COMPETITION FOR NUTRIENTS AND LIGHT: STABLE COEXISTENCE, ALTERNATIVE STABLE STATES, OR COMPETITIVE EXCLUSION?

Abstract. Competition theory has put forward three contrasting hypotheses: Competition for nutrients and light may lead to (i) stable coexistence of species, (ii) alternative stable states, or (iii) competitive exclusion. This paper presents a detailed investigation of competition among phytoplankton species to test these three different hypotheses. First, we developed a competition model combining competition for nutrients and light. Next, we ran monoculture experiments in phosphorus-limited and light-limited chemostats to estimate the model parameters for five freshwater phytoplankton species. Finally, we tested the model predictions in competition experiments, using phosphorus levels ranging from oligotrophic to eutrophic conditions. The population dynamics in the competition experiments were all in agreement with the model predictions. This demonstrates that competition for nutrients and light can be accurately predicted over a wide range of productivities. The experiments revealed that the intensity of competition remained constant or even decreased with increasing nutrient supply. Contrary to expectation, there were no trade-offs between competitive abilities for phosphorus and light. Species that were strong competitors for phosphorus were strong competitors for light as well. Hence, we found neither stable coexistence nor alternative stable states. All competition experiments led to competitive exclusion. Furthermore, the physiological traits of the species indicated that, if one would find trade-offs in competitive abilities, competition for phosphorus and light would lead to alternative stable states rather than to stable coexistence. These results suggest that stable coexistence mediated by competition for phosphorus and light is rare, and hence an unlikely explanation for the high biodiversity commonly found in phytoplankton communities.