Supply and demand of nutrients and dissolved organic matter at and across the NW European shelf break in relation to hydrography and biogeochemical activity

As part of the OMEX I project, nutrient determinations were made on 17 cruises in the region of the Goban Spur and La Chapelle Bank between 46 and 51°N, in all seasons of the year, between 1993-1995. Over this period no change was detectable in the structure of the water masses below the deep winter mixed layer. The N:P (dissolved nitrate-to-phosphate) ratio changed from 16 at 100-m depth to less than 15 at 3300-m depth. At intermediate depths nutrient and oxygen data indicate the presence of Mediterranean Outflow water overlying Labrador Sea Water at its most eastern extension. Estimated maximum levels of production in the spring bloom are the total N-limited new primary production equivalent between 24 and 41 gC m<sup>-2</sup>, the equivalent maximum diatom production is 11 gC m<sup>-2</sup>. Measurements during the spring bloom suggest a conversion factor of 1 muM nitrate to 1 µg l<sup>-1</sup> chlorophyll, at the shelf break, which is consistent with other recent measurements in European shelf seawaters. Sediment trap data suggest that 80% (5.4 gm<sup>-2</sup>) of the opal produced in the spring bloom dissolved before reaching the sediment trap at 600 m. A comparison of the winter and summer profiles for dissolved silicon suggests a similar dissolution of 9 +/- 3 g opal m<sup>-2</sup> above 300-m depth. Measurements of dissolved organic carbon (DOC) in September 1994 show an enrichment of 7 µM-C above the seasonal thermocline relative to the winter values (52 +/- 4 µM). In winter dissolved organic nitrogen represents 40% of the pool of total dissolved nitrogen. There is no consistent evidence of an increase in the concentration of DON during summer. Measurements of nitrate in surface waters in January 1994 show that concentrations off-shelf vary with the temperature of the water and are related to the depth of winter mixing. Mixing in surface waters is discontinuous at the shelf break, demonstrating the degree to which exchange across the shelf break is limited even in winter. OMEX winter measurements of nitrate concentrations can be used to estimate the flow of water across the shelf break that would be required to maintain the nitrogen balance in the North Sea at a steady state. The estimate is 0.6 Sv (1 Sv = 10<sup>6</sup>m<sup>3</sup>s<sup>-1</sup>), which is similar to an earlier estimate of a total flow of 1.7 Sv based on salt budgets.

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