Carbon dioxide emissions from the flat bottom and shallow Nam Theun 2 Reservoir: drawdown area as a neglected pathway to the atmosphere

Abstract

Abstract. Freshwater reservoirs are a significant source of CO2 to the atmosphere. CO2 is known to be emitted at the reservoir surface by diffusion at the air–water interface and downstream of dams or powerhouses by degassing and along the river course. In this study, we quantified total CO2 emissions from the Nam Theun 2 Reservoir (Lao PDR) in the Mekong River watershed. The study started in May 2009, less than a year after flooding and just a few months after the maximum level was first reached and lasted until the end of 2013. We tested the hypothesis that soils from the drawdown area would be a significant contributor to the total CO2 emissions. Total inorganic carbon, dissolved and particulate organic carbon and CO2 concentrations were measured in 4 pristine rivers of the Nam Theun watershed, at 9 stations in the reservoir (vertical profiles) and at 16 stations downstream of the monomictic reservoir on a weekly to monthly basis. CO2 bubbling was estimated during five field campaigns between 2009 and 2011 and on a weekly monitoring, covering water depths ranging from 0.4 to 16 m and various types of flooded ecosystems in 2012 and 2013. Three field campaigns in 2010, 2011 and 2013 were dedicated to the soils description in 21 plots and the quantification of soil CO2 emissions from the drawdown area. On this basis, we calculated total CO2 emissions from the reservoir and carbon inputs from the tributaries. We confirm the importance of the flooded stock of organic matter as a source of carbon (C) fuelling emissions. We show that the drawdown area contributes, depending on the year, from 40 to 75 % of total annual gross emissions in this flat and shallow reservoir. Since the CO2 emissions from the drawdown zone are almost constant throughout the years, the large interannual variations result from the significant decrease in diffusive fluxes and downstream emissions between 2010 and 2013. This overlooked pathway in terms of gross emissions would require an in-depth evaluation for the soil organic matter and vegetation dynamics to evaluate the actual contribution of this area in terms of net modification of gas exchange in the footprint of the reservoir, and how it could evolve in the future.

Carbon dioxide emission from drawdown areas of a Brazilian reservoir is linked to surrounding land cover

“…The drawdown area thus accounted for < 20% of the total reservoir area but contributed to > 80% of total reservoir CO 2 emissions upstream the dam. Our results are in line with a recent study conducted in a reservoir in Southeast Asia, which found that drawdown areas accounted for 50-75% of total annual reservoir CO 2 emission (Deshmukh et al 2018). Our findings indicate that drawdown areas are CO 2 emission hotspots in CDU reservoir, not only due to high emission rates in relation to reservoir water surface, but also because exposed sediments cover a large fraction of the total reservoir area over long periods of the year (Fig.…”

“…An important factor supporting enhanced CO 2 emission rates from exposed sediments is the increased microbial metabolism (e.g., enhanced enzyme activity of phenol oxidases and hydrolases) as sediment dries out (Hyojin et al 2016;Weise et al 2016). The importance of exposed sediments to reservoir carbon processing is clearly illustrated by a study in a Southeast Asian reservoir, which demonstrates that drawdown areas may contribute up to 75% of total annual CO 2 emissions (Deshmukh et al 2018). Globally, dry exposed sediments are estimated to emit ~ 200 Tg of carbon as CO 2 , which is equivalent to ~ 10% of global CO 2 emissions from inland waters (Marcé et al 2019).…”

“…Understanding the variability of CO 2 fluxes from drawdown areas over time and space is fundamental to support the definition of adequate sampling strategies and thus more realistic upscaling of CO 2 emissions from freshwater systems. While one study has reported limited spatial and annual variability in drawdown area CO 2 fluxes (Deshmukh et al 2018), the scarcity of data makes it difficult to draw general conclusions about spatial or temporal variability of drawdown area CO 2 emission. Here we investigate the spatial variation in CO 2 fluxes from the drawdown areas of a reservoir in southeastern Brazil.…”

“…Cycles of wetting-desiccation accelerate carbon losses from freshwater systems (Reverey et al 2016). Indeed, a growing number of studies in different types of aquatic ecosystems (reservoirs, intermittent streams, temporary ponds) suggest that exposed sediments emit substantially more CO 2 than adjacent water surfaces (Catalán et al 2014;Deshmukh et al 2018;Gilbert et al 2017;Gómez-Gener et al 2015;Hyojin et al 2016;Looman et al 2017;Obrador Fig. 3 Relative contribution of drawdown area (dark grey) and water surface area (light grey) to the total reservoir area of Chapéu D'Uvas (CDU) reservoir over time Fig.…”

Wetland methane emissions dominated by plant‐mediated fluxes: Contrasting emissions pathways and seasons within a shallow freshwater subtropical wetland

“…) and thus some labile sedimentary carbon stores were likely converted to CO 2 via aerobic pathways and evaded to the atmosphere, resulting in a net loss of labile carbon between C1 and C2. Even relatively small drawdown areas and drying of sediments have been shown to significantly enhance carbon losses and retard subsequent methanogensis upon reflooding (Chimner and Cooper ; Knorr et al ; Deshmukh et al ), with significant alteration to the microbial community following drought events (Rees et al ). After the initial suppression of methane production during water table drawn down, reflooding events eventually cause methane concentrations to increase with the timing of recovery ranging from a matter of days to months (Boon et al ; Knorr et al ; Tyler et al ).…”