Evaluating spatial variation in food web connectivity and energetics within the Lake Superior fish community

Abstract

Habitat coupling integrates energy between habitats, a process known to contribute to food web stability, and has been observed in Lake Superior. The degree of coupling differs among species, but the extent to which it may vary across Lake Superior is unknown. A change in the degree of coupling may alter regional trophic transfer efficiency (TTE), which is the amount of energy that is available to be passed on to a successive trophic level. Spatial differences in habitat coupling and TTE in Lake Superior were evaluated using common fish species and their prey, collected from four geographically distinct regions and along a depth gradient. For each species, habitat coupling was assessed using stable isotope analysis, and bioenergetics models were created to estimate conversion efficiency (as a proxy measure of TTE). Species that exhibit diel vertical migration (lake trout, smelt, herring and bloater) showed more variability in resource partitioning with depth, especially in the deeper regions. Overall, there was a large pelagic reliance, but some benthic specialization was observed in the medium and deep depth strata. Lake whitefish and sculpin spp. showed a common use of pelagic resources across all depth strata within a region. Bioenergetic estimates exhibited opposite trends in lake whitefish and smelt across sampled regions, but lake trout bioenergetics were generally well conserved. Conversion efficiency showed a positive relationship with percent benthic reliance in lake whitefish populations and a negative association with percent benthic reliance in the smelt populations, suggesting species are better able to utilize energy from which they are most commonly associated. This study contributes to a better understanding of how food webs large lakes are structured, and the inherent variation in ecosystem function that should be considered in whole-lake modelling or regional management. Because Lake Superior is a relatively undisturbed ecosystem, these features can be compared with other large lakes and used as an example to identify common properties of healthy systems to protect or re-establish the functionality of disturbed systems.