Elemental composition of dissolved organic matter and bacterioplankton production in the Faroe-Shetland Channel (North Atlantic)

The elemental composition of dissolved organic matter (DOM) as well as concentrations of dissolved amino acids and bacterial production was determined in the Faroe-Shetland Channel (FSC) of the North Atlantic. The FSC is characterized by a complex hydrography with near-surface North Atlantic waters and deeper water masses originating from the Arctic basin. This complex hydrography results in major deviations in the elemental composition of DOM from the commonly observed depth profiles. Total organic carbon (TOC) declined from surface water masses to the low mesopelagic waters (>600 m depth) only by approx. 16% while DON did not exhibit any trend with depth. In contrast, dissolved organic phosphorus (DOP) decreased from the surface (0.2 μM P) to the lower mesopelagic zone by about 50%. The molar C:N ratio of the DOM was approx. 13 in the surface water masses and 7.5 in the Faroe-Shetland Channel Bottom Water (FSCBW), indicating the presence of N-rich DOM in these deep water masses while the N:P ratio of the DOM pool increased from 60:1 in the Atlantic surface waters to 117:1 in the FSCBW. Also, the amino acid yield (in percent of amino acid-C and -N to the TOC and DON pool, respectively) did not show any trend with depth, which might indicate the presence of relatively young, unaltered DOM in these deeper water masses. Moreover, the % contribution of DON and DOP to the total dissolved nitrogen (TDN) and phosphorus (TDP) (33% DON of TDN, 8.9% DOP of TDP) was higher in the FSCBW than in the global ocean at these depths. Thus, we conclude that Arctic water masses export bioreactive DOM into the mesopelagic North Atlantic.