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 andsedimentation areas of the River Rhine in the Netherlands wasanalyzed using a simulation model describing the carbon andsilicate metabolism. This model is based on data derived froma sampling programme in which river water was followed duringdownstream 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 phytoplanktonand silicate regeneration in the river bottom, changes in silicateconcentrations were simulated and found to match observed changesin dissolved silicate. Low sìlicate concentrations wereshown to restrict the maximum population density of diatoms.Depth- and time-integrated rates of photosynthesis were shownto permit multiplication of the phytoplankton at a rate of upto one doubling day^–1 In the primary production periodApril-August 1988. values of 0.48–6.33 g C m^–2 day^–1,close to the few values reported for highly eutrophic riversand lakes, were observed. Model runs, including phytoplanktonproduction and losses, such as respiration, sedimentation andplanktonic grazing, were carried out to simulate the downstreamdevelopment of phytoplankton biomass. These simulations confirmthe view that a substantial part of the phytoplankton biomassand production is grazed or settles in the river delta despiteresidence times of only 52–97 h.