COMPETITION FOR NUTRIENTS AND LIGHT IN A MIXED WATER COLUMN - A THEORETICAL-ANALYSIS

Interactions between nutrient limitation and light limitation are fund amental for the dynamics and structure of phytoplankton communities. W e investigate a model that predicts the outcome of competition for nut rients and light in a mixed water column on the basis of monoculture c haracteristics. Growth in monoculture leads to a steady state. The nut rient availability and light penetration in this steady state characte rize the minimal resource requirements of a species. These minimal req uirements not only determine monoculture growth but also the outcome o f competition. We show that competition for nutrients and light can be investigated by means of a graphical isocline approach. In contrast t o earlier resource-based approaches, our model predicts that it is not only the ratio of nutrient supply to light supply that matters for th e outcome of competition but also their absolute supply rates. It is e ven possible that a species that competes successfully when there is a high or low light supply is displaced when the light supply is interm ediate. Factors such as mixing depth and background turbidity also aff ect the composition of phytoplankton communities. Still. our model pre dicts that at most two species can stably coexist in a mixed water col umn. Hence, the spatial heterogeneity imposed by a light gradient is n or sufficient to solve Hutchinson's paradox of the plankton.