Temporal Variability of Air-Sea CO2 flux in the Western Tropical North Atlantic Influenced by the Amazon River Plume

The partial pressure of carbon dioxide (pCO(2)) was surveyed across the Amazon River plume and the surrounding western tropical North Atlantic Ocean (15-0 degrees N, 43-60 degrees W) during three oceanic expeditions (May-June 2010, September-October 2011, and July 2012). The survey timing was chosen according to previously described temporal variability in plume behavior due to changing river discharge and winds. In situ sea surface pCO(2) and air-sea CO2 flux exhibited robust linear relationships with sea surface salinity (SSS; 15 < SSS < 35), although the relationships differed among the surveys. Regional distributions of pCO(2) and CO2 flux were estimated using SSS maps from high-resolution ocean color satellite-derived (MODIS-Aqua) diffuse attenuation coefficient at 490 nm (K(d)490) during the periods of study. Results confirmed that the plume is a net CO2 sink with distinctive temporal variability: the strongest drawdown occurred during the spring flood (-2.39 +/- 1.29 mmol m(-2) d(-1) in June 2010), while moderate drawdown with relatively greater spatial variability was observed during the transitional stages of declining river discharge (-0.42 +/- 0.76 mmol m(-2) d(-1) in September-October 2011). The region turned into a weak source in July 2012 (0.26 +/- 0.62 mmol m(-2) d(-1)) when strong CO2 uptake in the mid-plume was overwhelmed by weak CO2 outgassing over a larger area in the outer plume. Outgassing near the mouth of the river was observed in July 2012. Our observations draw attention to the importance of assessing the variable impacts of biological activity, export, and air-sea gas exchange before estimating regional CO2 fluxes from salinity distributions alone.