The benthic nepheloid layer in the offshore waters of the Great Lakes and its post-dreissenid disappearance

Prior to the appearance of Dreissena, pronounced benthic nepheloid layers (BNL) near the bottom characterized by elevated levels of both turbidity and total phosphorus (TP), were a consistent and extensive feature of the offshore, stratified waters of all the Laurentian Great Lakes, except Lake Superior. In recent (2010–2019) years, the BNL has disappeared from all areas except for central Lake Erie, where only a small decrease in bottom turbidity has occurred. All stratified regions which exhibited a pre-Dreissena BNL, including central Lake Erie, experienced substantial post-Dreissena reductions in near-bottom TP, although the forms of phosphorus (particulate or soluble) responsible for these reductions have varied from lake to lake. Notably, the arrival of Dreissena at offshore sites was not accompanied by an increase in soluble phosphorus. Initiation of changes in the BNL almost invariably preceded appearance of Dreissena in the offshore, suggesting both that dreissenid impacts on the reductions in the BNL were largely remote, and by extension that the source of the BNL was also at least in part remote. Previous researchers’ estimates of the importance of the benthic pool of phosphorus to offshore water column concentration suggest that the post-invasion reductions in bottom phosphorus during the stratified season could be contributing to the offshore oligotrophication of the lakes.