During the last two decades, our perception of lake and reservoir methane (CH₄) emissions have changed from being peripheral to increasingly important in global CH₄ budget and source attribution discussions. This is reflected by a 10-fold increase in the number of related scientific publications per year from 2001 to 2020. Earlier research provided a strong foundation that has been developed further. As a result of expanding reserach in the field, some of the early bottlenecks are getting resolved, while others remain, and new challenges have emerged.

It is now clear that lakes and reservoirs jointly contribute in the order of 5 -15 % och the global CH₄ emissions to the atmosphere, and thereby cannot be ignored if we want to understand the atmospheric CH₄ development. This has added challenges regarding data needs and methodologies to make more accurate large-scale CH₄ flux estimates. Key questions also include how lake and reservoir emissions are influenced by environmental change including climate. Critical challenges are nested across a range of scales, from microscale process regulation that shape spatiotemporal variability at the whole-system scale, in turn generating measurement challenges and data constraints influencing global assessments. 

This presentation aims to provide a brief overview, highlighting some learnings and challenges. In addition, predictions of future global lake and reservoir CH₄ emissions will be presented, exploring a data-driven approach to integrate existing knowledge on spatiotemporal flux variability with consideration of multiple emission pathways and their seasonal regulation and long-term response to climate change, as well as to projected changes in inland water area and nutrient load. The relative impacts of different potential flux change drivers was also investigated. Overall, the predicted future emission scenarios illustrate the sensitivity of one of the largest sources of atmospheric CH₄ to the ongoing global change.

How to cite: Bastviken, D. and Johnson, M. S.: Lake and reservoir methane (CH4) emissions – an underground past, a rising present, and scenarios for the future  , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12691, https://doi.org/10.5194/egusphere-egu24-12691, 2024.

Drainage ditches and irrigation canals are widespread across the globe, and have a high potential to emit greenhouse gases (GHG) to the atmosphere, contributing to climate change. Often located in agricultural or urban areas, ditches may receive high inputs of organic matter and nutrients, thereby stimulating GHG production. Previous work (Peacock et al., 2021) has calculated the global magnitude of methane emissions from ditches (~1% of all anthropogenic methane emissions). However, the relative contributions of carbon dioxide and nitrous oxide remain unknown at national and global scales, although field studies show emissions of these GHGs can be large. As anthropogenic features, GHG emissions from ditches must be reported to the United Nations Framework Convention on Climate Change under Intergovernmental Panel on Climate Change (IPCC) protocols, but current guidelines only exist for methane (in the 2019 Refinement). Here, we present the results of an (ongoing) review where we collate existing scientific literature to synthesize carbon dioxide and nitrous oxide emissions data from ditches and canals around the world, as well as identifying the principle driving variables. The results of this research will help inform IPCC guidelines for improved GHG emission accounting, and reveal if ditches and canals act as hotspots of non-methane GHGs.

How to cite: Silverthorn, T., Evans, C., and Peacock, M.: Greenhouse gas emissions from drainage ditches and irrigation canals, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-8036, https://doi.org/10.5194/egusphere-egu24-8036, 2024.

Stream ecosystems are actively involved in the biogeochemical cycling of carbon (C) and nitrogen (N) from terrestrial and aquatic sources. Streams hydrologically connected to peatland soils are suggested to receive significant quantities of particulate, dissolved, and gaseous C and N species, which directly enhance losses of greenhouse gases (GHGs), i.e., carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O), and fuel in-stream GHG production. However, riverine GHG concentrations and emissions are highly dynamic due to temporally and spatially variable hydrological, meteorological, and biogeochemical conditions. In this study, we present a complete GHG monitoring system in a peatland stream, which can continuously measure dissolved GHG concentrations and allows to infer gaseous fluxes between the stream and the atmosphere and discuss the results from March 31 to August 25 at variable hydrological conditions during a cool spring and warm summer period. Stream water was continuously pumped into a water-air equilibration chamber, with the equilibrated and actively dried gas phase being measured with two GHG analyzers for CO₂ and N₂O and CH₄ based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) and Non-Dispersive Infra-Red (NDIR) spectroscopy, respectively. GHG measurements were performed continuously with only shorter measurement interruptions, mostly following a regular maintenance program. The results showed strong dynamics of GHGs with hourly mean concentrations up to 9959.1, 1478.6, and 9.9 parts per million (ppm) and emissions up to 313.89, 1.17, and 0.40 mg C or N m⁻²h⁻¹ for CO₂, CH₄, and N₂O, respectively. Significantly higher GHG concentrations and emissions were observed shortly after intense precipitation events at increasing stream water levels, contributing 59% to the total GHG budget of 762.2 g m⁻² CO₂-equivalents (CO₂-eq). The GHG data indicated a constantly strong terrestrial signal from peatland pore waters, with high concentrations of dissolved GHGs being flushed into the stream water after precipitation. During drier periods, CO₂ and CH₄ dynamics were strongly influenced by in-stream metabolism. Continuous and high-frequency GHG data are needed to assess short- and long-term dynamics in stream ecosystems and for improved source partitioning between in-situ and ex-situ production.

