Molecular evidence for rapid dissolved organic matter turnover in Arctic fjords

Abstract

Dissolved organic matter (DOM) in the ocean comprises one of the largest active carbon pools on earth. Deep water formation at high latitudes carries DOM from the active surface layers to the deep ocean. However, information on sources and fate of DOM in the Arctic Ocean is limited. To reveal the relevance of autochthonous DOM production and transformation in Arctic fjord systems to the global deep ocean DOM pool, we performed a comprehensive study on the molecular composition of DOM and the composition of the associated microbial communities in four selected fjords of Svalbard. At various water depths, a total of 34 samples were taken in fall 2010 for the determination of bulk concentrations of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN), for the molecular characterization of solid-phase extractable DOM as well as for microbial community fingerprinting.While TDN concentration and the composition of the microbial community showed a clear distinction between surface and bottom water samples, bulk DOC (~60μmol C L−1) and dissolved black carbon (~1.8% of DOC) as a marker for terrestrial input were uniformly distributed. In-depth molecular-level analyses of the DOM composition using ultrahigh resolution mass spectrometry via Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) revealed insignificant variation of the relative abundance of 11630 molecular masses that were detected in the water samples.From these findings we conclude that DOM produced during the spring/summer bloom is rapidly transformed within the short, but productive warm season by the specialized resident microbial community. Thus, in fall the DOM pool mainly consists of semi-refractory and refractory material, most of which has been introduced from Arctic Ocean water inflow. Assuming that our findings are representative for high latitude marine systems in general, the contribution of autochthonous seasonal DOC production in plankton bloom situations to the DOC pool in regions of deep water formation might be marginal.