Abstract
Remineralization of organic matter at the seafloor is an important ecosystem function, as it drives carbon and nutrient cycling, supplying nutrients for photosynthetic production, but also controls carbon burial within the sediment. In the Arctic Ocean, changes in primary production due to rapid sea-ice decline and thinning affect the export of organic matter to the seafloor and thus, benthic ecosystem functioning. Due to the remoteness and difficult accessibility of the Arctic Ocean, we still lack baseline knowledge about patterns of benthic remineralization rates and their drivers in both shelf and deep-sea sediments. Particularly comparative studies across regions are scarce. Here, we address this knowledge gap by contrasting benthic diffusive and total oxygen uptake rates (DOU and TOU), both established proxies of the benthic remineralization function, between shelf and deep-sea habitats of the Barents Sea and the central Arctic Ocean, sampled during a RV Polarstern expedition in 2015. DOU and TOU were measured using ex situ porewater oxygen microprofiles and sediment core incubations, respectively. In addition, contextual parameters including organic matter availability and microbial cell numbers were determined as environmental predictors. Pan-Arctic regional comparisons were obtained by extending our analyses to previously published data from the Laptev and Beaufort Seas. Our results show that (1) benthic oxygen uptake rates and most environmental predictors varied significantly between shelf and deep-sea habitats; (2) the availability of detrital organic matter is the main driver for patterns in total as well as diffusive respiration, while bacterial abundances were highly variable and only a weak predictor of differences in TOU and DOU; (3) regional differences in oxygen uptake across shelf and deep-sea sediments were mainly related to organic matter availability and may reflect varying primary production regimes and distances to the nearest shelf. Our findings suggest that the expected decline in sea-ice cover and the subsequent increase in export of organic matter to the seafloor may particularly enhance remineralization in the deep seas of the Arctic Ocean, altering benthic ecosystem functioning in future climate scenarios.
Highlights
A major function of benthic ecosystems is the turnover of organic matter, mainly driven by microorganisms (Glud et al, 1998), which we refer to as the “benthic remineralization” function (Link et al, 2013b)
We address the following three questions: (1) How large are variations in TOUsed, diffusive oxygen uptake (DOU) and environmental parameters among the Barents Sea shelf and the central Arctic deep sea? (2) How much of the variability in TOUsed and DOU can be explained by environmental proxies and microbial abundance? (3) How do variations in benthic activity and environmental parameters among shelf and deep seas compare across different regions in the Arctic Ocean?
Even if stations at the bottom of the slope were at least an order of magnitude deeper compared to the Lomonosov Ridge, we found higher oxygen uptake rates as well as higher concentrations of chlorophyll pigments, total amount of organic carbon (TOC) and microbial cell numbers (Table 2)
Summary
A major function of benthic ecosystems is the turnover of organic matter, mainly driven by microorganisms (Glud et al, 1998), which we refer to as the “benthic remineralization” function (Link et al, 2013b) This remineralization is crucial for the replenishment of nutrients to the water column (Canfield, 1993) and determines the amount of buried carbon in the seafloor (Archer and Maier-Reimer, 1994). 15–50% (Glud, 2008) of the pelagic primary production reach the seafloor on the shelf, while the deep sea only receives about 1 – 5% of the exported organic matter (Klages et al, 2004; Smith et al, 2008) From these 1 – 5%, only 1 – 2% are remineralized within a few days, while the remaining proportion is turned over very slowly and gets buried in the seabed (Turley and Lochte, 1990; Jørgensen and Boetius, 2007)
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