English Abstract A simulation model was used to analyse quantitatively
the carbon and silicate metabolism of the phytoplankton population in the
lower reaches of the river Rhine. Field measurements were carried out five
times in 1988, by sampling a body of water during its transport downriver
through the Netherlands. Parameters of the model were chosen by
interpolation of these data. The potential photosynthetic productivity was
estimated from laboratory measurements of the carbon fixation in the samples
at saturating light conditions. The actual productivity was calculated by
incorporating the varying solar illumination, the extinction coefficient of
the water and the river depth in a 'plugflow' model. It was also assumed
that the productivity of the diatom-dominated phytoplankton and the silicate
uptake as relative to carbon fixation were limited by low silicate
concentrations. On the basis of the productivity, the silicate uptake of
the diatoms and silicate regeneration in the river bottom simulations of the
dissolved silicate concentration were made and found to match with the
observed silicate dynamics, thus corroborating the calculated productivity.
For a further verification of the model, and as a first step in the
construction of a foodchain model for the river Rhine, the phytoplankton
concentration was simulated. Loss factors such as respiration,
sedimentation and grazing were specified analogous to published models for
phytoplankton dynamics. After calibration, using the assumed grazer
densities as tuning parameters, these simulations matched well with the
observations in spring. In summer situations however, an overestimation of
the productivity and/or an underestimation of the loss factors seemed to
occur.
