The Influence of Terrestrial Matter in Marine Food Webs of the Beaufort Sea Shelf and Slope

Forecasted increases in terrestrial organic matter (OM_terr) inputs to the Beaufort Sea necessitate a better understanding of the contribution of this organic matter 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. Interannual variability in organism trophic level was assessed to confirm the persistent impact of these large-scale patterns in food source distribution on marine consumers. 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 values) of pelagic particulate organic matter (pPOM) and marine consumers from locations ranging from 20 to 1000 m bottom depth revealed a strong isotopic imprint of OM_terr in the eastern Beaufort Sea, which decreased westward from the Mackenzie River. Food web length, based on the nitrogen stable isotope ratios (δ¹⁵N values) 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 sources and lower trophic consumers. A large proportion of 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 support the concept that terrestrial matter is an important source in the Arctic marine food web, and compel a more specific understanding of energy transfer through the OM_terr-associated microbial loop.