Nitrogen and carbon cycling in the North Sea and exchange with the North Atlantic—A model study. Part I. Nitrogen budget and fluxes

The three-dimensional biogeochemical model ECOHAM was applied to the Northwest European Shelf (47°41′–63°53′N, 15°5′W–13°55′E) for the years 1993–1996. Nitrogen budgets were calculated for the years 1995 and 1996 for the inner shelf region, the North Sea (511,725 km2). Simulated temperatures as well as nitrate, oxygen, and chlorophyll concentrations are compared with observations.The mid-1990s were chosen because they exhibit a shift from a very high North Atlantic Oscillation Index (NAOI) in winter, 1994/1995, to an extremely low one in winter, 1995/1996, with consequences for the North Sea system: During the first-half of 1996, the observed mean sea surface temperature (SST) was about 1 °C lower than in 1995; in the southern part of the North Sea the difference was even larger. These observations could be reproduced by the model. Due to a different wind regime, the normally prevailing anti-clockwise circulation, also found in winter 1995, was replaced by more complicated patterns in winter 1996. Decreased precipitation over the drainage area of the continental rivers led to a reduction in the total riverine nitrogen loads to the North Sea from 76 Gmol N yr−1 in 1995 to 52 Gmol N yr−1 in 1996. In addition to these high loadings (additionally, atmospheric deposition supplied 27 Gmol N yr−1 of inorganic nitrogen), the system imported from the adjacent seas a net amount of 28 and 13 Gmol yr−1 of TN, in 1995 and 1996, respectively. As the main sink for nitrogen, we identified the coupled benthic nitrification/denitrification, which released 118 and 119 Gmol N yr−1 of molecular nitrogen into the atmosphere in the two years, respectively. This would account for the removal of the total amount of terrigenous (riverine and atmospheric) nitrogen inputs by denitrification in these two years. Additionally, allochthonous organic nitrogen, imported across the northern boundary, was transformed to inorganic nitrogen, part of which was also denitrified, the rest being exported as DIN to the North Atlantic.