Regional variability of resource use, trophic position and habitat coupling within Lake Superior

Abstract

Food web characterizations of large lakes have rarely considered spatial variation in resource use or trophic connections among species. This is also true of Lake Superior, the world’s second largest freshwater lake. While instances of habitat coupling in Lake Superior have been documented, the extent to which coupling or resource specialization in this system varies spatially, particularly within a species, remains unknown. To address this, stable isotopes of common fish species and prey were collected and analyzed along a depth gradient at four geographically and bathymetrically distinct regions of Lake Superior; 1099 fish and 60 composite invertebrate samples (separately for zooplankton and benthos) were collected from both deeper regions (Keweenaw and Western Arm) and shallower regions (Nipigon Bay and Whitefish Bay). Benthic and pelagic species of fish and invertebrates were collected to characterize different energy pathways. Within regions and taxa, benthic reliance and trophic position differences across depth strata were large and comparable to those previously observed among species lake wide. Across regions, large within-taxa differences in resource use patterns and trophic position existed at similar depth strata, as well as among taxa. Generally, there was a high reliance on pelagic resources across all fish species with greater benthic resource use observed at medium and deep strata. As expected, higher trophic organisms tended to have greater evidence of benthic-pelagic coupling. Our findings reinforce the need to consider regional variation in resource use and trophic position in large lake systems over broad approaches that can overgeneralize patterns of energy flow.