Freshwater bodies such as lakes release the greenhouse gas methane (CH4) into the atmosphere. Global emissions from lakes are estimated to emit more CH4 than oceans, despite that lakes occupies a much smaller global land area. Lakes are therefore significant components for global budgets of CH4. Accurate global estimations of lakes are troublesome, partly because of the spatial and temporal variability of CH4 fluxes, making regional and global assessments filled with uncertainties. Yet, few studies consider the spatial and temporal variability of CH4 fluxes. Therefore, this study investigates the spatial and temporal variability of CH4 fluxes in lakes at different scales. Measurements were made during two field campaigns in lake Venasjön and Parsen, located in the municipality of Söderköping, Sweden. We used the commonly used floating chamber (FC) method for CH4 flux measurements. In order to investigate the small-scale flux variability, we redeveloped the FC-method by constructing two grids consisting of seven FCs distributed approximately 1m apart from each other. One grid was placed at the shallow zone at the inflow of each lake and the other at the lakes deepest zone. By sampling the grid several times every field campaign, spatial and temporal variability of fluxes at different scales could be measured. Overall, we found a significant difference of CH4 fluxes in both lakes depending on field campaign and grid location. Our results also indicate that there is a small-scale variability of CH4 fluxes in lakes. Our hope is that these findings can illustrate the importance of investigating lake fluxes at small spatial and temporal scales. / Sjöar släpper ut växthusgasen metan (CH4) i atmosfären. Globala utsläpp från sjöar beräknas avge mer CH4 än havet, trots att sjöar har en mycket mindre global areal. Sjöar är därför viktiga komponenter för globala budgetar av CH4. Dessvärre är noggranna globala uppskattningar av sjöar svårt att göra, delvis på grund av den spatial och temporala variationen av CH4, vilket gör regionala och globala bedömningar fyllda med osäkerheter. Trots detta undersöker få studier metanflödets spatiala och temporala variabilitet. Denna studie undersöker den spatiala och temporala variabiliteten av CH4-flöden från sjöar i olika skalor. Mätningar genomfördes under två fältkampanjer i sjöarna Venasjön och Parsen som ligger i Söderköpings kommun, Sverige. För att undersöka variabiliteten i en liten skala utvecklade vi den redan använda floating chamber (FC) metoden för flödesmätningar genom att bygga två grids med sju FCs, ungefär 1m mellan varje kammare. En grid placerades vid den grunda delen vid inflödet av varje sjö och den andra vid respektive sjös djupaste del. Genom att mäta griden flera gånger varje fältkampanj kunde den spatiala och temporala variationen av flöden i olika skalor undersökas. Sammantaget fann vi signifikanta skillnader i båda sjöarnas CH4-flöden mellan fältkampanjer och grids. Våra resultat tyder också på småskaliga variationer av CH4-flöden i sjöar. Vårt hopp är att dessa resultat kan ytterligare bekräfta betydelsen av att undersöka sjöflödena i små spatiala och temporära skalor

The role of streams in the global carbon budget was for a long time neglected, since they were considered passive transporters of carbon from land to sea. However, studies have shown that streams are often supersaturated in carbon dioxide (CO2), making them sources of carbon to the atmosphere. The main sources of stream CO2 are in-stream mineralization of organic matter and transport of carbon from the catchment. The catchment derived CO2 could both be of biogenic (respiration) or geogenic (weathering) origin. Most studies regarding the topic rely on measurements carried out in forest-dominated catchments, while agricultural streams are under-represented. The objective of this study was to examine partial pressure of CO2 (pCO2) in streams in catchments dominated by agriculture. This was done to increase the knowledge about agricultural influence on stream pCO2, and to provide a basis for planning mitigation strategies for reducing CO2 emissions from the agriculture sector. Sampling was performed in ten streams draining agriculture-dominated catchments around Uppsala, Sweden, from June to November 2017. Measurements of pCO2 were carried out with floating chambers, equipped with CO2 sensors. Nutrients, organic carbon, discharge and different chemical variables were also measured. For correlation tests, the method Kendall’s Tau was used. Catchments were delineated in a geographic information system (GIS) and the CORINE Land Cover dataset was used to examine land use. Stream specific median pCO2 varied from 3000 to 10 000 μatm. In some streams, pCO2 exceeded 10 000 μatm, which was outside of the sensor’s measurement range. Values of pCO2 were high compared to similar studies in forested catchments, which could indicate that occurrence of agriculture in the catchment increases stream CO2. Correlation was found between pCO2 and discharge, with negative correlation in five streams and positive correlation in two. Negative correlation was found between pCO2 and pH and percentage of dissolved oxygen, respectively. No significant correlation was found between pCO2 and fraction of agricultural land use, nutrients or organic carbon. Further studies are needed to examine the sources of CO2, since it is possible that a large part of the CO2 has a geogenic origin. The floating chamber method should be revised to reduce the sensor’s sensitivity to condensation and cold temperatures, and to increase the measuring range.