An Integrated Approach to Assessing Multiple Stressors for Coastal Lake Superior

Abstract

This peer-reviewed article summarizes research conducted under the Great Lakes Environmental Indicators (GLEI) project initiated by the authors in 2001. The authors assessed the status of Lake Superior’s coastal ecosystem relative to over 200 environmental variables collected from GIS data sets for the enture US Great Lakes basin. These were assessed using gradients including atmosphereic deposition, agriculture, human population and development, land cover, point source pollution, soils and a cumulative stress index. Relationships of biological assemblages of birds, diatoms, fish and invertebrates, wetland plants, soils and stable isotopes to these gradients were then assessed. Key findings are extracted and reproduced below. Biological indicators can be used both to estimate ecological condition and to suggest plausible causes of ecosystem degradation across the U.S. Great Lakes coastal region. Here we use data on breeding bird, diatom, fish, invertebrate, and wetland plant communities to develop robust indicators of ecological condition of the U.S. Lake Superior coastal zone. Sites were selected as part of a larger, stratified random design for the entire U.S. Great Lakes coastal region, covering gradients of anthropogenic stress defined by over 200 stressor variables (e.g. agriculture, altered land cover, human populations, and point source pollution). A total of 89 locations in Lake Superior were sampled between 2001 and 2004 including 31 sites for stable isotope analysis of benthic macroinvertebrates, 62 sites for birds, 35 for diatoms, 32 for fish and macroinvertebrates, and 26 for wetland vegetation. A relationship between watershed disturbance metrics and 15N levels in coastal macroinvertebrates confirmed that watershed-based stressor gradients are expressed across Lake Superior’s coastal ecosystems, increasing confidence in ascribing causes of biological responses to some landscape activities. Several landscape metrics in particular—agriculture, urbanization, human population density, and road density—strongly influenced the responses of indicator species assemblages. Conditions were generally good in Lake Superior, but in some areas watershed stressors produced degraded conditions that were similar to those in the southern and eastern U.S. Great Lakes. The following indicators were developed based on biotic responses to stress in Lake Superior in the context of all the Great Lakes: (1) an index of ecological condition for breeding bird communities, (2) diatom-based nutrient and solids indicators, (3) fish and macroinvertebrate indicators for coastal wetlands, and (4) a non-metric multidimensional scaling for wetland plants corresponding to a cumulative stress index. These biotic measures serve as useful indicators of the ecological condition of the Lake Superior coast; collectively, they provide a baseline assessment of selected biological conditions for the U.S. Lake Superior coastal region and prescribe a means to detect change over time.” Key points: “In general, the U.S. Great Lakes coastal region of Lake Superior shows greater overall stress in the southern regions compared with relatively low overall stress in the northern regions. These patterns are primarily due to agricultural land use, higher human population densities, and point sources in the eastern and western portions on the south shore, while the north shore at the western end of Lake Superior is primarily forested with relatively sparse human population densities. Coastal regions of Lake Superior can be found at each of the extremes of the disturbance gradients. This includes relatively pristine watersheds in the northern regions with low human population densities and little agriculture that contrast with regions of relatively high populations with industrial activity such as Duluth-Superior in Minnesota-Wisconsin and Sault Ste. Marie Michigan at the other end of the gradient. The U.S. Lake Superior coastal region varies widely in the degree of human-related stress; generally, levels of stress decrease from south to north but with considerable variation, especially along the southern shore due to local agricultural activity and the presence of several population and industrial centers. In spite of a lack of latitudinal variation, there is human-induced, watershed scale variability across the Lake Superior coast. Compared to the other Great Lakes, Lake Superior coastal fish communities had more generally intolerant fish and more turbidity intolerant fish. Coastal fish community composition reflected the higher levels of suspended solids associated with human alteration to watersheds. The most disturbed sites on Lake Superior had greater proportions of non-native species and fewer bottom-feeding taxa.