Evaluation of land-use effects on coastal and marine ecosystems requires better understanding of the role of rivers in regulating mass transport from terrestrial to oceanic environments. Here we take advantage of the presence of a riverine lake to use paleoecological techniques to quantify impacts of logging, European-style agriculture, urbanization and continued terrestrial disturbance on mass transport and water quality in the northern drainage of the Mississippi River. Two 2-m sediment-cores recovered in 1999 from Lake St. Croix, a natural impoundment of the St. Croix River, were dated using 210Pb and 137Cs, and analyzed for historical changes (c. 1840-present) in sediment magnetic susceptibility, inorganic and organic matter content, biogenic silica, fossil pigments, and diatom microfossils. Inorganic sediment accumulation increased threefold between the mid-1800s and present, whereas clear signs of eutrophication were only evident after the mid-twentieth century when biogenic silica accumulation increased sixfold, diatom accumulation rates increased 20- to 50- fold, and the diatom community shifted from predominantly benthic species to assemblages composed mainly of planktonic taxa. Similarly, fossil pigment concentrations increased during the 1960s, and diatom-inferred total phosphorus (DI-TP) increased from ~30 μg TP l-1 c. 1910 to ~60 μg l-1 since 1990, similar to historical records since 1980. Together, these patterns demonstrate that initial land clearance did not result in substantive declines in water quality or nutrient mass transport, instead, substantial degradation of downstream environments was restricted to the latter half of the twentieth century.
Bibliographical noteFunding Information:
Acknowledgments This project was supported in part by the Minnesota Pollution Control Agency, the Metropolitan Council Environmental Services (MCES), and NSERC Canada. Thanks are due Scott Schellhaass (MCES) and Mike Perneil for coring assistance, Tara Bromenshenkel, Jill Coleman, Ellen Mallman, Diana Roen and Kelly Thommes for geochemistry, biogenic silica, and 210Pb dating, Joy Ramstack for providing access to her diatom training set, and Jasmine Saros and an anonymous reviewer for improving earlier versions of the manuscript. Doug Schnurrenberger performed magnetic susceptibility logging at the Limnological Research Center, Department of Geology and Geophysics, University of Minnesota-Twin Cities.
- Biogenic silica
- Fossil pigments
- Gulf of Mexico
- Mississippi River