Lakes play a significant role in the global carbon cycle, acting as sources and sinks of carbon dioxide (CO2). In situ measurements of CO2 flux (FCO2) from lakes have generally been collected during daylight, despite indications of significant diel variability. This introduces bias when scaling up to whole-lake annual aquatic carbon budgets. We conducted an international sampling program to ascertain the extent of diel variation in FCO2 across lakes. We sampled 21 lakes over 41 campaigns and measured FCO2 at 4-h intervals over a full diel cycle. Rates of FCO2 ranged from -3.16 to 4.39 mmol m-2 h(-1). Integrated over a day, FCO2 ranged from -381.68 to 878.49 mg C m(-2) d(-1) (mean = 76.54) across campaigns. We identified three characteristic diel patterns in FCO2 related to trophic status and show that for half of the campaigns, daily flux estimates were biased by > 50% if based on a single (daytime) measurement.

The CO( 2)flux (FCO2) from lakes to the atmosphere is a large component of the global carbon cycle anddepends on the air-water CO2concentration gradient (Delta CO2) and the gas transfer velocity (k). Both Delta CO2 and k can vary on multiple timescales and understanding their contributions toFCO(2)is important for explaining var-iability influxes and developing optimal sampling designs. We measuredFCO2 and Delta CO(2 )and derivedkforone full ice-free period in 18 lakes usingfloating chambers and estimated the contributions of Delta CO2 and k to FCO2 variability. Generally, kcontributed more than Delta CO2to short-term (1-9d) FCO2 variability. With in creased temporal period, the contribution of k to FCO2 variability decreased, and in some lakes resulted in Delta CO2 contrib-uting more thank to FCO2 variability over the full ice-free period. Increased contribution of Delta CO2 to FCO2 vari-ability over time occurred across all lakes but was most apparent in large-volume southern-boreal lakes and indeeper (>2m) parts of lakes, whereaskwas linked to FCO(2 )variability in shallow waters. Accordingly, knowing the variability of bothk and Delta CO(2 )over time and space is needed for accurate modeling of F CO2 from these vari-ables. We conclude that priority in FCO(2 )assessments should be given to direct measurements of FCO2 at multiplesites when possible, or otherwise from spatially distributed measurements of Delta CO(2 )combined with k- models that incorporate spatial variability of lake thermal structure and meteorology.

Lakes cover only ~2 % of the global land area, but their connections to the surrounding catchment make them important for the global carbon cycle. A considerable amount of the carbon input to lakes is emitted to the atmosphere as carbon dioxide (CO₂) through diffusive flux. This CO₂ flux varies with surface water CO₂ concentrations (C_wCO2) and the transfer velocity of CO₂ across the thin boundary layer between surface water and atmosphere (k), which both in turn depend on physical, biological, and chemical factors that interplay with lake and catchment characteristics over various time scales. Comprehensive studies of these interlinkages across lake types are rare, and current assessments of lake CO₂ emissions are therefore uncertain. In this thesis, the variability and regulation of lake CO₂ fluxes across a latitudinal gradient in Sweden is investigated. The thesis explores how C_wCO2 and k regulate lake CO₂ fluxes and how spatiotemporal patterns of CO₂ fluxes vary within and across lakes.

Regulation of CO₂ flux at shorter temporal scales (<1 week) was dominated by k. However, the contribution from C_wCO2 increased over time making it the dominant factor for seasonal CO₂ flux in some lakes. Furthermore, we show that ways of assessing k in lakes may lead to bias, possibly due to inadequate consideration of processes occurring at the upper surface layer of lakes. In the three lakes where daynight variability was studied, we found consistent patterns of higher fluxes of CO₂ at daytime during periods where lakes were emitting CO₂. Meanwhile, the period of lake water column turnover in autumn was crucial for both day-night variability and total lake CO₂ fluxes. Based on the patterns above, we have made recommendations on improved study design for representative measurements of CO₂ fluxes in lakes. In addition, we produced models for estimating CO₂ flux from combinations of climatic data, satellite imagery and national lake inventory data, i.e., information that is relatively easily available and thus simplify extrapolation of flux estimates to other lakes. Patterns observed across our models suggest strong climate feedbacks, which may lead to increasing CO₂ fluxes from lakes at northern latitudes along with precipitation and temperature increases there. Thus, results in this thesis urges forthcoming studies to better account for spatiotemporal variability to improve upon models that can be used for large-scale estimates and future predictions of lake CO₂ fluxes.

