Paleolimnological Reconstructions for the White Iron Chain of Lakes

Abstract

To quantify the environmental history of the White Iron Chain of Lakes (Lake and St. Louis Counties, Minnesota), five lakes were selected for retrospective analyses. Primary goals were to determine pre- European settlement conditions and track the timing and extent of anthropogenic impacts and remediation. Sediment cores were collected from each lake and sediment intervals were dated using isotopic analyses. Fossil remains, in concord with other stratigraphic indicators (organic and inorganic materials, sedimentation rates, other biological entities), were used to reconstruct the ~200-year history of each lake. Pollen analyses allowed for reconstruction of local and regional terrestrial conditions. Geochemical analyses provided data on historical flux of elemental trace metals to the sediments. Diatom assemblages were assessed from sediment intervals and inferred trophic conditions in the profiles were derived using a regional diatom-based model for Minnesota lakes. Eutrophication apparently occurred following settlement, particularly in White Iron Lake, but reconstructed phosphorus trends indicate more recent nutrient reductions. Pollen data track the decrease in pine abundance in the region and the rise of birch. Sedimentary metals largely reflect physical changes in the system, such as a change in sediment deposition regimes resulting from damming. Recent increases in metals are probably a result of increasing accumulation of soil and bedrock materials, a trend that is supported by increasing accumulation rates of overall organic and inorganic material. These recent increases in the last 30-40 years, which include increased algal deposition in Birch, Farm and Fall lakes, are not well explained at this time, but may be due to shifting water quality unrelated to phosphorus and possibly hydrological changes.