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Abstract
The Amazon River is the world’s largest river by discharge. Its massive delivery of freshwater, carbon, and nutrients to the ocean significantly impacts the ecosystem and air–sea fluxes of carbon dioxide (CO2) in the western tropical North Atlantic Ocean (WTNA). Spatial and temporal dynamics of the carbonate system in the Amazon River-ocean continuum remain poorly understood, however, due to under-sampling of the river plume and limited historical data from the river mouth. This study aims to explore these CO2 dynamics in the plume-influenced WTNA, their sensitivity to seasonal discharge and river chemistry, and their contribution to the global carbon cycle.We conducted six river cruises in the lower reach of the Amazon River and three oceanic cruises in the WTNA during multiple seasons between 2010–2012. At the river mouth,
dissolved inorganic carbon (DIC) and alkalinity (ALK) end members from the Amazon were determined and are now reported for the first time. DIC and ALK were ~230–300 μmol L-1 lower than those measured ~1000 km upriver from the mouth, historically used to represent the Amazon’s contribution to the ocean. Our measured DIC end member was ~100 μmol L-1 higher than that of a previous estimate, causing a ~25% increase in the net community production calculated for the mesohaline region of the plume. Inorganic nutrients at the mouth were also measured and compared with previous literature, showing differences in concentrations and seasonal variability that may indicate long term changes in river chemistry and human impact.
During the oceanic expeditions, we estimated the seasonal air–sea CO2 fluxes using in situ and remotely-sensed observations, and analyzed the deviation of the pCO2-salinity relationships from conservative mixing calculated by our newly-determined river end members. The plume was a major sink for atmospheric CO2 in June, and a weak source/sink in July and September. Differences in monthly CO2 fluxes were primarily due to seasonal discharge and salinity distributions of the plume. An eighteen-year (2003–2020) satellite-derived time series of CO2 fluxes revealed June-2010 as somewhat of an anomaly, although a strong June CO2 sink was consistently observed over the past two decades. Further exploration of other months and years will be necessary to confirm any significant climate-driven trends.