Algal palynomorphs as proxies of human impact on freshwater resources in the Great Lakes region

Management of freshwater resources requires an understanding of the response of lakes to human impact. The long sedimentary records in lake archives hold the key to accurate forecasting. The remains of algae in “pollen” slides record two distinct phases of cultural eutrophication and siltation/turbidity resulting from soil erosion in sediments from two lakes in southern Ontario, Canada: 1) agricultural settlements by Iroquoian (Wendat/Huron) people around the middle of the last millennium and 2) widespread land-clearing by European colonists in the mid-nineteenth century, followed by industrial expansion and urbanization in the Great Lakes watershed to the present day. The half-cells of benthic desmids were particularly sensitive to turbidity associated with land clearing. In contrast, planktonic algae adapted to eutrophic waters thrived in response to increased agricultural runoff and human and animal waste during both intervals in cores from Lake Simcoe and in the well-documented varved sediments from Crawford Lake. These under-utilized microfossils can be useful proxies of human impact, particularly where mineralized microfossils are sparse due to dissolution.