Temperature mediates competitive exclusion and diversity in benthic microalgae under different N:P stoichiometry

The dependence of competitive interactions on abiotic conditions is attracting increasing interest in the face of globally rising temperatures and altered biogeochemical cycles of major nutrients. In a microcosm experiment involving a natural inoculum of benthic microalgae, temperature and nutrient supply ratios were manipulated in order to test three main hypotheses: (1) temperature and nutrient supply ratios determine species composition and diversity of the assemblage, (2) the identity of the dominating species depends on nutrient supply and temperature, and (3) higher temperature leads to faster competitive exclusion and thus more rapid decline in species richness. Over a period of 7 weeks, algal biomass reached an equilibrium carrying capacity, with was higher at colder temperatures and intermediate N:P supply ratios (N:P = 16). Initial growth rate increased with temperature and under high P-supply. Species richness in the stationary phase of the experiment decreased with increasing temperature, reflecting a higher extinction rate in the warmer treatments, which were also characterized by higher dominance of single species. Thus, increasing temperature both altered the identity of the dominating species and accelerated competitive displacement. This experiment thus indicates that warming might influence outcome and temporal dynamics in species interactions, and thereby eventually local diversity.