Low Diffusive Methane Emissions From the Main Channel of a Large Amazonian Run-of-the-River Reservoir Attributed to High Methane Oxidation

Abstract

The global development of hydropower dams has rapidly expanded over the last several decades and has spread to historically non-impounded systems such as the Amazon River’s main low land tributaries in Brazil. Despite the recognized significance of reservoirs to the global methane (CH4) emission, the processes controlling this emission remain poorly understood, especially in Tropical reservoirs. Here we evaluate CH4 dynamics in the main channel and downstream of the Santo Antônio hydroelectric reservoir, a large tropical run-of-the-river (ROR) reservoir in Amazonia. This study is intended to give a snapshot of the CH4 dynamics during the falling water season at the initial stage after the start of operations. Our results show substantial and higher CH4 production in reservoirs’ littoral sediment than in the naturally flooded areas downstream of the dam. Despite the large production in the reservoir or naturally flooded areas, high CH4 oxidation in the main channel keep the concentration and fluxes of CH4 in the main channel low. Similar CH4 concentrations in the reservoir and downstream close to the dam suggest negligible degassing at the dam, but stable isotopic evidence indicates the presence of a less oxidized pool of CH4 after the dam. ROR reservoirs are designed to disturb the natural river flow dynamics less than traditional reservoirs. If enough mixing and oxygenation remain throughout the reservoir’s water column, naturally high CH4 oxidation rates can also remain and limit the diffusive CH4 emissions from the main channel. Nevertheless, it is important to highlight that our results focused on emissions in the deep and oxygenated main channel. High emissions, mainly through ebullition, may occur in the vast and shallow areas represented by bays and tributaries. However, detailed assessments are still required to understand the impacts of this reservoir on the annual emissions of CH4.

Dominance of Diffusive Methane Emissions From Lowland Headwater Streams Promotes Oxidation and Isotopic Enrichment

“…−67‰; Fisher et al, 2017). Similarly, δ 13 C-CH 4 values measured in fluvial portions of the Amazon River basin (Sawakuchi et al, 2016;Sawakuchi et al, 2021) are enriched relative to other tropical wetlands (approx. −60‰; Brownlow et al, 2017).…”

“…The most extensive data in rivers comes from the Amazon, where consideration of both ebullitive and diffusive emissions provides mean δ 13 C-CH 4 values for total emitted CH 4 of −49.6‰ (Sawakuchi et al, 2016) and −38.4‰ (Sawakuchi et al, 2021). In these large tropical rivers, diffusion comprises a smaller fraction of the total emission budget, but a greater proportion of dissolved CH 4 is oxidized and thus the emitted CH 4 is more isotopically enriched.…”

“…In these large tropical rivers, diffusion comprises a smaller fraction of the total emission budget, but a greater proportion of dissolved CH 4 is oxidized and thus the emitted CH 4 is more isotopically enriched. Our proposed conceptual model allows for these differences, where the higher efficiency of oxidation in larger rivers is enough to counter the higher prevalence of ebullition (Sawakuchi et al, 2021). Further, hyporheic flow is relatively less important in large rivers (Battin et al, 2008), potentially resulting in a greater proportion of CH 4 accumulating in bubbles in river sediments compared to streams.…”

Carbon dioxide partial pressures and emissions of the Yarlung Tsangpo River on the Tibetan Plateau

“…Thus, carbon remobilization, particularly through water-air interface exchange from freshwaters, including river systems, represents an important component of the global carbon cycle. A large number of studies on river outgassing have been undertaken in typical rivers, such as the Amazon (Richey et al, 2002;Alin et al, 2011;Sawakuchi et al, 2021), the Mississippi (Dubois et al, 2010), the Yangtze River (Zhao et al, 2013;Liu et al, 2016;Ni et al, 2019), the Yellow River (Ran et al, 2015) and the karst river system (Ni et al, 2020;Ni and Li, 2022). However, there is substantial uncertainty in current understanding about global carbon flux, in part due to a lack of direct measurements of rivers in the high-altitude cryosphere (Zhang et al, 2020).…”

Inorganic Nitrogen Stimulates Methane Oxidation in Coastal Lagoon Sediments

“…In addition, there is increasing evidence that CH 4 production in the water column contributes significantly to lakes CH 4 emissions (Tang et al 2014, DelSontro et al 2018, Bižić et al 2019. Despite the large methanogenesis in naturally flooded areas, high rates of CH 4 oxidation might keep concentration and fluxes low (Sawakuchi et al 2021).…”