Analysis of the vertical distribution in a model of phytoplankton dynamics

Abstract

Phytoplankton often faces the dilemma of living in contrasting gradients of two essential resources: the light that comes from above and nutrients that are often supplied from below. In poorly mixed water columns, algae can be heterogeneously distributed, with thin layers of biomass found on the surface, in depth, or on the sediment surface. Here, we show that these patterns can result from intraspecific competition between light and nutrients. First, we present numerical solutions of a reaction-diffusion-taxis model for phytoplankton, nutrients and light. We argue that motile phytoplankton can form a thin layer under poorly mixed conditions. The numerical solution of this model indicates the depth at which phytoplankton is equally limited by both resources. The layer becomes shallower with an increase in the nutrient supply and deeper with an increase in the light supply. These general patterns are in agreement with field observations. Thus, this model can explain many patterns of algal distribution found in poorly mixed aquatic ecosystems.