Combined neutral sugars as indicators of the diagenetic state of dissolved organic matter in the Arctic Ocean
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Dissolved organic matter (DOM) was sampled by tangential flow ultrafiltration from different basins in the central Arctic Ocean, the Greenland, Iceland and Norwegian (GIN) Seas, and the estuaries of the Ob and Yenisei Rivers. Dissolved organic carbon (DOC) concentrations ranged from more than 500 M C in the rivers to about 50 M C in the deep basins. Arctic Mediterranean Sea (AMS) surface water DOC concentrations ranged from 66 to 137 M C. High concentrations in surface waters were due mainly to river discharge of terrigenous DOC, while elevated concentrations in AMS deep water likely reflect downwelling of DOM in this system. Total hydrolyzable neutral sugar concentrations were 7-fold higher In ultrafiltered DOM (UDOM) from estuaries (1200 nM) than in Arctic ocean surface waters (160 nM), but carbon-normalized neutral sugar yields (% OC) were lower in the estuaries indicating that marine DOM sources were relatively rich in neutral sugars. Depth profiles of neutral sugar concentrations and carbon-normalized yields indicated that carbohydrates were produced and consumed largely in surface waters of the AMS. These observations were consistent with rapid bacterial degradation of neutral sugars during decomposition experiments. The molecular composition of neutral sugars was very similar among AMS surface samples and showed only minimal differences between terrestrial and marine UDOM, indicating that the neutral sugar composition is not useful for distinguishing these sources. It appears diagenetic processes rather than sources are primarily responsible for determining neutral sugar compositions in UDOM. Neutral sugar compositions in all of the UDOM samples analyzed in this study were indicative of heteropolysaccharides. The strong relationship between neutral sugars and the diagenetic state of DOM was best represented by neutral sugar yields (% OC), which were used as a quantitative indicator for the amount of labile DOM in the AMS. Based on the low average neutral sugar yield (5.5% OC) in UDOM from AMS surface waters, we estimated that only 2.2% of the DOC was of labile nature. This value was lower than for surface waters of the Ross Sea and Atlantic and Pacific Oceans suggesting DOM is less bioavailable and the microbial loop is less active in AMS surface waters. 2003 Elsevier Science Ltd. All rights reserved.
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