Microevolution can have consequences at higher levels of ecological organization. Although divergence among populations can be rapid and driven by anthropogenic changes to the environment, the ecological relevance of evolution induced by human activities remains poorly understood. 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 as Daphnia, we hypothesized that such adjustments should alter consumer–resource dynamics. We examined how cultural eutrophication in temperate lakes causes trait variation in the grazer Daphnia pulicaria. We tested for variation in Daphnia traits 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 that Daphnia from 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 source Daphnia. These results suggest that Daphnia may face a trade‐off in performance at low versus excess P that may be mediated in part by genetic variation at the Pgi locus. The variation observed in laboratory growth experiments scaled up to Daphnia populations in both mesocosm experiments and among lakes. In both the mesocosm experiment and in the lake survey, Daphnia from 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.

Consumer-driven nutrient cycling influences aquatic ecosystem functioning by altering energy flow as well as the movement and transformation of nutrients. In hypereutrophic lakes, zooplankton nutrient recycling has largely been considered negligible due to the high concentrations of available nutrients. A recent comparative analysis found that zooplankton community composition may influence nutrient availability, particularly phosphorus availability, in hypereutrophic lakes. However, the magnitude and timing of the effect of zooplankton nutrient recycling and grazing on phytoplankton in hypereutrophic lakes remains unclear. We quantified zooplankton, phytoplankton, and nutrient concentration dynamics during the summer of 2019 in a temperate, hypereutrophic reservoir. We found that zooplankton excretion contributed a substantial proportion (23-46%) to the inorganic phosphorus (P) pool in early summer when P concentrations were low and limiting phytoplankton growth. Zooplankton excretion of P also significantly (p=0.003) contributed to variation in phytoplankton community composition in early summer. Further, we found evidence that zooplankton affected phytoplankton size distributions through selective grazing of smaller phytoplankton cells. Overall, our results demonstrate the important role of zooplankton in hypereutrophic lakes as excretion helped drive springtime phytoplankton dynamics through nutrient recycling while grazing influenced phytoplankton size distributions later in the summer.

We observed episodic zooplanktivory by largemouth bass (Micropterus salmoides; LMB) ! 150 mm total length (TL) on the cladoceran Daphnia spp. over two decades in a small oligotrophic lake. While zooplanktivory is beneficial for young-of-year and juvenile LMB, dietary utilization of Daphnia by adults is unusual because zooplankton are not energetically favorable and the LMB buccal cavity is poorly adapted for filter feeding. We classified the threshold of ! 38 Daphnia per feeding event as an episode of zooplanktivory based on data from a zooplankton specialist, the bluegill sunfish (Lepomis macrochirus). In this study, 1718 LMB were caught, marked, sampled, and released. Gut contents at the time of capture were collected via gastric lavage. We captured the same 612 LMB 3 or more times. Of these, approximately 26% had at least one episode of zooplanktivory and approximately 6% had multiple episodes. There was no relationship between the population sizes of Daphnia and LMB, but the yearly percentage of individual LMB with at least one episode of zooplanktivory significantly increased with LMB population density. Episodes of zooplanktivory were more likely to occur later in the season when Daphnia were more abundant in the lake. The probability of an individual LMB having a repeat episode of zooplanktivory was not significantly different from expected; however, the probability of a LMB not having a single episode was significantly greater than expected by random chance signifying that some LMB were better than others at eliminating Daphnia from their diets. The probability of episodic zooplanktivory significantly decreased with increasing fish TL and weight but significantly increased with increasing Daphnia abundance. LMB without episodes of zooplanktivory achieved significantly greater maximum weight and body condition factor than those with them, but there was no statistical difference in their maximum TL or longevity (duration between first and final capture of the same LMB). In contrast, LMB with episodes of zooplanktivory had significantly greater instantaneous growth rates than those without. These results ARTICLE HISTORY

Consumer nutrient recycling influences aquatic ecosystem functioning by altering the movement and transformation of nutrients. In hypereutrophic reservoirs, zooplankton nutrient recycling has been considered negligible due to high concentrations of available nutrients. A comparative analysis ( Moody and Wilkinson, 2019) found that zooplankton communities in hypereutrophic lakes are dominated by nitrogen (N)-rich species, which the authors hypothesized would increase phosphorus (P) availability through excretion. However, zooplankton nutrient recycling likely varies over the course of a growing season due to changes in biomass, community composition and grazing pressure on phytoplankton. We quantified zooplankton, phytoplankton and nutrient concentration dynamics during the summer of 2019 in a temperate, hypereutrophic reservoir. We found that the estimated contribution of zooplankton excretion to the dissolved nutrient pool on a given day was equivalent to a substantial proportion (21–39%) of the dissolved inorganic P standing stock in early summer when P concentrations were low and limiting phytoplankton growth. Further, we found evidence that zooplankton affected phytoplankton size distributions through selective grazing of smaller phytoplankton cells likely affecting nutrient uptake and storage by phytoplankton. Overall, our results demonstrate zooplankton excretion in hypereutrophic reservoirs likely helped drive springtime phytoplankton dynamics through nutrient recycling while grazing influenced phytoplankton size distributions.