The Impact of the Amazon on the Biological Pump and the Air‐Sea CO₂ Balance of the Western Tropical Atlantic

Abstract

The Amazon River strongly modifies the biogeochemistry of the Western Tropical Atlantic (WTA). To disentangle the different mechanisms driving these modifications, we conduct a series of modeling experiments with a high‐resolution regional ocean model (ROMS) coupled to a biogeochemical/ecological model (BEC) that we augmented to include Diatom‐Diazotroph‐Assemblages (DDAs). In our model, the Amazon River increases net primary production (NPP) in the WTA by almost 10%, exceeding the stimulation expected from the supplied inorganic nitrogen and phosphorus by a factor of two. This amplification is fueled by new nitrogen stemming from DDA‐driven N2 fixation in the plume region, supported, in part, by the consumption of riverine dissolved organic phosphorus. The vertical export of organic carbon is enhanced by a shift of the phytoplankton community toward diatoms induced by the large amount of Si(OH)4 delivered by the Amazon. These changes in NPP and export production induce a strong uptake of atmospheric CO2. In contrast, the remineralization of the river‐delivered terrestrial organic matter leads to a release of CO2 over the WTA, which is partially offset by a net uptake induced by the riverine dissolved inorganic carbon and alkalinity. Overall, the Amazon reduces the strong outgassing of the WTA in our simulations by more than 50%. Our study demonstrates how rivers modify the marine biological pump and the air‐sea CO2 fluxes in the downstream ocean through a myriad of cascading effects, highlighting the need to fully consider the land‐ocean aquatic continuum in the modeling of the Earth System.

Historical increases in land‐derived nutrient inputs may alleviate effects of a changing physical climate on the oceanic carbon cycle

“…Despite these potential dramatic consequences regionally, our results suggest that open ocean inflows have a very prominent role in supplying global coastal waters with nutrients, as suggested by ocean inflows versus freshwater contributions to the global coastal ocean in Lacroix et al (2021). Inflows from the open ocean have been demonstrated to majorly dictate biogeochemical dynamics in a multitude of coastal regions (Outer North Sea: Mathis et al, 2019; Thomas et al, 2005, North America East Coast: Fennel and Wilkin, 2009, California Current System: Frischknecht et al, 2018; South China Sea: Dai et al, 2013) while others are clearly driven by riverine inputs (Amazon plume: Louchard et al, 2021, Louisiana Shelf: Große et al, 2019, Sylvan et al, 2006), or for which the dominant controls are still debated (East China Sea: Große et al, 2020; Zhang et al, 2019).…”

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

“…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 .…”

“…The annual average net CO 2 flux across the whole region drops considerably from −1.6 to −0.2 mmol m −2 day −1 , when we remove the ARP sub-region. Thus, the waters derived from ARP are responsible for 87% of the CO 2 uptake by the western Tropical Atlantic Ocean, a greater contribution than the 60% showed by biogeochemical models (Louchard et al, 2021). Hence, further observational efforts are needed to broaden our understanding of this biogeochemically complex region, and thus to improve ocean and climate model estimates.…”

Contrasting sea-air CO2 exchanges in the western Tropical Atlantic Ocean

“…The ocean dynamics of the surface current system (e.g., Johns et al, 1998;Salisbury, et al, 2011;Johns et al, 2021) and the large freshwater input from the Amazon River (Liang et al, 2020), coupled with precipitation dynamics (Ibánhez et al, 2015;Utida et al, 2019), also add a biogeochemical complexity to the western Tropical Atlantic Ocean (da Cunha & Buitenhuis, 2013). Such complexity is reflected in the pronounced spatial and temporal variability of the marine carbonate system and particularly in the sea-air CO2 exchanges (Körtzinger, 2003;Cooley et al, 2007;Lefèvre et al, 2010;Valerio et al, 2021;Louchard et al, 2021;Mu et al, 2021). As a result, some sea-air CO2 flux estimates indicate that the western Tropical Atlantic Ocean acted as a net CO2 source region (e.g., Cooley et al, 2007;Valerio et al, 2020;Louchard et al, 2021).…”

“…Such complexity is reflected in the pronounced spatial and temporal variability of the marine carbonate system and particularly in the sea-air CO2 exchanges (Körtzinger, 2003;Cooley et al, 2007;Lefèvre et al, 2010;Valerio et al, 2021;Louchard et al, 2021;Mu et al, 2021). As a result, some sea-air CO2 flux estimates indicate that the western Tropical Atlantic Ocean acted as a net CO2 source region (e.g., Cooley et al, 2007;Valerio et al, 2020;Louchard et al, 2021). Conversely, other studies revealed a net CO2 sink behaviour, mainly due to the high primary productivity driven by optimal nutrient availability supplied by the Amazon River waters (e.g., Körtzinger, 2003;Lefèvre et al, 2010;Ibánhez et al, 2015;Mu et al, 2021).…”