Spatial and Temporal Dynamics of Phytoplankton Communities in a Freshwater Tidal Estuary (Schelde, Belgium)

During the course of 1996, the brackish to freshwater tidal reaches of the Schelde Estuary were sampled monthly along a longitudinal transect to study the origin and composition of phytoplankton communities in the freshwater tidal estuary. Variation in the phytoplankton community as a whole and its relation with the abiotic environment was analysed using multivariate analysis with variation partitioning. Of the variation in the species data 23·4% was spatially structured and a large part (13·8%) was explained by salinity or factors co-varying with salinity, with a marked species succession observed along the salinity gradient. Spatial variation not related to salinity (9·6%) was mainly situated within the freshwater tidal reaches, where in the uppermost stations phytoplankton was imported from the riverine tributaries while in the more downstream situated stations it was produced in situ. Riverine phytoplankton declined after import into the estuary where it was replaced by autochthonous, estuarine populations which were adapted to the low light climate and/or resistant to rotifer grazing. Of the species variation 27·8% was temporally structured. While riverine blooms appeared to be initiated by an increase in irradiance a decrease in river discharge in spring, estuarine phytoplankton only developed in summer, when temperature increased. Some potentials and limitations of the use of multivariate analyses with variation partitioning in pelagic estuarine environments are discussed in view of our results.