While the occurrence and oversaturation of GHG across the water column and sediments in marine, estuarine, and lacustrine systems are well documented, the understanding of GHG dynamics in the aquatic realm remains a major scientific challenge, shaped by geological, oceanographic/limnological, biological, and anthropogenic factors. This session invites contributions addressing recent advances in understanding the biogeochemical cycling, microbial pathways, and fluxes of CH4, N2O and CO2 in aquatic environments. We welcome studies based on laboratory and mesocosm experiments, field observations, remote sensing and modelling approaches. Topics of interest include, but are not limited to:
• GHG cycling and emissions in estuarine and other transitional water bodies across all latitudes.
• The role of nutrient and pollutant transport across the land-ocean continuum, submarine groundwater discharge, and benthic-pelagic coupling in GHG cycling.
• CH4, N2O and CO2 formation, transport, and consumption, including specific microbial processes involved.
• Microbe-mineral and microbe-animal interactions (including symbioses) and how these affect CH4, N2O and CO2 turnover.
Orals: Fri, 8 May, 14:00–18:00 | Room 2.23
Lakes in cryospheric regions are increasingly recognized as important but uncertain contributors to global methane (CH4) budgets. Because CH4 production and release are highly temperature sensitive, rapid warming in cryospheric regions is expected to amplify lake emissions and play a substantial role in climate feedback mechanisms. However, the response of CH4 emissions in cryosphere lakes to warming across lake sizes remains underexplored. Using a large, standardized dataset spanning a broad range of lake sizes, we show that diffusive and ebullitive CH4 fluxes display higher apparent temperature dependences in larger and deeper lakes compared to smaller and shallower systems. These results demonstrate that lake surface area and depth amplify the temperature dependence of CH4 emissions. Our findings highlight the importance of accounting for lake-size structure when assessing future CH4 dynamics under accelerated cryosphere warming and shifting lake extent.
How to cite: Song, C., Wang, G., Gudasz, C., Woolway, R. I., Chen, H., Li, Y., and Karlsson, J.: Lake size shapes the temperature dependence of methane emissions in cryosphere lakes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4659, https://doi.org/10.5194/egusphere-egu26-4659, 2026.
Gas hydrate provinces along continental margins are major reservoirs of methane and play a key role in regulating carbon fluxes between the geosphere, hydrosphere, and biosphere. The Amazon Deep-Sea Fan, one of the largest sedimentary systems on Earth, hosts extensive gas-hydrate accumulations and widespread fluid-expulsion structures, yet the spatial and temporal dynamics of methane within this system remain poorly constrained. Here, we integrate organic geochemical biomarkers with micropaleontological and biostratigraphic data to assess methane occurrence and microbial processing within shallow sediments of the hydrate stability field. Seven piston cores recovered during the 2023 AMARYLLIS–AMAGAS expedition penetrated up to ~30 m below the seafloor across hydrate-rich areas of the Amazon Fan. Twenty sediment samples were selected based on total organic carbon content and stratigraphic position and were analyzed using solvent extraction, liquid chromatography, and GC–MS. In parallel, 367 samples from 17 piston and gravity cores were studied for planktonic foraminifera, supplemented by analyses of calcareous nannofossils and palynofacies, thereby providing a robust Quaternary stratigraphic framework. Biomarker distributions indicate a dominance of terrestrial organic matter, with long-chain odd-numbered n-alkanes (n-C27–n-C35) and immature hopane and sterane assemblages, reflecting rapid burial in a clay-rich, low-maturity depositional environment. Despite this strong terrigenous imprint, all analyzed samples contain 3-methylhopanoids, diagnostic lipids of aerobic methanotrophic or methylotrophic bacteria. Their ubiquitous occurrence demonstrates that methane is present and bioavailable throughout the shallow subsurface of the hydrate stability zone. Meanwhile, the absence of 2-methylhopanoids suggests that cyanobacterial or phototrophic inputs are negligible in this zone, emphasizing a subsurface microbial signal. In selected cores, pentamethylcosenes further indicate localized zones of elevated microbial lipid production, suggesting spatially heterogeneous methane oxidation associated with focused fluid flow. Micropaleontological data indicate that the upper tens of meters of sediment are entirely Quaternary but are strongly affected by sediment remobilization associated with mass-transport deposits and mud volcanism driven by gas hydrate dissociation. Biozonation based on the presence and absence of Globorotalia menardii reveals alternations between glacial and interglacial intervals, reflecting climatic control on sedimentation, productivity, and bottom-water properties. The frequent occurrence of reworked Cenozoic and even Cretaceous microfossils within Holocene and late Pleistocene strata provides independent evidence for upward sediment transport driven by methane-rich fluids. Together, these datasets reveal a tightly coupled system in which methane stored in hydrates is episodically mobilized, transported, and consumed by microbial communities within shallow Amazon Fan sediments. Biomarkers provide direct evidence for active methane cycling, while microfossils document the stratigraphic and depositional framework that modulates hydrate stability and fluid migration. This integrated approach highlights the Amazon Deep-Sea Fan as a dynamic methane system, sensitive to both climatic forcing and sedimentary processes, with implications for carbon cycling along tropical continental margins.
How to cite: Rizzi, M. A. M., Brocks, J., Silva, T. F., Cagliari, J., Petró, S. M., Medina-Silva, R., Girelli, T. J., Augustin, A. H., Rodrigues, L. F., Santos, A. D. S., Leandro, L. M., Bruno, M. D. R., Guerra, R. D. M., Martarello, N. S., Fauth, G., Miller, D. J., Cupertino, J. A., and Chemale Jr, F.: Biogeochemical and micropaleontological constraints on methane in the Amazon Fan gas hydrate province, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17939, https://doi.org/10.5194/egusphere-egu26-17939, 2026.
A previously unknown gas ebullition field has been identified at ~400 m water depth in the Landsort Deep, the deepest part of the Baltic Sea. Hydroacoustic mapping reveals persistent seabed gas release over an area of approximately 17 km2. Although no direct gas samples were collected, elevated methane concentrations in sediment pore waters and bottom waters suggest that the bubbles are methane-dominated. Stable carbon isotope signatures of dissolved methane indicate a predominantly microbial origin, consistent with in situ production from the degradation of organic matter rather than migration from deeper thermogenic reservoirs.
The seep field is spatially associated with a drift deposit characterized by enhanced sedimentation rates, pointing to a tight coupling between organic matter accumulation and methane production. Acoustic flux estimates indicate an average seabed methane release on the order of ~10 mol m-2 yr-1, comparable to fluxes reported from the well-studied Tommeliten seep area in the North Sea. However, the Landsort Deep seep field is roughly two orders of magnitude larger in areal extent, implying substantially higher integrated methane emissions.
These findings highlight the potential for deep, hypoxic basins in eutrophied marginal seas to host large, previously unrecognized methane sources. The Landsort Deep provides a natural laboratory for investigating how sedimentation, redox conditions, and water-column stratification regulate methane production, oxidation, and escape from the seabed in coastal and semi-enclosed marine systems.
How to cite: Stranne, C., Doñate, V., Ladroit, Y., Y. Y. Yau, Y., Yu, C., Humborg, C., Jakobsson, M., Chang, C., and Ketzer, M.: The discovery of a new ebullition field in the deep Baltic Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11561, https://doi.org/10.5194/egusphere-egu26-11561, 2026.
Natural, cold seeps are important processes affecting seafloor geochemistry, ecosystems and greenhouse gas cycling. However, spatial and temporal constraints on offshore seepage remains limited. Methane seeps can form a variety of structures at the seabed, including pockmarks (depressions), or carbonate crusts in the form of methane-derived authigenic carbonates (MDACs), which can therefore be indicators of methane pathways through the seabed sediments.
MDACs form through microbial mediated anaerobic oxidation of methane and provide a unique record of both relict and active seep-related carbonated formation. Here, we investigate the spatial distribution of MDACs, their morphology, relation to the subsurface geology, and whether they are still forming in the Danish offshore areas of Kattegat and Skagerrak. Their exposure at the seafloor is linked to cementation of unconsolidated sediment, combined with glacio-isostatic uplift and erosion, resulting in various morphologies such as pillars, mushroom-like structures or slabs. As the hard substrates occur at a predominantly sandy sea floor, MDACs can act as local ‘oases’ and provide a foundation for benthic ecosystems.
Occurrences and morphologies of MDACs are identified by a combination of side-scan sonar, multibeam echosounder and sub-bottom profiler data, and confirmed with videos by remotely operated vehicles (ROVs) or divers. The majority of the MDACs align with the Sorgenfrei–Tornquist Zone, suggesting a tectonic control on fluid migration pathways. Previously published data indicated a stable carbon isotope signature (δ¹³C) of predominantly microbial methane source, likely derived from Late Quaternary organic-rich marine sediments (Jørgensen et al., 1990).
