Incorporating turbulence into a plankton foodweb model

Small-scale fluid motions in the ocean affect the rate of nutrient uptake by bacteria and phytoplankton and the predation rates of zooplankton. The magnitude of the effect depends on the size and swimming speed of the organisms. Theory predicts that nutrient uptake will be increased by turbulence and that zooplankton–phytoplankton encounter and capture rates will be increased at low turbulent intensity but the capture rate will be decreased at high turbulent intensity. We present a mathematical model of an enclosure experiment carried out in Norway in July 2001. In the experiment the enclosed plankton communities were subjected to various levels of turbulence, generated using oscillating grids, and to different initial nutrient conditions. We predict that, for the experimental conditions, the rate of nutrient uptake by diatoms and grazing by copepods will increase with turbulence but other parameters will be unaffected. We present results of simulations in which these parameters are increased separately and together. Comparison of computational and experimental results suggests that the dominant effect of turbulence is the increase in nutrient uptake by diatoms.