Landscape-Scale Food Webs of Fish Nursery Habitat Along a River-Coast Mixing Zone

We used carbon and nitrogen stable isotope ratios to identify changes in organic matter (OM) sources supporting fish larvae along the river–lake mixing zone in three Lake Superior coastal tributaries. We used a stable isotope mixing model to quantify how the number and origin of OM sources supporting fish larvae changed with respect to the relative fraction of tributary and lake water along the coastal mixing zone. Fish stable isotope ratios, after adjusting for trophic fractionation, generally were intermediate between the measured OM sources, indicating widespread nutritional support from multiple OM sources. Mixing model results indicated that more OM sources supported fish larvae in river and mixed regions than in the lake; however, the number of OM sources did not change linearly with respect to tributary water fraction. Contributions from allochthonous OM were highest in the mixed and lake regions; most fish larvae obtained some energy from food sources originating outside the region where the fish larvae were collected. At the species level, principal component analysis revealed that the number of OM sources was positively correlated with percent terrestrial OM contribution and greatest in demersal fish larvae that were captured in the mixed region. Our findings demonstrate that these coastal habitats do function as a “mosaic”, wherein proximal and some more distal habitats and ecosystems contribute to fish growth during a critical life stage. By extension, the findings suggest the need for conservation measures to consider upland, wetland, and coastal habitat, whether or not fish directly occupy all of those habitats.