How to cite: Blok, C. N., Al-Hamdani, Z., S. Andersen, M., Ø. Hansen, L., R. Larsen, I., and B. Ernstsen, V.: Natural methane seepage in the Danish offshore area – spatial distribution and morphology of methane-derived authigenic carbonates (MDACs), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16780, https://doi.org/10.5194/egusphere-egu26-16780, 2026.
Deep marine cold seeps occurring along the seabed of continental margins are identified by the oasis-like ecosystems that are largely fueled by the chemical energy of the venting fluids. Seep site 2A-1, situated at ~2500 m water depth on the Scotian Slope of the North Atlantic was discovered in 2021. The seep hosts a large mussel encrusted, carbonate mound with biogenic methane bubbling up from a single vent. The emitted biogenic methane is primarily sourced from ~1 km below the seafloor within the basin bedrock that resides directly above the crest of an underlying salt diapir. A 600-m long transect composed of six push cores was collected across the seep structure. Downcore porewater ions and lipidomic profiles of twenty-four predominantly archaeal in origin lipid classes were tentatively identified and quantified across the transect. The resolved lipidomes comprised of intact polar lipids, core lipids, core lipid degradation products, and photosynthetic pigments. These data were compiled as two-dimensional heatmaps to spatially examine vertical and lateral changes in the subsurface geochemical and microbiological architecture of the seep. Microbially mediated metabolic zones of elevated heterotrophy, denitrification, microbial sulfate reduction, and anaerobic methane oxidation were then mapped across the seep structure based on an integrated analysis of porewater geochemistry, bulk organic matter and its carbon isotope compositions, lipidomic diversity and biomarker proxy patterns. Increased lipidomic diversity is shown to exist within the seep particularly at boundaries of high lateral geochemical gradients. Biomarker lipid proxies indicate a microbial community dominated by ANME-1 and -2/-3 archaea and high level of sulfate driven anaerobic oxidation of methane that is mixed with, but also surrounded by, an envelope of microbial sulfate reduction. Spatial changes in the stratified system highlight the complex interplay of micro- and macro-seepage and provide insights into the seep’s evolution and impact on microbial dynamics across the carbonate structure.
How to cite: Ventura, G. T., Redshaw, E., Oueslati, G., Umoh, U., MacAdam, N., Granados, P., Bentley, J., Ahangarian, N., Bennett, R., Baghalabadi, V., Fowler, M., and MacDonald, A.: Microbial and Geochemical Architecture of an Active Scotian Slope Cold Seep, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4073, https://doi.org/10.5194/egusphere-egu26-4073, 2026.
Enhanced methane cycling has been observed in various regions of the Arctic Ocean. Particularly, hotspots of high seawater methane concentrations in the surface waters of the Laptev and East Siberian seas highlight the risk of this methane to serve as an atmospheric source and further exacerbate climate warming. While seawater methane observations display the general distribution patterns within this region, the measurements are often compromised by the timing of expeditions and the frequent storm ventilation of the water column in these remote and shallow regions. Consequently, there is a need for an assessment of “long-term” methane cycling to better assess the geospatial patterns of methane cycling in the Arctic Ocean. Complementary tools such as compound-specific isotope analysis (CSIA) of hopanoid biomarkers has been suggested as a proxy to trace such regions of enhanced methane cycling. To investigate the long-term methane cycling over recent years in these areas, we quantified tracers of aerobic methane oxidation (C30 hopanoids; n=154) in surface sediments and, in a subset, their stable isotope compositions across the circum-Arctic shelf seas. The highest hopanoid concentrations were observed in the Laptev, East Siberian Seas and the Kara Sea together with methane indicating isotope compositions. Additionally, high hopanoid concentrations were widely accompanied by elevated concentrations of methane in the overlying seawater. However, local hotspots of elevated methane concentrations were also present in the Herald Canyon, Beaufort Sea, and in south-western Svalbard, yet C30 hopanoids in these regions did not corroborate the abundance of long-term enhanced methane cycling. Taken together, we display the first circum-Arctic assessment of seawater methane cycling through time-integrated measurements, highlighting regions and hotspots of enhanced methane cycling.
How to cite: Eriksson, A., Wild, B., Hong, W.-L., and Gustafsson, Ö.: Circum-Arctic Patterns of Proxy-Derived Methane Release from Shelf Sediments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6346, https://doi.org/10.5194/egusphere-egu26-6346, 2026.
Methane (CH4) is produced mostly in anoxic sediments through anaerobic degradation of organic matter. Here, we used both ex-situ sediment core and in-situ chamber lander incubations to quantify sediment-water CH4 fluxes under anoxic to oxic bottom water conditions in the Baltic Sea. Sediments acted as a source of CH4 into the water column with fluxes up to 13 mmol m-2 d-1. Strong spatial variability in sediment-water CH4 fluxes was observed with highest fluxes in the anoxic Western Gotland basin, followed by the Gulf of Finland and Gulf of Riga, and near-zero fluxes in the oxic Bothnian Bay. Sediment-water CH4 fluxes were negatively correlated with bottom water oxygen concentration, and positively correlated with sediment organic carbon content.
We incorporated observational data into a physical-biogeochemical model (BALTSEM-CH4 v1.0) to perform extrapolations. Sediments release 5 - 60 Gg CH4 yr-1 to the water column of the Baltic Sea. These large benthic CH4fluxes are largely counteracted by efficient CH4 oxidation in the water column (3 - 50 Gg CH4 yr-1). Both observations and model results indicate that water column oxidation prevents the high sediment-water CH4 fluxes from reaching the atmosphere.
How to cite: Yau, Y. Y. Y., Cheung, H. L. S., Santos, I. R., Hall, P., Bonaglia, S., Kononets, M., Henriksson, L., Politi, T., Gustafsson, E., Gustafsson, B., and Stranne, C.: Sediment-water CH4 fluxes across Baltic Sea bottom water oxygen gradients , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15572, https://doi.org/10.5194/egusphere-egu26-15572, 2026.
Coastal shallow ecosystems vary widely in physical conditions, habitat structure, and biodiversity, resulting in differences in biogeochemical processes and greenhouse gas (GHG) emissions. Benthic and pelagic processes, together with their interactions, regulate the production and consumption of carbon dioxide (CO2) and methane (CH4) across coastal ecosystems, potentially giving rise to intense, localized episodes of production or emission known as “hot moments” at specific sites, or “hot spots”. However, despite their importance for regional and global carbon cycles, these processes remain poorly characterized in many coastal environments due to their strong spatial and temporal heterogeneity.
We measured surface and bottom seawater biogeochemical properties using a state-of-the-art flow-through system equipped with a cavity ring-down system Picarro G2201-i, to measure CO2 and CH4 concentrations and stable carbon isotope composition, coupled with sensors for physical (temperature and salinity) and biogeochemical (chlorophyll-a (Chl-a), turbidity, colored dissolved organic matter (cDOM), dissolved oxygen (DO)) parameters. Sampling took place from June 2024 to May 2025, with 21 sites sampled in SW Finland, covering diverse soft-sediment habitats from sheltered to exposed areas along a salinity gradient.
Surface water partial pressure of CO2 (pCO2) and CH4 concentration ranged from 73.61 µatm to 3078.49 µatm, and from 4.48 nmol/L to 1104.77 nmol/L, respectively, with lower values observed for both parameters during spring, while higher values were observed during the summer months. Bottom water pCO2 ranged 89.09 µatm to 1969.67 µatm and the CH4 ranged from 4.68 nmol/L to 5145.44 nmol/L. For bottom waters, both minima appeared in spring, while the maxima appeared in summer months for the pCO2 and during autumn for CH4.
The lowest surface pCO2 values were associated with elevated Chl-a concentrations (57.94 µg/L), indicating a relation between low pCO2 and periods of high phytoplankton biomass and enhanced autotrophic activity. This pattern was observed during the spring bloom and persisted to a lesser extent during the summer, when Chl-a concentrations remained relatively high. In contrast, surface pCO2 and CH4 concentrations increased later in the season, with elevated values during summer and maxima generally occurring in autumn. This seasonal increase coincided with declining surface DO concentrations (minimum 215.92 µM) and increasing cDOM concentrations (up to 26.02 µg/L), reflecting pronounced seasonal changes in biogeochemical conditions.
In addition to seasonal variability, there was strong spatial heterogeneity across sites with different exposures. Sheltered locations consistently showed higher and more variable concentrations of pCO2 and CH4, especially during summer and autumn. In contrast, exposed sites had lower GHG levels and less seasonal fluctuation, while semi-exposed sites generally showed intermediate values. These spatial patterns were visible in both surface and bottom waters, with the largest contrasts observed in bottom-water CH4 concentrations, suggesting a key role of seafloor habitats.
