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Methane dynamics in northern lakes: Insights from multi-scale observations
Linköping University, Department of Thematic Studies, Tema Environmental Change. Linköping University, Faculty of Arts and Sciences.
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Methane (CH4) is a potent greenhouse gas which is emitted to the atmosphere from both natural and anthropogenic sources. Current evidence indicates that lakes account for a large part of the global emissions of CH4, but their contribution is difficult to quantify because of large temporal and spatial variability in processes leading to CH4 fluxes from lakes to the atmosphere. Making sense of the complexity and variability of CH4 emissions from lakes requires observations covering the range of temporal and spatial scales at which these processes occur, both within and between lakes. Northern regions are of particular interest for such studies because they contain a larger number of lakes than any other region in the world and they are disproportionately affected by climate change, with possible consequences for future CH4 emissions.

The aim of this thesis was to investigate patterns of CH4 dynamics and emissions in several lakes distributed in different climatic regions of Sweden, paying particular attention to spatial and temporal variability of CH4 fluxes and concentrations. Fluxes, concentrations, carbon stable isotope signature of CH4, and a range of commonly monitored lake characteristics were measured several times during one year at multiple locations in each lake. The measurements provided an extensive set of observations of CH4 concentrations and fluxes in lakes, together with possible environmental drivers. These observations were then used to investigate patterns of CH4 dynamics in northern lakes and to assess the ability of empirical and process-based models to predict CH4 concentrations and fluxes in lakes.

The results indicate that simple empirical models, consisting of linear regressions between explanatory variables and CH4 fluxes and concentrations averaged over the lake surface and ice-free period of the year, can be useful in some specific cases (for example describing ebullitive fluxes from total phosphorus or chlorophyll a concentrations). However, it was also noted that using such models for extrapolation can lead to large errors, especially if the observations do not account for temporal and spatial variability of CH4 fluxes and concentrations. An example of high variability was seen in day-night measurements of CH4 fluxes in four lakes over several months. To try to compensate for some of the shortcomings of empirical models, an established process-based and one-dimensional lake model was used to simulate CH4 concentration in the water column of the studied lakes. Predictions were in good agreement with observations in several of the investigated lakes, considering that the model was not pre-calibrated for any of the lake specifically. However, it was also clear that there can be key processes that require specific consideration in process-based models, and some degree of simplification is needed, especially when detailed information on the modelled systems is not available. The simplifications and assumptions that need to be made can be informed by the study and observation of relevant processes in situ. For example, groundwater was found to potentially contribute a major part of CH4 stored in one small boreal lake using measurements of stable isotope signature of CH4 in littoral sediment and deep water of that lake, as well as in the groundwater in the mire next to it. Stable isotope measurements in five other lakes also revealed consistent differences in CH4 sources to the surface and deep zones of lakes when they are separated by thermal stratification of the water column. Such knowledge could be used in the design of numerical models of lakes with the objective to improve predictions of current and future emissions of CH4 from these environments.

Overall, this thesis contributes to the current knowledge on assessment of CH4 emissions from lakes at several temporal and spatial scales. It also emphasizes critical aspects which must be considered to reduce bias in future empirical and process-based models of CH4 in lakes.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2022. , p. 37
Series
Linköping Studies in Arts and Sciences, ISSN 0282-9800 ; 837
Keywords [en]
Methane, Lakes, Variability, Sampling design, Models
National Category
Climate Research
Identifiers
URN: urn:nbn:se:liu:diva-185123DOI: 10.3384/9789179293871ISBN: 9789179293864 (print)ISBN: 9789179293871 (electronic)OAI: oai:DiVA.org:liu-185123DiVA, id: diva2:1658905
Public defence
2022-08-24, TEMCAS, Building T, Campus Valla, Linköping, 12:15 (English)
Opponent
Supervisors
Available from: 2022-05-18 Created: 2022-05-18 Last updated: 2022-05-18Bibliographically approved
List of papers
1. Diel variability of methane emissions from lakes
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2020 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 117, no 35, p. 21488-21494Article in journal (Refereed) Published
Abstract [en]

