Abstract/Summary

It has been hypothesized that allelopathy can prevent competitive exclusion and promote phytoplankton diversity in aquatic ecosystems, where numerous species coexist on a limited number of resources. However, no experimental proof-of-principle is available to support 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.) competing on a single limiting nutrient. By performing long-term competition experiments in nitrate-limited continuous cultures, and describing the population dynamics using a mechanistic model, we demonstrate that when allelopathy comes into play, one of the following outcomes is possible depending on the relative initial abundances of the species: dominance of the stronger competitor for nitrate (the non-allelopathic species), oscillatory coexistence, or dominance of the weaker competitor (the allelopathic species). Our model analysis reveals that sustained oscillatory coexistence of the two-species coexistence would be a common outcome of this experiment. Our study confirms for the first time, based on laboratory experiments combined with mechanistic models, that allelopathy can alter the predicted outcome of inter-specific competition in a nutrient-limited environment and increase the potential of coexistence of more species than resources, thereby contributing to find endogenous mechanisms explaining the extreme diversity of phytoplankton communities.