All in all, these findings demonstrate that seasonal ecosystem changes significantly influence coastal GHG variability, highlighting the role of spatial-temporal heterogeneity as a key factor for improving the understanding of coastal GHG dynamics.
How to cite: Menéndez García, A., Villnäs, A., Norkko, A., and Geilfus, N.-X.: Seasonal variability in changes in greenhouse gas dynamics in shallow coastal ecosystems , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17754, https://doi.org/10.5194/egusphere-egu26-17754, 2026.
Methane (CH4) is a potent greenhouse gas. Coastal systems account for a large fraction of marine CH4 emissions. This emphasizes the need to understand coastal sources and sinks of CH4. Most of the CH4 in coastal systems is produced in sediments during organic matter degradation and consumed through anaerobic oxidation of CH4 (AOM), a process predominantly mediated by anaerobic methanotrophic archaea (ANME). Typically, AOM coupled to sulfate reduction is considered the dominant CH4 sink. However, in iron (Fe)-rich sediments, CH4 can also be oxidized either directly via coupling to Fe(III) reduction or indirectly via an Fe-driven cryptic sulfur cycle that sustains sulfate-dependent AOM. Additionally, natural organic matter (NOM) may also act as an electron acceptor in AOM.
While Fe-dependent AOM has been demonstrated in surface sediments, experimental evidence for such processes in deeper sediment layers (>1 m) remains limited and is largely inferred from model studies. Furthermore, experimental evidence for NOM-dependent AOM in coastal sediments remains scarce. The Bothnian Sea is a brackish basin in the northern Baltic Sea that receives high inputs of reactive Fe oxides and organic matter, creating conditions that may favor Fe- and NOM-coupled AOM in its deep sediments.
In this study we assess whether there is potential for AOM coupled to Fe and NOM reduction in deep sediments (>1 m) of the Bothnian Sea. We present results from long-term incubation experiments using sediments retrieved from the Bothnian Sea, site US5B which we amend with Fe oxide and graphene oxide, a NOM analogue, to evaluate their effect on CH4 oxidation. Our incubations, using 13CH4, show that Fe oxide and graphene oxide both stimulate AOM. In the case of Fe oxide, this could potentially involve a cryptic sulfur cycle. Based on metagenomic sequencing, ANME-2a/b archaea and potential metal-oxide reducing bacteria were enriched over time in both treatments. These findings provide new experimental constraints on the occurrence and relevance of Fe oxide and natural organic matter as electron acceptors in AOM in Fe-oxide and organic rich coastal sediments.
How to cite: Baas, E., ter Horst, P., Klomp, R., Lenstra, W., Jetten, M., and Slomp, C.: Anaerobic oxidation of methane mediated by iron and graphene oxides in coastal sediments, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17867, https://doi.org/10.5194/egusphere-egu26-17867, 2026.
Coastal ecosystems are critical components of the global carbon cycle, exerting a disproportionate influence on the carbon budget despite their limited spatial extent. Shallow coastal ecosystems exhibit strong gradients in physical, biogeochemical, and biological processes. Yet, their effects on carbon cycling and greenhouse gas (GHG) dynamics, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), remain inadequately understood. This knowledge gap is compounded by substantial heterogeneity in marine biodiversity, further complicating the issue.
Surface seawater partial pressure of CO2 (pCO2), CH4, and N2O concentrations, along with seawater physical and biogeochemical properties, and air-sea gas exchange, were measured at 21 sites in southwest Finland (Baltic Sea). Sampling progressed from estuarine inner bays to the outer archipelago, covering diverse soft-sediment habitats, from sheltered to exposed areas, across a salinity gradient. Seawater pCO2 and N2O concentrations ranged from undersaturated (160 ppm and 9 nmol L-1, respectively) to supersaturated (2521 ppm and 25 nmol L-1, respectively), compared to the atmosphere, resulting in an uptake of -36 and -0.0021 mmol m-2 d-1, and a release up to 220 and 0.0383 mmol m-2 d-1, respectively. CH4 concentrations were consistently supersaturated (19 to 469 nmol L–1) compared to the atmosphere, resulting in a net source to the atmosphere from 0.014 to 1.39 mmol m–2 d–1.
Freshwater input and its mixing with seawater shaped the overall spatial patterns of GHGs. However, deviations from this salinity-driven control were seen in sheltered sites within the archipelago, where elevated pCO2 and CH4 concentrations likely reflected biological processes, including enhanced organic matter respiration and methanogenesis in warm, late-summer shallow waters, where limited oxidation favored CH4 accumulation. At exposed and semi-sheltered sites, mixing processes exerted greater control, resulting in lower GHG concentrations. Our results show that both physical mixing and biological processes influence coastal GHG dynamics, with benthic ecosystems potentially playing a key but still poorly constrained role. The overall budget of air–sea GHG exchanges was dominated by CO2 fluxes, with CH4 consistently acting as a source, and N2O alternating between source and sink. High environmental variability in shallow coastal systems leads to strong fluctuations in the balance between GHG production and consumption, which needs to be considered when evaluating their role in the global carbon budget.
How to cite: Geilfus, N.-X., Delille, B., Villnäs, A., and Norkko, A.: Spatial heterogeneity of GHG dynamics across an estuarine ecosystem, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-16438, https://doi.org/10.5194/egusphere-egu26-16438, 2026.
Inland water bodies and rivers can be important sources of the greenhouse gases (GHG) nitrous oxide and methane, which are particularly high in managed water bodies receiving high nutrient loads. GHG emissions derived from such input are an important subset to national greenhouse gas inventories, but are difficult to parameterize. Within the national monitoring network ITMS, we thus investigated the seasonal variability of GHG concentration in the Elbe River. To support the parametrization of GHG fluxes and emissions, we measured nitrous oxide and methane concentrations along with key biogeochemical properties in the water column at a sampling station at the entrance to the Elbe Estuary in Geesthacht, Germany, in 2024.
Methane and nitrous oxide concentrations appear to be governed by surface water input and organic matter decomposition. Nitrous oxide remains close the equilibrium for most of the study period, with little autochthonous production in the river, and only increases during a flood event driven by elevated discharge and soil water inflow. In contrast, the limnic Elbe shows intense methane production, with strongly increasing concentrations over the course of the vegetation period, fuelled by organic matter turnover in the riverine water column and sediments. High chlorophyll and high methane concentrations with low nutrient concentrations and long residence times at high temperatures suggest intense internal recycling in the water column over the summer. Especially in summer, we also see a strong inverse correlation of water discharge and methane concentration in the river.
Our data show the interplay of water sources, discharge patterns and biological productivity in river and catchment on GHG concentration and underscore the complex interplay of processes that make the eutrophic Elbe River an important source of GHG under global change.
How to cite: Dähnke, K., Gök, I., Schulz, G., Sanders, T., Rewrie, L., and Bussmann, I.: Seasonal variation and controls of nitrous oxide and methane concentrations at the entrance to the Elbe Estuary, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14448, https://doi.org/10.5194/egusphere-egu26-14448, 2026.
Estuaries convert organic carbon and total nitrogen inputs from land to emissions of greenhouse gases such as carbon dioxide (CO₂) and nitrous oxide (N₂O). Many estuarine air-water flux budgets report CO₂ but omit N₂O, which has a much higher global warming potential. N₂O may have more elusive spatial and temporal emission patterns and exhibit short-lived high-emission events. A tide-resolving biogeochemical model for the tidal Elbe estuary, which flows through the German city of Hamburg, shows that CO₂ fluxes vary strongly through the year and often change sign between net uptake and net release. N₂O is always a net source, is highest in the tidal freshwater reach near Hamburg, and remains relevant in the outer estuary where CO₂ fluxes are small. The annual N₂O budget is driven by short events: within each estuary section, the top 10% of emission days contribute 27–38% of the annual flux. Similar N₂O maxima have been reported for other nutrient-rich and urban estuaries like the Scheldt and Humber. To observe and manage greenhouse gas emissions better, it is essential to identify the spatial and temporal pattern (the “hotspots” and "hot moments") of episodic nitrous oxide emission events, as has been done in our model study. This enables mitigation measures, such as temporary load reduction or artificial water oxygenation be effective.
How to cite: Saadaoui, Y., Pein, J., Preußler, N., Schulz, G., Dähnke, K., Sanders, T., and Lemmen, C.: “Hot moments” dominate nitrous oxide emissions in the Elbe estuary, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-11896, https://doi.org/10.5194/egusphere-egu26-11896, 2026.
