Seasonal shifts in the relative importance of local versus upstream sources of phosphorus to individual lakes in a chain
Water quality in the Yahara chain of lakes in southern Wisconsin has been degraded significantly since European settlement of the region, primarily as a result of anthropogenic nutrient inputs. While all four main lakes (Mendota, Monona, Waubesa, and Kegonsa) have undergone eutrophication, elevated phosphorus and chlorophyll concentrations are particularly pronounced in the smaller lakes at the bottom of the chain (Waubesa and Kegonsa). Due to their short water residence times (2–3 months), these lakes are more responsive to seasonal variability in magnitude and source of phosphorus loading compared with the larger upstream lakes. In 2014, more than 80 % of the phosphorus load to Lake Waubesa passed through the outlet of Lake Monona (situated immediately upstream). However, between mid-May and late October when phosphorus concentrations in Lake Monona were reduced as a result of thermal stratification the upstream load dropped to ~40 % of the total, with the majority of loading during this period coming from Lake Waubesa’s local watershed. Correspondingly, seasonal phosphorus trends in Lake Waubesa during summer are correlated with precipitation, rather than phosphorus concentrations leaving upstream lakes. While phosphorus export from the local watersheds of Lakes Waubesa and Kegonsa is relatively small on an annual time scale, targeted loading reductions in these areas during the summertime will most effectively reduce summertime phosphorus concentrations in these fast-flushing lakes. Understanding the interaction of landscape position, water residence time, and mixing regime can help guide watershed management for water quality improvements in lake chains.
KeywordsLake chain Landscape limnology Phosphorus Eutrophication
This work was funded by the Wisconsin Department of Natural Resources and the National Science Foundation, North Temperate Lakes Long-Term Ecological Research (DEB-0822700). The authors would like to thank Stephen Carpenter and two anonymous reviewers for helpful comments on earlier versions of this manuscript.
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