Maria Ll. Calleja

Control of air‐sea CO2 disequilibria in the subtropical NE Atlantic by planktonic metabolism under the ocean skin

The air-sea CO 2 gradient at the subtropical NE Atlantic was strongly dependent on the metabolism of the planktonic community within the top cms, but independent of that of the communities deeper in the water column. Gross primary production (GPP) and community respiration (R) of the planktonic community within the top cms exceeded those of the communities deeper in the water column by >10-fold and >7 fold, respectively. Net autotrophic metabolism (GPP > R) at the top cms of the water column in some stations drove CO 2 uptake by creating a CO 2 deficit at the ocean surface, while net heterotrophic metabolism (GPP < R) at the top cms of the water column in other stations resulted in strong CO 2 supersaturation, driving CO 2 emissions. These results suggest a strong control of the air-sea pCO 2 anomaly by intense biological processes.

Evidence for surface organic matter modulation of air-sea CO 2 gas exchange

Air-sea CO2 exchange depends on the air-sea CO2 gradient and the gas transfer velocity (k), computed as a function of wind speed. Large discrepancies among re- lationships predicting k from wind suggest that other pro- cesses also contribute significantly to modulate CO 2 ex- change. Here we report, on the basis of the relationship be- tween the measured gas transfer velocity and the organic car- bon concentration at the ocean surface, a significant role of surface organic matter in suppressing air-sea gas exchange, at low and intermediate winds, in the open ocean, confirming previous observations. The potential role of total surface or- ganic matter concentration (TOC) on gas transfer velocity (k) was evaluated by direct measurements of air-sea CO2 fluxes at different wind speeds and locations in the open ocean. Ac- cording to the results obtained, high surface organic matter contents may lead to lower air-sea CO2 fluxes, for a given air-sea CO2 partial pressure gradient and wind speed below 5 m s 1 , compared to that observed at low organic matter contents. We found the bias in calculated gas fluxes resulting from neglecting TOC to co-vary geographically and season- ally with marine productivity. These results support previ- ous evidences that consideration of the role of organic matter in modulating air-sea CO2 exchange may improve flux esti- mates and help avoid possible bias associated to variability in surface organic concentration across the ocean.