Piatka DR, Nánási RL, Mwanake RM, Engelsberger F, Willibald G, Neidl F and Kiese R (2024) Precipitation fuels dissolved greenhouse gas (CO2, CH4, N2O) dynamics in a peatland-dominated headwater stream: results from a continuous monitoring setup. Front. Water 5:1321137. doi: 10.3389/frwa.2023.1321137

How to cite: Piatka, D., Nánási, R., Mwanake, R., Engelsberger, F., Willibald, G., Neidl, F., and Kiese, R.: Precipitation fuels dissolved greenhouse gas (CO2, CH4, N2O) dynamics in a peatland-dominated headwater stream: results from a continuous monitoring setup, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5944, https://doi.org/10.5194/egusphere-egu24-5944, 2024.

Small thermokarst lakes, formed by the thawing of ice-rich permafrost, can emit significant amounts of methane (CH₄) and carbon dioxide (CO₂) to the atmosphere. The physical processes behind diurnal variations in greenhouse gas (GHG) emissions from thermokarst lakes remain poorly understood due to a lack of observational data in subarctic regions. This study focuses on the dynamics of GHG emissions from two small lakes (< 200 m²) located in the Tasiapik Valley, near the village of Umiujaq, Nunavik, Canada (56°33'28.8"N 76°28'46.5"W). One lake is characterized by a more humic and sheltered environment, while the other is characterized by greater transparency and exposure to wind. Continuous measurements of temperature, conductivity, and oxygen in the water column, as well as meteorological conditions (wind, pressure, heat exchanges) have been conducted since October 2021. CO₂ fluxes measured using a floating chamber and dissolved gases (CO₂, CH₄, N₂O) at the surface were measured on a daily cycle for 2-week periods in July 2022 and August 2023, with bubble traps quantifying ebullition rates. Diffusive CO₂ fluxes are in line with estimates for other thermokarst lakes, ranging from –2 to 17 mmol m^–2 d^–1 in July 2022 (during a particularly cold period) and from 8 to 66 mmol m^–2 d^–1 in August 2023, a period of stronger stratification. Turbulence, characterized by the gas transfer coefficient k₆₀₀, was higher in 2023 (0.4 to 10.4 cm h^–1) than in 2022 (1.1 to 4.7 cm h^–1). CH₄ emission through ebullition was more than 6 times higher than through diffusion in the more humic and sheltered lake (13 ± 5.5 mol m^–2 d^–1) and almost 7 times higher than in the more transparent and exposed lake (2 ± 1.5 mmol m^–2 d^–1), where ebullition was of the same order of magnitude than diffusion. Diurnal cycles were characterized by the nocturnal mixing of surface waters with deeper waters enriched in CO₂, leading to a peak in CO₂ fluxes in the morning, which gradually decreased over the course of the day with the establishment of thermal stratification due to solar radiation, and the potential uptake by primary production. Overall, these results highlight the complex interactions between environmental factors influencing GHG emissions in thermokarst lakes, and the major differences that can exist between adjacent lakes.

How to cite: Pouliot, A., Nadeau, D., and Laurion, I.: Diurnal variations in greenhouse gas emissions from two contrasting thermokarst lakes in Nunavik, Canada, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2172, https://doi.org/10.5194/egusphere-egu24-2172, 2024.

