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Gas bubble emission (ebullition) from lake sediment is a prominent source of the greenhouse gas methane (CH₄) to the atmosphere, but the stochastic nature of bubble release and thus high variability in space and time makes the estimation of lake CH₄ ebullition challenging. Several studies have identified major drivers of CH₄ ebullition variability over time, while fewer studies have identified what modulates CH₄ ebullition spatially. Here, we measured CH₄ ebullition in a Swedish eutrophic lake for 2 years over a longitudinal gradient between the main inlet and outlet, hypothesizing that differences in sediment properties across the lake are related to CH₄ ebullition. We found that CH₄ ebullition decreased with increasing distance from the main inlet. Elevated CH₄ ebullition was observed in areas with high sediment organic carbon density. These regions experience temporary sediment deposition but are not sites of long-term sediment accumulation due to their proximity to the inlet and susceptibility to erosion during high discharge events. Major CH₄ ebullition events were synchronous across sites and related to water level changes, indicating that changes in hydrostatic pressure regulate the timing of CH₄ ebullition. Even though the timing of major CH₄ ebullition events was very different between years, the spatial variability pattern in CH₄ ebullition was very similar both within and between years, further indicating that site-specific sediment characteristics drive CH₄ ebullition. The links between sediment properties and the spatial variability of CH₄ ebullition shown here can contribute to reducing the uncertainty in estimates of CH₄ emission from lakes.

Measurements of surface-atmosphere carbon dioxide (CO2) and methane (CH4) fluxes have been relatively sparse across the Arctic tundra and boreal biomes, causing significant uncertainties in carbon budget estimates from the region. While the availability of Arctic-boreal carbon flux data has increased substantially over the past decade, the data have remained spread across different repositories, scientific articles, and unpublished sources, making it difficult to leverage. Here we present a new dataset of monthly Arctic-boreal carbon fluxes (ABCFlux v2) across terrestrial (wetlands and uplands) and freshwater (lakes and rivers) ecosystems compiled from previous syntheses including the Arctic-boreal CO2 flux database (ABCFlux v1), the Boreal-Arctic Wetland and Lake Methane Dataset (BAWLD-CH4), and the Global River Methane Database (GRiMeDB). In addition, we consider data from general-purpose (e.g., Zenodo) and flux network repositories, literature, and site principal investigators. The dataset includes surface-atmosphere CO2 fluxes of gross primary production (GPP), ecosystem respiration (Reco), and net ecosystem exchange (NEE), alongside CH4 fluxes. For aquatic ecosystems, we split CH4 fluxes into diffusive and ebullitive flux pathways, and included potential emissions from transient storage in the water column ("storage fluxes"), alongside CO2 and CH4 concentrations dissolved in the surface water. Fluxes are measured through a variety of methods including chamber and eddy covariance techniques alongside bubble traps, ice-surveys, and concentration-based turbulence-driven modelling in aquatic ecosystems. The monthly flux data are reported together with supporting methodological and environmental metadata. The resulting ABCFlux v2 has 23 847 flux site-months, 8182 concentration site-months, and 199 seasonal observations from 1024 sites, and includes 56 139 reported fluxes (i.e. sum of GPP, Reco, NEE, and CH4 fluxes) from the years 1984 to 2024. The majority of monthly observations occurred after 1999. Wetlands had the highest number of site-month observations (8758), followed by boreal forest (6981), lotic ecosystems (6275), lentic ecosystems (3799) and upland tundra (3308). Measurements of CO2 dominated the dataset across most ecosystem types (25 222) except for lentic ecosystems, where CH4 flux site-months (3098) were more frequent than CO2 flux site-months (2915). Overall, ABCFlux v2 includes 160 % more site-months for terrestrial CO2 flux data compared to ABCFlux v1. Integrating and updating BAWLD-CH4 flux data from growing season averages to monthly fluxes resulted in 5671 site-months of chamber CH4 data compared to 762 site-years. This collaborative initiative, involving contributions from over 260 researchers, provides a comprehensive overview of the current state of the Arctic-boreal carbon flux network and its data, and serves as an important step in reducing uncertainties in Arctic-boreal carbon budgets and in enhancing our understanding of climate feedbacks. The data can be accessed at ORNL DAAC at 10.3334/ORNLDAAC/2448 (Virkkala et al., 2026).

