Twin Cities WRS Seminar
A Paired Paleolimnological Study on Contrasting Large Shallow Lakes Provides Perspective on Land Use, Point Source Pollutants, Climate, and Lake Recovery
The mid-continent USA-Canada border is characterized by several large, shallow, relict basins of Glacial Lake Agassiz, which are important resources for regional economies. Only 70 km apart, Lake of the Woods and Upper and Lower Red Lakes are limnologically similar with respect to size, depth, and frequent cyanobacteria blooms. Upper and Lower Red Lake are still surrounded by natural forest and peatland, whereas Lake of the Woods has a long history of human-mediated nutrient loading and remediation. Multiple Pb-210-dated cores were taken from each lake to contrast their post-European histories. Multiproxy analysis targeted sediment character, P inventory and cycling, and algal communities. Lake of the Woods showed increased productivity and cyanobacterial blooms through the 1970s when point sources were curtailed, however the lake has continued to have high nutrient levels from cycling of legacy sediment P. Algal communities reorganized after the 1970s, but did not return to pre-European structure. In contrast, large areas of Upper and Lower Red Lake had nonconformable sedimentation, likely a consequence of physical mixing of sediments by wind. Total P and Fe-bound P concentrations increased upcore, indicating increased burial and high potential for internal loading. Corresponding to the increased nutrients, biogenic Si and chlorophyll-a increased while 13 C and C:N decreased, indicating greater algal productivity upcore. Diatom communities were dominated by tychoplanktonic taxa, though planktonic diatoms increased subtly with damming, and a recent shift from tychoplanktonic to benthic diatoms was observed. Corresponding to the recent shift in the diatoms, pigment analysis revealed that nitrogen-fixing and toxin-producing cyanobacteria increased. Comparisons between lakes tell us that we have to select carefully where to recover undisturbed stratigraphies in large shallow lakes. Upper and Lower Red Lake paleoecology provides a unique opportunity to understand trends among the remnant Agassiz lakes, trends that may be obscured by anthropogenic disturbance.