The relative impacts of nutrient loads and invasive species on a Great Lakes food web: An Ecopath with Ecosim analysis

Abstract

Excessive nutrient loads and species invasions pose significant threats to productivity and function of Great Lakes aquatic ecosystems. We used an Ecopath and Ecosim model to analyze impacts of changes in phosphorus loads, and dreissenid mussels and alewife biomass on the Saginaw Bay food web, Lake Huron. We configured the food web model in Ecopath with pre-dreissenid (1990) data on organism biomass, production, consumption, and diet from federal and state agency surveys and other sources. We conducted 70-year simulations in Ecosim of single factors (nutrients, alewives, and dreissenids) and their combinations. Phosphorus load scenarios were run with high (1970s), average (current), and low (target) levels; alewife scenarios were run with double the 1990–2003 average biomass, 1990–2003 average biomass, and alewife absence; dreissenid scenarios were run with the 1990–1996 average biomass, current (2009–2010) biomass, and dreissenid absence. Results indicated that phosphorus loads were positively correlated with simulated biomass of most food web groups, and alewife biomass was negatively correlated with biomass of most fish groups and macrozooplankton. Dreissenid impacts were most severe on lower trophic levels but were relatively minor for fish groups compared with nutrient and alewife scenarios. Dreissenids had little effect on fish because Chironomids, which feed on detritus and are the main component of fish diets, were not affected by dreissenids. Our results suggest that, under current conditions of absence of alewives and reduced dreissenid biomass, the target nutrient loads established in 1978 would not sustain current fishery harvests in Saginaw Bay given food web changes caused by invasive species.