Physically controlled CO₂ effluxes from a reservoir surface in the upper Mekong River Basin: a case study in the Gongguoqiao Reservoir

• Main Authors: L. Lin, X. Lu, S. Liu, S.-Y. Liong, K. Fu
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-05-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/2205/2019/bg-16-2205-2019.pdf
• ISSN: 1726-4170; 1726-4189

<p>Impounding alters the carbon transport in rivers. To quantify this effect, we measured <span class="inline-formula">CO₂</span> effluxes from a mountainous valley-type reservoir in the upper Mekong River (known as Lancang River in China). <span class="inline-formula">CO₂</span> evasion rates from the reservoir surface were <span class="inline-formula">408±337</span>&thinsp;mg&thinsp;<span class="inline-formula">CO₂</span>&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> in the dry season and <span class="inline-formula">305±262</span>&thinsp;mg&thinsp;<span class="inline-formula">CO₂</span>&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> in the rainy season much lower than those from the riverine channels (<span class="inline-formula">1567±2312</span>&thinsp;mg&thinsp;<span class="inline-formula">CO₂</span>&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> at the main stem and <span class="inline-formula">905±1536</span>&thinsp;mg&thinsp;<span class="inline-formula">CO₂</span>&thinsp;m<span class="inline-formula">⁻²</span>&thinsp;d<span class="inline-formula">⁻¹</span> at the tributary). Low effluxes in the pelagic area were caused by low allochthonous organic carbon (OC) inputs and photosynthetic uptake of <span class="inline-formula">CO₂</span>. A negative relationship between <span class="inline-formula">CO₂</span> efflux and water temperature suggested <span class="inline-formula">CO₂</span> emissions at the pelagic area were partly offset by photosynthesis in the warmer rainy season. <span class="inline-formula">CO₂</span> emissions from the reservoir outlet and littoral area, which were usually considered hotspots of <span class="inline-formula">CO₂</span> emissions, had a low contribution to the total emission because of epilimnion water spilling and a small area of the littoral zones. Yet at the river inlets effluxes were much higher in the dry season than in the rainy season because different mixing modes occurred in the two seasons. When the river joined the receiving waterbody in the dry season, warmer and lighter inflow became an overflow and large amounts of <span class="inline-formula">CO₂</span> were released to the atmosphere as the overflow contacted the atmosphere directly. Extended water retention time due to water storage might also help mineralization of OC. In the wet season, however, colder, turbid and heavier inflow plunged into the reservoir and was discharged downstream for hydroelectricity, leaving insufficient time for decomposition of OC. Besides, diurnal efflux variability indicated that the effluxes were significantly higher in the nighttime than in the daytime, which increased the estimated annual emission rate by half.</p>