LAUSR

Abstract To reduce uncertainty regarding the contribution of continental shelf areas in low latitude regions to the air-sea CO 2 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 Maranhao (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 CO 2 (fCO 2 ) 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 CO 2 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 fCO 2 are observed during the wet season when the winds are the weakest. As a result, the CO 2 flux does not show a clear seasonal pattern. Further offshore, fCO 2 is significantly lower than on the continental shelf. However, the oceanic CO 2 flux, with an annual mean of 2.32±1.09 mmol m −2  d −1 , is not statistically different from the CO 2 flux at the continental shelf because the wind is stronger in the open ocean.