Due to the high nitrogen (N) and phosphorus (P) input and the weak self-cleaning ability of waterbodies, lakes are more prone to be eutrophication. The continuous input of N and P in enriched lakes regulates the carbon (C) cycle process, which affects the production of greenhouse gases such as CO₂ and CH₄. Therefore, we assessed the water CO₂ and CH₄ emission fluxes and their response to ongoing N and P inputs based on data from 707 globally distributed lakes. We found that CO₂ and CH₄ emission fluxes were higher in the tropics than in the temperate zone, with Antarctica acting as a methane sink. The emission fluxes of CH₄ and CO₂ increased with the increase in N and P concentration, and the effect of total phosphorus (TP) on the emission fluxes of CO₂ and CH₄ was the highest. When the TP load is increased by 3 times, the CO₂ emission reaches 443.99 mg m^-2 d^-1, and the CH₄ emission reaches 205.47 mg m^-2 d^-1, which is 1.48 and 3.85 times of the normal condition respectively. If the TP load is reduced by 3 times, it reduces 597.83 mg m^-2 d^-1 C emissions. This study shows that lake C emissions are highly dependent on continuous N and P input, which provides a scientific reference for the C cycle process of eutrophic lakes, and provides an important basis for the study of lake response to climate.

How to cite: wu, F., Gao, Y., Jia, J., Wen, X., and Yu, G.: Constantly nitrogen and phosphorus input will increase methane and carbon dioxide emissions from global lakes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1930, https://doi.org/10.5194/egusphere-egu24-1930, 2024.

 Since the impoundment in 2003, the flux of CO₂, CH₄ in the Three Gorges Reservoir has changed significantly compared with the pre-impoundment status. How to understand and evaluate the influence of the construction and operation of the Three Gorges Reservoir on the CO₂, CH₄ and other greenhouse gas fluxes has attracted much attention. In this paper, we reviewed the experience of monitoring and analysis of CO₂, CH₄ fluxes in the reservoir since 2009. At present, air-water diffusion was the major pathway for carbon emissions in the reservoir. Terrigenous organic carbon input was the main carbon source leading the production of CO2 and CH4 in the reservoir. Yet, the contribution of autochthonous organic carbon seemed to be with growing significant. Compared with pre-impoundment status, a net increase of greenhouse gas emissions in the Three Gorges Reservoir is evident. Flooding accounted for about 20% of the net increase of the reservoir formation. Anthropogenic pollution in the reservoir region did not significantly to the net increase of CO₂ emissions. In addition, the dam acting as barriers and reservoir aquatic ecosystem reconstruction were major contributors for the net greenhouse gas emissions. The past decade sampling campaigns and research promoted the improvement and optimization of monitoring system of the greenhouse gas emissions in the Three Gorges Reservoir. Application of new monitoring methods and technologies also provided support and reimbursement. However, the hydro-ecological mechanism driving the carbon cycle in the reservoir under complex hydrological environment is still unclear, which is a difficulty in the long-term trend prediction of the reservoir carbon flux. In the future, innovation of monitoring technology will be applied to promote the accurate calculation of the carbon flux of the Three Gorges Reservoir. It is still urgent to put forward more scientific and effective models or methods to support the long-term trend prediction and serve the reservoir carbon management.

How to cite: Li, Z. and Liu, Y.: The field monitoring and analysis of carbon emissions in the Three Gorges Reservoir: Review and outlook, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-20453, https://doi.org/10.5194/egusphere-egu24-20453, 2024.

Quantifying headwater stream carbon emissions is important for our understanding of the global carbon cycle because these emissions (an estimated 0.93-1.15 Pg C year) can be substantial compared to the terrestrial flux. Headwater stream networks can have high emissions due to their coupling with the terrestrial environment and high turbulence with some estimates predicting headwater stream networks can contribute 70% of the global riverine stream emissions. These carbon emissions are challenging to predict, especially with regards to headwater stream network spatiotemporal heterogeneity. The majority of headwater streams exhibit changes in stream network area on a seasonal basis, and these locations and extents are not often well documented because they are based on topographic maps with limited spatial accuracy. Research suggests 50-80% of river networks are comprised of non-perennial stream segments.  Physically based models are a potential solution to both mapping streamflow permanence and carbon dioxide emissions by accounting for the spatiotemporal heterogeneity that can occur in stream networks.

In this study, we modeled stream permanence at three streams across the United States in different ecosystems using the Watershed Erosion Prediction Project (WEPP) hydrological model to simulate changes in stream network area over the year. We then used these results to inform a process-based stream network model to predict carbon emission from these networks throughout the year. We calibrated these network model with longitudinal data collected at the three sites during both low and high flow. Our results show the importance of considering stream permanence when predicting stream network carbon emissions, and how some ecosystems may emerge as hotspots for these emissions during high flow periods.