Submarine groundwater discharge (SGD) is a major yet often overlooked driver of coastal carbon cycling. We examined two contrasting coastal basins: a bay dominated by karstic groundwater discharge and a fjord influenced by riverine inputs in the west coast of Ireland. Radioisotope-based (Ra and Rn) models and seasonal measurements were used to quantify water and carbon fluxes. We hypothesize that high-alkalinity groundwater from karstic aquifers delivers elevated dissolved carbon (DIC and DOC) and modulates air-sea CO2 exchange and carbon outwelling to the ocean. We further assess whether groundwater inputs act to buffer or intensify coastal acidification along the land–ocean continuum, providing new insights into how groundwater chemistry regulates carbonate equilibria and CO2 dynamics in contrasting coastal environments. Groundwater discharge was about 30% lower than river discharge yet contributed ~34 times more DIC (899 ± 453 vs 26 ± 22 mmol m2 d-1) and similar DOC fluxes (62 ± 31 vs 65 ± 51 mmol m2 d-1, respectively) to the coastal basins. Rivers (TA/DIC = 0.5 ± 0.2) showed a stronger acidifying effect than groundwater (TA/DIC = 0.9 ± 0.1) due to the strong buffering capacity of karst aquifers derived from carbonate dissolution. Bicarbonate outwelling from the coastal basins to the ocean and CO2 emissions to the atmosphere from the SGD influenced bay (155 ± 63 and 155 ± 121 mmol m2 d-1, respectively) exceeded those from the river fed fjord (87 ± 109 and 67 ± 33 mmol m2 d-1), highlighting the disproportionate role of groundwater-derived alkalinity in regulating carbon fluxes across the land-ocean interface.
How to cite: Cabral, A., Savatier, M., and Rocha, C.: Submarine groundwater discharge as a major driver of coastal carbon fluxes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13180, https://doi.org/10.5194/egusphere-egu26-13180, 2026.
Coastal bays are dynamic environments where both natural processes and human activities influence greenhouse gas (GHG) cycling. We investigated spatial and temporal variability of surface water CO₂, CH₄, and N₂O across coastal systems in northern New Zealand, spanning a gradient of ecological condition. Spatial surveys in Mahurangi Harbour, the Firth of Thames, the Hauraki Gulf and Auckland Harbour revealed pronounced heterogeneity in GHG distributions. Elevated CO₂ and CH₄ concentrations were consistently observed in upper bay reaches, particularly near mangrove-dominated areas, underscoring the role of tidal wetlands in coastal carbon dynamics. Distinct local hotspots of CH₄ and N₂O were detected in the Firth of Thames, associated with mussel aquaculture, suggesting aquaculture operations may enhance localized emissions. Complementary tidal investigations in Mahurangi and Whangateau Harbours highlighted higher CO₂ and CH₄ concentrations during low tide, linked to mangrove export, tidal pumping, and water-column processing. Notably, the persistence of elevated CO₂ at low tide under fully marine conditions highlights the strong influence of tidal wetlands and benthic processes, even in the absence of a salinity gradient. These measurements also demonstrated significant export of dissolved inorganic carbon (DIC) and alkalinity under fully marine conditions, indicating strong coupling between carbon cycling and exchange with the coastal ocean. To quantify these dynamics, a box-model approach incorporating DIC, total alkalinity, air–sea exchange, and export fluxes was applied to estimate carbon production and transformation. Together, these findings demonstrate how natural habitats and aquaculture activities jointly shape GHG fluxes and provide new insights into the spatial and tidal controls governing emissions in temperate coastal environments.
How to cite: Zinke, J., Geilfus, N.-X., Thrush, S., Villnäs, A., Norkko, A., and Humborg, C.: Spatial and Tidal Controls on Greenhouse Gas Dynamics in Temperate Coastal Bays of Northern New Zealand, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19027, https://doi.org/10.5194/egusphere-egu26-19027, 2026.
Seagrass meadows are highly productive systems that act as important nitrogen (N) sinks in the coastal zone. However, this service can be impacted by anthropogenic nutrient enrichment, which can increase the release of the greenhouse gas nitrous oxide. Anthropogenic climate change and coastal development will make moderate nutrient enrichment more commonplace in the future. Unlike high nutrient enrichment, our understanding of the impact of this moderate nutrient enrichment on N pathways in seagrass meadows is limited. In this study the water column above Australian seagrass patches (Cymodocea serrulata) was experimentally enriched in situ with N-P-K fertilizer, delivering 0.12 g N m-2 day-1 for 75 days. From days 35 to 75 of nutrient enrichment, an in situ 15N pulse-chase experiment was conducted to compare N uptake and capture by seagrass, epiphytes, and sediment within nutrient-enriched and ambient patches. Simultaneously, the effect of nutrient enrichment on dissimilatory N pathways was measured using three incubation methods: sediment slurries, intact sediment cores, and in situ benthic chambers. 15N-labelling indicated that moderate nutrient enrichment enhanced N uptake by epiphytes (median increased from 9.9 ± 1.5% to 19.9 ± 5.2% of 15N label) and lowered N storage in belowground tissue (median decreased from 5.9 ± 0.5% to 3.9 ± 1.9% of 15N at experiment end). Slurry incubations revealed that the potential denitrification rate in sediment was enhanced by nutrient enrichment. However, the more representative, intact sediment core and in situ incubations showed no change in denitrification rate due to nutrient-enrichment. Furthermore, denitrification was of minor importance in both the core and in situ incubations, while dissimilatory reduction of nitrate to ammonium (DNRA) was the dominant NO3- consuming pathway regardless of nutrient treatment. Moderate nutrient enrichment did not alter the rate of nitrous oxide production in the sediment, nor did it increase nitrous oxide flux from the sediment to the water. Our findings support the idea that seagrass functions as a buffer against nutrient enrichment, preventing drastic changes to dissimilatory N pathways. While moderate nutrient enrichment does not induce additional nitrous oxide release, it does decrease the long-term N storage efficiency of seagrass meadows.
How to cite: Joling, T., Andskog, M. A., Wells, N. S., Middelburg, J. J., and Oakes, J. M.: How Resilient is the Seagrass Nitrogen Cycle to Moderate Nutrient Enrichment?, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-2756, https://doi.org/10.5194/egusphere-egu26-2756, 2026.
The intertidal system of the back-barrier area of Spiekeroog in the East Frisian Wadden Sea is biogeochemically dynamic and essential for understanding regional dissolved inorganic carbon (DIC) budgets. However, DIC fluxes in this region remain poorly quantified compared to total alkalinity (TA), limiting our understanding of coastal carbon cycling. From October 2021 to December 2022, a range of discrete and in-situ measurements were conducted. Using these data, a seasonal linear regression model was developed to estimate lateral DIC fluxes continuously and to investigate seasonal DIC source–sink dynamics.
Results indicate that the Wadden Sea acted as a net DIC sink to the adjacent North Sea, with an import of 0.711 ± 1.48 mol m⁻² d⁻¹ (equivalent to 3.58 Gmol yr⁻¹) in 2022. The strongest import rates occurred in winter 2021 and spring 2022, likely driven by sediment–water exchange, remineralization, and biological uptake. During summer, import rates were lower, although intensified photosynthetic activity and elevated TA continued to modulate DIC dynamics, promoting CO₂ uptake. In contrast, during autumn, the Wadden Sea episodically exported DIC to the North Sea, driven by enhanced remineralization of organic matter following the summer production peak, intensified sediment–water exchange, and physical processes such as wind-induced mixing and storm events.
Air-sea CO₂ exchange and submarine groundwater discharge (SGD) were integrated into the seasonal carbon budget, revealing significant internal retention and transformation of DIC within the system. The findings highlight the function of the Wadden Sea as a coastal carbon sink and demonstrate substantial seasonal variability. SGD also represents a major knowledge gap, emphasizing the need for integrated, high-resolution measurements and modelling to constrain regional carbon budgets and inform climate change mitigation strategies.
How to cite: Meyer, J., Luebben, A., Thomas, H., Voynova, Y. G., and Van Dam, B.: Seasonal Dissolved Inorganic Carbon Dynamics in the East Frisian Wadden Sea: From Net Sink to Episodic Source, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18682, https://doi.org/10.5194/egusphere-egu26-18682, 2026.
Microbial methane oxidation in freshwater sediments can substantially reduce methane emissions to the atmosphere, yet the processes regulating this activity in the methanogenic zone have remained poorly constrained. In iron-rich sediments, methane cycling may overlap with microbial iron reduction, suggesting potential coupling between these processes. Lake Kinneret sediments exhibit such conditions at depth, where methanogenesis dominates in the presence of high concentrations of reactive iron and limited availability of alternative electron acceptors. Previous studies from Lake Kinneret methanogenic sediments pointed to the coupling between methane oxidation and iron reduction, as well as to the unexpected presence of aerobic methane-oxidizing bacteria within; however, the microbial interactions are not clear.
