Species diversity of resident green algae slows the establishment and proliferation of the cyanobacterium Microcystis aeruginosa

As harmful algal blooms (HABs) have expanded in size and become more frequent, there have been increased efforts to predict when and why blooms occur, as well as to identify the ecological factors that limit their formation. Here we report the results of a laboratory experiment in which we examined how the establishment and proliferation of Microcystis aeruginosa, a common HAB species, was influenced by competition with resident green algae. We hypothesized that competition with green algae would reduce the final biomass of M. aeruginosa, and that the effects of competition would be most pronounced in communities of high species richness and when nutrients are scarce. We found that competition with green algae did, in fact, consistently reduce the final biomass of M. aeruginosa. The effects of competition were more pronounced in species-rich than in species-poor communities, but this was only true when nutrients were abundant. When nutrients were scarce, resident diversity did not matter because competition with any resident species was sufficient to limit the growth of M. aeruginosa. Our results confirm that biotic interactions with phytoplankton are important in limiting the establishment and proliferation of HAB species like M. aeruginosa. But our work goes on to suggest that, under some nutrient conditions, diverse communities of resident phytoplankton can be more resistant to proliferation of cyanobacteria that can cause HABs.