The influence of terrestrial matter in marine food webs of the Beaufort Sea shelf and slope

Terrestrial organic matter (OMterr) can function as a food source for Arctic marine consumers, though the relative contribution of OMterr to the structure and efficiency of marine food webs compared to marine production is unclear. Forecasted increases in OMterr inputs to the Arctic Beaufort Sea necessitate a better understanding of the proportional contribution of this organic matter source to the trophic structure of marine communities. This study investigated the relative ecological importance of OMterr across the Beaufort Sea shelf and slope by examining differences in community trophic structure concurrent with variation in terrestrial versus marine organic matter influence. Hydrogen stable isotope ratios (δD) of surface water, surface sediment particulate organic matter (sPOM), and selected benthic consumers were used as an exploratory assessment of freshwater and OMterr distribution in the Beaufort Sea. δD values of surface water confirmed the widespread influence of Canada's Mackenzie River plume across the Beaufort Sea; however, δD values of terrestrial and marine production were not sufficiently distinguishable to differentiate organic matter sources in consumers. Carbon stable isotope ratios (δ¹³C values) of pelagic particulate organic matter (pPOM) and marine consumers confirmed a significant decrease in OMterr presence and utilization by consumers with increasing distance from the Mackenzie River outflow. Food web length, based on the nitrogen stable isotope ratios (δ¹⁵N values) of marine consumers, was longer closer to the Mackenzie River outflow both in shelf and slope locations due to relatively higher δ¹⁵N values of pelagic and benthic primary consumers. The absence of macrofaunal consumers at the lowest trophic levels of OMterr-influenced food webs was interpreted to result from the prior metabolic turnover of OMterr by the microbial loop, which was not sampled in this study. The inferred presence of strong microbial processing of OMterr in the eastern regions of the Beaufort Sea resulted in a higher proportion of relative epifaunal biomass occupying higher trophic levels, suggesting that OMterr 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 contributions to the Arctic marine food web, and compel a more specific understanding of energy transfer through the OMterr-associated microbial loop.

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