Modeling phytoplankton growth rates

Mathematical models of planktonic ecosystems use a variety of different formulations to relate phytoplankton growth rates to environmental conditions. Does the formulation influence the model result? We have modified the model of Fasham, Ducklow and McKelvie (J. Mar. Res., 34, 591-639, 1990) to test how its results would respond to changes in algal growth rate formulations. The original model uses a Monod relationship between nutrient concentration and relative growth rate, and a multiplicative rule to combine light and nutrient effects. Use of a Droop formulation for algal growth rate or a threshold (Blackman's law) mechanism to combine light and nutrient limitation produced significant changes in simulation results. One important effect was to increase zooplankton population and, as a result, the regenerated production. While there are aesthetic reasons to prefer these alternate formulations, a more accurate formulation will require more laboratory work on algal physiology. Such laboratory work should be encouraged as an adjunct to modeling work.