Effects of an invasive consumer on zooplankton communities are unaltered by nutrient inputs

Interactions between multiple anthropogenic stressors can have unexpected synergistic or antagonistic effects, making it difficult to predict their combined effect using single stressor studies. The interaction between invasive consumers and nutrient enrichment is particularly important as both of these stressors frequently co-occur and their respective bottom-up and top-down effects have the potential to interact across multiple trophic levels. We conducted a mesocosm experiment that crossed an increasing nutrient addition gradient against an increasing zebra mussel invasion gradient. Native zooplankton communities were added to the mesocosms, and after three months we examined how the single stressor effects on available resources and the zooplankton community were altered by their multiple stressor interaction. Added nutrients had no effect on primary producer abundance, but increased the abundance and dominance of the top consumer, which likely increased predation pressure on the producers and so prevented their response to increased nutrients. Zebra mussels reduced total phytoplankton abundance by ~75%, rotifer abundance by ~80%, and shifted communities towards dominance of cladocerans and adult/juvenile copepods. When combined, the top-down control exerted by the mussels interacted antagonistically to prevent any bottom-up influence of nutrient enrichment on the zooplankton community. These results provide insight into the potential outcomes of nutrient and invasive consumer stressor interactions, and illustrate the need for researchers to consider single stressor problems in a multiple stressor context.