Interannual variability of carbon fluxes in the North Sea from 1970 to 2006 – Competing effects of abiotic and biotic drivers on the gas-exchange of CO2

The three-dimensional biogeochemical model ECOHAM was applied to the Northwest European Continental Shelf (NECS) (47° 41′ – 63° 53′ N, 15° 5′ W – 13° 55′ E) for the years 1970–2006. The development of annual carbon fluxes was analysed for the North Sea as the inner shelf region. We divided the North Sea into several regions, the northern North Sea, the southern North Sea, the German Bight and the Southern Bight for a more detailed analysis. To separate the effect of physical and biological processes a second simulation without biology was performed. The results of our method for calculating the biological pCO2 were in good agreement with the biological pCO2 calculated after the method of Takahashi et al. (2002). While in the standard run the North Sea acted as sink for atmospheric CO2, in the run without biology the North Sea was a continuous source for atmospheric CO2.The main drivers of the air-sea flux variability were identified as being temperature, net ecosystem production and pH. The eutrophication due to high riverine nutrient inputs during the 1980s had no significant effect on the air-sea flux of CO2 because in contrast to net primary production, net ecosystem production did not respond to the period of higher phosphate input. The increase of sea surface temperature of 0.027 °C yr−1 over the simulation period and the pH decline of 0.002 yr−1 led to a decline of the uptake of atmospheric CO2 by the North Sea of about 30% in the last decade of the simulation period. A special feature occurred in the year 1996, where a cold sea surface temperature anomaly led to an additional (physical) uptake of atmospheric CO2 and corresponded with a low primary and net ecosystem production, which on the other hand led to less biologically induced uptake of CO2.Our results indicate an ongoing decline of the uptake capacity for atmospheric carbon dioxide of the North Sea for future scenarios.

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► North Sea is a sink for atmospheric CO2 from 1970 to 2006.
► Uptake of atmospheric CO2 is declining over last decade.
► Decline is due to pH and SST.
► Biological air-sea flux controlled by NEP.
► No effect of eutrophication on air-sea flux.