PALEOLIMNOLOGY OF A LARGE RIVER SYSTEM USING SEDIMENT DIATOMS AND MULTIPLE DATING TECHNIQUES

The St. Croix River (Minnesota-Wisconsin, USA), a major tributary to the upper Mississippi River, has undergone significant land-use changes since European settlers arrived in the 1840s. The river continues to face major ecological threats from increased recreational use, population growth, and development in the watershed. For guiding management decisions on the river, an understanding of the timing and magnitude of change in river conditions before and since European settlement is crucial and well suited to paleolimnological investigation. However, paleolimnology of rivers provides unique challenges. Sediment transport is often too episodic and complex to allow accumulation of continuous sedimentary sequences. Therefore, previous paleolimnological studies on the river have targeted Lake St. Croix (a natural impoundment of the river) and demonstrated that, despite its perception of being a pristine river, significant increases in nutrients, sedimentation rate, and algal productivity have occurred since European settlement.

Environmental impacts on the river are not limited to Lake St. Croix; for that reason, we have analyzed sediment cores from three floodplain wetlands on the lower reaches of the St. Croix River. Results from our work on floodplain wetlands, as well as work from Lake St. Croix, demonstrate that dating of sediment cores from riverine systems is extremely challenging. To provide reliable dating models for these cores, careful site selection was coupled with a combination of lead-210, cesium-137, magnetics, and pollen analyses. Once dating models were established for the floodplain wetland cores, sediment diatom communities were analyzed with decadal scale resolution for the past 250-300 years. Shifts in the diatom community structure were then correlated with the land use history of the St. Croix watershed. To identify some of the drivers of change in the diatom communities we compared the changes in the cores to a large diatom calibration set for MN lakes.