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Ecosystems can undergo abrupt regime shifts as a result of many factors. Shifts between turbid and clearwater states are well understood for human‐impacted shallow lakes, but are not well understood in other types of lakes. Here we use long‐term data to describe abrupt shifts in water clarity in Trout Lake, an oligotrophic lake with a largely undeveloped watershed. For several decades mean summer water clarity averaged 4.5 m, but then around 2007 water clarity sharply increased and the “clear water regime” persisted for nearly a decade. Nutrient availability did not explain these changes, but rather they were explained by a classic top‐down trophic cascade. Around 2007, the population of the apex pelagic predator, Lake Trout, substantially increased. This was accompanied by a sharp decline in the lake's major pelagic prey fish, the zooplanktivorous Cisco. In turn, there was an increase in large‐bodied zooplankton taxa (calanoids, Daphnia), which reduced algal biomass. This clear water regime was then disrupted in 2014 by the invasion of a predatory zooplankton, Bythotrephes cederstroemi. This invasion corresponded to strong impacts on lower trophic levels (decrease in large‐bodied zooplankton and decreased water clarity), but more minor impacts on higher trophic levels (increased Cisco, decreased Lake Trout abundances)—in effect reversing the trophic cascade and shifting Trout Lake to a novel ecosystem state. Our study provides a long‐term, empirically based example of successive ecological regime shifts that occurred from the rise of an apex predator and a mid‐trophic level invasion in an undeveloped, oligotrophic lake.

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‐bodied Daphnia pulicariaand density of invasive Bythotrephes) 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 of D. pulicariaaffected all clear‐water phase metrics in both lakes. When D. pulicariaproportion was higher, the clear‐water phase occurred earlier, lasted longer, and the water was clearer. Moreover, high Bythotrephesdensity 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.

AbstractMagee, MR, Hein CL, Walsh JR, Shannon PD, Vander Zanden MJ, Campbell TB, Hansen GJA, Hauxwell J, LaLiberte, GD, Parks TP, Sass GG, Swanston CW, Janowiak MK. 2019. Scientific advances and adaptation strategies for Wisconsin lakes facing climate change. Lake Reserv Manage. 35:364–381.Climate change threatens inland lakes, which are highly valued for their ecological and economic benefits. Here, we synthesize adaptation strategies that could offset climate impacts on Midwestern lakes. Our synthesis is based on results from the Wisconsin Initiative on Climate Change Impacts lake adaptation workshop, in which 48 researchers and managers with expertise on Wisconsin’s inland lakes gathered to provide input on climate adaptation strategies. We identified recent scientific advances, knowledge gaps, and examples of successful climate adaptation strategies with respect to four key themes: lake levels, water quality, aquatic invasive species, and fisheries. While adaptation strategies for each theme differed, there was consensus around the need for a multifaceted approach that incorporates communication and outreach, policy and regulation changes, traditional resource conservation approaches, and novel engineering designs. Managers should focus on protecting high-quality lakes, building lake resilience, and retaining beneficial ecosystem services. Most importantly, thoughtful and strategic interactions with stakeholders, policymakers, and researchers across multiple disciplines will be key to implementing climate adaptation strategies.

Human impacts on freshwater ecosystems are pervasive, but the short and discontinuous nature of most datasets limits our ability to understand the controls on water quality and effectively manage freshwater resources. We examine change in Lake Mendota (Madison, Wisconsin) over the last two centuries by pairing analyses of a sedimentary archive with the site's > 100 yr limnological record. We show that eutrophication of the lake, evident as an abrupt shift in sediment composition, began in the late 19thcentury following the intensification of urban and agricultural land use in the watershed. Efforts to address deterioration of lake water quality, including the removal of point‐source pollutants and biomanipulation, have had a measurable influence on sediment composition and water clarity. Since the early 1980s, quasi‐seasonal cycles of phytoplankton blooms have induced calcite precipitation, leaving distinct laminations in the sedimentary record. These “whiting events” evidently did not accumulate in lake sediments until the late 20thcentury, indicating that efforts to remediate water quality have shifted the lake to a new ecosystem state. Calcite whitings can improve water quality in eutrophic lakes by coprecipitation with phosphate, increasing phosphorus (P) burial in lake sediments. Using long‐term limnological records, we report negative correlations between calcite saturation indices and P in lake surface waters and show that calcite whitings could partially explain recent P decline in Lake Mendota surface waters. Our study reveals a previously uncharacterized potential control on water quality in this eutrophic lake and demonstrates the benefit of coupling long‐term limnological data with sedimentary records.

Cultural eutrophication of lakes due to excess loading of nutrients is a global problem affecting the provisioning of freshwater ecosystem services. Lake water quality is often described by trophic state indicators such as water clarity or concentrations of nutrients, chlorophyll, and cyanobacteria toxins. While these indicators are widely assumed to covary, capturing the same basic aspects of water quality, they can be uncoupled by changes in lake food webs. Outbreak of the invasive predatory zooplankton, Bythotrephes longimanus, in Lakes Mendota and Monona, Wisconsin (U.S.A.) in 2009 led to a decline in the herbivore, Daphnia pulicaria, and in turn, water clarity. We use 20‐yr datasets for Lakes Mendota and Monona to evaluate interactions among nutrients, zooplankton, and phytoplankton that resulted in this trophic cascade. After invasion, diatoms increased in both lakes resulting in reduced water clarity, whereas cyanobacteria did not change in either lake. Multivariate time series analysis revealed that D. pulicariagrazing controlled edible diatoms in both lakes. Cyanobacteria biomass was positively associated with both D. pulicariaand summer nutrient concentrations. These results further clarify the limitations of top‐down control in eutrophic lakes: grazing by zooplankton can control edible phytoplankton taxa, but largely inedible cyanobacteria may be more limited by nutrient concentrations. Accordingly, despite declining clarity in Lake Mendota, the number of beach closures due to cyanobacteria blooms declined following Bythotrephesinvasion. Our study reveals how these two indicators of water quality—water clarity and cyanobacteria blooms—can become uncoupled during a food web shift.

Trophic cascades can determine the structure of aquatic food webs, a role often used to manage water quality in lakes. However, trophic cascades are subject to multiple interacting drivers of environmental change. For example, invasive species can dramatically alter food webs, though we know less about invasive species effects on managed trophic cascades. Here, we investigate the changing food web dynamics of Lake Mendota (Wisconsin, U.S.A.) over a 40‐yr time period. Piscivore stocking (biomanipulation) beginning in 1988 coupled with a 1987 die‐off of cisco (the dominant zooplanktivore) led to decreased zooplanktivory that cascaded to increases in the large‐bodied herbivorous zooplankton Daphnia pulicariaand greater water clarity. In 2009, the invasive spiny water flea Bythotrephes longimanuswas first detected in Lake Mendota and soon reached record densities. As a result of this invasive invertebrate predator, zooplanktivory increased to 47% of the earlier cisco period. Concomitant with the spiny water flea population explosion were declines in D. pulicariaand water clarity as both exhibited a strong negative relationship to zooplanktivory across the 40‐yr time series. We argue that the trophic cascade of 1987–1988 created an environment of high food resources and low fish planktivory that promoted the proliferation of Bythotrephesas it functionally replaced cisco in Lake Mendota's food web. Our results highlight how a manipulated, eutrophic lake was vulnerable to adverse impacts by invasive species, calling attention to the importance of understanding interactions among drivers of environmental change.