Understanding ecosystem change in upper Mississippi River backwaters through geochemical and biological analyses of sediment cores
Belby, Colin S.; Rivera Pérez, Carmen; Gerrish, Gretchen A.
Anthropogenic activities have altered the natural ecology and hydrology of the upper Mississippi River (UMR). Land-clearance for agriculture during mid-19 th century European-American settlement changed regional plant communities and increased overland sediment transport. Locks and dams were constructed in the UMR to facilitate commercial navigation in the late 1930s, raising water level at mid-pool by approximately two meters. This increase in water level altered floodplain geomorphology, expanding wetlands and increasing connectivity between backwater lakes and the river's main channel. Biogeochemical properties, pollen, and zooplankton composition were analyzed from sediment cores from upper Lawrence Lake, a backwater lake, in Pool 8 near La Crosse, Wisconsin. Integration of these data provides an understanding of how landscape level changes affected UMR aquatic habitats. Land clearance in the mid-1800s caused high turbidity, drastically altering the aquatic ecosystem. During the post-settlement period sedimentation in the backwater increased by more than an order of magnitude. Magnetic susceptibility and ragweed pollen more than doubled at the same time point that zooplankton diapause egg abundance began decreasing. Recovery of zooplankton communities was not observed until dam closure in the 1930s. Based on the relative abundance of diapause eggs from the pre- and post-settlement sediments, species composition has changed from large Daphnia in the pre-settlement period to more Calanoid and small-bodied Cladocera (Chydoridae , Ceriodaphnia) in recent times. In addition to changes in native species abundances, it is clear that a non-native bryozoan, Lophopodella carteri, was introduced and has been abundant in the ecosystem since the mid-1940s. Both lock and dam closure, and modified agricultural practices likely contributed to generating an environment where aquatic biota can persist post-1930s. If sedimentation in the backwater continues at the post-1964 rate, upper Lawrence Lake will be filled to the flat pool elevation within the next 25–50 years.