Drivers of change for lakewater clarity

Lakes in the Upper Midwest have undergone extensive lakeshore development over the past 30 years, raising concerns about eutrophication. We examined 11 case study lakes in Minnesota that had undergone substantial shoreline development over the past 30 years to evaluate drivers of change in clarity. Relationships between current Secchi disk transparency (SDT) and the density of permanent equivalent houses (PEHs) and between change in SDT and change in density of PEHs were not statistically significant. For lakes with large watershed area-to-lake area (WSA:LA) ratios, modeled worst-case scenarios for impacts of shoreline housing show that phosphorus (P) inputs may not be sufficient to reduce SDT. For sensitive lakes, improved P management policies may counteract increased shoreline development, at least in part. For lakes with large WSA: LA ratios, activity outside the shoreline area, particularly agricultural activity, is probably more important than shoreline development in affecting SDT. Although policies considered lake management operate at fairly small scales, drivers of change in SDT operate at various temporal and spatial scales, from household to global.