Organic matter sources on the Chukchi Sea shelf in a changing Arctic

Abstract

Climate-change induced alterations of the organic matter flow from various primary production sources to the benthic system in the Arctic Chukchi Sea could have major implications on carbon cycling, sequestration, and benthic food web structure sustaining upper trophic levels. In particular, the role and contribution of terrestrial matter and bacterial matter could become more prominent, with increasing erosion and permafrost melt being discharged from land, and warming water temperatures raising bacterial metabolism. In this study, I used essential amino acid (EAA) specific stable isotope analysis to trace the proportional contributions of bacterial, phytoplankton, and terrestrial organic matter in sediments, as well as benthic invertebrates on the Chukchi Sea shelf. Across the upper 5 cm of sediments, most organic matter sources were equally distributed, except for a slight decrease with depth in phytoplankton EAA. Terrestrial sources contributed the majority of EAA (~76 %) in all sediment layers, suggesting a potential accumulation of this material due to slow degradation processes. These results indicate a well-mixed upper sediment horizon, possibly due to bioturbation activity by the abundant benthos. Experimental observations of increases in bacterial production, measured as phospholipid fatty acid (PLFA) production, at water temperatures 5 °C above ambient (0 °C) and under sufficient substrate conditions suggest that bacterial organic matter in sediments could become a greater organic matter source in the sediments of a future, warmer Arctic. EAA source contribution to various benthic invertebrate feeding types (FT) were similar but showed significant differences among genera within the same FT, suggesting that feeding habits are more genus-specific rather than FT-specific. These differences were attributed to variations in other characteristics such as mobility, selectivity, and assimilation efficiency. Terrestrial EAA contributed high amounts to all benthic genera, supporting other recent findings that this source is readily utilized by benthic invertebrate consumers. These results of organic matter source contributions across sediments and benthic invertebrate feeding types were then used to better resolve the detrital pathways in an Ecopath mass-balance model of the Chukchi Sea. The incorporation of terrestrial matter as an organic matter source to Chukchi Sea food webs and updated organic matter use in benthic invertebrate diets balanced energy flow from phytoplankton and bacterial production through the food web. Simulations of potential future reductions of the pelagic production to the benthos negatively impacted benthic feeding taxa, which could be partially compensated by a simulated increase in terrestrial and bacterial organic matter supply.