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Hotspots of diffusive CO2 and CH4 emission from tropical reservoirs shift through time
Department of Biology, Institute of Biological Sciences, Federal University of Juiz de Fora, Juiz de Fora, Brazil .
Department of Biology, Institute of Biological Sciences, Federal University of Juiz de Fora, Juiz de Fora, Brazil .
Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, USA.
Department of Biology, Institute of Biological Sciences, Federal University of Juiz de Fora, Juiz de Fora, Brazil .
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2021 (English)In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 126, no 4, article id e2020JG006014Article in journal (Refereed) Published
Abstract [en]

The patterns of spatial and temporal variability in CO2 and CH4 emission from reservoirs are still poorly studied, especially in tropical regions where hydropower is growing. We performed spatially resolved measurements of dissolved CO2 and CH4 surface water concentrations and their gas-exchange coefficients (k) to compute diffusive carbon flux from four contrasting tropical reservoirs across Brazil during different hydrological seasons. We used an online equilibration system to measure dissolved CO2 and CH4 concentrations; we estimated k from floating chamber deployments in conjunction with discrete CO2 and CH4 water concentration measurements. Diffusive CO2 emissions were higher during dry season than during rainy season, whereas there were no consistent seasonal patterns for diffusive CH4 emissions. Our results reveal that the magnitude and the spatial within-reservoir patterns of diffusive CO2 and CH4 flux varied strongly among hydrological seasons. River inflow areas were often characterized by high seasonality in diffusive flux. Areas close to the dam generally showed low seasonal variability in diffusive CH4 flux but high variability in CO2 flux. Overall, we found that reservoir areas exhibiting highest emission rates (‘hotspots’) shifted substantially across hydrological seasons. Estimates of total diffusive carbon emission from the reservoir surfaces differed between hydrological seasons by a factor up to 7 in Chapéu D’Úvas, up to 13 in Curuá-Una, up to 4 in Furnas, and up to 1.8 in Funil, indicating that spatially-resolved measurements of CO2 and CH4 concentrations and k need to be performed at different hydrological seasons in order to constrain annual diffusive carbon emission.

Abstract [en]

Reservoirs are key for flood control, water supply, and hydropower generation. However, reservoirs are usually not carbon neutral. Studies worldwide point to reservoirs as important net sources of anthropogenic carbon emission to the atmosphere. Carbon emission from reservoirs derives from the decomposition of organic matter. Although carbon emission from reservoirs has been increasingly studied over the past two decades, most studies do not sufficiently describe emissions across space and time. Our study applies highly-resolved spatial coverage of dissolved surface water concentrations and gas-exchange coefficients of CO2 and CH4 to compute rates of CO2 and CH4 diffusion to the atmosphere across distinct hydrological seasons in four contrasting tropical reservoirs. We found that emissions varied substantially over both space and time. More specifically, we found that reservoir areas exhibiting highest emission rates ('hotspots') shifted substantially between dry and rainy seasons. Overlooking the spatial within-reservoir variability across seasons may result in serious under- or overestimations of total diffusive carbon emission from reservoirs, depending on the time and space that studies focus their sampling on. Our work may support scientists in adopting more comprehensive sampling strategies relevant for better constrained upscaling, and, consequently, support informed policy decisions and management actions.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2021. Vol. 126, no 4, article id e2020JG006014
National Category
Climate Research Environmental Sciences Geosciences, Multidisciplinary Oceanography, Hydrology and Water Resources
Identifiers
URN: urn:nbn:se:uu:diva-393437DOI: 10.1029/2020JG006014ISI: 000645001600025OAI: oai:DiVA.org:uu-393437DiVA, id: diva2:1353262
Available from: 2019-09-22 Created: 2019-09-22 Last updated: 2024-01-15Bibliographically approved
In thesis
1. Greenhouse gas emission from tropical reservoirs: Spatial and temporal dynamics
Open this publication in new window or tab >>Greenhouse gas emission from tropical reservoirs: Spatial and temporal dynamics
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The emission of methane (CH4) and carbon dioxide (CO2) from reservoirs has been estimated to make up for about 1.3% of the global anthropogenic greenhouse gas emission. The impoundment of a river leads to the accumulation of sediment that is brought in from inflowing rivers, and the sediment organic matter is degraded to CH4 and CO2. CH4 is of particular concern as its global warming potential is 34 times stronger than that of CO2. In the tropics, high temperatures and high availability of fresh organic matter from high net primary production fuel CH4 and CO2 production. As the construction of hydropower plants is currently undergoing a boom, especially in the tropics, reservoir emission is probably bound to increase.

The emission of CH4 and CO2 from reservoir surfaces is, however, highly variable, which makes current estimates uncertain. This thesis is built on the hypothesis that the spatial and temporal variability of greenhouse gas emission in tropical reservoirs, particularly of CH4 ebullition (the emission via gas bubbles), is so large that the sampling strategy affects whole-system estimates of greenhouse gas emission.

This thesis shows that greenhouse gas emission from the four studied tropical reservoirs in Brazil varied greatly at different timescales – over 24 hours, between days and between seasons. Seasonal variability was identified as the most important temporal scale to be covered for CH4 ebullition inventories. In addition, the spatial variability of gas emission was large for all pathways. The variability of CH4 ebullition across space, for example, was estimated to be almost as large as its variability between seasons, and patterns of spatial variability in diffusive CH4 and CO2 emission differed between seasons. For both ebullition and diffusion, river inflow areas were prone to elevated greenhouse gas emission.

This thesis shows that for retrieving solid emission estimates, there is no alternative to time-consuming measurements in the field. Measurements should be repeated at least once during each hydrological season (i.e. falling and rising water level). The seasonal surveys should cover space at a high resolution, including areas with and without river inflows, and different water column depths. CH4 ebullition made up for 60–99% of the total CO2-equivalent emission from the whole water surface of the studied reservoirs, with the highest contribution in the most productive reservoir. The most variable greenhouse gas emission pathway is therefore the most important one to be measured at appropriate resolution, particularly in productive reservoirs.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2019. p. 79
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1859
Keywords
methane, carbon dioxide, climate, carbon cycle, lake, limnology, inland water
National Category
Environmental Sciences Climate Research Oceanography, Hydrology and Water Resources Geosciences, Multidisciplinary
Identifiers
urn:nbn:se:uu:diva-393433 (URN)978-91-513-0757-2 (ISBN)
Public defence
2019-11-08, Friessalen, Norbyvägen 18, Uppsala, 09:15 (English)
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Supervisors
Available from: 2019-10-18 Created: 2019-09-22 Last updated: 2019-11-12

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Linkhorst, AnnikaSobek, Sebastian

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Climate ResearchEnvironmental SciencesGeosciences, MultidisciplinaryOceanography, Hydrology and Water Resources
Paranaíba, J. R. (2020). Supplemental data: Hotspots of diffusive CO2 and CH4 emission from tropical reservoirs shift through time.

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