Quantifying spatial subsidies of freshwater fish in large lakes

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Trophic interactions of species are a key driver that structure food webs and ultimately ecosystem function and health. In combination with resource availability, these interactions contribute to defining the ecological role of a species. When resources are insufficient, mobile species need to move to new regions or habitats to meet energetic requirements. These movements can be short, such as when individuals move to a new habitat for a short period to feed, or long, when they are linked to seasonal changes in ecosystem productivity or environmental conditions. In both cases, movements can have consistent temporal and spatial patterns. When a species or population uses a second or multiple habitats for energetic needs, or to increase its productivity, it can be referred to as a spatial subsidy. Spatial subsidies are often critical for supporting higher trophic level species, such as shark species on coral reefs, and often explain confusing observed ecological phenomena, such as inverted food-web biomass pyramids. Spatial subsidies are recognized as important components of marine ecosystems but are poorly studied or understood in freshwater ecosystems, despite their significant effects on nutrient cycling, productivity, and food webs. Freshwater ecosystems are vital for human and environmental health but are under intense pressure from multiple stressors, and there is a need to better understand their structure and function. My program will quantify the feeding and movement ecology of freshwater fish species to better understand the prevalence and importance of spatial subsidies in large lakes. This research will use chemical tracers, stomach contents, and acoustic telemetry to define diet, habitat use, and movements of different fish species in three regions of the Laurentian Great Lakes. These data will populate Ecopath models to quantify trophic pathways and food-web structure to estimate the importance of spatial subsidies for these systems. The first field project will be in the Detroit River and will assess the importance of metabolic activity variation across two predator fish species over smaller areas. The second project, in Lake Erie, will quantify the importance of different lake basins on the diet and movement of less and highly mobile fish species. The third project, in Lake Ontario, will assess the importance of movement and diet on growth rates in four predator fish. These projects will use eight fish species with different life histories. The three study systems vary in characteristicse.g., productivity (Erie > Ontario); and depth (Ontario > Erie > Detroit)which will allow this research to identify key mechanisms and processes that influence spatial subsidies in freshwater ecosystems. This project will also include laboratory studies to quantify chemical tracer (amino acid and sulfur stable isotopes) turnover and diet tissue discrimination factors in fish to better inform the food web projects.