The effects of nutrient enrichment on aquatic ecosystems have been well documented, but less is known about how these effects vary in response to additional stressors. I used experimental mesocosms to assess how nutrient enrichment affected zooplankton density and community structure in the presence and absence of invasive zebra mussels (Dreissena polymorpha). The density of several pelagic zooplankton taxa increased in response to nutrient enrichment, but these increases were offset or reduced in mesocosms that contained zebra mussels. At least at the beginning of the experiment, zebra mussels reduced the amount of algae that developed in response to nutrient enrichment and potentially were in competition with pelagic zooplankton. In contrast, zebra mussels did not reduce the total density of zooplankton in the absence of nutrient enrichment. Instead, the density of several littoral taxa increased in mesocosms with zebra mussels, presumably because they were able to use attached algae that also developed in mesocosms with zebra mussels. Combined, these results suggest that the overall effects of nutrient enrichment on plankton communities should be considered in the context of multiple stressors including the presence or absence of invasive species.