Analysis of the vertical distribution in a model of phytoplankton dynamics

Phytoplankton often faces the dilemma of livingin contrasting gradients of two essential resources: the lightthat comes from above and nutrients that are often suppliedfrom below. In poorly mixed water columns, algae can beheterogeneously distributed, with thin layers of biomass foundon the surface, in depth, or on the sediment surface. Here, weshow that these patterns can result from intraspecific competitionbetween light and nutrients. First, we present numerical solutionsof a reaction-diffusion-taxis model for phytoplankton, nutrientsand light. We argue that motile phytoplankton can form a thinlayer under poorly mixed conditions. The numerical solution ofthis model indicates the depth at which phytoplankton is equallylimited by both resources. The layer becomes shallower with anincrease in the nutrient supply and deeper with an increase inthe light supply. These general patterns are in agreement withfield observations. Thus, this model can explain many patternsof algal distribution found in poorly mixed aquatic ecosystems.