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 crui ses in the region of the Goban Spur and La Chapelle Bank between 46 and 51 degreesN, in all seasons of the year, between 1993-1995. Over this period n o change was detectable in the structure of the water masses below the deep winter mixed layer. The N: P (dissolved nitrate-to-phosphate) ratio change d from 16 at 100-m depth to less than 15 at 3300-m depth. At intermediate d epths nutrient and oxygen data indicate the presence of Mediterranean Outfl ow water overlying Labrador Sea Water at its most eastern extension. Estima ted maximum levels of production in the spring bloom are the total N-limite d new primary production equivalent between 24 and 41 gCm(-2), the equivale nt maximum diatom production is 11 gC m(-2). Measurements during the spring bloom suggest a conversion factor of 1 muM nitrate to 1 mu gl(-1) chloroph yll, at the shelf break, which is consistent with other recent measurements in European shelf seawaters. Sediment trap data suggest that 80% (5.4 gm(- 2)) 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(-2) ab ove 300-m depth. Measurements of dissolved organic carbon (DOC) in Septembe r 1994 show an enrichment of 7 muM-C above the seasonal thermocline relativ e to the winter values (52 +/- 4 muM). In winter dissolved organic nitrogen represents 40% of the pool of total dissolved nitrogen. There is no consis tent evidence of an increase in the concentration of DON during summer. Mea surements of nitrate in surface waters in January 1994 show that concentrat ions off-shelf vary with the temperature of the water and are related to th e depth of winter mixing. Mixing in surface waters is discontinuous at the shelf break, demonstrating the degree to which exchange across the shelf br eak is limited even in winter. OMEX winter measurements of nitrate concentr ations can be used to estimate the flow of water across the shelf break tha t 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(6)m(3)s(-1)), which is simi lar to an earlier estimate of a total flow of 1.7 Sv based on salt budgets. (C) 2001 Elsevier Science Ltd. All rights reserved.