How to cite: Conroy, H. D., Hotchkiss, E. R., Cawley, K. M., Goodman, K., Jones, J. B., Wollheim, W. M., and Butman, D.: Examining the Impact of Stream Permanence on Headwater Stream Carbon Emissions , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17025, https://doi.org/10.5194/egusphere-egu24-17025, 2024.

It has long been recognized that terrestrial ecosystems are not isolated from other earth systems with all the absorbed carbon being permanently sequestered on land. Inland water systems (e.g., streams, rivers, lakes, and reservoirs) are an important component of the global carbon cycle, functioning as active reactors that transport and transform large quantities of terrestrially derived carbon. Strong interactions between terrestrial ecosystems and inland waters indicate that a portion of the carbon sequestered on land by vegetation can be transported to the ocean through inland waters, the land-to-ocean aquatic continuum (LOAC). Therefore, the transport, transformation, and redistribution of terrestrial carbon along this continuum will change the land carbon sink strength. A comprehensive understanding of the magnitude and significance of carbon transfer in the LOAC in modulating the net landscape carbon balance is of paramount importance for an accurate assessment of carbon budget. In this work, we systematically examined the carbon transport in the LOAC of the entire China, including carbon export into the ocean, carbon burial within inland waters, and carbon emissions into the atmosphere. Our results show that the flux of carbon transported into the ocean and buried within Chinese inland waters was 40-45 Tg C yr^-1 and 10-15 Tg C yr^-1, respectively. In addition, the flux of carbon emissions (as CO₂ and CH₄) from Chinese inland waters was in the range of 100-105 Tg C yr^-1. The total carbon flux entering Chinese inland waters was estimated at 150-160 Tg C yr^-1 with carbon emissions being the largest transport pathway (63-70% of the total). Compared with the simultaneous terrestrial carbon sink in China, this terrestrial-aquatic carbon export could offset China’s terrestrial carbon sink capacity by up to 25%. Our results highlight that the terrestrial-aquatic carbon export must be integrated into future national-scale carbon budgets.

How to cite: Ran, L., Chen, S., and Yang, Q.: Carbon export in the land-to-ocean aquatic continuum (LOAC) of China, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2742, https://doi.org/10.5194/egusphere-egu24-2742, 2024.

Urban ponds are significant emitters of methane (CH₄) to the atmosphere. Methane cycling in these ecosystems is influenced by a multitude of factors, including redox conditions, organic carbon and nutrient supplies, pollutant loads, as well physical environmental factors such as temperature. However, the quantitative relationships between CH₄ production and temperature remain insufficiently known. Our aim in the present study was to quantify the impacts of CH₄ production in urban pond sediments as a critical prerequisite to projected impacts of global warming on CH₄ emissions from freshwaters. We collected intact sediment cores from eight ponds located in the city of Berlin, Germany, and incubated them over a broad range of temperatures (2 to 44 °C) to determine the thermal dependencies of CH₄ production. The selected ponds represent a variety of urban land-use types, including residential areas, industrial areas, protected forest areas, and recreational green spaces. Our results indicate a clear dependency of CH₄ production on temperature, showing that methanogenesis was consistently driven by mesophilic microorganisms, with optimal temperatures ranging between 28 and 36 °C, despite sediment temperatures that are mostly much lower throughout the year. Our findings indicate that methanogenesis in sediments of urban ponds occurs through both the heterotrophic and hydrogenotrophic pathway, with the prevailing temperatures in these environments being conducive to producing the essential precursors needed for syntrophic hydrogen production. We also integrated these temperature relationships into global warming scenarios, specifically within the Representative Concentration Pathway (RCP) framework, to make projections for the year 2100. The results of the analysis spanning a range of scenarios, from the very stringent RCP 2.6 to the continually rising RCP 8.5 indicate that even with moderate global warming, CH₄ production and thus emissions to the atmosphere from urban ponds will markedly increase in the future.

How to cite: Sepulveda-Jauregui, A., Martinez-Cruz, K., Peeters, F., and Gessner, M. O.: Projecting Impacts of Climate Warming on Methane Production and Emissions from Urban Ponds, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17006, https://doi.org/10.5194/egusphere-egu24-17006, 2024.

