Browsing by Subject "boreal lake"

Methane is a powerful greenhouse gas that is released to the atmosphere in many ways from natural sources. While wetlands are seen as major sources, open water systems, including small boreal lakes, should also be considered when estimating methane emissions locally and globally. Methane is produced the sediments and has several oxidation processes and emission pathways. In this master’s thesis, the sediments of Lake Pääjärvi, southern Finland, were studied using two different porewater sampling methods to analyze methane concentrations and geochemical properties of the porewater. The sampling methods, the Rhizons filter and the cut syringe method, were both performed to total of five sampling sites. A sediment profile was made from two locations, PAME1 and PAME2, and single sampling at 10 cm depth from the other sites. The sampling sites were located on different parts of the lake at depths of 3–16 m. Additionally, the water column was sampled for methane and water quality, and sediment for grain size, organic matter, and the C:N-ratio. As a result, the sampling methods were successful, and the sediment profiles and the sites could be compared. While there was difference in the methane concentrations, reliability of the methods was not concluded. The Rhizons filter method gave higher methane concentrations in only one sampling site, whereas the cut syringe was dominant in all others. Influential factors on the differences could be the use of different sampling cores, or the different duration of sampling between the sampling sites. The sulfate-methane transition zone was recognized from the depth of 4.5–6.5 cm in PAME1 and circa 3 cm in PAME2. The only sediment feature that coincided with high methane concentrations was larger grain size, although the variation between sampling sites was not large. The methane concentrations in the epilimnion were higher than near the sediment-water interface, which suggested that the methane in Lake Pääjärvi originates mainly from the catchment area and not from the sediments.