Net heterotrophy in the Amazon continental shelf changes rapidly to a sink of CO₂ in the outer Amazon plume

The Amazon continental shelf and adjacent oceanic area were sampled for inorganic and organic carbon parameters in order to improve data coverage and understanding of carbon cycling dynamics within this important region. Seasonal coverage of the Amazon plume on the French Guiana continental shelf further north, was provided by CO₂ monitoring using a merchant ship sailing from France to French Guiana (2006-2016). Salinity ranged from 1 to 36 (transects in April 2013, and May 2014). At salinity below 10, strong outgassing was observed with fugacity of CO₂ (fCO₂) over 2,000μatm. This region displayed net heterotrophy, fueled by organic matter with terrestrial origin, as shown by γ¹³C and γ¹⁵N values of suspended particles. A γ¹³C cross shelf average of -31% was measured during May 2014, contrasting with oceanic values in excess of -20%. The reactivity of this terrestrial material resulted in the local production of dissolved inorganic and organic carbon as well as fluorescent humic compounds. Further offshore, the dilution of freshwater by ocean waters created a sink for CO₂, enhanced by biological activity. The strongest CO₂ drawdowns, associated with high chlorophyll a concentrations, were observed on the French Guiana continental shelf in the outer Amazon plume, with fCO₂ values below 150 μatm. Here, a CO₂ sink was present almost throughout the year, with a seasonal maximum of -9.2 mmol CO₂ m-2d-1 observed in June 2015. However, both the CO₂ and salinity distributions could vary significantly within a few days, confirming the presence of many eddies in this region. The Amazon continental shelf hence behaved as a transition zone between an inshore source of CO₂ to the atmosphere and an offshore sink. Some marine phytoplankton production was detected but occurred mainly close to the French Guiana shelf. A mean net CO₂ outgassing of 44 ± 43.6mmol m^-2d^-1 was estimated for the area. Quantifying the CO₂ flux for the entire Amazon shelf area led to a revised annual estimate of the net ocean carbon export of 2.86 10¹² mol C from the region.