Ökad växthuseffekt har länge varit i fokus för dess inverkan på framtida klimat. Det är i huvudsak mänskliga utsläpp som är orsaken till ökade mängder växthusgaser i atmosfären och stora ansträngningar görs för att utsläppen på sikt ska minska. I klimatmodeller beskrivs växthusgasbalansen utifrån både mänsklig och naturlig påverkan. Förståelsen för naturlig påverkan har länge varit begränsad och mer forskning behövs inom området. Flera studier visar på att sötvattensystem (sjöar, vattendrag osv.) avger växthusgaser som koldioxid, CO2 och metan, CH4, i större proportioner än vad som tidigare varit känt. Denna studie syftade till att undersöka CO2 och CH4-flöden från Erssjön i Skogaryd, Västergötland (en typisk svensk skogssjö) och få förståelse för hur den omgivande skogen påverkar såväl flöden som turbulens (utbyteskoefficienter) över sjön. Dessa resultat skulle jämföras med tidigare studier från den betydligt större sjön Tämnaren i norra Uppland. Resultaten visar på ett upptag av CO2 som en följd av fotosyntes hos skogen under dagtid, vilket leder till slutsatsen att omgivande skog påverkar växthusgasflödena. En ökning av både CO2 och CH4-flöden över sjön var tydlig nattetid, vilket enligt tidigare studier beror på konvektion i vattnet (liknande resultat för Tämnaren). Ökningen är speciellt utmärkande för CH4-flöden, vilket tros bero på att konvektionen ökar omblandningen och därmed löser upp sedimenterat CH4 och inducerar flerCH4-bubblor. Det sistnämnda kan styrkas med denna studie eftersom svaga vindar (1-2 m/s) varit dominerande i nattmätningarna och därmed har vindpåverkan på flödena varit låg. Resultaten visade likt Tämnaren att omgivningen påverkar turbulensen över sjön. Turbulensen uppstår troligen till följd av friktion vid skogskanten snarare än inverkan från hela skogsomgivningen, då beräknade utbyteskoefficienter inte visade på några större skillnader i spridning när endast data från sjön fanns representerade. Sammanfattningsvis är det intressant att i vidarestudier göra turbulensmätningar runt sjön för att sedan undersöka eventuella samband med utbyteskoefficienter och växthusgasflöden. / Increasing amounts greenhouse gases and its impacts on future climate has been in focus quite some time. The increase is mainly due human sources, and hugh efforts to decrease the emissions are made continuously. Climate models describe the greenhouse gas balance including both human and natural sources. In contrast to human sources, knowledge about natural sources is limited and requires further investigations. Recent studies show that greenhouse gases, such as Carbon dioxid, CO2, and methane, CH4, are emitted from freshwater systems (lakes, rivers etc.) in much larger proportions than what has been estimated earlier. The purpose of this study was to analyze CO2 and CH4 fluxes from a small lake with forest surroundings in southwestern Sweden, and also investigate if the forest affects both fluxes and turbulence (exchange coefficients) above the lake. The results from this study were to be compared with studies of a much larger lake (in eastern Sweden) but with similar surroundings. Results from the present study show an uptake of CO2 due to photosynthesis in the forest during daytime, leading to the conclusion that the surrounding forest affects the greenhouse gas fluxes. Both CO2 and CH4 fluxes increased above the lake during nighttime. According to similar studies (e.g. the larger lake in eastern Sweden), increased nighttime fluxes is an effect of increased convection in the water. The diurnal differences is most distinctive for CH4 fluxes. As suggested in earlier studies, the convection increases mixing in the water, dissoliving sedimented CH4 and inducing ebullition. In this study, low windspeed (1-2 m/s) dominated the nighttime measurements suggesting that ebullition is the main source of increased CH4 fluxes since wind-effects on the fluxes were low. Like the larger lake in eastern Sweden, results in this study indicate that the surroundings affect the turbulence above the lake. A large distribution in the calculated exchange coefficients that doesn’t decrease when only data above the lake is represented, suggests that turbulence above the lake is induced by friction at the forest edge, rather than the whole forest surroundings. Additional turbulence measurements around the lake for comparing with exchange coefficients and greenhouse gas fluxes could be of interest for future studies.

Turbulent fluxes govern the exchange of momentum, heat and moisture between the Earth’s surface and the overlying air. Computations of these fluxes are crucial, particularly over lakes and seas because most of the earth’s surface consists of water. One of the most common methods of calculating turbu- lent fluxes is the bulk method, where the fluxes are expressed with exchange coefficients. With more knowledge of these coefficients, the fluxes can be determined with a higher accuracy. Consequently, the turbulence structure and the exchange of moisture, momentum and heat between the surface and the overlying air can be better understood. The goal of this study was to compute the neutral exchange co- efficients for drag (CDN), heat (CHN) and moisture (CEN) and investigate their dependency on various atmospheric conditions, based on four years of measurements from Lake Erken, located about 70 km east of Uppsala. The coefficients were evaluated against the wind speed, stratification and time over water TOW (the time that the air is above the water before it reaches the tower). A special analysis was done by studying the variation of the coefficients with the wind speed during the UVCN-regime. Another analysis was done to see if the coefficients may have been influenced by non-local processes, e.g. advection from the surroundings. Additionally, normalized standard deviations for the temperature and humidity were evaluated for different stabilities. The results were compared with estimations by the COARE3.0 algorithm (for the dependency on the wind speed and the stability) in a previous report and other earlier studies.  The results indicated that the neutral exchange coefficients were higher and more dispersed during near neutral stratification and low TOWs. The normalized standard deviations also increased during neutral conditions. The explanation for this could be related to the presence of the UVCN-regime or non-local effects such as advection or entrainment from the surroundings. The wind speed had no ob- vious impact on the coefficients. However, the drag coefficient was larger and more spread out in the wind speed range 1-3 m/s. In comparison to earlier studies, the exchange coefficients were higher and scattered to a greater extent. This may be because of a strong UVCN-regime, sustainable non-local influences, relatively steeper waves than open-sea conditions or outliers in the data.