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

Henrique O Sawakuchi,Jeffrey E Richey,Plínio B Camargo,Nicholas D Ward,Alex Enrich-Prast,David Bastviken

doi:10.3389/fenvs.2021.655455

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.

Highlights

Hydroelectric reservoirs are recognized as an important anthropogenic source of the greenhouse gas methane (CH4) to the atmosphere, with an estimated global flux ranging from 3 to 14 Tg CH4 year−1 (Deemer et al, 2016)
It has been shown that CH4 emissions and oxidation may vary in space and time within Amazonian rivers (Sawakuchi et al, 2014, 2016)
Such variability is expected in the Madeira River, and because our study covered only the falling water season and a few sites, we highlight that the findings presented here are a snapshot of the CH4 dynamics at the early stage after reservoir construction

Summary

Introduction

Hydroelectric reservoirs are recognized as an important anthropogenic source of the greenhouse gas methane (CH4) to the atmosphere, with an estimated global flux ranging from 3 to 14 Tg CH4 year−1 (Deemer et al, 2016). Recent studies have indicated minimal changes in water chemistry, thermal structure, and fine sediment transport in the Santo Antonio ROR reservoir in the Maderia River (Almeida et al, 2019a; Rivera et al, 2019), and de Araújo et al (2019) found that CO2 emissions in the Belo Monte ROR reservoir’s main channel in the Xingu River remained similar to non-affected river areas. These findings suggest that the main channel of ROR reservoirs can keep the original river characteristics

Methods

Results

Conclusion