 Climate change impacts both lakes and their sur-rounding catchments, leading to altered discharge and nu-trient loading patterns from catchments to lakes, as well asmodified thermal stratification and mixing dynamics withinlakes. These alterations affect biogeochemical processes andwater quality in lakes. Coupled catchment-lake modelingprovides both a holistic evaluation of the effects of climatechange on lakes and a framework for explicitly assessingthe importance of how catchments effect lakes. The Inter-Sectoral Impact Model Intercomparison Project (ISIMIP)provides a framework for projecting the impacts of cli-mate change across multiple sectors (e.g. water, lakes, en-ergy, health) of the Earth System consistently, enabling inte-grated cross-sectoral assessments. However, climate impactson lake dynamics are modeled in ISIMIP without consider-ation of the links between lakes and the surrounding catch-ments. This is a significant limitation, as it restricts assess-ments to only the direct impacts of climate change on lakes,overlooking the critical interactions between lakes and theircatchment areas. In this study, we establish the first dynamicconnection between the Global Water and Lake Sectors inISIMIP, achieved by scaling the gridded modeled outputs ofwater fluxes from the Global Water Sector to the catchmentsof the representative lakes of the Lake Sector. The stream-flow to the representative lake of each grid cell, as definedby the ISIMIP Global Lake Sector, was calculated based onrunoff proportional to the catchment area of each representa-tive lake. If the lake surface area was larger than the grid cellarea, water from upstream grid cells was included as the cor-responding proportion of river discharge. The methodologywas applied to 70 lakes across Sweden covering a wide rangeof sizes, hydrological settings and catchment characteris-tics. The estimated streamflow was validated against both thestreamflow outputs from the hydrological model HYPE andobserved data. The comparison demonstrated good agree-ment in terms of long-term streamflow mean and seasonalpattern, indicating that the proposed approach is capableof producing reliable streamflow estimates without requir-ing high-resolution local models. This estimated streamflow,representing water flow into lakes, will provide a valuabledataset for the scientific community within the ISIMIP LakeSector supporting hydrological and water quality modelingefforts aimed at understanding the impacts of climate changeon lakes.

In the littoral zone, at the land–water interface of lakes, the areal productivity of aquatic vegetation rivals that of rainforests, resulting in a potentially very high carbon (C) turnover. Whereas tidal wetlands at the land–ocean interface are included in global C budgets, lake littoral zones are currently not accounted for, despite the total shoreline of lakes being estimated at four times longer than that of the global ocean. Here we quantify the littoral net atmospheric C sink using mass balance and a model of C export from the littoral to the pelagic zone. We argue that ignoring littoral C turnover in lakes potentially results in biased estimates of continental C cycling. In our global estimate, we show that the estimated global C balance of lakes may reverse from a net C source to a net C sink (that is, net C burial > net C outgassing). In addition, a large part of the C outgassed in the pelagic might originate from the littoral, implying that previous estimates of terrestrial C inputs to inland waters were too high. We argue that quantifying and modelling lake littoral C fluxes are essential to more accurately estimate the feedback between the continents and climate.

The Three Gorges Dam (TGD), as one of the world's largest water conservancy projects, plays an important role in flood control and disaster resistance. This study evaluated the accumulation rate, sources and influencing factors of organic carbon in the Eastern Chongming Wetland (ECW) before and after the construction of the TGD. The organic carbon accumulation rate (OC-MAR) was estimated using 210Pb radionuclide dating, delta 13C and C/N ratio mixing model, and the contribution of rivers, vegetation and ocean to organic carbon in the ECW was quantified. Research shows that the mass accumulation rates (MAR) in the Phragmites australis zone (CM-1), Scirpus mariqueter zone (CM-2), and mudflat (CM-3) are 2.88 f 1.32 g cm-2 yr-1, 1.95 f 0.45 g cm-2 yr-1 and 1.51 f 1.22 g cm-2 yr-1 and OC-MAR are 185 f 97 g C m-2 yr-1, 80 f 34 g C m-2 yr-1, and 33 f 15 g C m-2 yr-1, respectively. Integrated analysis of MAR, OC-MAR, and the three-end-member mixing model indicates that, although the construction of the TGD and other reservoirs/dams in the Yangtze River basin has reduced sediment discharge and particulate organic carbon (POC) flux in the Yangtze River, MAR and OC-MAR in the study area did not respond promptly in the short term. This may be attributed to: (1) sampling area being situated within a sedimentary belt formed by flood-dominant tidal currents; (2) a vegetation-dominated sediment trapping mechanism; and (3) continuous sediment supply from the submerged delta (below the spring low tide line). Results from the three-end-member model demonstrate that the increase in riverine OC input driven by finer sediment grain size and the rise in plant OC due to expanded vegetated area are the primary reasons for the OC-MAR increase. The current global annual carbon loss from mudflats (4.8 Tg C yr-1) has reached 330 times the annual carbon burial in the ECW (1.5 x 10-2 Tg C yr-1).

Purpose: Insight into the distribution and sedimentation patterns of organic and inorganic carbon (OC and IC) in urban lake sediments is essential for understanding their role in the carbon (C) cycling of inland waters and supporting effective ecosystem management.

Methods: This study investigated the spatial variability of sediment OC and IC accumulation in a mesotrophic human-made urban lake (Lake Berendonck; 45 ha) by combining high-resolution hydroacoustic sub-bottom profiling surveys and sediment coring.

