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  1. This repository includes the setup and output from the analysis ran on Lake Mendota to explore the trophic cascade caused by invasion of spiny water flea in 2010. Scripts to run the model are located under /src, and the processed results for the discussion of the paper are located under /data_processed.

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  2. Abstract

    Species invasions can disrupt aquatic ecosystems by re‐wiring food webs. A trophic cascade triggered by the invasion of the predatory zooplankter spiny water flea (Bythotrephes cederströmii) resulted in increased phytoplankton due to decreased zooplankton grazing. Here, we show that increased phytoplankton biomass led to an increase in lake anoxia. The temporal and spatial extent of anoxia experienced a step change increase coincident with the invasion, and anoxic factor increased by 11 d. Post‐invasion, anoxia established more quickly following spring stratification, driven by an increase in phytoplankton biomass. A shift in spring phytoplankton phenology encompassed both abundance and community composition. Diatoms (Bacillaryophyta) drove the increase in spring phytoplankton biomass, but not all phytoplankton community members increased, shifting the community composition. We infer that increased phytoplankton biomass increased labile organic matter and drove hypolimnetic oxygen consumption. These results demonstrate how a species invasion can shift lake phenology and biogeochemistry.

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  3. ABSTRACT It is well established that nonnative species are a key driver of global environmental change, but much less is known about the underlying drivers of nonnative species outbreaks themselves. In the present article, we explore the concept and implications of nonnative sleeper populations in invasion dynamics. Such populations persist at low abundance for years or even decades—a period during which they often go undetected and have negligible impact—until they are triggered by an environmental factor to become highly abundant and disruptive. Population irruptions are commonly misinterpreted as a recent arrival of the nonnative species, but sleeper populations belie a more complex history of inconspicuous occurrence followed by an abrupt shift in abundance and ecological impact. In the present article, we identify mechanisms that can trigger their irruption, and the implications for invasive species risk assessment and management. 
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  4. <italic>Abstract</italic>

    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.

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  5. Abstract

    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.

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