JOURNAL OF PLANKTON RESEARCH | VOLUME 15 | NUMBER 6 | PAGES 659-682 | 1993
© Oxford University Press
research-article |
A model of phytoplankton production in the lower River Rhine verified by observed changes in silicate concentration
National Institute of Public Health and Environmental Protection PO Box 1, 3720 BA Bilthoven, The Netherlands 1Present address: Aquatic Ecotoxicology Section, University of Amsterdam Kruislaan 320, 1098 SM Amsterdam, The Netherlands 2Present address: Institute of Theoretical Biology, Leiden University PO Box 9516, 2300 RA Leiden, The Netherlands 3Present address: International Institute for Hydraulic and Environmental Engineering (IHE), Aquatic Ecology Section PO Box 3015, 2601 DA Delft, The Netherlands
Received on September 1, 1992; accepted on February 16, 1993 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 sìlicate 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.