The role of allelopathy in determining the competitive ability of phytoplankton species and shaping plankton dynamics has been documented through field observations, laboratory experiments, and theoretical models. It has been hypothesized that in aquatic ecosystems, where numerous species of phytoplankton survive on a few resources (which Hutchinson termed as the Paradox of the Plankton), the effects of allelopathy would be important in maintaining community diversity through regulation of competitive exclusion which is normally an inevitable outcome of resource competition. However, no experimental results are available yet to confirm this hypothesis. Here we present the first experimental evidence supporting this hypothesis by demonstrating that allelopathy promotes the coexistence of two phytoplankton species (Ankistrodesmus falcatus and Oscillatoria sp.) on a single limiting nutrient.
Results/Conclusions
By performing short-term experiments to parameterize the competitive ability for nitrate and allelopathic effect of several phytoplankton species, we obtained a pair of species that trade-off nitrate affinity and allelopathic effect (the chlorophyte Ankistrodesmus falcatus and the cyanobacteria Oscillatoria sp.) and a third one that is the best nitrate competitor and non-sensitive to allelopathy (the chlorophyte Chlamydomonas reinhardtii). According to theoretical models of interspecific competition for a single limiting resource including allelopathic effect, the former trade-off will generate contrasting outcomes of competition, that we could test with our experimental system. Then, by performing long-term competition experiments in nitrate-limited continuous cultures, and constructing a mechanistic model of population dynamics, we demonstrate that, with allelopathy, one of the following outcomes is possible depending on the relative initial abundances of the species: dominance of the best competitor for nitrate (the non-allelopathic species), oscillatory coexistence, or dominance of the weakest competitor (the allelopathic species). Our model analysis reveals that sustained oscillations leading to two-species coexistence would be the most probable outcome of this experiment, provided that the initial abundances were varied within a reasonable range. According to the conditions in our experiments and the match between model predictions and data, interspecific competition for nitrate in interplay with allelopathy of Oscillatoria against A. falcatus is the only likely explanation for our results, compared to other alternatives. Therefore, allelopathy can alter the predicted outcome of inter-specific competition for nutrients, and increase the potential of phytoplankton coexistence.