CH₄ production in freshwaters has been considered to occur primarily in anoxic sediments. However, recent discovery of oxic CH₄ production in some lake waters makes the origins and controls over the fate of cumulative CH₄ in water column elusive. Especially, whether CH₄ can be produced in oxic water column of river systems remains unclear, and the role of water column in CH₄ emissions is poorly understood across different river sizes. Here, we present water column contributions to riverine CH₄ emissions based on 4-year national-wide in situ measurements across six large river networks with stream order from 3^rd to 8^th. We find water column acts as a contributor of CH₄ net production in 58% observations, indicating the occurrence of CH₄ production in oxic water column, which is probably attributed to CH₄ production in anoxic interface of suspended particles and production by phytoplankton. Water column can account for 7% of riverine CH₄ emissions on average across all observations. Water-air CH₄ fluxes decreased exponentially with stream order. In contrast, water column contribution increased with stream order and its roles vary with river size and shift from CH₄ sinks in streams to CH₄ sources in large rivers. Water column can consume 6% of CH₄ released from sediment in rivers of size lower than 6^th, while contribute 12% to water-air CH₄ fluxes in higher-order rivers. This shift is mainly attributed to the increase of river depth and higher concentrations of suspended particles, which may facilitate net CH₄ production in water column of large rivers. Our findings suggest oxic CH₄ production in water column represents a hitherto overlooked source of methane and can be important for CH₄ cycling and emissions in river systems, especially for large rivers.

How to cite: Wang, J. and Xia, X.: Water column can shift from sinks to sources of CH4 with increasing river size, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4478, https://doi.org/10.5194/egusphere-egu24-4478, 2024.

Lotic ecosystems traversing mixed land-use landscapes are sources of GHGs to the atmosphere, but their emission strength is uncertain due to longitudinal GHG heterogeneities. In this study, we quantified N₂O (as well as CO₂ and CH₄ concentrations) and N₂ concentrations and several water quality parameters along the Rhine river and the Mittelland canal, two critical inland waterways in Germany in the summer of 2023. Our main objectives were to compare N₂O concentrations along the two ecosystems and to identify the main drivers responsible for their longitudinal heterogeneities. The results indicated that N₂O concentrations in both ecosystems were oversaturated relative to equilibrium concentrations (116 – 782 % saturation), particularly in the Mittelland canal. We also found significant longitudinal variability in % N₂O saturation along the mainstems of both lotic ecosystems (CV = 43 – 68 %), with the highest variability in the Mittelland canal, suggesting that single N₂O measurements along large lotic ecosystems are not representative of the entire reach. Overall, these significant longitudinal N₂O heterogeneities were driven by differences in biogeochemical processes between the two lotic ecosystems. N₂O was strongly related to N₂ concentrations, with a negative relationship in the Rhine river and a positive relationship in the Mittelland canal. Based on these findings, we concluded that denitrification drives the N₂O hotspots in the Canal, while coupled biological N₂-fixation and nitrification accounted for N₂O hotspots in the Rhine. These findings also highlight the need to include N₂ concentration measurements in GHG sampling campaigns, as it has the potential to help better constrain nitrogen cycling in lotic ecosystems.

How to cite: Mwanake, R., Imhof, H., and Kiese, R.: Contrasting processes involving denitrification and biological N2-fixation drive N2O hotspots along two large lotic ecosystems in Germany, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6215, https://doi.org/10.5194/egusphere-egu24-6215, 2024.

Inland water was considered an important sources of nitrous oxide (N2O) production and emissions. However, assessment of indirect emission factors (EF5r) and contribution of different water body N2O on N2O budgets remains challenging, and results are uncertain due to limited data availability. In this study, floated chamber and boundary layer model method were conducted on four diurnal days, and two year observations at high temporal resolution in a subtropical forest catchment in Southeast China. The results showed that there was clearly diurnal characteristics of N2O emissions, and the CC98 model was more fit for estimating stream N2O emissions. The N2O fluxes from different water body was in the order: ponds (35.99±33.58 μg m^-2 h^-1) > main stream (17.09±4.48 μg m^-2 h^-1)> tributary (14.78±12.79 μg m^-2 h^-1). All of their EF5r (0.050%±0.058% to 0.249%±0.456%) were significantly higher than the IPCC 2006 default value 0.025%, suggesting that N2O emissions from China and world inland water may be grossly underestimated. Multiple regression model selected the dissolved oxygen and NH4-N concentrations as the crucial factors influencing the N2O emission fluxes from streams, whereas dissolved oxygen and NO3-N in ponds. Although the water body's surface area only occupied 2% in this catchment, where the N2O emissions fluxes were approximately contribute 1.4% of N2O budget in whole catchment. These findings will draw attentions to the role of inland water N2O productions and emissions in the contributions of N2O budgets, especially in the large scale N2O estimated.

