Carbon Dioxide Emissions along the Lower Amazon River

Abstract

A large fraction of the organic carbon derived from land that is transported through inland waters is decomposed along river systems and emitted to the atmosphere as carbon dioxide (CO 2 ). The Amazon River outgasses nearly as much CO 2 as the rainforest sequesters on an annual basis, representing ∼25% of global CO 2 emissions from inland waters. However, current estimates of CO 2 outgassing from the Amazon basin are based on a conservative upscaling of measurements made in the central Amazon, meaning both basin and global scale budgets are likely underestimated. The lower Amazon River, from Óbidos to the river mouth, represents ∼13% of the total drainage basin area, and is not included in current basin-scale estimates. Here, we assessed the concentration and evasion rate of CO 2 along the lower Amazon River corridor and its major tributaries, the Tapajós and Xingu Rivers. Evasive CO 2 fluxes were directly measured using floating chambers and gas transfer coefficients (k 600 ) were calculated for different hydrological seasons. Temporal variations in pCO 2 and CO 2 emissions were similar to previous observations throughout the Amazon (e.g., peak concentrations at high water) and CO 2 outgassing was lower in the clearwater tributaries compared to the mainstem. However, k 600 -values were higher than previously reported upstream likely due to the generally windier conditions, turbulence caused by tidal forces, and an amplification of these factors in the wider channels with a longer fetch. We estimate that the lower Amazon River mainstem emits 0.2 Pg C year −1 within our study boundaries, or as much as 0.48 Pg C year −1 if the entire spatial extent to the geographical mouth is considered. Including these values with updated basin scale estimates and estimates of CO 2 outgassing from small streams we estimate that the Amazon running waters outgasses as much as 1.39 Pg C year −1 , increasing the global emissions from inland waters by 43% for a total of 2.9 Pg C year −1 . These results highlight a large missing gap in basin-scale carbon budgets along the complete continuum of the Amazon River, and likely most other large river systems, that could drastically alter global scale carbon budgets.

A review of CO2 and associated carbon dynamics in headwater streams: A global perspective

“…This discrepancy results from carbon burial and aquatic carbon turnover, such as the mineralization of organic matter to CO 2 and its outgassing to the atmosphere. Excluding the amount of CO 2 outgassing from lakes and reservoirs as determined by Raymond et al [] yields a recent estimate of 2.58 petagrams (Pg) CO 2 yr −1 for rivers and streams alone [ Sawakuchi et al , ]. This constitutes 45% of the global carbon transfer from terrestrial ecosystems to streams and rivers [ Raymond et al , ], with potentially large amounts of carbon outgassing from headwater regions.…”

“…This 9 year‐old estimate incorporates uncertainties of spatial variations, and if investigated at more detail, this number is likely underestimated. Another study included the total surface area of the lower reaches of the Amazon River and estimated a CO 2 outgassing rate of 1.39 Pg C yr −1 [ Sawakuchi et al , ]. They stated a 43% higher CO 2 outgassing from global inland waters (streams, rivers, lakes, and reservoirs) than estimated by Raymond et al [] .…”

Patterns of Bacterial and Archaeal Gene Expression through the Lower Amazon River

“…The main channel is well mixed and highly turbid, and high levels of CO 2 as well as poor light penetration support an environment assumed to be dominated by heterotrophic bacteria. These microbes remove, transform, and stabilize riverine organic matter during transit, with their activities leading to river outgassing of as much as 1,390 Tg C y −1 to the atmosphere (Richey et al, 2002;Sawakuchi et al, 2017) Within the mainstem of the Amazon, heterotrophic bacteria rely on allochthonous input of carbon and nutrients from the surrounding rainforest and drainage basins. Humic and fulvic acids likely derived from lignin and other terrestrial plant components account for ∼60% of riverine dissolved organic carbon (DOC) (Ertel et al, 1986).…”

Groundwater data improve modelling of headwater stream CO<sub>2</sub> outgassing with a stable DIC isotope approach

“…In the process of transport, large amounts of carbon -mostly in the form of CO 2 -outgas from the water surface to the atmosphere (Cole et al, 2007;Aufdenkampe et al, 2011;Regnier et al, 2013;Wehrli, 2013). Globally, this form of CO 2 contributions to the atmosphere was estimated between 0.6 to 2.6 petagrams (Pg) of carbon per year 30 (Raymond et al, 2013;Lauerwald et al, 2015;Sawakuchi et al, 2017;Marx et al, 2017a). The lower value of this range by Lauerwald et al (2015) excluded streams of Strahler stream numbers (Strahler, 1952) below 3.…”