A source of CO₂ to the atmosphere throughout the year in the Maranhense continental shelf (2°30'S, Brazil)

To reduce uncertainty regarding the contribution of continental shelf areas in low latitude regions to the air-sea CO₂ exchange, more data are required to understand the carbon turnover in these regions and cover gaps in coverage. For the first time, inorganic carbon and alkalinity were measured along a cross-shelf transect off the coast of Maranhão (North Brazil) in 9 cruises spawning from April 2013 to September 2014. On the last 4 transects, dissolved organic matter and nutrients were also measured. The highest inorganic and organic carbon concentrations are observed close to land. As a result of low productivity and significant remineralization, heterotrophy dominates along the transect throughout the year. Although the temporal variability is significantly reduced at the offshore station with carbon concentrations decreasing seaward, the fugacity of CO₂ (fCO₂) at this station remains significantly higher, especially during the wet season, than the open ocean values measured routinely by a merchant ship further west. Overall, the continental shelf is a weak source of CO₂ to the atmosphere throughout the year with an annual mean flux of 1.81±0.84 mmol m⁻² d⁻¹. The highest magnitudes of fCO₂ are observed during the wet season when the winds are the weakest. As a result, the CO₂ flux does not show a clear seasonal pattern. Further offshore, fCO₂ is significantly lower than on the continental shelf. However, the oceanic CO₂ flux, with an annual mean of 2.32±1.09 mmol m⁻² d⁻¹, is not statistically different from the CO₂ flux at the continental shelf because the wind is stronger in the open ocean.