Abstract
The Prior Lake-Spring Lake system in Scott County, MN, consists of three connected suburban lakes near the Twin Cities: Spring Lake, Upper Prior Lake (UPL), and Lower Prior Lake (LPL). These lakes are vital to residents and nearby businesses for fishing and aesthetics. Spring Lake is hypereutrophic with phosphorus levels averaging 125.2 mg/L between 1996-2006, more than triple the targeted water quality standards. As a result of increased primary productivity, lower dissolved oxygen in Spring Lake has caused periods of anoxia in the last few decades. UPL has been eutrophic since 1998, with phosphorus exceeding the state standard of 60 mg/L. LPL is considered mesotrophic to eutrophic, with current phosphorus levels averaging 30 mg/L since 2003. In this study, we used geochemical methods (x-ray fluorescence (XRF), loss on ignition (LOI), Pb-210 age dating) and sediment microscopy to characterize sediments from a multi-core transect from Spring Lake and a deepwater core in LPL to examine historical changes in sedimentation and nutrient levels in both lake basins. Additionally, this allowed us to compare the histories of the two ends of the system that currently are very different in trophic status. Spring Lake has undergone a drastic shift in geochemistry since European settlement. Not only was there an increase in heavy metals, there was also a significant change in lake ecology as indicated by changes in the biogenic fraction, including a shift in the diatom community. Spring Lake has crossed an environmental threshold in the last decade, as indicated by laminations highly concentrated with centric diatoms and calcite, the result of increased biological productivity and the onset of deepwater anoxia. In contrast, our results indicate that LPL has been the more stable of the two lakes, showing little evidence of the extreme changes that have occurred in Spring Lake. However, there was a change in the dominant species in the diatom community in the uppermost portion of the core, and we believe this may be the early signal of a shift towards a less desirable condition.