Distribution and C / N stoichiometry of dissolved organic matter in the North Sea in summer 2011–12

We present the distribution and C : N stoichiometry of dissolved organic matter (DOM) in the North Sea in 2 summers (August 2011 and August 2012), with supporting data from the intervening winter (January 2012). This data demonstrates local variability superimposed on a general pattern of decreasing DOM with increasing salinity, suggesting strong control over broad scale concentrations by mixing between riverine sources and the open North Atlantic. We observe differences in DOC and DON concentrations and gradients between the two summers, leading to an estimated 10–20 Tg difference in the DOM carbon inventory between the 2 years, which is of the same order as the annual uptake of atmospheric CO2 by the North Sea system, and thus significant for the carbon budget of the North Sea. Differences were particularly pronounced in the bottom layer of the seasonally stratifying Northern North Sea, with higher DOC and lower DON in 2011 and lower C : N ratio and more moderate concentrations of DOC and DON in 2012. Using other data we consider the extent to which these differences in the concentrations and C : N ratio of DOM could be due to differences in the biogeochemistry or physical circulation in the 2 years, or a combination of both. We discuss the implications of these observations for the shelf sea carbon pump and the export of carbon rich organic matter off the shelf.