Variability of the partial pressure of CO2 on diel to annual time scales in the Northwestern Mediterranean Sea

Continuous underway measurements of the partial pressure of CO2 in surface water were performed on a monthly basis from February 1998 to February 2000 along a transect between Nice and the Dyfamed site located in the open central zone of the Ligurian–Provençal basin of the Northwestern Mediterranean. Hourly measurements of pCO2 were also performed with Carioca buoys moored at the Dyfamed site from February to May 1999 and in October 1999. The shipboard measurements allowed the spatial variability of pCO2 from the coastal area to the central part of the basin over diel and annual time scales to be evaluated. The variability over scales of a few hours or a few kilometers was highest in winter in the central part of the basin, when the surface temperature was close to 13 °C. During this period, the pCO2 variations were correlated to salinity and variations in pCO2 of up to 70 μatm over a period of 16 h were observed. This winter variability can be explained by vertical mixing. The annual variations in pCO2 range from 300 to 420 μatm at the Dyfamed site and from 315 to 450 μatm in the Ligurian current. They are mostly due to temperature variations (13 to 26 °C), but the effect of temperature is counterbalanced mainly by biological production in spring and by vertical mixing in autumn. The results obtained at the Dyfamed site confirm those obtained by Hood and Merlivat [J. Mar. Res. 59 (2001) 113] with a Carioca buoy, but show interannual variability. Over short time scales, the variations of pCO2 normalized to 13 °C are linked to the wind speed variations or to the chlorophyll fluorescence during periods of high biological productivity. As well in the central part of the basin as in the coastal Ligurian current, this region of the Mediterranean Sea is a medium sink for the atmospheric CO2.