Here we explored whether interactions between methane-oxidizing and iron-reducing bacteria can stimulate iron reduction under the methanogenic conditions. Controlled laboratory experiments were conducted using an aerobic methane-oxidizing bacterium and an anaerobic iron-reducing bacterium incubated with porewater from the methanogenic sediment zone of Lake Kinneret, amended with ¹³C-labeled methane and amorphous ferric iron, under 1% O₂ conditions. Our findings demonstrate that methane-oxidizing bacteria are linked to microbial iron reduction through indirect interactions, likely mediated by soluble metabolites or electron-shuttling compounds. The results highlight the role of microbial interactions in regulating sedimentary redox processes and methane cycling under low-oxygen conditions.
How to cite: Rosenblatt, A., Sivan, O., and Rubin-Blum, M.: Microbial interactions between iron reducing and methane oxidizing bacteria in methanogenic sediments of freshwater lake, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14290, https://doi.org/10.5194/egusphere-egu26-14290, 2026.
Aquaculture ponds are significant hotspots of methane (CH4) emissions, yet the mechanisms regulating CH4 production and emissions across diverse pond types remain poorly understood. By investigating 20 aquaculture ponds with diverse culture systems in (sub)tropical southern China, we examined CH4 emissions (FCH4) and their underlying drivers. Our preliminary results reveal substantial CH4 emissions from (sub)tropical aquaculture ponds, with significant variations among pond types. Fish ponds exhibited the highest FCH4 (226 ± 441 mg m–2 d–1), followed by shrimp ponds (68 ± 159 mg m–2 d–1) and crab ponds (49 ± 112 mg m–2 d–1). Ebullition was the dominant pathway of CH4 emissions, accounting for over 70% of the total CH4 flux. CH4 emissions were collectively regulated by management practices, environmental variables, and methane-cycling microbial communities. Salinity suppressed FCH4 by inhibiting methanogen metabolism and restructuring methanogenic community, while elevated organic substrates could offset the salinity-driven inhibitory effect. Furthermore, rising temperature could substantially stimulate CH4 emissions, especially ebullition, with an 11% increase in FCH4 per 1 °C rise in water temperature. This thermal sensitivity of FCH4 was further amplified in ponds with higher organic substrates, revealing a synergistic effect between temperature and substrate availability in promoting CH4 production. Notably, low-latitude aquaculture ponds exhibited greater temperature sensitivity. Our study highlights the considerable CH4 emission potential of (sub)tropical aquaculture ponds and identifies salinity, organic matter, and temperature as key regulators of FCH4. These findings provide a framework for scaling the contribution of aquaculture ponds to the global CH4 budgets.
How to cite: Ran, L. and Yang, Q.: Biogeochemical and microbial controls on methane emissions from (sub)tropical aquaculture ponds, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9879, https://doi.org/10.5194/egusphere-egu26-9879, 2026.
Thermokarst lakes, formed by permafrost thaw in the Arctic, are ubiquitous with abrupt permafrost thaw and large atmospheric source "hotspots" of methane (CH4) and carbon dioxide (CO2) emissions, which are expected to double permafrost carbon emissions by the end of the century. While the implications of ongoing permafrost thaw on CH4 dynamics within these lakes have been modeled, here we provide empirical data on CH4 production dynamics as lakes evolve from young recently formed lakes to older lakes that have been present for hundreds of years. Sediment cores were collected from the centers and thermokarst margins of a new thermokarst lake and from an older thermokarst lake from the same interior Alaskan watershed. The highest CH4 production rates were observed in the uppermost sediments near the sediment-water interface at the thermokarst margins of both lakes, with a steep decrease with sediment depth into the talik. The young lake exhibited elevated CH4 production rates, correlated with higher carbon lability. The integrated sediment-column CH4 production rates were similar, primarily due to the thinner talik at the young lake. Our data support the predictions that formation and expansion of thermokarst lakes over the next centuries will increase CH4 production in newly thawed Yedoma permafrost sediments, while CH4 production will decrease as taliks mature and labile organic carbon is used up. Our results also suggest important controls of methane production and oxidation in the sediments.
How to cite: Sivan, O., Pellerin, A., Gerera, Y., Eilani Russak, E., and Walter Anthony, K.: The Evolution of Methane Emissions in Thermokarst Lakes , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14247, https://doi.org/10.5194/egusphere-egu26-14247, 2026.
The salinization of inland waters, driven by sea-level rise and anthropogenic activities, poses increasing threats to aquatic ecosystems and the human communities that depend on them. This process can alter the functioning of tidal rivers, particularly their biogeochemical cycles and their role as sources or sinks of carbon. The Gambia River, in West Africa, represents a key system for study: first, because the contribution of inland waters in this region—and in equatorial dry climates more broadly—to the global carbon cycle remains poorly quantified; second, due to its relatively intact hydrogeomorphology; and third, because its integrity is increasingly threatened by salinization linked to sea-level rise (~4 mm yr⁻¹ in the region), climate-driven changes in precipitation patterns, and upstream dam construction.
Within this context, we investigated how salinity influences key parameters of aquatic carbon cycling by combining seasonal and spatial sampling along the river–estuary continuum. We tested whether increasing salinity affects the concentrations, sources (δ¹³C isotopic signatures), and atmospheric emissions of dissolved greenhouse gases (CO₂ and CH₄), and how salinity gradients influence the availability and partitioning of carbon pools, including dissolved organic carbon (DOC), particulate organic carbon (POC), dissolved inorganic carbon (DIC), and alkalinity.
To measure these variables, along with ancillary parameters such as chlorophyll-a, total suspended solids, and nutrients, we conducted three sampling campaigns between 2024 and 2025 under dry, wet, and transitional seasonal conditions, covering 12 sites from freshwater reaches (~400 km inland) through the estuary to the coastal ocean. Each campaign also included intensive spatial sampling across salinity transition zones (67, 16, and 15 additional sites, respectively).
Preliminary analyses indicate that nutrients and ions exhibit relatively stable concentrations in freshwater reaches, increase at the onset of salinity intrusion, and stabilize again under fully saline conditions, with overall higher values in saline sections compared to freshwater. In contrast, CO₂ concentrations increase downstream in the river but decrease again across the salinity transition zone, remaining slightly higher in saline sections than in freshwater reaches, which can occasionally be undersaturated relative to the atmosphere, resulting in negative CO₂ fluxes. CO₂ patterns appear to be primarily associated with organic matter availability, closely following DOC distributions rather than salinity gradients, and showing an inverse relationship with chlorophyll-a, suggesting an important role of biological uptake in the upper river.
CH₄ dynamics, in contrast, show a stronger sensitivity to salinity, likely reflecting enhanced microbial competition with sulfate under saline conditions, which may reduce CH₄ production and emissions. Isotopic signatures indicate shifts in dominant methanogenic pathways, highlighting the role of organic matter composition and availability in controlling methane production pathways rather than absolute production rates.
Alkalinity was generally higher than DIC along the river–estuary continuum, and both variables deviated from conservative mixing at intermediate salinities (10–20), indicating the presence of in situ DIC production within the estuary. Together with the observed patterns in other carbon pools, these results demonstrate that salinity gradients exert differential controls on carbon species and associated biogeochemical processes along the Gambia River.
How to cite: Bisbal Regidor, E., Stegehuis, A., von Schiller, D., Minaudo, C., Faal, M. (., Nkamnebe, A., Rodríguez-Lozano, P., and Catalán García, N.: Salinity-driven controls on carbon cycling along the Gambia River (West Africa), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-21243, https://doi.org/10.5194/egusphere-egu26-21243, 2026.
Posters on site: Fri, 8 May, 08:30–10:15 | Hall X1
Methane seeps along active continental margins record the interaction between fluid flow, biogeochemical processes, benthic ecosystems, and tectonic forcing. Here we present an integrated analysis of methane seep environments from the Concepción Methane Seep Area (CMSA) on the central Chile margin (~36°S), combining seafloor imagery, multibeam bathymetry, sediment biogeochemistry, authigenic carbonate geochemistry, and U–Th chronology from the R/V Sonne 210 expedition in 2010. High-definition ROV surveys reveal a mosaic of seep habitats ranging from soft-sediment sites with active seepage and chemosynthetic fauna to extensive carbonate chemoherms representing older, largely inactive seep stages, dominated by background deep-sea fauna that utilize the carbonates as hard substrate and refuge.
