Competition for Nutrients and Light in a Mixed Water Column: A Theoretical Analysis

Interactions between nutrient limitation and light limitation are fundamental for the dynamics and structure of phytoplankton communities. We investigate a model that predicts the outcome of competition for nutrients and light in a mixed water column on the basis of monoculture characteristics. Growth in monoculture leads to a steady state. The nutrient availability and light penetration in this steady state characterize the minimal resource requirements of a species. These minimal requirements not only determine monoculture growth but also the outcome of competition. We show that competition for nutrients and light can be investigated by means of a graphical isocline approach. In contrast to earlier resource-based approaches, our model predicts that it is not only the ratio of nutrient supply to light supply that matters for the outcome of competition but also their absolute supply rates. It is even possible that a species that competes successfully when there is a high or low light supply is displaced when the light supply is intermediate. Factors such as mixing depth and background turbidity also affect the composition of phytoplankton communities. Still, our model predicts that at most two species can stably coexist in a mixed water column. Hence, the spatial heterogeneity imposed by a light gradient is not sufficient to solve Hutchinson's paradox of the plankton.