| Summary: | For their size, shelf seas play a disproportionately large role in the oceanic carbon pump. While accounting for only 7% of the surface ocean, the amount of carbon exported from shelf seas contributes to between 20 and 50% of total oceanic CO2 storage (Tsunogai et al. 1999). In terms of their socio economic importance, shelf seas support ~ 90% of global fish catches (Pauly et al. 2002), and future projections estimate that up to 4.2 billion people will live within 200 km of the coast by 2030 (Kummu et al. 2016). Shelf seas are the interface between land and ocean and high nutrient inputs and intense physical energy provided mainly by tidal mixing, help to maintain high biological activity on the shelf. Indeed, rates of primary production are up to 3 times greater in shelf seas than in the open ocean (Simpson and Sharples 2012). Through a process known as the continental shelf pump, over 40% of particulate organic matter produced on-shelf is exported to the adjacent ocean (Muller-Karger et al. 2005). The role of dissolved organic matter (DOM) in this pump, in particular, dissolved organic carbon (DOC), is less well understand, despite DOC concentrations in the ocean being 50 times more than carbon in the particulate pool (Eglinton and Repeta 2006), and the amount of carbon in the DOC pool being similar to that of carbon as CO2 in the atmosphere (Hansell and Carlson 1998). Here, to add to the growing body of work on the distribution of DOM in shelf seas, the main goals of this thesis were to (a) map the distribution of DOC and dissolved organic nitrogen (DON) across 5 shelf regions across the Northwest European shelf, and the adjacent North Atlantic; (b) to assess the distribution of DOC and DON in different regions over a seasonal cycle to see if they followed a typical seasonal cycle of production, consumption and loss; (c) to characterise the source of DOM in the Celtic Sea and determine how much of the DOC pool was of terrestrial origin; (d) to assess multi-year trends in DOC and DON in the North Sea specifically and finally (e) for the first time, estimate DOC fluxes between the Celtic Sea and North Atlantic, and compare them with DOC fluxes across the Malin-Hebrides Shelf. DOC and DON distributions across the Northwest European seas were in accordance with global trends, and concentrations decreased with increasing distance from land, highlighting the influence of terrestrially-derived DOM in this region. However, there was large variability within and between regions, highlighting the importance of local controls on DOM production and distribution. In the North Sea, multi-year differences in DOC and DON concentrations showed an overall decline between 2011 and 2015 alongside declines in concentrations of inorganic nutrients, indicating a combination of changing nutrient regimes and variability in the strength of exchange with the North Atlantic. Finally, annual net and springtime surface fluxes from the Celtic Sea to the North Atlantic were significant and higher compared to the Malin-Hebrides Shelf. The estimates were large but within global ranges (Barron and Duarte 2015).
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