David Rudberg

Lakes play a significant role in the global carbon cycle, acting as sources and sinks of carbon dioxide (CO ₂ ). In situ measurements of CO ₂ flux (FCO ₂ ) 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 FCO ₂ across lakes. We sampled 21 lakes over 41 campaigns and measured FCO ₂ at 4‐h intervals over a full diel cycle. Rates of FCO ₂ ranged from −3.16 to 4.39 mmol m ⁻² h ⁻¹ . Integrated over a day, FCO ₂ 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 FCO ₂ 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 CO₂ flux () from lakes to the atmosphere is a large component of the global carbon cycle and depends on the air–water CO₂ concentration gradient (ΔCO₂) and the gas transfer velocity (k). Both ΔCO₂ and k can vary on multiple timescales and understanding their contributions to is important for explaining variability in fluxes and developing optimal sampling designs. We measured and ΔCO₂ and derived k for one full ice‐free period in 18 lakes using floating chambers and estimated the contributions of ΔCO₂ and k to variability. Generally, k contributed more than ΔCO₂ to short‐term (1–9 d) variability. With increased temporal period, the contribution of k to variability decreased, and in some lakes resulted in ΔCO₂ contributing more than k to variability over the full ice‐free period. Increased contribution of ΔCO₂ to variability over time occurred across all lakes but was most apparent in large‐volume southern‐boreal lakes and in deeper (> 2 m) parts of lakes, whereas k was linked to variability in shallow waters. Accordingly, knowing the variability of both k and ΔCO₂ over time and space is needed for accurate modeling of from these variables. We conclude that priority in assessments should be given to direct measurements of at multiple sites when possible, or otherwise from spatially distributed measurements of ΔCO₂ combined with k‐models that incorporate spatial variability of lake thermal structure and meteorology.

Lakes contribute 9%–19% of global methane (CH₄) emissions to the atmosphere. Dissolved molecular oxygen (DO) in lakes can inhibit the production of CH₄ and promote CH₄ oxidation. DO is therefore often considered an important regulator of CH₄ emissions from lakes. Presence or absence of DO in the water above the sediments can affect CH₄ production and emissions by (a) influencing if methane production can be fueled by the most reactive organic matter in the top sediment layer or rely on deeper and less degradable organic matter, and (b) enabling CH₄ accumulation in deep waters and potentially large emissions upon water column turnover. However, the relative importance of these two DO effects on CH₄ fluxes is still unclear. We assessed CH₄ fluxes from two connected lake basins in northern boreal Sweden where one was experimentally oxygenated. Results showed no clear difference in summer CH₄ emissions attributable to water column DO concentrations. Large amounts of CH₄ accumulated in the anoxic hypolimnion of the reference basin but little of this may have been emitted because of incomplete mixing, and effective methane oxidation of stored CH₄ reaching oxic water layers. Accordingly, ≤24% of the stored CH₄ was likely emitted in the experimental lake. Overall, our results suggest that hypolimnetic DO and water column CH₄ storage might have a smaller impact on CH₄ emissions in boreal forest lakes than previous estimates, yet potential fluxes associated with water column turnover events remain a significant uncertainty in lake CH₄ emission estimates.

Large greenhouse gas emissions occur via the release of carbon dioxide (CO₂) and methane (CH₄) from the surface layer of lakes. Such emissions are modeled from the air-water gas concentration gradient and the gas transfer velocity (k). The links between k and the physical properties of the gas and water have led to the development of methods to convert k between gases through Schmidt number normalization. However, recent observations have found that such normalization of apparent k estimates from field measurements can yield different results for CH₄ and CO₂. We estimated k for CO₂ and CH₄ from measurements of concentration gradients and fluxes in four contrasting lakes and found consistently higher (on an average 1.7 times) normalized apparent k values for CO₂ than CH₄. From these results, we infer that several gas-specific factors, including chemical and biological processes within the water surface microlayer, can influence apparent k estimates. We highlight the importance of accurately measuring relevant air-water gas concentration gradients and considering gas-specific processes when estimating k.

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 ...

Lakes are major global sources of atmospheric methane (CH4), representing an important greenhouse gas. Dissolved molecular Oxygen (DO) in lakes hinders production of CH4 while promoting CH4 oxidation. Consequently, it has been suggested as an important regulator of CH4 emissions from lakes. Presence or absence of DO at the sediment-water interface could therefore influence the extent of CH4 production in top sediment layers, and the amount of CH4 that is stored in the anoxic layer of the water column and potentially emitted during water column mixing events. However, the quantitative importance of DO on CH4 fluxes remains unknown. We studied CH4 fluxes from two lake basins in northern boreal Sweden of which DO was experimentally added to the deep waters in one of the basins (experimental basin) while the other basin was left in a natural state (reference basin). We used spatial and temporally distributed flux chambers to measure CH4 fluxes while the lake basins were stratified (from June to October) and found that there was no significant difference in CH4 fluxes between the two lake basins attributable to the water column experiment. Moreover, we found that the oxygenation of the hypolimnion resulted in a large decrease in CH4 concentration in the experimental basin, in contrast to the reference basin. However, our monthly lake profile measurements indicated that only a small amount of this CH4 may have been emitted during the open-water season. First, the two lake basins were subject to incomplete spring and fall mixing events. Second, our CH4 emission and oxidation model, based on bathymetry, CH4 concentrations, depth distributed 13C/12C measurements, and gas transfer velocities, indicated that 0 – 24 % of the stored CH4 may be emitted on a yearly basis. This shows that the overall impact on CH4 emissions from boreal forest lakes, due to CH4 storage and DO, may be smaller than previously believed. However, Fluxes during the mixing events still represent a large uncertainty for the total yearly lake CH4 emissions.

