Air-sea CO 2 fluxes for the Brazilian northeast continental shelf in a climatic transition region

Abstract Oceanographic cruises were carried out in October 2012 (3°S–5°S and 38,5°W–35,5°W) and in September 2014 (1°S–4°S and 43°W–37°W), measuring atmospheric and sea surface CO 2 fugacity ( f CO 2 ) underway in the northeast coast of Brazil. Sea surface water samples were also collected for chlorophyll a , nutrients and DOC analysis. During the second cruise, the sampling area covered a transition between semi-arid to more humid areas of the coast, with different hydrologic and rainfall regimes. The seawater f CO 2 sw , in October 2012, was in average 400.9 ± 7.3μatm and 391.1 ± 6.3 μatm in September 2014. For the atmosphere, the f CO 2 air in October 2012 was 375.8 ± 2.0 μatm and in September 2014, 368.9 ± 2.2 μatm. The super-saturation of the seawater in relation to the atmosphere indicates a source of CO 2 to the atmosphere. The entire study area presents oligotrophic conditions. Despite the low concentrations, Chl a and nutrients presented significant influence on f CO 2 sw , particularly in the westernmost and more humid part of the northeast coast, where river fluxes are three orders of magnitude larger than eastern rivers and rainfall events are more intense and constant. f CO 2 sw spatial distribution presented homogeneity along the same transect and longitudinal heterogeneity, between east and west, reinforcing the hypothesis of transition between two regions of different behaviour. The f CO 2 sw at the eastern portion was controlled by parameters such as temperature and salinity. At the western portion, f CO 2 sw was influenced by nutrient and Chl a . Calculated instantaneous CO 2 flux ranged from + 1.66 to + 7.24 mmol m − 2  d − 1 in the first cruise and + 0.89 to + 14.62 mmol m − 2  d − 1 in the second cruise.