Modelling of cyanobacterial blooms in the mixed layer of lakes and reservoirs

Abstract

The chemical and biological components of existing water quality models are in general described by simple first-order rate equations in which the production and uptake coefficients are fixed functions of the other variables of the model. Thus although, for example, in a photosynthesis model the specific algal growth rate may be a function of light intensity, nutrient concentration and temperature, the form of this dependence on these variables is assumed to be fixed. In this paper, the effect on the performance of a water quality model of removing this assumption for the specific case of photosynthesis is examined. An existing coupled motion-photosynthesis model has been installed in the water quality model DYRESM-WQ and the result compared with the original model, which assumes a fixed functional dependence. The resulting model has been applied to two reservoirs, without recalibration. The result shows that the removal of the assumption of a fixed functional form for the photosynthetic growth rate may have a significant effect on the magnitude and timing of predicted cyanobacterial blooms in lakes and reservoirs, which would have important implications for reservoir and lake management. In addition, the result suggests that, in general, the validity of the assumption of fixed functional form for the rate coefficients in water quality models is not assured.