Lakes are responsible for substantial emissions of methane (CH4) to the atmosphere globally, but their contribution is poorly constrained and current estimates vary widely. One of the reasons for this large uncertainty lies in the practical challenges involved in collecting CH4 concentration and flux data at the suitable temporal and spatial scales to capture their natural variability. Here, we present the results of an attempt to account for the spatial and temporal variability of CH4 concentrations and fluxes when deriving whole-lake and yearly average values. We used these average values to investigate the main environmental drivers of CH4 concentrations in the surface water of lakes and CH4 emissions from lakes. We made surface water CH4 concentration and CH4 fluxes measurements using headspace equilibration and floating chambers, respectively, in a set of boreal lakes located in different parts of Sweden and with various morphological and biogeochemical properties. Measurements covered different periods of the open-water season at multiple locations covering various depths/distances to shore in each lake. Individual CH4 flux measurements (150-300 measurements per lake) were interpolated based on relationships with local spatial and temporal variables. Relationships between mean open water season CH4 concentrations and emissions, and one to three independent environmental variables were tested as different models. The results suggest that the frequency and spatial coverage of the measurements is critical for identifying reliable quantitative empirical models of CH4 concentrations and emissions from lakes.

Lakes emit CO2 to the atmosphere at magnitudes significant for the global carbon cycle, in the form of diffusive CO2 flux (FCO2). As direct FCO2 measurements are time-consuming, FCO2 is often estimated from the air-water CO2 concentration gradient (ΔpCO2) and the gas transfer velocity (k), representing the two components considered to regulate FCO2. However, extrapolating measurements of ΔpCO2 and k to whole-year estimates require understanding of their variability in time and across different types of lakes, which is often insufficient. As a result, simple linear interpolations are typically used in extrapolations which risk producing bias as spatiotemporal variability is not included. Further insight to the variability of ΔpCO2 and k may contribute to more representative extrapolations and provide guidance for focusing sampling campaigns on capturing times of high variability. We used floating flux chambers and surface water samples to measure FCO2 and ΔpCO2, respectively, both within-weeks and over seasons during the open water period at 12 locations in each of 15 boreal lakes across a latitudinal gradient in Sweden. We combined these measurements to derive spatially resolved values of k in order to identify: i) the contributions of ΔpCO2 and k to FCO2 variability over time; and ii) if differences in the contributions of ΔpCO2 and k to FCO2 variability can be related to lake characteristics. The results presented are relevant for improved modelling of lake CO2 emissions.

Lakes are generally supersaturated in carbon dioxide (CO₂) and emitters of CO₂ to the atmosphere. However, estimates of CO₂ flux () 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 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 in daytime during periods of surface water CO₂ supersaturation in all lakes. Day‐night differences in wind speed were correlated with the day‐night differences in the two larger lakes, but in the smallest and most wind‐sheltered lake peaks of coincided with low‐winds at night. During stratification in the eutrophic lake, CO₂ was near equilibrium and diel variability of insignificant, but after autumn mixing 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 by up to 30%, whereas extrapolating measurements from the stratified period to annual rates in the eutrophic lake underestimated by 86%. This shows the importance of accounting for diel and seasonal variability in lake CO₂ emission estimates.

An increasing number of rivers is being dammed, particularly in the tropics, and reservoir water surfaces can be a substantial anthropogenic source of greenhouse gases. On average, 80% of the CO₂‐equivalent emission of reservoirs globally has been attributed to CH₄, which is predominantly emitted via ebullition. Since ebullition is highly variable across space and time, both measuring and upscaling to an entire reservoir is challenging, and estimates of reservoir CH₄ emission are therefore not well constrained. We measured CH₄ ebullition at high spatial resolution with an echosounder and bubble traps in two reservoirs of different use (water storage and hydropower), size and productivity in the tropical Brazilian Atlantic Rainforest biome. Based on the spatially most well‐resolved whole‐reservoir ebullition measurements in the tropics so far, we found that mean CH₄ ebullition was twice as high in river inflow areas as in other parts of the reservoirs, and more than 4 times higher in the eutrophic compared to the oligotrophic reservoir. Using different upscaling approaches rendered similar whole‐reservoir CH₄ ebullition estimates, suggesting that highly spatially‐resolved measurements may be more important for constraining reservoir‐wide CH₄ estimates than choice of upscaling approach. The minimum sampling effort was high (>250 and >1,700 thirty‐meter segments of hydroacoustic survey to reach within 50% or 80% accuracy, respectively). This suggests that traditional manual bubble‐trap measurements should be abandoned in favor of highly resolved measurements in order to get spatially representative estimates of CH₄ ebullition, which accounted for 60% and 99% of total C emission in the two studied reservoirs.