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

Sinclair, James S., Arnott, Shelley E.
Freshwater biology 2015 v.60 no.1 pp. 161-173
Copepoda, Dreissena polymorpha, Rotifera, adults, mussels, nutrients, phytoplankton, predation, researchers, zooplankton
Interactions between multiple anthropogenic stressors can have unexpected synergistic or antagonistic effects, making it difficult to predict their combined effects using single‐stressor studies. The interaction between invasive consumers and nutrient enrichment is particularly important as both 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 3 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 small phytoplankton, but increased the abundance and dominance of copepods and reduced the density of large phytoplankton, probably due to increased top‐down predation pressure. Zebra mussels reduced large phytoplankton concentration by about 80%, rotifer abundance by about 75%, and shifted communities towards dominance by 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 environmental change in a multiple‐stressor context.