The balance of supply of nutrients and demands of biological production and denitrification in a temperate latitude shelf sea - a treatment of the southern North Sea as an extended estuary

Data from the NERC North Sea Programme (1988-1989) and the OMEX project (1993-1997), and subsequent numerical modelling based on that data, enables budgets to be drawn up which allow the scales of a number of processes which are significant in shelf seas to be quantified, In winter the N:P ratio is lower in many areas of the southern North Sea than in any of the source waters. The deficit in the load of nitrate-N relative to that which would be present assuming conservative mixing of river and ocean waters is 580 ktonnes. This is probably due to denitrification. Considering the flushing rate it is equivalent to a maximum rate of loss of 0.7 mnol N m(-2) day(-1). A loss of this magnitude confirms the suggestion of Seitzinger and Giblin (Seitzinger, S.P., Giblin, A.E., 1996. Biogeochemistry 35, 235-260,) that the relatively high productivity in shelf seas such as the North Sea fuels significant levels of denitrification. The wide area of the shelf involved makes these areas net sinks for nitrogen in global budgets. A net flow of ocean water onto the shelf equivalent to 0.6 Sv (1 Sv = 10(6) m(3) s(-1)) containing nitrate at the winter concentration of 7.5 mu M is required to maintain the nitrogen balance in the North Sea. An analysis of the sources of dissolved inorganic nitrogen (DIN) (and silicon) contributing to the change in dissolved nitrogen load between November 1988 and January 1989 (526 ktonnes N, 242 ktonnes Si) shows that between November and January the principal source of dissolved nitrate over the southern North Sea is probably from recycling of detrital or dissolved organic nitrogen (DON) already present in the system (383 ktonnes N) and not from direct inputs of new nitrogen. In one region, the German Eight, inputs of new nitrogen from rivers and the atmosphere approach 50% of the observed change. The ratio of annual primary production in different areas of the North Sea to the estimate of available nitrogen varies between five off the Dutch and German coasts and two off the UK coast. This indicates that the high productivity of the North Sea is maintained by both the total amount of nitrate that is supplied to the system and recycling in its shallow waters. Both the ratio and the efficiency of-recycling are lower off the UK coast where light limitation of production is more likely. (C) 1999 Elsevier Science B.V. All rights reserved.