Plankton dynamics in the river Rhine during downstream transport as influenced by biotic interactions and hydrological conditions

Two water parcel studies were performed in the river Rhine from Maxau (Germany) to Maassluis (the Netherlands) in the spring and summer of 1995. Chlorophyll-a levels were low both in spring and in summer, probably due to the high discharge in 1995 as compared to other years. Nutrients were never depleted. Zooplankton densities were very low. Diatoms and green algae had positive net apparent growth rates of 0.38 d-1 and 1.01 d-1, respectively, as calculated from cell counts. Grazing experiments estimated the zooplankton grazing rate in the order of 0.05 d-1 or 8% of the chlorophyll-based growth rate. As in previous years, the plankton densities decreased in the lower part of the Rhine. The loss rate could not be explained by zooplankton grazing, respiration and sedimentation. From earlier published data the potential impact of benthic filter feeders was calculated. This exercise suggests that filtration by zoobenthos can explain the observed decrease in plankton density in the Rhine.