A MODEL OF PHYTOPLANKTON PRODUCTION IN THE LOWER RIVER RHINE VERIFIED BY OBSERVED CHANGES IN SILICATE CONCENTRATION

The production of phytoplankton in the three main branches and sedimentation areas of the River Rhine in the Netherlands was analyzed using a simulation model describing the carbon and silicate metabolism. This model is based on data derived from a sampling programme in which river water was followed during downstream transport. A 'plug-flow model' was developed, including sky irradiance and light attenuation in the water, and integrating photosynthetic rates determined in the laboratory. On the basis of the silicate content of diatom-dominated phytoplankton and silicate regeneration in the river bottom, changes in silicate concentrations were simulated and found to match observed changes in dissolved silicate. Low silicate concentrations were shown to restrict the maximum population density of diatoms. Depth- and time-integrated rates of photosynthesis were shown to permit multiplication of the phytoplankton at a rate of up to one doubling day-1. In the primary production period April-August 1988, values of 0.48-6.33 g C m-2 day-1, close to the few values reported for highly eutrophic rivers and lakes, were observed. Model runs, including phytoplankton production and losses, such as respiration, sedimentation and planktonic grazing, were carried out to simulate the downstream development of phytoplankton biomass. These simulations confirm the view that a substantial part of the phytoplankton biomass and production is grazed or settles in the river delta despite residence times of only 52-97 h.