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.
Terrestrial dissolved organic matter distribution in the North Sea
