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Implications of phosphorus reduction for sustainable Great Lakes fisheries.

Marten Koops, Monir Hossain (DFO), Ed Rutherford and Hongyang Zhang (NOAA), Lars Rudstam, Randy Jackson (Cornell), Willie Fetzer (Univ Wyoming) and others. (Funded by the Great Lakes Fisheries Commission)

The Great Lakes Water Quality Agreement (GLWQA) has been successful in reducing total phosphorus (TP) loadings, thereby initiating an oligotrophication process across the Great Lakes. Emerging evidence suggests that the Great Lakes are experiencing both an ‘offshore desertification’, whereby low ambient TP concentrations undermine pelagic ecosystem integrity, and in some nearshore areas, blooms of toxic blue-green algae resulting from excess TP. TP is a determinant of ecosystem productivity related to observed biomass at all trophic levels, and has been recognized as a predictor of fish biomass and production for many decades. However, TP reductions have not occurred in isolation; a number of anthropogenic stressors may have affected the energy dynamics that determine how TP is converted into fish biomass. One important stressor is the invasion of dreissenid mussels that have engineered changes in nutrient cycling and lake productivity, and have affected many components of the food web including the production of fish. In 2019, we started to examine the phosphorus to fishes (P2F) relationship across multiple systems in the Great Lakes basin to test the hypothesis that recent anthropogenic stressors have changed Great Lakes energy dynamics reducing the efficiency of nutrient conversion to fish biomass. Across systems, TP has also been related to species diversity, and different types of fish communities may respond differentially to TP. Further, the altered biotic interactions associated with dreissenid-invaded food webs will elicit distinct shifts in energy dynamics which may differentially decouple the P2F relationship between pre and post-invasion periods. We will also test the hypothesis that the synergistic effect of TP reduction and dreissenid invasion will vary across habitat types (a spatial effect) and biotic communities (an interaction effect). Activities in 2021continued on the data sets and the construction of the Ecopath/Ecosim models.