Applying the reference condition approach to Lake of the Woods: sediment and benthic invertebrate community assessment for lake-wide management

Lake of the Woods (LOW) is a large, international lake recently designated as impaired by the State of Minnesota due to excess nutrients and nuisance algal blooms. Concerns regarding the impacts of eutrophication have prompted the need for management tools to help to defines areas of ecosystem impairment and to monitor changes in trophic status. The goal of this study was to assess areas of potential anthropogenic impacts in LOW using a benthic macro-invertebrate reference condition approach model and identify factors correlating with these impacts. We also sought to provide baseline information on sediment chemistry prior to the initiation of increased mining activity in the basin. A Canadian Aquatic Biomonitoring Network (CABIN) reference model was developed for LOW to compare the benthic community structure at a number of potentially stressed or impaired sites. Concentrations of both nutrients and metals in sediments at many sites in LOW exceeded Ontario provincial and Canadian federal effect levels for aquatic life. The benthic community at some locations was found to be divergent from reference sites, with substantial reductions in diversity and abundance associated with stress to the benthic community. As expected, benthic invertebrate diversity appeared to be most affected at sites that were deep, thermally stratified and high in nutrients thus making them prone to hypoxia. Benthic diversity was also negatively associated with higher concentrations of metals such as lead and arsenic. The CABIN approach can provide a useful tool in lake management for the identification of stressed sites.