How to cite: Huang, T. and Lu, L.: Nitrous oxide emissions from water bodies was a no-negligible contribution to nitrous oxide budgets in a subtropical forest catchment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4121, https://doi.org/10.5194/egusphere-egu24-4121, 2024.

Nitrous oxide (N₂O) is a strong greenhouse gas and ozone-destroying compound, whose atmospheric concentrations have been increasing over the past decades. Lakes play a relatively uncertain role with regards to their contribution to the global natural N₂O emissions, and a better understanding of the environmental controls on lacustrine N₂O production and consumption is needed.

We investigated N₂O production pathways in the anoxic deep hypolimnion of the South Basin of eutrophic Lake Lugano (Switzerland) during summer stratification. Temporary high concentrations of N₂O (up to 3000 nmol/L) were observed in the oxygen-depleted near-bottom waters, accompanied by a site preference (SP, indicating the intramolecular ¹⁵N distribution) of +32 ‰. Incomplete heterotrophic denitrification is commonly thought to be the main N₂O production pathway in low-oxygen environments, but it is typically characterized by a low SP of -5-0 ‰ (Sutka et al. 2006). High SP values, as observed here, rather point to an oxidative N₂O production mechanism such as (micro-aerobic) nitrification, yet they may also be caused by partial reduction of N₂O to N₂ (Ostrom et al. 2007). We performed incubation experiments with ¹⁵N-labeled NH₄⁺ to investigate oxidative N₂O production through (micro-aerobic) nitrification, and ¹⁵N-labeled NO₃^- to investigate reductive N₂O production through denitrification. Our results point indeed to a reductive mode of N₂O production in the anoxic bottom water. Alternative denitrification pathways known to produce N₂O with potentially higher SP, such as fungal denitrification (SP = >30 ‰, Rohe et al. 2014) and chemo-denitrification (SP = 0-27 ‰, Li et al. 2022), were investigated in additional incubation experiments using bacterial and fungal inhibitors. These experiments revealed that bacterial denitrification contributes most to N₂O production in the sediment and the bottom water layer, while fungal- and chemo-denitrification were much less important.The elevated SP values in the bottom-water N₂O during summer stratification are most likely due to the fact that much of the produced N₂O has been reduced to N₂. Thus, N₂O reduction can completely mask primary N₂O isotopic source signatures in redox transition zones, complicating the use of N₂O stable isotope measurements to disentangle reductive and oxidative N₂O production and to reveal alternative denitrification pathways.

REFERENCES

Li, S., Wang, S., Pang, Y., & Ji, G. 2022: Influence of electron donors (Fe, C, S) on N₂O production during nitrate reduction in lake sediments: Evidence from isotopes and functional genes, ACS ES&T Water, 2(7), 1254–1264.

Ostrom, N. E., A. Pitt, R. Sutka, P. H. Ostrom, A. S. Grandy, K. M. Huizinga, and G. P. Robertson (2007), Isotopologue effects during N₂O reduction in soils and in pure cultures of denitrifiers, J. Geophys. Res., 112, G02005.

Rohe, L., Anderson, T.-H., Braker, G., Flessa, H., Giesemann, A., Lwicka-Szczebak, D., Wrage-Mönnig, N., & Well, R. 2014: Dual isotope and isotopomer signatures of nitrous oxide from fungal denitrification – a pure culture study, Rapid Communiciations in Mass Spectrometry, 28, 1893-1903.

Sutka, R. L., Ostrom, N. E., Ostrom, P. H., Breznak, J. A., Gandhi, H., Pitt, A. J., & Li, F. 2006: Distinguishing nitrous oxide production from nitrification and denitrification on the basis of isotopomer abundances, Applied and Environmental Microbiology, 72(1), 638–644.

How to cite: Einzmann, T., Lehmann, M. F., Zopfi, J., and Frey, C.: Importance of alternative denitrification pathways in a seasonally stratified lake basin, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15511, https://doi.org/10.5194/egusphere-egu24-15511, 2024.

Since the beginning of the industrial era, the atmospheric greenhouse gases (GHG) have increased continuously (around +50% for carbon dioxide (CO₂) and +150% for methane (CH₄), for the two most important), causing the current climate change. In November 2023, the World Meteorological Organization (WMO) highlighted once again there are still significant uncertainties about the carbon cycle, its fluxes, and they stressed the importance to follow the non-CO₂ GHG with greater global warming potential.