Lakes are considered the second largest natural source of atmospheric methane (CH4). However, current estimates are still uncertain and do not account for diel variability of CH4 emissions. In this study, we performed high-resolution measurements of CH4 flux from several lakes, using an automated and sensor-based flux measurement approach (in total 4,580 measurements), and demonstrated a clear and consistent diel lake CH4 flux pattern during stratification and mixing periods. The maximum of CH4 flux were always noted between 10:00 and 16:00, whereas lower CH4 fluxes typically occurred during the nighttime (00:00-04:00). Regardless of the lake, CH4 emissions were on an average 2.4 higher during the day compared to the nighttime. Fluxes were higher during daytime on nearly 80% of the days. Accordingly, estimates and extrapolations based on daytime measurements only most likely result in overestimated fluxes, and consideration of diel variability is critical to properly assess the total lake CH4 flux, representing a key component of the global CH4 budget. Hence, based on a combination of our data and additional literature information considering diel variability across latitudes, we discuss ways to derive a diel variability correction factor for previous measurements made during daytime only.

Place, publisher, year, edition, pages
Washington, DC 20001 United States: NATL ACAD SCIENCES, 2020
Keywords
methane fluxes; diel variability; automated flux chambers; lake greenhouse gas emissions
National Category
Physical Geography
Identifiers
urn:nbn:se:liu:diva-170661 (URN)10.1073/pnas.2006024117 (DOI)000572974300014 ()32817550 (PubMedID)2-s2.0-85090508855 (Scopus ID)
Note

Funding Agencies|European Research Council under the European Unions Horizon 2020 research and innovation programmeEuropean Research Council (ERC) [725546]; Swedish Research CouncilSwedish Research Council [2016-04829]; FORMAS (Swedish research council for sustainable development) [2018-01794]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [2016.0083]

Available from: 2020-10-19 Created: 2020-10-19 Last updated: 2022-05-18Bibliographically approved
2. Methane in Lakes: Variability in Stable Carbon Isotopic Composition and the Potential Importance of Groundwater Input
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2021 (English)In: Frontiers in Earth Science, E-ISSN 2296-6463, Vol. 9, article id 722215Article in journal (Refereed) Published
Abstract [en]

Methane (CH4) is an important component of the carbon (C) cycling in lakes. CH4 production enables carbon in sediments to be either reintroduced to the food web via CH4 oxidation or emitted as a greenhouse gas making lakes one of the largest natural sources of atmospheric CH4. Large stable carbon isotopic fractionation during CH4 oxidation makes changes in 13C:12C ratio (δ13C) a powerful and widely used tool to determine the extent to which lake CH4 is oxidized, rather than emitted. This relies on correct δ13C values of original CH4 sources, the variability of which has rarely been investigated systematically in lakes. In this study, we measured δ13C in CH4 bubbles in littoral sediments and in CH4 dissolved in the anoxic hypolimnion of six boreal lakes with different characteristics. The results indicate that δ13C of CH4 sources is consistently higher (less 13C depletion) in littoral sediments than in deep waters across boreal and subarctic lakes. Variability in organic matter substrates across depths is a potential explanation. In one of the studied lakes available data from nearby soils showed correspondence between δ13C-CH4 in groundwater and deep lake water, and input from the catchment of CH4 via groundwater exceeded atmospheric CH4 emissions tenfold over a period of 1 month. It indicates that lateral hydrological transport of CH4 can explain the observed δ13C-CH4 patterns and be important for lake CH4 cycling. Our results have important consequences for modelling and process assessments relative to lake CH4 using δ13C, including for CH4 oxidation, which is a key regulator of lake CH4 emissions.

Place, publisher, year, edition, pages
Lausanne, Switzerland: Frontiers Media S.A., 2021
Keywords
General Earth and Planetary Sciences, stable carbon isotope, methane, lake, groundwater, endmember
National Category
Environmental Sciences
Identifiers
urn:nbn:se:liu:diva-181136 (URN)10.3389/feart.2021.722215 (DOI)000717647200001 ()2-s2.0-85118920484 (Scopus ID)
Funder
EU, European Research CouncilSwedish Research CouncilSwedish Research Council FormasKnut and Alice Wallenberg Foundation
Note

Funding: European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programmeEuropean Research Council (ERC) [725546]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [201604829]; FORMASSwedish Research Council Formas [2018-01794]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [2016.0083]

Available from: 2021-11-17 Created: 2021-11-17 Last updated: 2023-08-28Bibliographically approved

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