Sinks and sources of CO2 in coastal seas: the North Sea

Alkalinity, ∑ 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 μm, the CO₂ flux entering the areas with a PCO₂ below that of ambient air (346 ppmv in 1986) amounted to 3.1 × 109 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 × 109 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 × 109 moles d^-1 7.5 × 10⁴ moles km^-2 d^-1 or 24 × 10³ t C d^-1 in summer 1986. It is estimated that a total of 4.3 × 10⁶ t C enters the North Sea in the warm season. This size is comparable to the additional CCO₂ uptake by phytoplankton induced by the anthropogenic input of nutrients which amounts to 5—6 × 10⁶ t C a^-1.