Carbonates from multiple sites show strongly depleted δ¹³C values (down to ~–50‰ VPDB), confirming methane-derived carbon sources. U–Th ages span from very young (<5 ka) carbonates associated with active seepage to late Pleistocene and older structures (>100 ka), documenting long-lived and multi-phase seep activity. In contrast, some massive carbonate blocks exhibit complex internal architectures and anomalous U–Th systematics, indicating open-system behavior and requiring cautious age interpretation. Sediment biogeochemical data reveal high rates of benthic methane oxidation at active seep sites, characterized by shallow sulfate depletion and elevated sulfide concentrations. In contrast, carbonate-dominated sites lack comparable sedimentary biogeochemical signatures, primarily due to the limited presence of soft sediments, although methane oxidation may still partially occur within the carbonate framework. Water-column methane measurements indicate active methane release from the seafloor, with highest concentrations near the bottom and a pronounced decrease within the first 100–200 m above the seafloor.
By comparing multiple subregions within the CMSA, we identify distinct successional stages of seepage, progressing from sediment-hosted sites through mixed sediment–carbonate settings to predominantly fossil chemoherms. We discuss how these stages reflect temporal variability in methane flux, carbonate precipitation, and biological colonization, potentially modulated by episodic tectonic activity along the Chilean margin. Our results highlight the value of combining geomorphological, geochemical, and ecological data to reconstruct the life cycle of methane seep systems on active margins.
How to cite: Treude, T., Linke, P., Liebetrau, V., Scholz, F., Steeb, P., Geersen, J., Schmidt, M., Sommer, S., Rovelli, L., Bryant, L., and Scholten, J.: Successional stages of methane seep systems off central Chile: from active sediment-hosted seepage to fossil carbonate chemoherms, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8152, https://doi.org/10.5194/egusphere-egu26-8152, 2026.
Methane (CH4) is a potent greenhouse gas with a global warming potential by mass 25 times greater compared to carbon dioxide. Its atmospheric concentration has tripled since the industrial revolution, and half of the global CH4 emissions can be attributed to aquatic ecosystems. However, there is a large spatiotemporal heterogeneity in river and estuary CH4 emissions leading to challenges in precise quantification.
This study presents CH4 diffusive fluxes from three temperate estuaries discharging into the German Bight of the southern North Sea: the Ems, Weser and Elbe, which are all subject to anthropogenic perturbations. During a campaign in autumn 2024 on the RV Heincke, continuous measurements of CH4 were obtained using cavity ring-down spectroscopy (Picarro G2508 coupled with an equilibrator system). For quality control purposes, discrete water samples were collected, preserved and later measured with gas chromatography analysis. Ancillary biogeochemical variables were measured continuously using a FerryBox system installed on board.
Preliminary results show varied CH4 diffusive fluxes across all three estuaries ranging between 6 µmol d-1 m2 and 763 µmol d-1 m2. In the Weser and Elbe, the CH4 fluxes were elevated (701 µmol d-1 m2 and 499 µmol d-1 m2, respectively) in the lower estuaries with salinities of > 17. Concentrations decreased in the mid-regions and then increased in the upper freshwater region to 763 µmol d-1 m2 and 360 µmol d-1 m2 with salinities 0.2 – 0.5. In the Ems, the highest CH4 flux up to 627 µmol d-1 m2 was observed in the lower estuary with salinity of 28. We postulate that site specific characteristics, such as organic matter degradation and CH4 production in the actively dredged Hamburg Harbour (upper Elbe Estuary), as well as stronger winds at 16 m s-1 in the lower Elbe Estuary promoted elevated CH4 fluxes. We aim to further disentangle the impacts of human alterations to coastal environments on CH4 production and emissions, by incorporating and assessing the accompanying FerryBox biogeochemical variables along with discrete nutrient samples in these temperate estuaries.
How to cite: Rewrie, L., Bußmann, I., Camillini, N., Dähnke, K., Macovei, V., Sanders, T., Schulz, G., and Voynova, Y. V.: Methane emissions from three European temperate estuaries, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12057, https://doi.org/10.5194/egusphere-egu26-12057, 2026.
About one-third of the world’s coastline is classified as permafrost. Increased erosion promotes abrupt thaw along these shorelines, leading to remobilization and enhanced microbial degradation of previously stored organic matter. With rising sea levels, coastal thermokarst lakes may be inundated by seawater, causing a gradual transformation into marine-influenced lagoons. Depending on the connectivity of the lagoon to the open sea, methanogenic communities are exposed to high concentrations of ions, especially sulfate, which promotes the establishment of competitive anaerobic methanotrophic archaea and sulfate-reducing bacteria consortia and thus leads to shifts in the composition and activity of the microbial community (Yang et al., 2023). Previous research focusing on surface sediments from lagoon systems in the Canadian Arctic found the highest total greenhouse gas production in the initial stage of the transition (Jenrich et al., 2025) while surface soil samples from a land-sea transect showed highest methane production rates in the active layer of the intertidal zone (Roy-Lafontaine et al., 2025). However, questions remain regarding the effects of marine inundation on the microbial community and the associated carbon dynamics of erosion-affected coastal environments. Here, we use vertical sediment profile incubations from thermokarst lakes, the coastal ocean, and soils from an intertidal zone near the community of Tuktoyaktuk, located in the Inuvialuit Settlement Region (NWT, Canada). We performed anoxic long-term incubation experiments under in situ (freshwater) and marine conditions to simulate saltwater intrusion. Corresponding methane and carbon dioxide production rates were monitored by monthly measurements. In addition, we analyzed sedimentary pore-water nutrient and metal concentrations, along with bulk organic matter characteristics (TOC, δ13C, lability), to examine potential relationships between initial redox conditions, organic matter quantity and quality, and greenhouse gas production. Additionally, we investigated potential shifts in the microbial community during the incubation by 16S rRNA sequencing. Preliminary results of the first months of incubation indicate that freshwater lakes located further away from the coastline show higher production rates under in situ compared to marine conditions. In contrast, negligible production rates were found for a marine-influenced lagoon. This pattern suggests a shift in the microbial community from a dominance of methanogens in freshwater lakes to the establishment of methanotrophs as a consequence of increased marine influence. As a result, rising sea levels may decrease methane emission rates from coastal lakes.
References:
Jenrich, M., et al. (2025). Biogeosciences, 22, 2069–2086.
Roy-Lafontaine, A., et al. (2025) EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-2570.
Yang, S., A et al. (2023). Global Change Biology, 29, 2714–2731.
How to cite: Hellmann, J., Pellerin, A., Whalen, D., Bröder, L., Brinkmann, I., Heintzman, P., and Lattaud, J.: Transgression and Transformation: Methane Cycling in Thawing Arctic Coastal Landscapes, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-745, https://doi.org/10.5194/egusphere-egu26-745, 2026.
As an important and rapidly developing agricultural industry worldwide, oyster aquaculture helps mitigate the coastal eutrophication by denitrification, a process which produces nitrous oxide (N2O). Yet it is not presently possible to quantify the N2O flux associated with oyster farming in transitional water bodies. Incubations using nitrate and nitrite as the substrates complemented with functional gene screening confirm net anaerobic N2O production in oyster digestive tract. The N2O production rate could be positively regulated by nitrate and nitrite availabilities and temperature. Surprisingly, oyster’s digestive tract is an unexpected N2O source due to the inability of N2O reduction to N2. In comparison, the oyster shell-associated biofilm and attached particulate matter (APM) can perform complete denitrification, thus offsetting net N2O production by digestive tract. Such a net N2O consumption is more effective under lower oxygen condition. However, high availabilities of nitrite and nitrate in the water column may lower the N2O sink capacity, even ceasing N2O consumption. This study elucidates a modular N2O turnover that is specific to the compartments inside and outside of oysters, providing insights into the environmental controls about dynamic N2O source or sink associated with shellfish-microbe interactions.
How to cite: Liu, R. and Ji, Q.: Component-specific oyster-mediated modular nitrous oxide turnover, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6836, https://doi.org/10.5194/egusphere-egu26-6836, 2026.
