Nitrogen and carbon cycling in the North Sea and exchange with the North Atlantic—A model study, Part II: Carbon budget and fluxes

The 3-d coupled physical–biogeochemical model ECOHAM (version 3) was applied to the Northwest-European Shelf (47°41′–63°53′N, 15°5′W–13°55′E) for the years 1993–1996. Carbon fluxes were calculated for the years 1995 and 1996 for the inner shelf region, the North Sea (511,725km2). This period was chosen because it corresponds to a shift from a very high winter-time North Atlantic Oscillation Index (NAOI) in 1994/1995, to an extremely low one in 1995/1996, with consequences for the North Sea physics and biogeochemistry.

During the first half of 1996, the observed mean SST was about 1°C lower than in 1995; in the southern part of the North Sea the difference was even larger (up to 3°C). Due to a different wind regime, the normally prevailing anti-clockwise circulation, as found in winter 1995, was replaced by more complicated circulation patterns in winter 1996.

Decreased precipitation over the drainage area of the continental rivers led to a reduction in the total (inorganic and organic) riverine carbon load to the North Sea from 476GmolCyr−1 in 1995 to 340GmolCyr−1 in 1996. In addition, the North Sea took up 503GmolCyr−1 of CO2 from the atmosphere. According to our calculations, the North Sea was a sink for atmospheric CO2, at a rate of 0.98molCm−2yr−1, for both years.

The North Sea is divided into two sub-systems: the shallow southern North Sea (SNS; 190,765km2) and the deeper northern North Sea (NNS; 320,960km2). According to our findings the SNS is a net-autotrophic system (net ecosystem production NEP>0) but released CO2 to the atmosphere: 159GmolCyr−1 in 1995 and 59GmolCyr−1 in 1996.

There, the temperature-driven release of CO2 outcompetes the biological CO2 drawdown. In the NNS, where respiratory processes prevail (NEP<0), 662 and 562GmolCyr−1 were taken up from the atmosphere in 1995 and 1996, respectively. Stratification separates the productive, upper layer from the deeper layers of the water column where respiration/remineralization takes place.

Duration and stability of the stratification are determined by the meteorological conditions, in relation to the NAO. Our results suggest that this mechanism controlling the nutrient supply to the upper layer in the northern and central North Sea has a larger impact on the carbon fluxes than changes in lateral transport due to NAOI variations.

The North Sea as a whole imports organic carbon and exports inorganic carbon across the outer boundaries, and was found to be net-heterotrophic, more markedly in 1996 than in 1995.