New primary production in northwest European shelf seas, 1960–2003

Spatial and temporal patterns from 1960 to 2003 in annual potential new primary production (PNP) of the NW European shelf seas were derived from general additive models of nitrate concentrations and from data on riverine and atmospheric fluxes of oxidized nitrogen. Average PNP was highest in the seasonally stratified outer shelf regions (>70 gC m-2 yr-1), where the proportion of PNP accounted for by vertical fluxes from deep water (>65%) was correlated with the North Atlantic Oscillation (NAO) index. PNP was lowest in the central North Sea (~30 gC m-2 yr-1) and in the southern North Sea was correlated with river inputs that accounted for 24% of the annual total (average ~50 gC m-2 yr-1). Atmospheric deposition accounted for ~3% of annual PNP region-wide, but in the northern North Sea this was higher than the contribution from rivers. Tidal fronts are traditionally considered to be highly productive zones, but we find them to have characteristically low PNP and conclude that they must be loci of high recycled production. The results indicate an exceptional flux of nitrate-rich ocean water onto the shelf in the early 1990s, which resulted in a pulse of PNP coincident with a well-documented 'regime shift' in the pelagic food web. North Sea-wide, long-term average PNP was approximately equal to production by all higher trophic levels combined, though trophic propagation of inter-annual variations was weakly defined. Nevertheless, there is a case for proposing that harvesting in areas and periods of low PNP should be managed more conservatively to minimize the risk of detrimental effects on the food web