The ocean, as a sink of anthropogenic CO₂, plays a crucial role in climate regulation, whereas the surface seawater is naturally supersaturated in CH_4, and shallow coastal waters are a source of CH₄ to the atmosphere. However, the air-sea CO₂ and CH₄ fluxes are driven by different key processes depending on the region of the open or coastal ocean.

To improve the understanding of the processes driving the air-sea exchange of GHG, we investigate the CO₂ and CH₄ concentrations and fluxes in open ocean and coastal areas affected by sea ice, glacier runoff and riverine inputs within the context of the European project GreenFeedBack. To do so, we measured CO₂ and CH₄ concentrations and calculated the fluxes, in surface water during a summer cruise (July-August 2023) conducted on board the RV Belgica in the subpolar North Atlantic Ocean, between Iceland and Southern Greenland Fjords. The data were obtained using a custom-made air-water equilibration system, that was connected to the vessel’s non-toxic seawater supply (equilibrator and Cavity Ring Down Spectrometer) and discrete sampling.

Our first results show a pronounced gradient of CO₂ and CH₄ concentration between open ocean and the fjords. The oceanic CO₂ concentration is minimal in the fjords where the CH₄ concentration is maximal, indicating a potential impact of freshwater discharge on the GHG exchanges.

How to cite: Leseurre, C., Delille, B., Roobaert, A., Boone, W., Crabeck, O., Ponsoni, L., Theetaert, H., T'Jampens, M., Verbrugge, S., and Gkritzalis, T.: Greenhouse gases gradients from Southern Greenland Fjords to subpolar North Atlantic Ocean, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7427, https://doi.org/10.5194/egusphere-egu24-7427, 2024.

The steady rise in global temperature is likely to perturb the cycling and transfer of carbon in the coastal Arctic. Given the high carbon content of Arctic soils, this region may become an increasingly important source of methane (CH4) and carbon dioxide (CO2) in response to permafrost loss in coming decades. We present targeted observations along a lake to bay system during the spring thaw around Cambridge Bay, in the coastal Canadian high Arctic. In this system, greenhouse gases produced in the freshwater environment are transported to the bay, where they may undergo further transport, transformation, or ventilation to the atmosphere. As warming alters the timing and dynamics of the ice retreat, carbon sources may shift and the magnitude of outgassing may change. We investigate the sources and pathways of the dissolved CO2 and CH4 via radio- and stable carbon isotope analyses, and conduct a spatial survey using a sensor suite containing a dissolved gas extractor, greenhouse gas analyzer, conductivity-temperature-depth probe, and oxygen optode. Across the transect, observed CH4 ranged from 1-5900 ppm, with δ13C values ranging from -70 to -47‰ (mean: -61.1±10.5‰); CO2 ranged from 100-3350 ppm, with δ13C values of -12 to +38‰ (mean: 6.1±13.1‰). Isotopic depletion of 13C correlated with lower CH4, while enrichment consistent with a primary productivity signal was seen in lower CO2 concentrations. Isotopic signatures additionally clustered with habitat type, revealing spatial variability in the processes controlling the production and transformation of CH4. Radiocarbon dating of the dissolved gases indicated predominantly modern carbon sources at all locations, with the high-CH4 melt ponds containing the youngest carbon (mean age of CH4: 148±16 years; mean age of CO2: 252±16 years). This work aims to enhance our understanding of interannual variability in the carbon cycle dynamics at this site, and to assess the system’s response to a changing climate.

How to cite: Traylor, S., Youngs, S., Pohlman, J. W., Kessler, J. D., Manganini, K., Pardis, W., Michel, A. P. M., and Nicholson, D. P.: Isotopic Analysis of Dissolved CO2 and CH4 in a Coastal Permafrost Zone, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2714, https://doi.org/10.5194/egusphere-egu24-2714, 2024.

