AUTHOR=Curbelo-Hernández David , Santana-Casiano J. Magdalena , González Aridane González , González-Dávila Melchor 

TITLE=Air-Sea CO2 Exchange in the Strait of Gibraltar

JOURNAL=Frontiers in Marine Science

VOLUME=8

YEAR=2021

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2021.745304

DOI=10.3389/fmars.2021.745304

ISSN=2296-7745

ABSTRACT=<p>The seasonal and spatial variability of the CO<sub>2</sub> system and air-sea fluxes were studied in surface waters of the Strait of Gibraltar between February 2019 and March 2021. High-resolution data was collected by a surface ocean observation platform aboard a volunteer observing ship. The CO<sub>2</sub> system was strongly influenced by temperature and salinity fluctuations forced by the seasonal and spatial variability in the depth of the Atlantic–Mediterranean Interface layer and by the tidal and wind-induced upwelling. The changes in seawater CO<sub>2</sub> fugacity (<italic>f</italic>CO<sub>2,sw</sub>) and fluxes were mainly driven by temperature despite the significant influence of non-thermal processes in the southernmost part. The thermal to non-thermal effect ratio (T/B) reached maximum values in the northern section (&gt;1.8) and minimum values in the southern section (&lt;1.30). The <italic>f</italic>CO<sub>2,sw</sub> increased with temperature by 9.02 ± 1.99 μatm °C<sup>–1</sup> (<italic>r</italic><sup>2</sup> = 0.86 and ρ = 0.93) and 4.51 ± 1.66 μatm °C<sup>–1</sup> (<italic>r</italic><sup>2</sup> = 0.48 and ρ = 0.69) in the northern and southern sections, respectively. The annual cycle of total inorganic carbon normalized to a constant salinity of 36.7 (NC<sub><italic>T</italic></sub>) was assessed. Net community production processes described 93.5–95.6% of the total NC<sub><italic>T</italic></sub> change, while air-sea exchange and horizontal and vertical advection accounted for &lt;4.6%. The <italic>f</italic>CO<sub>2,sw</sub> in the Strait of Gibraltar since 1999 has been fitted to an equation with an interannual trend of 2.35 ± 0.06 μatm year<sup>–1</sup> and a standard error of estimate of ±12.8 μatm. The seasonality of the air-sea CO<sub>2</sub> fluxes reported the behavior as a strong CO<sub>2</sub> sink during the cold months and as a weak CO<sub>2</sub> source during the warm months. Both the northern and the southern sections acted as a net CO<sub>2</sub> sink of −0.82 and −1.01 mol C m<sup>–2</sup> year<sup>–1</sup>, respectively. The calculated average CO<sub>2</sub> flux for the entire area was −7.12 Gg CO<sub>2</sub> year<sup>–1</sup> (−1.94 Gg C year<sup>–1</sup>).</p>