The St. Louis River Estuary (SLRE), a freshwater estuary bordering Duluth, Minnesota, Superior, Wisconsin, and the most western point of Lake Superior (46.74°, − 92.13°), has a long history of human development since Euro-American settlement ~ 200 years ago. Due to degradation from logging, hydrologic modification, industrial practices, and untreated sewage, the SLRE was designated an Area of Concern in 1987. Action has been taken to restore water quality including the installation of the Western Lake Superior Sanitary District in 1978 to help remove beneficial use impairments. A better understanding of historical impacts and remediation is necessary to help document progress and knowledge gaps related to water quality, so a paleolimnological study of the SLRE was initiated. Various paleolimnological indicators (pigments, diatom communities, and diatom-inferred phosphorus) were analyzed from six cores taken throughout the SLRE and another from western Lake Superior. Reductions in eutrophic diatom taxa such as Cyclotella meneghiniana and Stephanodiscus after 1970 in certain cores suggest an improvement in water quality over the last 40 years. However, in cores taken from estuarine bay environments, persistence of eutrophic taxa such as Cyclostephanos dubius and Stephanodiscus binderanus indicate ongoing nutrient problems. Sedimentary pigments also indicate cyanobacteria increases in bays over the last two decades. Diatom model-inferred phosphorus and contemporary monitoring data suggest some of the problems associated with excess nutrient loads have been remediated, but modern conditions (internal phosphorus loading, changing climate) may be contributing to ongoing water quality impairments in some locations. The integrated record of biological, chemical, and physical indicators preserved in the sediments will aid state and federal agencies in determining where to target their resources.
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Acknowledgements This work was made possible by two grants. (1) Research sponsored by the Minnesota Sea Grant College Program supported by the NOAA office of Sea Grant, United States Department of Commerce, under grant No. R/CE-05-14. The U.S. Government is authorized to reproduce and distribute reprints for government purposes, notwithstanding any copyright notation that may appear hereon. (2) Project funding with support from the Minnesota Pollution Control Agency with funds from the Minnesota Clean Water Legacy Amendment. Thanks go to Kitty Kennedy, Meagan Aliff, Lisa Estepp, and the Fond du Lac Band of Lake Superior Chippewa for their help with field and lab work; Dr. Daniel Engstrom of the Science Museum of Minnesota for completing 210Pb analysis and interpretation; Molly Wick (WDNR) provided helpful reviews of earlier drafts of this manuscript.
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- Beneficial use impairment
- Great lakes
- Lake Superior
- St. Louis River