During the late Austral summer of 2023, we carried out three surveys in the West Antarctic Peninsula (WAP) from Horseshoe Island (67° 514 south) to the Northern tip of the Peninsula to document the distribution of CH₄ in surface waters. We observed a striking feature in Dodman Island in the Grand Didier Channel with a marked supersaturation of methane (up to 400%) in the bay of the island, whereas saturation (maximum of 260%) was observed elsewhere. Our main hypothesis is that this supersaturation is linked to meltwater from the glacier on the island, which acts as a source of methane in the water column. This hypothesis is supported by vertical profiles of CH₄ concentration, field observations of sub-glacial water flowing to the surface of the water column, as well as by variations in salinity showing a freshwater inflow. This phenomenon has already been suggested in the Arctic (Lamarche-Gagnon et al., 2019) but does not yet seem to have been demonstrated in the Antarctic. These data show that it would be worthwhile investigating areas with active glaciers to determine whether melting glaciers can be a source of methane for the Antarctic water column.

How to cite: Brusselman, A., Crabeck, O., Muller, S., Araujo, P. A., Dogniez, M., Lepoint, G., Michel, L., Danis, B., Dall'Osto, M., Fripiat, F., and Delille, B.: Glacier meltwater, a potential source of methane in West Antarctica Peninsula, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-18144, https://doi.org/10.5194/egusphere-egu24-18144, 2024.

Coastal margins play a crucial role for greenhouse gases (GHG) budgets because biological cycling and sediment-water-air exchanges are more intense than in the open ocean. Given the tight connection between marine GHG cycling (production, consumption, fluxes) and dissolved oxygen dynamics, ongoing deoxygenation is of particular concern in coastal systems that experience seasonal or perennial hypoxia, and could therefore have a large impact on regional GHG budgets. N₂O stands out among other long-lived GHG because of its role as ozone-depleting compound and its effectiveness in enhancing Earth’s warming. Based on the vast majority of studies, marine coastal margins are expected to be hotspots of N₂O emissions to the atmosphere, with nitrification and partial denitrification as the main sources. However, despite significant advances in constraining the marine budget of N₂O over the last decade, the magnitude and seasonal variability of coastal emissions are still highly uncertain. While N₂O depletion in marine settings is usually associated to complete denitrification, recent evidence indicates the possibility of consumption at oxic-hypoxic interfaces or under fully oxic conditions. Moreover, a mechanistic understanding on the benthic-pelagic coupling of N₂O fluxes and its potential changes with deoxygenation is still rudimentary. To amend this deficit, we conducted a comprehensive study in Lake Grevelingen (Netherlands), a marine coastal reservoir characterized by seasonally hypoxic/anoxic conditions resulting from limited water exchange with the North Sea and eutrophication. Our study combined biogeochemical and microbial analyses and comprised multiyear (2020–2023) shipboard observations. We observed that unlike most coastal systems, surface waters of Lake Grevelingen are a rather weak source of atmospheric N₂O, with annual sea-air fluxes that represent <0.1% of the global marine emissions, and are below the regional climatological mean of the adjacent North Sea. Overall, the water column distribution of N₂O across the lake showed enhanced concentrations towards the bottom, which intensified during summer (stratified, low-oxygen period) at the deepest part of the basin (~45m). Nevertheless, computed gas saturations ranged between 20 and 100%, suggesting the occurrence of N₂O consumption which cannot be explained by solubility changes alone. Quasi-monthly observations in 2021 showed a clear seasonal variability with comparatively enhanced N₂O throughout the water column during summer. Although overall low, collocated measurements of amoA (gene marker of ammonia oxidation during nitrification) abundances within the oxycline showed a similar seasonal pattern, explaining part of the temporal N₂O variability in the lake. Cross-lake observations showed low spatial variability in the distribution of N₂O albeit ubiquitous low-oxygen conditions, suggesting N₂O production/accumulation to also occur in the shallowest (~10m) parts of the basin. Benthic micro-profile measurements showed enhanced concentrations within bottom waters and a rapid decline within the sediments. Analysis of sediments from different sites indicated this pattern to be consistent, such that the sediment-water interface of the lake acted as a source of N₂O. During this presentation we discuss these results, provide the first N₂O budget for the lake and put forward the potential implications of our study for the future representation of coastal fluxes in modelling studies.

How to cite: Arévalo-Martínez, D. L., Bange, H. W., Dotsios, N., van Erk, M. R., van Helmond, N. A. G. M., ter Horst, P. A. G., van Kessel, M. A. H. J., Lenstra, W., Lücker, S., Rigutto, I., Schrammeck, S., Zygadlowska, O., Jetten, M. S. M., and Slomp, C. P.:  Dynamics of N2O under coastal hypoxia: the case of Lake Grevelingen (The Netherlands) , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16142, https://doi.org/10.5194/egusphere-egu24-16142, 2024.