SINKS AND SOURCES OF CO2 IN COASTAL SEAS - THE NORTH-SEA

Alkalinity, SIGMA CO2 and pH were measured and the respective PCO2 was calculated for over 1000 water samples from the North Sea. Distribution of these parameters in the surface waters in May-June, 1986, shows that three principle water masses exist in the North Sea: North Atlantic water, Skagerrak water and German Bight water. The carbonate parameters of the German Bight water are indicative of an intense plankton bloom which reduced the PCO2 to values as low as 88 ppmv. Assuming an average thickness of the stagnant boundary layer of 40-mu-m, the CO2 flux entering the areas with a PCO2 below that of ambient air (346 ppmv in 1986) amounted to 3.1 x 10(9) moles CO2 d-1. However, at the same time, areas with a PCO2 up to 518 ppmv and therefore higher than the atmospheric PCO2 co-existed in the central North Sea and near the British coast. These areas formed a source for CO2 to the atmosphere in the size of 1.1 x 10(9) moles CO2 d-1. Taken together, the North Sea had an area-averaged PCO2 of 323 ppmv and formed a net sink for CO2 in the range of 2.0 x 10(9) moles d-1 or 7.5 x 10(4) moles km-2 d-1 or 24 x 10(3) t C d-1 in summer 1986. It is estimated that a total of 4.3 x 10(6) t C enters the North Sea in the warm season. This size is comparable to the additional CO2 uptake by phytoplankton induced by the anthropogenic input of nutrients which amounts to 5-6 x 10(6) t C a-1.