Evaluation of the carbonate system and air-sea CO2 fluxes in coastal waters of the Macaronesia archipelagos.

The CO₂ system, anthropogenic carbon (C_ant) inventory and air-sea CO₂ fluxes (FCO₂) were analysed in the archipelagic waters of the Macaronesia. The (sub)surface data were collected during POS533 (February and March, 2019) in coastal and open-ocean areas leeward of Cape Verde, Canary Islands and Madeira. The observed variability was controlled by changes in temperature, biological activity and advection processes forced by spatial heterogeneities in the Canary Upwelling System, the mixed layer depth, the mesoscale activity and the circulation patterns. The surface fCO_2,sw variability was driven by biological production and CO₂-rich water injection in tropical waters while temperature fluctuations controlled the fCO_2,sw changes in subtropical waters. The factors controlling the upper ocean changes in the normalized total inorganic carbon (NC_T) were assessed. The uptake and storage of anthropogenic carbon described > 60% (>90% above the MLD) of the NC_T increase from preformed values. The organic carbon pump accounted for 36.6-40.9% in tropical waters and lose importance in subtropical waters (7.5-11.6%), while the carbonate pump has a minimal contribution (<4.2%). The upper-ocean C_ant inventory in Cape Verde (8,570 Km²), Canary Islands (7.960 Km²) and Madeira (1,250 Km²) was 7.57 x 10³, 9.26 x 10³ and 8.86 x 10³ µmol kg^-1, respectively (0.51, 0.58 and 0.09 Tg C, respectively). In terms of FCO₂, these archipelagos behaved as a winter CO₂ sink (-4.74, -3.90 and -8.34 mmol m^-2d^-1, respectively) while a strong outgassing was detected over the Cape Blanc filament (20-25 mmol m^-2d^-1). The total average FCO₂ for the ocean area of the three archipelagos (371,250 Km²) was -28.27 Gg CO₂ d^-1. The POS533 data were compared and combined with SOCAT and GLODAP data and a new set of equations was provided to calculate the fCO_2,sw, C_ant and FCO₂ in the Macaronesian region based on physical and biogeochemical fluctuations.