Recent FerryBox observations reveal a strong increase in surface seawater pCO2 in the North Sea

Abstract

<p>Surface seawater carbon dioxide partial pressure (pCO<sub>2</sub>) in the North Sea, a large temperate shelf sea, was measured between 2014 and 2018 using FerryBox-integrated membrane sensors on ships of opportunity. The use of commercial vessels ensured a high spatio-temporal resolution, with data available year-round in areas belonging to all the stratification regime types found in the North Sea. Average annual cycles revealed a dominant biological control on pCO<sub>2</sub> variability, with thermal effects modulating its amplitude. In the regions of freshwater influence, the biogeochemical characteristics of the riverine end-member also influenced the pCO<sub>2</sub> measured near shore. Deseasonalized winter trends of seawater pCO<sub>2</sub> were positive (ranging from 4.4 ± 2.0 µatm yr<sup>-1</sup> to 8.4 ± 2.9 µatm yr<sup>-1</sup> depending on the region), while the trends calculated including all deseasonalized monthly averages were even higher (ranging from 9.7 ± 2.8 µatm yr<sup>-1</sup> to 12.2 ± 1.4 µatm yr<sup>-1</sup>). All these trends were stronger than the atmospheric pCO<sub>2</sub> trend. Consequently, during our study period, the southern North Sea became a stronger source and the northern North Sea became a weaker sink for atmospheric carbon with implications for the Northwestern European Shelf carbon uptake capacity.</p>