Climate and Food Web Effects on the Spring Clear‐water Phase in Two North‐temperate Eutrophic Lakes

Although climate change has shifted the phenological timing of plankton in lakes, few studies have explicitly addressed the relative contributions of climate change and other factors, including planktivory and nutrient availability. The spring clear-water phase is a period of marked reduction in algal biomass and increased water transparency observed in many lakes. Here, we quantified the phenological patterns in the start date, maximum date, duration, and magnitude of the clear-water phase over 38 yr in Lakes Mendota and Monona, and examined the effects of water temperature, total phosphorus, and food web structure (proportion of large-bodiedDaphnia pulicariaand density of invasiveBythotrephes) and interactions between temperature and other predictors on these clear-water phase metrics. We found that climate and food web structure affected the clear-water phase, but the effects differed among the metrics. Higher water temperature led to earlier clear-water phase start dates and maximum dates in both lakes. The proportion ofD. pulicariaaffected all clear-water phase metrics in both lakes. WhenD. pulicariaproportion was higher, the clear-water phase occurred earlier, lasted longer, and the water was clearer. Moreover, highBythotrephesdensity delayed clear-water phase start dates (both lakes), and decreased clear-water phase duration (Lake Mendota) in the following year. These results suggest that variation in food web structure changes the full phenological dynamics of the clear-water phase, while variation in climate condition affects clear-water phase timing only. Our findings highlight the importance of large-bodied grazers for managing water quality under climate change.