Measuring ecosystem recovery to disturbance using zooplankton egg banks

Background/Question/Methods

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 caused landscape level disturbances in regional plant communities and increased sediment transport to aquatic systems. Locks and dams for flood control and navigation were constructed in the UMR in the late 1930's, raising water level at mid-pool by approximately 2 meters to maintain a 2.74 meter navigation channel. This increase in water level altered floodplain geomorphology, resulting in the expansion of wetlands and backwater lakes as well as increased connectivity to the river’s main channel. The closure of the locks and dams altered flow corridors and wash-out frequencies throughout the floodplain. Sediment cores collected from multiple backwater lake sites in Pool 8 of the UMR near La Crosse, Wisconsin, were analyzed for biogeochemical properties, pollen, and zooplankton composition.  Integration of these data provides a better understanding of how landscape level changes affect large river ecosystems. 

Results/Conclusions

In the mid-1800’s magnetic susceptibility and Ambrosia pollen more than doubled at the same time that zooplankton diapause eggs were no longer observed. Each of these lines of evidence support that land clearance throughout the region was causing high turbidity, drastically altering the aquatic ecosystem. Recovery of zooplankton communities was not observed until after the 1930’s (dates based on Pb-210 and Cs-137) when dam closure took place.  Both lock and dam closure, and modified agricultural practices likely contributed to the re-establishment of the aquatic biota post-1930’s.  Based on the relative abundance of diapause eggs in the sediment cores from pre- and post-settlement, species composition has changed from large-bodied Cladocera (Daphnia) dominated in the 1800’s to Calanoid and small-bodied Cladocera (Chrydora/Bosmina) in recent times. In addition to landscape change, it is clear that the invasive bryozoan (L. carteri) has been abundant in the ecosystem since the late 1930’s.