Sjöar täcker endast 2 % av jordens landyta, men deras kopplingar till det omgivande avrinningsområdet gör dem viktiga för den globala kolcykeln. En betydande del av koltillförseln till sjöar släpps ut i atmosfären som koldioxid (CO₂), genom diffusion. Detta CO₂-flöde varierar med ytvattnets koncentration av CO₂ (C_WCO2) och överföringshastigheten för CO₂ över det tunna gränsskiktet mellan ytvatten och atmosfär (k), som båda i sin tur beror på fysikaliska, biologiska och kemiska faktorer som samverkar inom sjön och dess avrinningsområde över olika tidsskalor. Omfattande studier av dessa kopplingar mellan sjötyper är sällsynta, och som följd är bedömningar av sjöars koldioxidutsläpp osäkra. I denna avhandling undersöks variabiliteten och regleringen av sjöars CO₂-flöden över en latitudinell gradient i Sverige. Avhandlingen undersöker hur C_WCO2 och k reglerar sjöars CO₂-flöden och hur rumsliga och tidsmässiga mönster av CO₂-flöden varierar inom och mellan sjöar.

Regleringen av CO₂-flöde vid kortare tidsskalor (<1 vecka) dominerades av k. Bidraget från C_WCO2 ökade dock över tiden, och utgjorde den dominerande faktorn för säsongsbetonat CO₂-flöde i vissa sjöar. Vi visar att metoder som används för att bedöma k i sjöar kan leda till fel uppskattningar, möjligen på grund av otillräcklig hänsyn till processer som sker vid sjöns övre ytskikt. I de tre sjöarna där dag-natt variabilitet studerades, fann vi tydliga mönster av högre CO₂-flöden på dagtid under perioder där sjöar avgav CO₂. Samtidigt var sjöars omsättningsperiod på hösten avgörande för både dag-natt-variabilitet och totala CO₂-flöden. Baserat på mönstren ovan har vi gjort rekommendationer som syftar till hur mätningar av CO₂-flöden i sjöar kan bli mer representativa. Vi har även tagit fram modeller för att uppskatta CO₂-flöde från kombinationer av klimat- och satellitdata samt nationella sjöinventeringsdata, d.v.s. data som är relativt lättillgängliga och därmed underlättar extrapolering av flödesuppskattningar till andra sjöar. Mönster som observerats i våra modeller tyder på stark klimatåterkoppling, som kan leda till allt större CO₂-flöden från sjöar vid norra breddgrader i takt med ökad nederbörd och temperatur där. Vi uppmanar kommande studier till ökad hänsyn av rumsliga och tidsmässiga variationer för att förbättra modeller som kan användas för storskaliga uppskattningar och framtida förutsägelser av sjöars CO₂-flöden.

Lakes are generally supersaturated in carbon dioxide (CO2) and emitters of CO2 to the atmosphere. However, estimates of CO2 flux (FCO2) from lakes are seldom based on direct flux measurements and usually do not account for nighttime emissions, yielding risk of biased assessments. Here, we present direct FCO2 measurements from automated floating chambers collected every 2-3 hr and spanning 115 24 hr periods in three boreal lakes during summer stratification and before and after autumn mixing in the most eutrophic lake of these. We observed 40%-67% higher mean FCO2 in daytime during periods of surface water CO2 supersaturation in all lakes. Day-night differences in wind speed were correlated with the day-night FCO2 differences in the two larger lakes, but in the smallest and most wind-sheltered lake peaks of FCO2 coincided with low-winds at night. During stratification in the eutrophic lake, CO2 was near equilibrium and diel variability of FCO2 insignificant, but after autumn mixing FCO2 was high with distinct diel variability making this lake a net CO2 source on an annual basis. We found that extrapolating daytime measurements to 24 hr periods overestimated FCO2 by up to 30%, whereas extrapolating measurements from the stratified period to annual rates in the eutrophic lake underestimated FCO2 by 86%. This shows the importance of accounting for diel and seasonal variability in lake CO2 emission estimates.

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.