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.

Abstract Bottom‐water oxygen (O 2 ) possibly constitutes the strongest regulating factor on ecosystem function of the seafloor environment. The conventional view of bottom‐water O 2 regulation in temperate regions is a strong seasonal variability imposed by deposition of fresh, labile organic carbon following the spring and fall plankton bloom generating changing sediment O 2 demand. The supply of O 2 is considered mainly regulated by seasonally‐varying thermal stratification or large‐scale lateral advection of water masses. However, benthic O 2 also varies over much shorter timescales, but the spatiotemporal regulation of this O 2 variability is not well resolved. We conducted a 10‐month‐long in situ study (2022–2023) of bottom‐water O 2 dynamics at a 40‐m‐deep coastal site in the Baltic Sea. Following the spring bloom and the onset of thermal stratification, the gradual decline in O 2 was regularly interrupted by rapid (<24 hr) oxygenation events in which O 2 levels spiked, subsided, but remained elevated by between 25 and 180 μmol L −1 relative to conditions before the event. Without these events, extrapolation of the observed O 2 decline implied hypoxic or anoxic bottom‐water conditions by mid‐May. Similarly, reoxygenation of the lower water column in the late fall was controlled by a few rapid and strong reoxygenation events. These events are consistent with regional‐scale ocean circulation and mixing processes, potentially involving a downwelling mixing front and/or coastal trapped/internal waves. However, we lack data to unambiguously distinguish between these mechanisms. We suggest that rapid benthic oxygenation events are likely a common characteristic of coastal seafloors and critical for benthic ecosystem functioning.

Abstract Background Cable bacteria are filamentous sulphide-oxidisers capable of cm-scale electron transport. They are generally considered restricted to the upper few oxic–suboxic cm of marine sediments, where they couple sulphide oxidation to oxygen or nitrate reduction. Despite their influence on redox gradients, trace metal mobility, and nutrient cycling, their presence and activity in deeper anoxic sediment layers remain unknown. The presence and activity of marine cable bacteria (Candidatus Electrothrix) were investigated at four stations in Sweden and Finland, including deep vertical profiles of anoxic sediment layers, to assess their presence and activity under different environmental contexts. Results Using metatranscriptomic data for rRNA-based community profiling and gene expression combined with porewater geochemistry, evidence of abundant and active cable bacteria was found, peaking below 20 cm depth in deep anoxic sediment layers of Koljö Fjord on the Swedish West Coast. This zone coincided with elevated gene expressions related to sulphide oxidation (including sqr) and nitrate reduction (napA), as well as an abundant presence of sulphide and a sharp nitrate peak. Phylogenetic analyses revealed a diverse assemblage of Ca. Electrothrix includes several potential novel taxa. The co-occurrence of cable bacteria activity, sulphide availability, and a nitrate peak at depth suggests that these organisms may be supported by local nitrate production under anoxic conditions. Conclusions Our findings challenge the prevailing view that cable bacteria are restricted to shallow sediment horizons and demonstrate their activity and diversity in deep, anoxic layers. This expands the known ecological niche of cable bacteria and suggests that locally produced nitrate under anoxic conditions may facilitate their activity at depth. This discovery advances our understanding of ecology in anoxic marine environments, providing new insights into marine cable bacteria, sediment biogeochemistry, and analogues of early Earth microbial ecosystems.

Glacial meltwater is a major contributor to stratification in polar waters, particularly in glacial fjords where it is contained by fjord topography. Stratification from glacial meltwater input impacts both light and nutrient availability, altering the timing and magnitude of phytoplankton blooms and peak in secondary productivity. Ice conditions can further impede near-surface circulation and trap low-density meltwater plumes, amplifying stratification. Whilst stratification is a critical process in the initiation of phytoplankton blooms, reduced mixing can impede nutrient resupply in the euphotic zone, reduce productivity, and alter the formation processes of marine snow. Here, using a combination of optical and acoustic instrumentation, we investigated how different stratification conditions in two adjacent fjords of northwest Greenland (Petermann Fjord, PF, and Sherard Osborn Fjord, SOF) impact the vertical distribution of two key components of Arctic pelagic ecosystems: marine snow and copepods. We show that the amplified stratification caused by ice damming outside SOF was associated with lower indices of primary and secondary production. Stratification also reduced concentrations of marine snow and resulted in an altered vertical distribution of small sphere particles that were likely fecal pellets in the top 100 m of SOF. Zooplankton distributions in both fjords were centered below the fluorescence peak but were more tightly coupled with the chlorophyll maximum in SOF than in the well-mixed PF. Feeding conditions in SOF were poorer, while in the more productive PF zooplankton were distributed deeper where risks of predation are likely reduced. Although small and large copepod densities were comparable between fjords, the low numbers of nauplii in SOF further suggest mismatch conditions not suitable for their survival. We demonstrate that sea ice conditions are linked to local physical water column stratification that has cascading effects on productivity and the abundance, distribution, and types of marine snow and copepods. Future conditions in glacial fjords are not clear because thermal stratification and glacier runoff will increase, but the number of ice damming events could decrease.

