CAUGHT IN THE MIDDLE: EVALUATING THE HISTORICAL CONDITION AND POTENTIAL IMPAIRMENT OF UPPER PRIOR LAKE, SCOTT COUNTY, MN

Spring Lake, Upper Prior Lake, and Lower Prior Lake comprise a connected three-lake system (PLSL system) in the Prior/Spring Lake Watershed District, Scott County, Minnesota. Spring Lake is a hypereutrophic lake at the head of the PLSL system characterized by high nutrient levels resulting in large algal blooms and deepwater anoxia during the summer. Consequently, several ecological and geochemical changes have taken place, including decreased diatom diversity, increased oxygen stress on certain organisms, and phosphate release from sediments. At the other end of the system, Lower Prior Lake has been much more stable and shows little evidence of major ecological or geochemical shifts. The hydrology of the PLSL system moves nutrient-rich waters down the system from Spring Lake to Upper Prior Lake (UPL), with the potential for impairment of UPL. In this study, I explored the paleoecology of UPL using x-ray fluorescence, loss-on-ignition, and sediment microscopy from a 2.6 m- long sediment core that spans several centuries. Preliminary results indicate significant biological, physical, and chemical changes in UPL over the last two centuries. Calcium concentration increases 500% from the beginning of the record, while iron concentration drops nearly 300%. I interpret this to mean that UPL has increased in depth and area over the course of this record, and the main source of deposition has shifted from external clastics to internal biological sources. Sediment microscopy shows that biological productivity has increased markedly since the start of the record, most likely as a result of deepening of the lake and increased nutrient input. Diatom species have shifted from predominantly benthic species to planktonic species, including the genus Stephanodiscus and Asterionella formosa, two types of diatoms known to exist in waters containing plentiful phosphate. Millimeter-thick laminations of lighter colored, calcite-rich sediments persist through the top 1.3m of sediment. Laminations were previously observed in recent Spring Lake sediments where the bottom waters became increasingly anoxic over the last few decades. Laminations in UPL may indicate that a transition to an impaired system occurred earlier than we suspected. This study shows how the impairment of one lake has caused similar changes in a downstream lake.