The Limnological Units of the Lower Great Lakes - St. Lawrence River Corridor and Their Role in the Source and Aquatic Fate of Toxic Contaminants

Abstract

Abstract The waterways of the lower Great Lakes and St. Lawrence River, between Sarnia and the Saguenay Fiord, are made up of four limnological units. The first comprises the high discharge, rapid flow rivers, namely the St. Clair, Detroit, Niagara and St. Lawrence. Second are the four shallow, short residence time, riverine lakes, namely St. Clair, St. Francois, St. Louis and St. Pierre. Third are the two, relatively deep, long residence time, lower Great Lakes Erie and Ontario. Lastly, there is the freshwater-salt water mixing zone of the upper St. Lawrence Estuary. The rivers are essentially sources and transport systems of toxic contaminants on a grand scale. The riverine lakes provide only temporary storage or sinks even for contaminants associated with sediments because these are eventually resuspended and moved on downstream. The major sinks, where long-term effects are most evident are the two lower Great Lakes and the St. Lawrence Estuary. These sites are also where sediment associated contaminants can be permanently removed by deep burial in bottom sediments. However, even here, a proportion of the contaminant load passes on downstream and eventually out to the Gulf of St. Lawrence. The distinctive characteristics of the four limnological units are discussed in relation to sources and fate of toxic contaminants. Understanding the role of the units is critical to development of toxic chemicals control strategies and reduction in aquatic ecosystem contamination.