Methane (CH4) oxidation in Arctic shelf waters plays a critical role in regulating subsea CH4 emissions, yet remains difficult to quantify due to strong spatial heterogeneity and complex transport processes. Here, we investigate CH4 oxidation in the outer Laptev Sea using vertical and lateral profiles of CH4 concentration together with dual-stable isotopic compositions (δ13C–CH4 and δD–CH4). Across the study area, dissolved CH4 exhibits large concentration gradients accompanied by pronounced enrichment in the heavy isotopes. The observed δ13C–CH4 and δD–CH4 values increase synchronously, yielding dual-isotope slopes (Λ=7.9–13.7) that fall within the characteristic range of aerobic CH4 oxidation. Isotopic enrichment is most clearly expressed below the pycnocline, indicating substantial oxidation of sediment-derived CH4 during its residence in sub-pycnocline waters. For samples that show Rayleigh-type isotope–concentration relationships, we quantify the fraction of CH4 oxidized (fox) using site-specific isotopic source signatures and incubation-derived fractionation factors. Station-integrated results yield regional median fox values of 22% (interquartile range, IQR: 11–32%) based on δ13C–CH4 and 36% (IQR: 17–42%) based on δD–CH4. These estimates likely represent conservative lower bounds as it is difficult to measure CH4 addition by bubble dissolution and to capture the complete trajectory of CH4 through the sub-pycnocline waters. The exceptionally high δ13C–CH4 (up to +21‰) and δD–CH4 values (up to +573‰) occur at relatively low CH4 concentrations (~80 nM) at stations located between active seep hotspots (~3000 nM), suggesting advanced oxidation under open-system conditions during continued transport. Comparison of isotope systems shows that δD–CH4 provides a more robust constraint on oxidation than δ13C–CH4 in heterogeneous shelf environments. Overall, our results demonstrate that water-column structure and CH4 residence time primarily control the extent of CH4 oxidation in the outer Laptev Sea. This study provides new quantitative and process-based constraints on water-column CH4 oxidation in Arctic shelf seas and demonstrates the utility of dual-stable isotope approaches for resolving CH4 cycling in complex marine systems.
How to cite: Yan, F., Brussee, M., Holmstrand, H., Wild, B., Semiletov, I., Shakhova, N., Kang, S., and Gustafsson, Ö.: Dual-stable isotope constraints on aerobic methane oxidation in the water column of the outer Laptev Sea, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9633, https://doi.org/10.5194/egusphere-egu26-9633, 2026.
The methane flux from coastal water areas such as fjords and the underlying control mechanisms have been little studied to date. Fjords are characterized by a complex hydrography that is shaped by marine and limnic interactions and leads to a pronounced stratification of the water column. The resulting low ventilation of the deep water together with high primary production rates in the surface water and the subsequent transport of the organic material to the seabed often lead to high methane releases from the seabed. In our study, we analyzed a fjord system in the Chilean part of Patagonia, the Golfo Almirante Montt. The investigation is based on studies of water column methane concentration and stable carbon isotopes, the distribution and activity of methane-oxidizing bacteria, and oceanographic and geological observations. Our results indicate that methane is of biogenic origin is released from gas-rich sediments at the entrance of the main fjord basin, which is characterized by pockmarks and gas flares. Tidal currents and turbulent mixing at the sill cause a methane plume near the surface to spread into the main fjord basin and mix with the methane- and oxygen-depleted deep water. The wind-induced mixing at the sea surface controls the methane flux from the methane plume into the atmosphere. The methane plume is consumed mainly by methanotrophic bacteria. An enrichment of the signature gene particulate methane monooxygenase (pmoA) in the methane-poor deep water, and a conspicuously high δ13C-CH4 signature of the methane suggest that methane-rich intrusions are periodically introduced into the deep water, which are subsequently converted microbially. Our interdisciplinary study offers a comprehensive insight into the complex physical and biological processes that modulate methane dynamics in fjords and thus help to better assess how methane emissions from these systems will change under anthropogenic influence.
How to cite: Jürgens, K., Schmale, O., Mohrholz, V., Papenmeier, S., Blumenberg, M., Feldens, P., Jordan, S., Ruiz-Fernández, P., Meeske, C., Fabian, J., Iwe, S., and Umlauf, L.: The control of physical and biological drivers on pelagic methane fluxes in a Patagonian fjord (Golfo Almirante Montt, Chile), EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22346, https://doi.org/10.5194/egusphere-egu26-22346, 2026.
Ongoing anthropogenic-driven climate change patterns prompt for better constraining drivers of coastal methane (CH4) and nitrous oxide (N2O) emissions, potent greenhouse gases (GHG) with greater warming potential than carbon dioxide. The temperate Elbe Estuary (Germany) is heavily influenced by anthropogenic activities including high agricultural nitrogen loads and intense dredging in the Hamburg Port, overall contributing to this ecosystem acting as a net source of CH4 and N2O. However, interactions between local microbial activity and transport processes translate into pronounced spatial and temporal variability in estuarine CH4 and N2O dynamics, thus challenging assessments at time-scales relevant to episodic events. This study presents spatio-temporal CH4 and N2O dynamics in the Elbe estuary based on discrete samples collected during three campaigns (in 08/2024, 09/2024 and 08/2025) that captured the effects of an episodic flood event in autumn following dry summer.
During low river flow conditions (08/2024 and 08/2025), CH4 and N2O concentrations showed distinct spatial gradients along the estuary, indicative of riverine input and CH4 production in the mid-estuary (salinities <20), while N2O production was mainly restricted to the upper and hypoxic estuary. During a river flood event (09/2024), the Elbe discharge rate rapidly increased 4-fold up to 1240 m3 s-1 within 10 days, which lowered CH4 and N2O concentration along the estuary. Consequently, the CH4 and N2O dilution during the flood event decreased estuarine diffusive water-air flux rates, while increasing export to the German Bight. While these preliminary findings need to be further evaluated against ancillary environmental data, capturing the effects of episodic flood events across different seasons has the potential to influence estuarine CH4 and N2O emission budgets.
How to cite: Camillini, N., Gök, I., Rewrie, L., Schulz, G., Voynova, Y., Brix, H., and Sanders, T.: Influence of an episodic flood event on methane and nitrous oxide dynamics in the Elbe estuary, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-17724, https://doi.org/10.5194/egusphere-egu26-17724, 2026.
Large fractions of the global subsea permafrost are located on the shallow East Siberian Arctic Shelf (ESAS), characterized by high methane ebullition and concentrations found over extensive scales, with atmospheric methane release estimated to be of the same scale as emissions from the rest of the World Oceans. Current research generally focuses on pinpointing the sources of subsea methane and understanding the release processes, by which methane migrates from the sediments, through the shallow water column and in part escapes to the atmosphere. A potentially neglected, yet critical question regarding the fate of the methane released from the subsea permafrost system is: can dissolved methane in the water column diffuse back down into sediment porewater with lower dissolved methane concentration?
Here, we investigate methane concentration gradients and estimate diffusive fluxes across the sediment–water interface on the ESAS. We compiled an extensive dataset of CH4 in sediments and overlying bottom water (n>100 stations), including both unpublished and published measurements collected during multiple expeditions between 2012 and 2020. Approximately 25% of the stations exhibit reversed concentration gradients, with higher CH4 concentrations in bottom waters than in surface sediments, indicating the potential for downward CH4 diffusion.
We propose that a substantial fraction of methane is released from the sediments to the seawater as bubbles, which then dissolves in the bottom water. A fraction of this methane diffuses back down into the surface sediment, where it may possibly be degraded by microbes – a process that mitigates how much of the total initial sediment release of methane that escapes to the atmosphere. We asses the magnitude of diffusive CH4 fluxes across the sediment water interface using Fick’s law. Aside from gradient strength, flux magnitude also depends on site specific conditions and properties such as sediment porosity, temperature, salinity, and bottom shear stress which controls the diffusive boundary layer thickness. The flux calculations indicate that the observed reversed gradients can result in a net diffusive flux of methane from the water column into the sediments, with the highest reflux estimated to be 11 mmol m-2 day-1.
Our results suggest that ESAS sediments can alternately function as both a source and a sink for methane, challenging the prevailing view of a one-directional sediment-to-water flux. These findings highlight the need to further explore the potential of Arctic Shelf sediments acting as both a sink and source of methane as part of the dynamic sediment–water methane exchange processes.
How to cite: Lundqvist, L., Yan, F., and Gustafsson, Ö.: Is There Methane Reflux from Bottom Waters into Sediments on the East Siberian Arctic Shelf? , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18215, https://doi.org/10.5194/egusphere-egu26-18215, 2026.
Methane emissions from offshore non-producing oil and gas wells (OGWs) at the global scale remain poorly understood. Most published studies on offshore non-producing OGWs have focused on the North Sea, and there are questions as to whether these findings can be extrapolated at the global scale. One important consideration for offshore OGW methane emissions into the atmosphere is the extent to which methane biodegrades in seawater above OGWs, for which the vertical extent of the water column is viewed as a key factor. Additionally, there is a lack of consensus on whether the knowledge from onshore wells is transferable to offshore wells, as well as how best to quantify emissions to the atmosphere, including practical limitations of current technologies. Therefore, we are developing a database of offshore wells globally, including water depth data, to facilitate analysis of potential for methane emissions and its mitigation. An accompanying literature review will identify knowledge gaps related to methane emissions from offshore non-producing wells, their role in national emissions estimates, and quantification approaches. Our results will be helpful in the understanding of potential contributions of offshore non-producing OGWs to methane emissions to the atmosphere, thereby informing methane mitigation strategies and policies.
