Transformation of organic matter in a Barents Sea sediment profile: coupled geochemical and microbiological processes

Abstract

Process-based, mechanistic investigations of organic matter transformation and diagenesis directly beneath the sediment–water interface (SWI) in Arctic continental shelves are vital as these regions are at greatest risk of future change. This is in part due to disruptions in benthic–pelagic coupling associated with ocean current change and sea ice retreat. Here, we focus on a high-resolution, multi-disciplinary set of measurements that illustrate how microbial processes involved in the degradation of organic matter are directly coupled with inorganic and organic geochemical sediment properties (measured and modelled) as well as the extent/depth of bioturbation. We find direct links between aerobic processes, reactive organic carbon and highest abundances of bacteria and archaea in the uppermost layer (0–4.5 cm depth) followed by dominance of microbes involved in nitrate/nitrite and iron/manganese reduction across the oxic-anoxic redox boundary (approx. 4.5–10.5 cm depth). Sulfate reducers dominate in the deeper (approx. 10.5–33 cm) anoxic sediments which is consistent with the modelled reactive transport framework. Importantly, organic matter reactivity as tracked by organic geochemical parameters ( n -alkanes, n -alkanoic acids, n -alkanols and sterols) changes most dramatically at and directly below the SWI together with sedimentology and biological activity but remained relatively unchanged across deeper changes in sedimentology. This article is part of the theme issue ‘The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning’.

Climate-driven benthic invertebrate activity and biogeochemical functioning across the Barents Sea polar front

“…Hence, our findings indicate that bioturbation activity is dependent on the interactive effects of season and sea ice condition which, in turn, are influenced by latitudinal position and local adjustments to circumstance. Furthermore, since the inventory of sediment organic material indicates more efficient carbon processing (lower organic material values) during extended sea ice conditions [73], the increased reworking activities of infauna during these periods may offer a mechanistic explanation for likely/potential greater carbon burial rates, at least at the most northerly stations in the transect [53]. If true, interspecific differences in community bioturbation should lead to variations in the vertical distribution of sediment organic matter, a conclusion that does appear to be consistent with observations of organic material profiles [72] and other sediment processes (Fe/Mn reduction, [119]).…”

“…If true, interspecific differences in community bioturbation should lead to variations in the vertical distribution of sediment organic matter, a conclusion that does appear to be consistent with observations of organic material profiles [72] and other sediment processes (Fe/Mn reduction, [119]). Direct links between aerobic processes, reactive organic carbon and highest abundances of bacteria and archaea have recently been shown for the uppermost sediment layers, and organic matter reactivity changes most dramatically at, and directly below, the sediment-water interface alongside sedimentology and biological activity [73]. However, invertebrate utilization of carbon can occur at the biochemical level [120] and/or depend on species-specific differences in adsorption efficiency and feeding behaviour [121], suggesting that multiple traits that each interact with climatic forcing will be important for resource exploitation and ecosystem functioning.…”

“…Our study also highlights the paucity of available information within this region on how species (or communities) moderate important ecosystem functions in relation to a changing climate, biotic re-organization, and their interactions with one another [70]. Furthermore, biogeochemical pathways and processes are poorly understood, and little is known about the relative importance of different components of organic material at an ecosystem level [73,122]. In order to establish generality and generate projections of the threats and opportunities of future change on biological and biogeochemical processes, process and experimental studies focused on developing mechanistic understanding of the interactive effects of different components of change (and any of their correlates) on organism-sediment relations are urgently needed and must be prioritized.…”

“…To minimize the effect of non-climatic drivers of change, stations were selected with comparable water depths (200–400 m), sediment type and bottom fishing activity [69,70]. Bottom fishing activity was minimized by selecting locations that showed low levels of activity (based on VMS tracking data, visualized at: ) and we verified that there was no recent activity at the point of station occupancy using sediment surface imagery [71] and geochemical profiles [72,73].…”

The changing Arctic Ocean: consequences for biological communities, biogeochemical processes and ecosystem functioning

“…Increased organic carbon fluxes on the Barents Sea slope may result in large fluxes of P from sediments to bottom waters, as a large stock of P has been accumulated in surface sediments. Stevenson et al [46] demonstrate mechanistic links between microbial processing and changes in organic and inorganic parameters that are coupled to biological mixing and the reactivity of organic material. They find direct links between aerobic processes, reactive organic carbon and highest abundances of bacteria and archaea in the uppermost sediment layer followed by dominance of microbes involved in nitrate/nitrite and iron/manganese reduction across the oxic-anoxic redox boundary and sulphate reducers at depth.…”