Paleolimnology of the Lake of the Woods southern basin

Abstract

To quantify the environmental history of the southern basin of Lake of the Woods (Ontario, Manitoba and Minnesota), seven core locations 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. Sediments were dated using isotopic analyses and fossil remains, in concord with other stratigraphic indicators (organic and inorganic materials, sedimentation rates, other biological entities), were used to reconstruct the ~150-year history of the lake. 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. Nutrient reconstructions indicated a period of cultural eutrophication throughout much of the 20th century. Despite a known reduction in anthropogenic nutrient flux to the lake in recent decades, there has been no apparent reversal in eutrophication in the pelagic system. Contemporary observations indicate that blooms of blue-green algae are becoming a greater problem. It appears that legacy nutrient recycling and other environmental drivers are maintaining the current condition of pelagic nutrient enrichment. Sedimentary analyses also indicated that physical changes to the lake resulting from warming may be contributing in small part to the recent reorganization of algal assemblages.