Historical niche partitioning and long-term trophic shifts in Laurentian Great Lakes deepwater coregonines

Over the last 100yr, anthropogenic stressors have decimated the assemblage of deepwater coregonines that once underpinned the food webs of the Laurentian Great Lakes. As a part of ongoing restoration efforts, fisheries managers are interested in reintroducing deepwater coregonines from remnant populations to reestablish historical food web connections. However, little is known about historical trophic position and niche partitioning among deepwater coregonines in the Great Lakes. We used nitrogen stable isotope analysis of amino acids to compare trophic position of museum-preserved (1920s) and present-day forage fishes in Lakes Michigan and Superior. In the 1920s, deepwater coregonines exhibited clear trophic niche partitioning, with trophic positions spanning a full trophic level. Additionally, species trophic positions were tightly conserved between lakes. In Lake Superior, trophic niche partitioning has been maintained over the last 100yr, but trophic position has shifted downward by similar to 0.5 trophic level. The more dramatic species loss in Lake Michigan corresponds with a sharp reduction in trophic niche breadth over time. Our study reveals remarkable trophic niche breadth among deepwater coregonines prior to the major anthropogenic impacts on the Laurentian Great Lakes and provides a food web benchmark for restoring the historical trophic diversity of this iconic species flock.