoplankton abundance, biomas s, diversity and species composition. In order to clarify the underlyi ng causes of differences in phytoplankton communities, the results wer e related to some important abiotic variables. 2. Although species ric hness and diversity diet not differ significantly between the riverine and the freshwater tidal stations, multivariate ordination techniques based on species abundances differentiate between these two ecosystem s. While in the rivers phytoplankton standing stocks were as high in s ummer as in spring, standing stocks in the freshwater tidal estuary we re significantly higher in the August samples. 3. It is postulated tha t due to the resuspension of suspended solids by estuarine currents, l ight is limiting phytoplankton development in the freshwater tidal rea ches in spring. At that stage, phytoplankton populations have already developed in the rivers. In summer, zooplankton prevent any further in crease of riverine phytoplankton populations. In the freshwater tidal estuary, however, increased light levels, a higher residence time comp ared to rivers and the absence of zooplankton due to low oxygen concen trations permit phytoplankton populations to bloom.