Algal blooms as a reactive dynamic response to seasonal perturbation in an experimental system

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Algal blooms are typical of many aquatic freshwater ecosystems in seasonal environments. Such blooms could derive from transient reactive dynamics of algae and limiting nutrients following seasonal perturbation events. Linking parameter estimates derived from previously published lab experiments with empirical estimates of algal density dependence, we modeled dynamic interactions between nutrients and the green algal species Chlorella vulgaris and tested model predictions in a dozen 140 L mesocosms supplied with bi-weekly inputs of liquid fertilizer. Consistent with the reactive nutrient-driven model, Chlorella populations exhibited an initial surge in abundance over the first month followed by collapse as they rapidly converged on stable equilibria. The reactive model suggests that the magnitude of transient blooms is positively related to the augmentation of nutrients and depression of algae over the winter period. The magnitude of both algal peaks and equilibrium abundance was positively related to fertilizer loading, as predicted by the reactive model. Our results suggest that transient reactive responses to climate-driven perturbation events can be an important contributor to seasonal algal blooms observed in many temperate freshwater ecosystems. Controlled experimental studies such as ours may be helpful in understanding and potentially mediating the impact of fertilizer run-off on freshwater systems in temperate agricultural landscapes.