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Comparing methane ebullition variability across space and time in a Brazilian reservoir
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.ORCID iD: 0000-0002-3609-5107
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Limnology.
Aquatic Physics Group, Department F.-A. Forel for Environmental and Aquatic Sciences, Faculty of Science, University of Geneva, Geneva, Switzerland.
Department of Biology, Institute of Biological Sciences, Federal University of Juiz de Fora, Juiz de Fora, Brazil.
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2020 (English)In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 65, no 7, p. 1623-1634Article in journal (Refereed) Published
Abstract [en]

The potent greenhouse gas methane (CH4) is readily emitted from tropical reservoirs, often via ebullition (bubbles). This highly stochastic emission pathway varies in space and time, however, hampering efforts to accurately assess total CH4 emissions from water bodies. We systematically studied both the spatial and temporal scales of ebullition variability in a river inflow bay of a tropical Brazilian reservoir. We conducted multiple highly resolved spatial surveys of CH4 ebullition using a hydroacoustic approach supplemented with bubble traps over a 12‐month and a 2‐week timescale to evaluate which scale of variation was more important. To quantify the spatial and temporal variability of CH4 ebullition, we used the quartile coefficients of dispersion at each point in space and time and compared their frequency distributions across the various temporal and spatial scales. We found that CH4 ebullition varied more temporally than spatially and that the intra‐annual variability was stronger than daily variability within 2 weeks. We also found that CH4 ebullition was positively related to water temperature increase and pressure decrease, but no consistent relationship with water column depth or sediment characteristics was found, further highlighting that temporal drivers of emissions were stronger than spatial drivers. Annual estimates of CH4 ebullition from our study area may vary by 75–174% if ebullition is not resolved in time and space, but at a minimum we recommend conducting spatially resolved measurements at least once during each major hydrologic season in tropical regions (i.e., in dry and rainy season when water levels are falling and rising, respectively).

Place, publisher, year, edition, pages
2020. Vol. 65, no 7, p. 1623-1634
Keywords [en]
CH4, carbon cycle, lake, biogeochemistry, spatial, temporal scale
National Category
Climate Research Environmental Sciences Geosciences, Multidisciplinary Oceanography, Hydrology and Water Resources
Research subject
Biology with specialization in Limnology
Identifiers
URN: urn:nbn:se:uu:diva-393435DOI: 10.1002/lno.11410ISI: 000547708600012OAI: oai:DiVA.org:uu-393435DiVA, id: diva2:1353260
Funder
EU, FP7, Seventh Framework Programme, 336642Available from: 2019-09-22 Created: 2019-09-22 Last updated: 2020-09-17Bibliographically 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, AnnikaHiller, CarolinSobek, Sebastian

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