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
Dj. Hydes et al., 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, MAR CHEM, 68(1-2), 1999, pp. 117-131
Data from the NERC North Sea Programme (1988-1989) and the OMEX project (19
93-1997), and subsequent numerical modelling based on that data, enables bu
dgets 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 wat
ers. The deficit in the load of nitrate-N relative to that which would be p
resent assuming conservative mixing of river and ocean waters is 580 ktonne
s. This is probably due to denitrification. Considering the flushing rate i
t is equivalent to a maximum rate of loss of 0.7 mnol N m(-2) day(-1). A lo
ss of this magnitude confirms the suggestion of Seitzinger and Giblin (Seit
zinger, S.P., Giblin, A.E., 1996. Biogeochemistry 35, 235-260,) that the re
latively high productivity in shelf seas such as the North Sea fuels signif
icant 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 w
ater onto the shelf equivalent to 0.6 Sv (1 Sv = 10(6) m(3) s(-1)) containi
ng 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 dissol
ved inorganic nitrogen (DIN) (and silicon) contributing to the change in di
ssolved nitrogen load between November 1988 and January 1989 (526 ktonnes N
, 242 ktonnes Si) shows that between November and January the principal sou
rce of dissolved nitrate over the southern North Sea is probably from recyc
ling 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 atmosp
here approach 50% of the observed change. The ratio of annual primary produ
ction in different areas of the North Sea to the estimate of available nitr
ogen 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 maint
ained by both the total amount of nitrate that is supplied to the system an
d recycling in its shallow waters. Both the ratio and the efficiency of-rec
ycling are lower off the UK coast where light limitation of production is m
ore likely. (C) 1999 Elsevier Science B.V. All rights reserved.