Results: The results revealed strong spatial variations in sediment C accumulation rates. Deep central and southeastern areas of the lake exhibited relatively high C deposition, even though deep areas with low C content were also found. Lake Berendonck had a mean +/- standard deviation sediment accumulation rate of 0.7 +/- 0.5 cm year- 1, with areal OC and IC accumulation rates ranging between 24 and 557 and 3-37 g m- 2 year- 1, respectively. Lake Berendonck's mean sediment OC accumulation rate (155 g m- 2 year- 1) was approximately four times higher than the mean OC accumulation rate of global lakes (37 g m- 2 year- 1), while Lake Berendonck's mean IC accumulation rate (12 g m- 2 year- 1) falls in the mid-range for global lakes and seas.

Conclusions: Our findings indicate that C accumulation is highly variable in space and that spatially integrated data are needed to estimate C stocks and unravel within-lake C processes reliably. Furthermore, this study highlights that the OC accumulation in Lake Berendonck ranks among the highest rates observed in global lakes with similar surface areas (0.4-0.5 km2). This underscores the global importance of small urban water bodies in C cycling, particularly as key C storage systems.

Lake shores are fringed with aquatic plants, but their very high productivity has been overlooked in global lake carbon budgets. We estimate the carbon fluxes of lake littoral zones and show that the carbon balance of lakes can reverse from a carbon source to a carbon sink when these zones are included. 

Freshwater lakes and reservoirs cover a small fraction of the Earth, however their emission of the greenhouse gas methane (CH₄) from the sediment to the atmosphere is disproportionately high. Currently, there is still a limited understanding of the links between sediment characteristics and CH₄ formation. Earlier studies have indicated that sediment age and nitrogen content are related to sediment CH₄ formation rates, but it is uncertain such relationships are valid across gradients of sediment characteristics. We therefore measured potential CH₄ formation rates in multiple layers of sediment sampled from nine lakes situated in the temperate, boreal and alpine biogeographic regions of Sweden, thus differing in productivity, catchment and climate properties. Potential CH₄ formation varied over 3 orders of magnitude, and was broadly related to the quantity and reactivity of organic matter, and generally decreased with sediment depth. Sediment age and total nitrogen content were found to be the key controlling factors of potential CH₄ formation rates, together explaining 62% of its variability. Moreover, the model developed from the Swedish lake sediment data was able to successfully predict the potential CH₄ formation rates in reservoirs situated in different biogeographic regions of Brazil (R² = 0.62). Therefore, potential CH₄ formation rates in sediments of highly contrasting lakes and reservoirs, from Amazonia to alpine tundra, could be accurately predicted using one common model (RMSE = 1.6 in ln-units). Our model provides a valuable tool to improve estimates of CH₄ emission from lakes and reservoirs, and illustrates the fundamental regulation of microbial CH₄ formation by organic matter characteristics.

Lakes and reservoirs are important emitters of methane, a strong greenhouse gas, to the atmosphere. Methane is produced in absence of oxygen by specific microbes that degrade the organic matter in the sediment. Currently, it is still uncertain which specific sediment properties control the production of methane, and if such properties are the same across lakes and reservoirs located in different ecosystem. To test this, we collected sediment cores from several lakes across different ecosystems in Sweden, and we measured potential methane formation rates. Methane formation rates varied greatly among lakes and was related to the quality and quantity of organic matter in the sediment. From this experiment, we calculated an empirical model that can predict methane formation rates as a function of sediment age and nitrogen content. Moreover, we found that our model could well predict potential methane formation rates in tropical reservoirs. In conclusion, sediment age and nitrogen content are universal controlling factors of methane formation rates across lakes and reservoirs in different ecosystems, from tropics to arctic tundra. Our findings provide a valuable tool to improve estimates of methane emission from lakes and reservoirs and illustrates how sediment characteristics play a crucial role in regulating methane formation rates.

A growing body of literature points to drawdown areas as important sources of atmospheric CO2 2 within reservoirs. Yet seasonal and temporal patterns of CO2 2 flux from periodically exposed sediments in drawdown areas remain poorly understood. Here we evaluate the annual and diel (24-hour cycle) variations in CO2 2 emissions from sediments periodically exposed to the atmosphere. We sampled sediments in the drawdown area of a tropical reservoir, which encompassed two primary adjacent land covers-grassland and forestland-in the watershed of a reservoir located in southeastern Brazil. We also experimentally assessed the effect of rewetting on CO2 2 emissions from exposed sediments. We found large variations in emissions during all hydrological periods (from 10 to 10116 mg C/m-- 2 (- |-) d-1),-1 ), except for the late rainy period. Land use and how distant dry sediments were from the open water significantly affected drawdown CO2 2 emissions, with higher emissions occurring in areas surrounded by forest than those adjacent to grassland. Our diel-cycle analysis did not show significant variation of emissions over daily cycles. Furthermore, a rewetting experiment indicated a significant increase in emissions 30 min after the onset of the rewetting event. Although drawdown areas only cover 20 % of the reservoir's area, they account for 80 % of the reservoir's total CO2 2 emissions. Ultimately, single-time measurements can lead to considerable underestimation (up to 52 %) or overestimation (up to 190 %) of whole- reservoir CO2 2 emissions.