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A frequent way in which human activities drive microevolution is the increase in supply of nutrients such as phosphorus (P) that are required for fitness‐relevant traits such as growth and reproduction. Because higher P concentrations decrease P‐use efficiency and feeding rate in heterotrophic consumers such asDaphnia, we hypothesized that such adjustments should alter consumer–resource dynamics.

We examined how cultural eutrophication in temperate lakes causes trait variation in the grazerDaphnia pulicaria. We tested for variation inDaphniatraits and genetic variation in the metabolic enzyme phosphoglucose isomerase (Pgi) which are each known to respond to eutrophication. We then examined the impact of this variation on consumer–resource dynamics using a combination of experiments and a multi‐lake survey.

We found thatDaphniafrom hypereutrophic lakes responded to experimental hypereutrophic conditions with increased growth rates and fecundity when raised on P‐fertilized seston, but had reduced performance on P‐poor seston relative to eutrophic sourceDaphnia. These results suggest thatDaphniamay face a trade‐off in performance at low versus excess P that may be mediated in part by genetic variation at thePgilocus.

The variation observed in laboratory growth experiments scaled up toDaphniapopulations in both mesocosm experiments and among lakes. In both the mesocosm experiment and in the lake survey,Daphniafrom hypereutrophic source lakes reached high biomass while phytoplankton biomass also remained high.

Given the prevalence and rapid eutrophication of freshwater ecosystems worldwide, these results indicate that considering the potential effects of evolutionary change in ecosystem models could be useful in forecasting the effects of anthropogenic environmental change on pivotal ecosystem services.