How to cite: Prado, P., Boutot, J., France, J., Coleman, M., Peltz, A., Gomez, N., Procesi, M., Etiope, G., van Oort, E., Vincent, P.-E., de Oliveira Bredariol, T., Hubert, C., de Bruin, G., Griffioen, J., Leifer, I., Reichart, G.-J., Socolofsky, S., Socolofsky, S., Wilpshaar, M., and Kang, M. and the Database of offshore non-producing oil and gas wells for quantifying methane emissions potential : Database of offshore non-producing oil and gas wells for quantifying methane emissions potential , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15463, https://doi.org/10.5194/egusphere-egu26-15463, 2026.
The Arctic is warming leading to rapid environmental changes including permafrost thaw, glacier retreat, and altered hydrology. These changes can lead to disintegration of cryospheric caps and mobilize previously stored greenhouse gases (GHGs) of both geogenic and biogenic origin, that represent a little known climatic feedback of permafrost melting. This release mechanism of CH4 and CO2 have been detected along glacier margins of the Greenland Ice Sheet and in Svalbard. Here the partitioning of CH4 and CO2 is strongly dependent on oxidation processes and hydrological connectivity1,2. However, the presence and relative contributions of geogenic and biogenic CH4 and CO2 emissions from ice-free Arctic permafrost landscapes located on top of known geological oil and gas resources remains understudied.
To study this, we apply an integrated isotopic approach combining bulk stable isotopes (δ13C(CH4), δ2H(CH4), δ13C(CO2)), clumped isotopes of CH4, and radiocarbon (¹⁴C) analyses of CH4 and CO2, including concentrations of C2/C3 gases, to disentangle gas sources, formation pathways, and cycling processes in an Arctic permafrost. Over a 10-day period gas samples were collected from in situ gas seeps in lakes on top of geological fault zones, natural springs, and permafrost thaw ponds to capture the variation in CH4 and CO2 concentration and isotopic compositions in areas with different geogenic impacts. The field work was carried out on the western tip of the Nuussuaq peninsula in West Greenland (70°29′57.16″ N, 54°10′35.91″ W). Here the landscape is characterized by active permafrost with known geogenic gas and oil seeps.
We will present the full isotopic composition of CH4 and CO2 from these gas seeps to show that the combined use of clumped isotopes and radiocarbon enables a clear distinction between microbial, thermogenic and geogenic gas sources, as well as oxidation and mixing processes. Results provide insight on the origin, turnover and fate of CH4 and CO2 in Arctic landscapes to help understand the role of subsurface geology to GHG emissions.
References
1: Adnew GA, Röckmann T, Blunier T, et al (2025) Clumped isotope measurements reveal aerobic oxidation of methane below the Greenland ice sheet. Geochim Cosmochim Acta 389:249–264. https://doi.org/10.1016/J.GCA.2024.11.009
2: Adnew GA, Schroll M, Röckmann T, et al (2025) Radiocarbon and bulk isotope composition of subglacial methane and carbon dioxide emitted at the western margin of the Greenland ice sheet. Geochim Cosmochim Acta
How to cite: Jørgensen, C. J., Adnew, G. A., Schroll, M., Roslund, K., Eckhardt, H. P., Röckmann, T., Szidat, S., van der Veen, C., Keppler, F., and Christiansen, J. R.: Tracing the geogenic and biogenic genesis and age of CH4 and CO2 in gas seeps across an Arctic permafrost landscape, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9526, https://doi.org/10.5194/egusphere-egu26-9526, 2026.
The Land–Ocean Aquatic Continuum (LOAC) is a critical and complex component of the global carbon cycle, regulating the transfer, transformation, and evasion of carbon from terrestrial ecosystems to the coastal ocean. Last layer of this continuum before the coastal ocean, estuaries play a dual role: 1) as reactors that process organic and inorganic carbon and 2) as sometimes significant sources of CO₂ emissions to the atmosphere compared to their modest surface areas. These processes are tightly controlled by a wide array of external factors including upstream land-based emissions (influenced by lithological context, land use, etc.), riverine transport and in-stream biogeochemical transformations, as well as internal estuarine morphology, hydrodynamics and metabolism. With over 50 000 estuaries worldwide and a wide heterogeneity between systems, performing an exhaustive carbon budget analysis, even regionally is a major but necessary challenge to better constrain the carbon land-ocean exchange and the contribution of estuaries to CO2 budgets.
To investigate the coupling between lateral carbon fluxes and atmospheric CO₂ exchanges over a continuous stretch of coast, batch simulations of the 1D depth integrated generic estuarine hydrological/biogeochemistry C-GEM has emerged as a suitable solution because of its design build on limited data and computing demand. In an application on the Atlantic French coast, estuarine dynamics were explicitly represented in time and space for 35 selected macro-tidal estuaries. This regional application quantifies the cascading fluxes of Organic Carbon (OC), Dissolved Inorganic Carbon (DIC), from the upstream influence of tides to the estuarine outlets, while simulating air–water CO₂ exchanges within estuaries. This exercise is based on a structured database that compile an exhaustive inventory of all aquatic measurements at the upstream boundary of the estuarine modelling domain (for all watersheds larger than 300 km²), as well as along the estuarine longitudinal profiles themselves for model validation.
Our integrated approach allows the establishment of a consistent regional carbon budgets that account for terrestrial inputs and estuarine processing, coastal exports, and CO₂ evasion to the atmosphere. Our simulations indicate that estuaries along the French Atlantic coast act predominantly as net sources of CO₂, with strong spatial variability driven by size, watershed characteristics, riverine carbon loads, and estuarine residence times. The fraction of riverine carbon loads that is outgassed towards the atmosphere as CO2 within the estuary ranges from a few percents to 20% from the smaller systems to the largest. By detailing the nature and intensity of carbon fluxes in the LOAC estuarine compartment, this work highlights the importance of proposing integrated land-sea modelling approaches that explicitly include estuarine interfaces in order to constrain regional carbon budgets and national and continental greenhouse gas inventories. Moreover, it opens to door to longer time scale simulations to disentangle the natural component of the global estuarine carbon budget from its anthropic perturbation, partitioning that is currently virtually unknown.
How to cite: Laruelle, G. G., Capet, A., Casquin, A., Thieu, V., Silvestre, M., and Regnier, P.: Carbon cycling and CO₂ emissions from French Atlantic estuaries: a regional modeling approach using a generic estuarine model, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-22802, https://doi.org/10.5194/egusphere-egu26-22802, 2026.
Posters virtual: Tue, 5 May, 14:00–18:00 | vPoster spot 2
EGU26-12306 | ECS | Posters virtual | VPS5
A computationally efficient framework for modelling estuarine biogeochemistryTue, 05 May, 15:21–15:24 (CEST) vPoster spot 2
Estuarine systems are crucial in deciphering coastal ocean dynamics and biogeochemistry, including the vital role they play as ecological sequesters of greenhouse gases. We present a modelling framework that combines the Estuary Box Model (EBM) with the Biogeochemical Flux Model (BFM) to simulate the interplay between physical dynamics and biogeochemical processes. The EBM is a robust, yet simplified model that represents estuarine hydrodynamics, addressing salinity, temperature, and freshwater discharge variations. The BFM simulates nutrient cycling, microbial interactions, phytoplankton dynamics, organic matter mineralization and particulate sedimentation across chemical functional families and living functional groups. To realistically simulate estuarine scenarios, the passive tracer transport equation was adapted to include explicit biogeochemical reaction terms within a time-varying estuarine simplified control volume, furthermore, accounting for riverine nutrient inputs, vertical mixing, tidal exchange and various biological feedback. Additional alterations were made to accommodate burial and sequestration parameters better representing estuarine zones.
The coupled framework was applied to the Po di Goro estuary in northern Italy, and the simulations were conducted for the period 2010 to 2023. The results were validated by comparing the Chlorophyll concentration outputs against satellite and in-situ buoy observations. The outcomes show a strong correlation between phytoplankton biomass and residence time during periods of algal blooms, whereas a rapid shift to zooplankton propelled top-down grazing control during prolonged periods of stable conditions. The model effectively replicates the organic matter sedimentation dynamics typical of deltaic environments, offering insights into the scale and factors controlling the burial and sequestration of organic matter in these ecosystems. The coupled EBM-BFM system is a highly computationally efficient and scalable framework for understanding the estuarine ecosystem drivers, with important potential applications in biogeochemical variability, nutrient retention, and climate-driven changes in coastal zones.
How to cite: Santhoshkumar, S., Verri, G., Vigiak, O., Niroumand, M., Riminucci, F., Silvestri, S., and Mentaschi, L.: A computationally efficient framework for modelling estuarine biogeochemistry, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12306, https://doi.org/10.5194/egusphere-egu26-12306, 2026.
