Eutrophication has become one of the most widespread anthropogenic forces impacting freshwater biological diversity. One potentially important mechanism driving biodiversity changes in response to eutrophication is the alteration of seasonal patterns of succession, particularly among species with short, synchronous life cycles. We tested the hypothesis that eutrophication reduces seasonally driven variation in species assemblages by focusing on an understudied aspect of biodiversity: temporal beta diversity . We estimated the effect of eutrophication on by sampling benthic macroinvertebrate assemblages bimonthly for 2 years across 35 streams spanning a steep gradient of total phosphorus (P) and benthic algal biomass (as chlorophyll). Two widely used metrics of β diversity both declined sharply in response to increasing P and chl, regardless of covariates. The most parsimonious explanatory model for included an interaction between P and macroinvertebrate biomass, which revealed that was lower when macroinvertebrate biomass was relatively high. Macroinvertebrate biomass explained a greater amount of deviance in at lower to moderate concentrations of P, providing additional explanatory power where P concentration alone was unable to fully explain declines in. Chl-explained similar amounts of deviance in comparison to the best P model, but only when temperature variability, which was positively related to, also was included in the model. Declines in suggest that nutrient enrichment decreases the competitive advantage that specialists gain by occupying particular temporal niches, which leads to assemblages dominated by generalists that exhibit little seasonal turnover. The collapse of seasonal variation in assemblage composition we observed in our study suggests that treating dynamic communities as static assemblages is a simplification that may fail to detect the full impact of anthropogenic stressors. Our results show that eutrophication leads to more temporally homogenous communities and therefore degrades a fundamental facet of biodiversity.