| Summary: | Degradation of marine organic matter under anoxic conditions involves microbial communities working in concert to remineralize complex substrates to CO2. In order to investigate the coupling between the initial and terminal steps of this sequence in permanently cold sediments, rates of extracellular enzymatic hydrolysis and sulfate reduction were measured in parallel cores collected from 5 fjords on the west and northwest coast of Svalbard, in the high Arctic. Inventories of total dissolved carbohydrates were also measured in order to evaluate their potential role in carbon turnover. Polysaccharide hydrolysis rates exhibited substrate-related and, to a lesser extent, depth-related differences (p <0.0001); laminarin hydrolysis was consistently most rapid at nearly all depths and sites, and fucoidan hydrolysis was least rapid. Although there was a high degree of variability in parallel cores, sulfate reduction rates also exhibited statistically significant depth-and station-related differences. A comparison with data from previous investigations in Svalbard sediments suggests that this variability is linked to substrate availability rather than to organism distribution. Total dissolved carbohydrate concentrations were comparable to those measured in more temperate sediments, and likely comprise a considerable fraction of porewater dissolved organic carbon. A comparison of dissolved carbohydrate inventories with hydrolysis and sulfate reduction rates suggests that the turnover of carbon through the dissolved pool occurs quite rapidly, on the order of a few days to weeks. The transformation of particulate to dissolved organic matter must also be sufficiently rapid to maintain the measured rates of terminal remineralization.
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