The North Sea: source or sink for nitrogen and phosphorus to the Atlantic Ocean?

Annual nitrogen and phosphorus budgets for the whole North Sea taking into account the most recent data available were established. The area considered has a total surface of approximately 700,000 km2 and corresponds to the definition by OSPARCOM (Oslo and Paris Commission) with the exclusion of the Skagerrak and Kattegat areas. Input and output fluxes were determined at the marine, atmospheric, sediment and continental boundaries, and riverine inputs based on river flows and nutrient concentrations at the river–estuary interface were corrected for possible estuarine retention. The results showed that the North Sea is an extremely complex system subjected to large inter-annual variability of marine water circulation and freshwater land run-off. Consequently, resulting total N (TN) and P (TP) fluxes are extremely variable from 1 year to another and this has an important influence on the budget of these elements. Total inputs to the North Sea are 8870 ± 4860 kT N year−1 and 494 ± 279 kT P year−1. Denitrification is responsible for the loss of 23 ± 7% of the TN inputs while sediment burial is responsible for the retention of only of 2 ± 2% of the TP input. For TN, due to the large variability on marine and estuarine fluxes, and to the uncertainty related to the denitrification rate, it was concluded that the North Sea could either be a source (1930 kT N year−1) or a sink (1700 kT N year−1) for the waters of the North Atlantic Ocean. For TP it was concluded that the North Sea is mostly a source (−4 to 52 kT P year−1) for the waters of the North Atlantic Ocean.