Rates and pathways of sediment organic-matter processing across the northern Bering and southern Chukchi Sea shelf

Labile organic matter deposited at the seafloor may be respired or assimilated into biomass; rates of these key processes are needed to characterize organic matter cycling under current conditions, and to project changes in ecosystem function with warming and sea-ice loss. Relative roles of bacteria and metazoan infauna may shift with rising temperatures and changes in quantity and quality of particulate flux if more efficient bacterial metabolism at warmer temperatures leads to increased carbon consumption, affecting food availability for detritivores. Intact sediment cores collected in June 2017 and 2018 from 10 locations in the Bering and Chukchi seas were incubated at 0ºC (ambient) and 4ºC (projected warming), with and without addition of an isotopically labeled pulse of phytodetritus. Sediment community oxygen demand, a proxy for organic carbon consumption, was ~30% higher in warmer treatments, but did not vary significantly between years. Substrate type, productivity, and particulate flux rates varied across the study area, resulting in spatial differences in microbial and metazoan biomass; these patterns were also reflected in oxygen and nutrient fluxes in incubated cores. Nutrient fluxes indicate net uptake of nutrients by sediments in some areas, and efflux from sediments in other areas. Respiration of labeled detritus was measured as production of DIC-13; assimilation of labeled algae by benthic fauna will also be evaluated. Individual respiration rates were measured for nematode communities and for dominant macrofaunal taxa. These results will be combined with whole-core data and estimates of biomass for microbes, meio- and macrofauna to partition contribution of each food-web component to organic carbon turnover.