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 (pCO2) was surveyed across the Amazon River plume and the surrounding western tropical North Atlantic Ocean (15–0°N, 43–60°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 pCO2 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 pCO2 and CO2 flux were estimated using SSS maps from high‐resolution ocean color satellite‐derived (MODIS‐Aqua) diffuse attenuation coefficient at 490 nm (Kd490) 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. Plain Language Summary: The Amazon River is the world's largest river with a watershed covering ∼3 million square miles. Its delivery of massive amounts of freshwater, organic material, and nutrients to the salty ocean produces an extensive 30–50 feet thick layer of fresher seawater, called the "plume", floating on the surface of the tropical Atlantic. This plume is spread by winds and currents to cover an additional ∼1 million square miles of the "river‐ocean continuum". We studied how this plume impacts the air‐sea exchange of the greenhouse gas, carbon dioxide (CO2). We combined shipboard measurements with satellite data to estimate the full regional impact (beyond what we could observe from the ship) and examined temporal variation in this exchange. We observed rapid atmospheric CO2 uptake by the plume in regions dominated by algal blooms. We also observed a large CO2 release from the plume near the river mouth, likely due to microbial community respiration of riverine material. Differences in the size and salinity distribution of the plume can greatly modify the plume's impact on CO2 exchange. Our observations draw attention to understanding these variable mechanisms before using the plume extent alone to estimate future impacts on carbon uptake by this key ocean region. Key Points: The Amazon River plume is a net sink for atmospheric CO2Plume size and salinity distribution affect the magnitude of the regional sinkTemporal variations in the relationship between salinity and CO2 flux occur due to changing wind speed, biological activity, and export [ABSTRACT FROM AUTHOR]