MODELING PHYTOPLANKTON GROWTH-RATES

Mathematical models of planktonic ecosystems use a variety of differen t formulations to relate phytoplankton growth rates to environmental c onditions. Does the formulation influence the model result? We have mo dified the model of Fasham, Ducklow and McKelvie (J. Mar. Res., 34, 59 1-639, 1990) to test how its results would respond to changes in algal growth rate formulations. The original model uses a Monod relationshi p between nutrient concentration and relative growth rate, and a multi plicative rule to combine light and nutrient effects. Use of a Droop f ormulation for algal growth rate or a threshold (Blackman's law) mecha nism to combine light and nutrient limitation produced significant cha nges in simulation results. One important effect was to increase zoopl ankton 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 p hysiology. Such laboratory work should be encouraged as an adjunct to modeling work.