Paleolimnological Investigation Of The St. Louis River Estuary To Inform Area Of Concern Delisting Efforts

Abstract

The St. Louis River Estuary (SLRE) 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 in the region including two developing cities (Duluth, MN and Superior, WI), the Environmental Protection Agency (EPA) designated the SLRE as an Area of Concern in 1987. Prior to 1987, actions had begun to restore water quality including the enactment of the Clean Water Act in 1972 and startup of the Western Lake Superior Sanitary District (WLSSD) in 1978 to help remove some beneficial use impairments (BUIs). A better understanding of the historical significance of these contributions over the years is necessary to help document both progress and knowledge gaps related to water quality. Therefore, 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 estuary and western Lake Superior. Reductions in eutrophic diatom taxa such as Cyclotella meneghiniana and Stephanodiscus after 1970 in certain cores suggest a recovery of 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 continuing nutrient loading and increased production. Sedimentary pigments indicate increases in cyanobacteria in some bays over the last two decades. Diatom model-inferred phosphorus and contemporary monitoring data suggest some of the problems associated with excess nutrient discharges have been remediated, but modern conditions (internal phosphorus loading, changing climate) may be contributing to ongoing water quality problems in some locations. The integrated biological, chemical, and physical indicators from the sediments will aid agencies in determining where to target resources and BUI removal efforts.