As harmful algal blooms (HABs) have expanded in both size and scope, there have been increased efforts to design models that try to predict which ecosystems are at the greatest risk for a bloom. However, predicting blooms is a difficult process that has had limited success which we believe partly stems from the fact that efforts to date have considered a relatively small set of variables that influence algal growth. In particular, the focus on hydrology and abiotic conditions fails to consider biotic interactions that ultimately control the final composition of ecological communities.In this study we examined how species richness of resident green algae influenced the growth and carrying capacity of Microcystis in both low and high nutrient environments. We found that when either nitrogen or phosphorous were reduced, competition with green algae had a significant negative impact on the growth rate and final biomass of Microcystis, but the species richness of the green algal assemblage had no discernible effect on outcomes. However, in a high nutrient environment, a species rich community of green algae had a greater negative impact on the invasion success of Microcystis than a species-poor community.These results suggest that biotic interactions with phytoplankton can be important in limiting the establishment and proliferation of HAB species like Microcystis, and diverse communities can be more resistant to proliferation of HABs under nutrient conditions that favor blooms. In the future, the inclusion of biotic variables that influence bloom formation, in addition to the physical and abiotic requirements of a HAB species, could more accurately predict HAB events allowing for more effective management of our freshwater resources in the future.
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