Phosphorus addition reverses the positive effect of zebra mussels (Dreissena polymorpha) on the toxic cyanobacterium, Microcystis aeruginosa

We tested the hypothesis that zebra mussels (Dreissena polymorpha) have positive effects on the toxin-producing cyanobacterium, Microcystis aeruginosa, at low phosphorus (P) concentrations, but negative effects on M. aeruginosa at high P, with a large-scale enclosure experiment in an oligotrophic lake. After three weeks, mussels had a significantly positive effect on M. aeruginosa at ambient P (total phosphorus, TP ∼10 μg L−1), and a significantly negative effect at high P (simulating a TP of ∼40 μg L−1 in lakes). Positive and negative effects were strong and very similar in magnitude. Thus, we were able to ameliorate a negative effect of Dreissena invasion on water quality (i.e., promotion of Microcystis) by adding P to water from an oligotrophic lake. Our results are congruent with many field observations of Microcystis response to Dreissena invasion across ecosystems of varying P availability.

169Highlights► We tested whether the positive effect of Dreissena on toxic Microcystis can be mitigated by adding P. ► The hypothesis was tested with an enclosure experiment in an oligotrophic lake. ► Dreissena's effect was positive at low P, not significant at medium P and negative at high P. ► We suggest that Microcystics may decline in low P lakes with Dreissena if P loading is increased.