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Browsing by Subject "järven metaanipäästöt"

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  • Haverinen, Samuel (2021)
    Methane is an important greenhouse gas in the global atmosphere and its concentration has more than doubled compared to preindustrial times. Fresh water lakes and streams are substantial sources of methane. However, the estimations of their role in the global methane budget vary significantly and not until the 21st century has the understanding of their role as substantial methane sources increased. In the boreal zone, lakes produce as much as 30 % of the methane emissions. In this study, we examine spatial and temporal changes in methane fluxes from northern boreal lake Pallasjärvi and from a small stream located in its catchment area between June and November of 2020. We also examine the factors that explain the spatial and temporal changes in the methane fluxes, both lake and stream. Lake and stream methane fluxes were measured every other week between June and August, once a week in September, and once in the beginning of November using the chamber method. During the chamber measurements we also measured water surface temperature. Furthermore, at the stream sites we measured the water flow rate. At the lake measurement points we measured the water depth in November, which we calibrated to apply to the entire measurement period using water pressure logger. In March 2021, we measured the depth of the sediment layer at the lake sites. We also used CORINE-landcover data to model stream methane fluxes and for the lake, we used Finnish Meteorological Institute’s wind speed and direction dataset. We used these variables in order to explain the spatial and temporal variation of the lake and the stream methane fluxes using correlation analysis and linear mixed models. In this study, we find that water surface temperature, water depth, and wind were significant variables in explaining the lake methane fluxes. Respectively, landcover and surface water temperature explained the stream methane flux. From a temporal perspective, the strongest fluxes were measured between June and July at the lake sites and August and the beginning of September at the stream sites. Methane fluxes were divided spatially in two different groups at both lake and stream sites. At the lake sites, the strongest fluxes were measured in the shallow Pallaslompolo area and the weakest at the larger and deeper main basin of the lake. At the stream sites, the fluxes were also divided in two groups, the upstream’s weak fluxes and the downstream’s strong fluxes. According to the results, the temporal change of flux in the lake area is controlled by the changes in the factors underlying the methane production, and the differences in the lake basin depth control the spatial change of flux. The temporal change of stream methane flux depends on the changes in the methane production in the stream and its catchment area, while the spatial change depends on the changes in the landcover along the stream. However, more research and data are needed about the lake sediment layer temperature and oxygen levels, the water methane concentration, and stream catchments with different landcovers, which all impact the methane fluxes.