We collected two sediment cores and modern submerged aquatic plants and phytoplankton from two sub-basins of Lake Christina, a large shallow lake 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 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 shows 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 50years, 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.
Bibliographical noteFunding Information:
We thank Thomas Carlson and Nicole Hansel-Welch of the Minnesota DNR for providing useful data on observed changes in lake plants for the past several decades and information on land-use changes around Lake Christina. We also thank an anonymous reviewer for comments that improved the manuscript. Benjamin Czeck and Sean Hagen prepared a significant number of our core samples in the laboratory, and we thank them as well. This research was supported by generous grants from the National Science Foundation (grant DEB-0919070 ) and the University of Minnesota's Initiative for Renewable Energy and the Environment (grant LG-S4-2005 ).
- Alternative stable regimes
- Historical change
- Lake sediments
- Sediment geochemistry (δC, δN, C/N, BSi)
- Shallow lakes