Stable isotope analysis of the Laurentian Great Lakes food webs: Quantifying spatial and temporal food web differences

Anthropogenic activities over the last century have dramatically changed the food webs of the Laurentian Great Lakes. I explored several questions relating to food web changes across temporal and spatial scales.;Ecologists use stable isotopes (delta13C, delta 15N) to better understand food webs and are increasingly comparing food webs across time and space. However, the traditional delta13 C vs. delta15N bi-plots used to describe food web structure are not sufficient for testing hypotheses of spatial or temporal food web differences. I demonstrate the utility of circular statistics and hypothesis tests for quantifying directional food web differences using two case studies. The quantitative analysis revealed significant insight into patterns and changes in food web structure that were not evident from qualitative comparisons.;Restoration and rehabilitation of native species in the Great Lakes is a priority for fisheries management agencies. I used stable isotope analysis and museum preserved specimens to describe and quantify 100 years of food web changes in the Lake Superior and Lake Michigan fish community. This historical information about ecological niches of native and non-native species can help guide management and benchmark restoration goals in the Great Lakes.;The Lake Superior and Lake Michigan food webs differed in responses to non-native species introductions. In Lake Superior, species introductions have created a complex and somewhat more trophically diverse food web than historical conditions. The introductions, however, have not caused any community-wide changes in the food web. Unlike Lake Superior, two measures of community-wide food web dynamics changed over time in Lake Michigan. Quantitative measures indicate a shift from pelagic to benthic production in the past two decades and a food web structure that has expanded in trophic niche space since the 1920's.;Of particular interest to Great Lakes fisheries managers is the reintroduction of several extirpated deepwater coregonines (Coregonus spp.). To improve our understanding of the deepwater coregonines, I investigated the historical and current trophic niche partitioning of coregonine species. Using a new food web centroid approach, MANOVA, and ANOVA I found that different species occupy distinct trophic niches and that the species maintain distinctness through time and across lakes.