Understanding Declining Productivity in the Offshore Regions of the Great Lakes

The Great Lakes Water Quality Agreement recognizes the complexity of the Great Lakes basin ecosystem and its interacting components of air, land, water and living organisms in lakes comprised of many different types of habitat in the nearshore and offshore regions. [...] Perspective The success of point source nutrient controls in the 1970s and 1980s resulted in a remarkable recovery of the lakes, with continued declines in TP concentrations in the open waters of all of the lakes (except Lake Erie) to well below target levels. [...] The emergence of these nearshore issues while the offshore was meeting or exceeding targets led to the call for a Nearshore Framework and also for a review of the nutrient objectives for the lakes (Canada and the United States, 2012), with the possible consequence of lowering target concentrations below the already historically low concentrations in the lakes. [...] The responses of fish abundance to phosphorus levels in the European Lakes (Figure 3) and the experimental lakes (Figure 4) occurred over the same ranges of TP concentrations evident in the Great Lakes, suggesting that similar responses in fish abundance to changes in nutrients in the Laurentian Great Lakes could be expected. [...] Overexploitation in the first half of the last century—which overlapped with and then was followed by invasion and establishment of predatory sea lamprey—diminished important native fish populations nearly to extinction while removing the top predators from the food web and led to dominance of lower-valued prey species, such as alewife, in the lakes through the 1950s to the 1960s.