ABSTRACT:

Forecasted increases in terrestrial organic matter (OM_terr) inputs to the Arctic Beaufort Sea necessitate a better understanding of the proportional contribution of this potential food source to the trophic structure of marine communities. This study investigated the relative ecological importance of OM_terr across the Beaufort Sea shelf and slope by examining differences in community trophic structure concurrent with variation in terrestrial versus marine organic matter influence. Oxygen stable isotope ratios (δ¹⁸O) of surface water confirmed the widespread influence of Canada’s Mackenzie River plume across the Beaufort Sea. Carbon stable isotope ratios (δ¹³C) of pelagic particulate organic matter (pPOM) and marine consumers indicated a significant decrease in OM_terr presence and utilization by consumers with increasing distance from the Mackenzie River outflow. Food web length, based on the nitrogen stable isotope ratios (δ¹⁵N) of marine consumers, was greater closer to the Mackenzie River outflow both in shelf and slope locations, due to relatively higher δ¹⁵N values of pelagic and benthic primary consumers. Strong microbial processing of OM_terr in the eastern regions of the Beaufort Sea is inferred based on a trophic gap between assumed end members and lower trophic consumers. A greater proportion of relative epifaunal biomass occupying higher trophic levels suggests that OM_terr as a basal food source can provide substantial energetic support for higher marine trophic levels. These findings challenge the current conception of low terrestrial matter usage in the Arctic marine food web, and indicate the need for a more specific understanding of energy transfer through the OM_terr-associated microbial loop.