A 200-YEAR RECORD OF REGIME SHIFTS, LAND-USE CHANGE, AND MANAGEMENT FROM A PRAIRIE WETLAND

We collected sediment cores and modern submerged aquatic plants and phytoplankton from two sub-basins of Lake Christina, a class V wetland in west-central Minnesota, and used stable isotopic and elemental proxies from sedimentary organic matter to explore questions about the pre- and post-settlement ecology of the lake. The cores are ²¹⁰Pb age-dated allowing for an accurate chronology of the past two centuries and the calculation of mass accumulation rates. The two morphologically distinct sub-basins vary in their sensitivities to internal and external perturbations offering different paleoecological information. The record from the shallower and much larger western sub-basin reflects its strong response to internal processes, while the smaller and deeper eastern sub-basin record primarily reflects external processes including important post-settlement land-use changes in the area. A significant increase in organic carbon accumulation (3-4 times pre-settlement rates) and long-term trends in δ¹³C, organic carbon to nitrogen ratios (C/N), and biogenic silica concentrations show that primary production has increased and the lake has become increasingly phytoplankton-dominated in the post-settlement period. Significant shifts in δ¹⁵N values reflect land-clearing and agricultural practices in the region and support the idea that nutrient inputs have played an important role in triggering changes in the trophic status of the lake. Our examination of hydroclimatic data for the region over the last century suggests that natural forcings on lake ecology have diminished in their importance as human management of the lake increased in the mid-1900s. In the last 50 years, three chemical biomanipulations have temporarily shifted the lake from the turbid, algal-dominated condition into a desired clear water regime. Two of our proxies (δ¹³C and BSi) measured from the higher resolution eastern basin record responded significantly to these known regime shifts.