Abstract
Abstract. We present the distribution and C:N stoichiometry of dissolved organic matter (DOM) in the North Sea in two summers (August 2011 and August 2012), with supporting data from the intervening winter (January 2012). These data demonstrate local variability superimposed on a general pattern of decreasing DOM with increasing distance from land, suggesting concentrations of DOM are controlled on large spatial scales by mixing between the open North Atlantic and either riverine sources or high DOM productivity in nearshore coastal waters driven by riverine nutrient discharge. Given the large size and long residence time of water in the North Sea, we find concentrations are commonly modified from simple conservative mixing between two endmembers. We observe differences in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations and land–ocean gradients between the two summers, leading to an estimated 10–20 Tg difference in the DOC inventory between the two years, which is of the same order of magnitude as the annual uptake of atmospheric CO2 by the North Sea system, and thus significant for the carbon budget of the North Sea. This difference is not consistent with additional terrestrial loading and is more likely to be due to balancing of mixing and in situ production and loss processes across the North Sea. Differences were particularly pronounced in the bottom layer of the seasonally stratifying northern North Sea, with higher DOC and C:N ratio in 2011 than 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 changes in the biogeochemistry or physical circulation in the two years, or a combination of both. The evidence we have is consistent with a flushing event in winter 2011/12 exchanging DOM-rich, high C:N shelf waters, which may have accumulated over more than 1 year, with deep North Atlantic waters with lower DOC and marginally higher DON. We discuss the implications of these observations for the shelf sea carbon pump and the export of carbon-rich organic matter off the shelf and hypothesise that intermittent flushing of temperate shelf systems may be a key mechanism in the maintenance of the continental shelf pump, via the accumulation and subsequent export of carbon-rich DOM.
Highlights
Coastal and shelf seas are generally more productive than the open ocean (Jickells, 1998; Simpson and Sharples, 2012), and through various processes may be disproportionately important for the drawdown of atmospheric carbon to the deep ocean (Bauer et al, 2013; Regnier et al, 2013; Thomas et al, 2005b; Tsunogai et al, 1999)
The data presented above demonstrate that mixing between high-dissolved organic matter (DOM), lower salinity coastal waters and low-DOM, higher-salinity ocean waters is an important component of DOM dynamics in the North Sea
We can consider the data in two distinct subsets based on salinity range: all salinities of less than 33.5 observed during this study were associated with surface waters in the Norwegian Coastal Current (NCC), which is strongly influenced by Norwegian rivers and Baltic outflow (Winther and Johannessen, 2006); salinities greater than 33.5 are all outside the NCC and can be considered to synthesise the high-salinity portion of the river–ocean continuum across the wider North Sea
Summary
Coastal and shelf seas are generally more productive than the open ocean (Jickells, 1998; Simpson and Sharples, 2012), and through various processes may be disproportionately important for the drawdown of atmospheric carbon to the deep ocean (Bauer et al, 2013; Regnier et al, 2013; Thomas et al, 2005b; Tsunogai et al, 1999). Our understanding of the mechanisms of carbon pump processes on the shelf and their relative importance is limited by observational data, the complexities of shelf circulation, and interannual variability of both biological processes and physical drivers This is the case in shelf sea systems such as the North Sea, which has a complex physical circulation, involving large water exchange with surrounding seas and ocean, and strong anthropogenic forcing from surrounding land masses, especially via substantial river discharge It intentionally does not attempt to elucidate distinct sources or types of DOM, or determine process rates, given only prima facie evidence
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