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

Abstract

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.

Contrasting Sea‐Air CO2 Exchanges in the Western Tropical Atlantic Ocean

“…Thus, both the increase in DIC from remineralization and the decrease in TA from dilution lead to an increase in sea surface pCO 2 , but this is not sufficient to counteract the pCO 2 drawdown by primary production. Therefore, photosynthesis is the main driver of sea surface pCO 2 drawdown in the ARP (Figure S8 in Supporting Information S1), as has been suggested in a number of previous studies (Bonou et al, 2016;Körtzinger, 2003;Lefèvre et al, 2017;Louchard et al, 2021;Mu et al, 2021). Moreover, both CO 2 solubility and calcium carbonate dissolution by riverine waters likely exert some influence on the marine carbonate system.…”

“…This leads to a net increase in p CO 2 over the whole NEC (Figure 3) during maximum NBC retroflection, despite the nutrient‐ and carbon‐enriched waters from the Amazon River (Lefèvre et al., 2020). During the dry season, instead of being transported to the east, the waters from the Amazon River are mainly advected to the northwest, decreasing p CO 2 in the NEC sub‐region due to high primary productivity (Bonou et al., 2016; Ibánhez et al., 2015; Louchard et al., 2021; Mu et al., 2021), hence intensifying the biological uptake of CO 2 (Figure 2a). Indeed, model estimates suggested that without the influence of the Amazon River, much of the western Tropical Atlantic Ocean, including part of the NEC sub‐region, would be a CO 2 source year‐round (Louchard et al., 2021).…”

“…During the wet season, there is a greater influence of the waters from the Amazon River over the western Tropical Atlantic Ocean (Lefèvre et al, 2020;Louchard et al, 2021;Mu et al, 2021). This is coupled with increased precipitation due to the northward displacement of the ITCZ (Ibánhez et al, 2015;Ibánhez et al, 2016;Utida et al, 2019), hence leading to a net pCO 2 drawdown in the NBC-NECC sub-region, although it remains supersaturated with respect to atmospheric pCO 2 .…”

“…For example, estimates of sea‐air CO 2 exchanges based on satellite data show an average annual CO 2 release of 4.6 mmol m −2 day −1 (Valerio et al., 2021), while complex biogeochemical models indicate a CO 2 source an order of magnitude lower (0.5 mmol m −2 day −1 ; Louchard et al., 2021) in a vast area of the western Tropical Atlantic. Although most studies indicate that the area under the influence of the Amazon River plume is an important atmospheric CO 2 sink, the magnitude of ocean CO 2 uptake also varies greatly among them (e.g., Louchard et al., 2021; Mu et al., 2021).…”

Impact of North Brazil Current rings on air-sea CO&lt;sub&gt;2&lt;/sub&gt; flux variability in winter 2020

“…Figure 3 shows that waters with a SST of 26.5°C and SSS between 35-36 can either be rich in Chla and have a high fCO2 or low in Chla and have a low fCO2. Waters rich in detrital material tends to limit the phytoplankton growth and microbial respiration of riverine material on the continental shelf likely dominates (Aller & Blair, 2006;Medeiros et al, 2015;Mu et al, 2021). This relationship is analyzed in more detail for the different water masses identified in the region in parts 3.2. and 3.3.…”