Allelopathy prevents competitive exclusion and promotes phytoplankton biodiversity

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, experimental proof-of-principle is not available to support this hypothesis. Here we present the first experimental evidence to support this hypothesis by demonstrating that allelopathy promotes the coexistence of two phytoplankton species, Ankistrodesmus falcatus and Oscillatoria sp., that compete for a single limiting nutrient. By performing long-term competition experiments in nitrate-limited continuous cultures, and by 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 (t...