Abstract. The multi-scale composition, structure, and dynamics of seasonal ice floating on freshwater lakes are influenced by ambient conditions. Here we describe a comprehensive geoscience experiment for lake system imaging and monitoring of spatiotemporal ice property variations. We explore the resolution of meteorological and environmental driving mechanisms that can include the quantification of methane degassing from boreal lakes. The project centerpiece is a seismic array of 210 geophones arranged in an aperiodic tiling configuration that was deployed in February 2025 on the ~25 cm thick ice of Lake Pääjärvi in southern Finland. The 10-km scale lake array is complemented by three dense circular arrays, 31 land-based sensors, eight broadband seismometers, three accelerometers, a rotational seismometer, a Distributed Acoustic Sensing system with a 1 km-long fibre optic cable, an underwater echosounder, a microphone, a Ground Penetrating Radar (GPR) survey, water chemistry measurements, manual ice thickness sampling and ice coring, and meteorological observations. We observe the strongly dispersive QS flexural mode and the weakly or non-dispersive QS₀ and HS₀ modes excited by hammer shots, icequakes, and environmental sources and reconstruct the average propagation using beamforming and noise correlations. Propagation speed estimates for the three modes range approximately between 20–100 m s⁻¹, 3000–3400 m s⁻¹, and 1650–1800 m s⁻¹, respectively. High values for the Poisson's ratio ν = 0.42 and Young's modulus E = 8.59 GPa reflect the overall competent characteristics of the ice referred to as teräsjää (steel ice). Seismic activity in the 0.03–0.2 Hz band increases during high wind speed episodes, and signals above 0.1 Hz correlate with rapid air-temperature cooling events. The GPR profile images the spatial ice variability across the lake that is compatible with the in-situ measurements, and we show that seismo-acoustic observations can be inverted for similarly compatible thickness estimates. The geochemical water and ice sample analysis suggests Lake Pääjärvi is a source of methane, and localized ebullition can potentially be resolved from echosounder data. This synthesis demonstrates that the application of environmental seismology concepts can form a bridge between bottom-up ebullition monitoring and remote-sensing approaches.

Different aquatic ecosystems may have opposing effects on global climate acting as sources and/or sinks of greenhouse gases. The conversion of sediment organic carbon to the powerful greenhouse gas methane is a key indicator of the potential climate impact of a given ecosystem. Here, we assess the lifetime climatic role of aquatic ecosystems by contrasting methane (CH 4 ) emissions with long-term organic carbon burial. Global compilations revealed that the ecosystem-specific ratio of organic carbon burial (kg CO 2 ) to CH 4 emissions (kg CH 4 ) ranged from 20:1 in lakes to 2500:1 in continental shelves. Using 100-year sustained-flux global warming potential and a radiative balance model, we show that most ecosystems currently exhibit a net negative radiative balance (i.e., net cooling or a CO 2 -equivalent sink). The lifetime cooling effect per area was strongest in the coastal ocean and fjords. Freshwater ponds, lakes, and inland wetlands had lowest organic carbon burial to CH 4 emission ratios, requiring ~ 0-700 years after their establishment to shift from a positive to net negative radiative balance. All aquatic ecosystems gradually increase their net climate benefit via long-term enhancement of the ratio of carbon sequestration to CH 4 emissions. Most aquatic ecosystems will thus achieve negative radiative balance over their lifetime.

Abstract Ice–ocean interactions at Greenland Ice Sheet outlet glaciers exert critical control on ice-sheet mass loss and, consequently, global sea-level rise. The GEOEO North of Greenland 2024 icebreaker expedition acquired a breadth of data in the unsurveyed Victoria Fjord, north-western Greenland, to assess the causes of the rapid break-up of the C.H. Ostenfeld ice tongue in 2002, and the environmental changes since. The observations reveal inflows of warm (>0 °C) Atlantic water reaching the grounded ice margin. We propose that Atlantic warm water pulses entering the Arctic Ocean reach north-western Greenland, where they interact with glacier fronts, enhance ice discharge, and amplify the climate-driven retreat of marine-terminating glaciers. The collapse of C.H. Ostenfeld’s ice tongue, unlike the slower retreats of the ice tongues of neighbouring Petermann and Ryder glaciers, reflects unimpeded Atlantic water inflow due to the absence of a shielding bathymetric sill, a function of the region’s bedrock geology.

The Southern Ocean is undergoing rapid transformations, marked by significant regional shifts in salinity that carry widespread and irreversible consequences. While the most noticeable changes are observed in the upper ocean, changes in deeper water masses have been identified and are expected to intensify over time. Changes in upper-ocean water mass salinity can be influenced by multiple drivers, and play a crucial role in changing ocean dynamics. However, the underlying causes of these characteristic changes remain poorly understood. In this study, we present a unique three-decade time-series focusing on salinity and oxygen isotopes in the upper 1200 m of the Indian sector of the Southern Ocean. Two regions emerge with pronounced surface ocean salinity trends: freshening of subpolar waters and salinification of subtropical waters. These robust changes in surface salinity are associated with an observed freshening of intermediate and winter waters in the subpolar sector of the Indian sector over the past three decades. Our findings reveal salinity changes of comparable magnitude to those reported in other regions of the upper-ocean water masses in the Southern Ocean. The oxygen isotope data allows for discriminating between different freshwater processes, showing that in the subpolar region, surface freshening is largely caused by the increase in net precipitation, while the decrease in sea ice melt is largely offset by the contribution of glacial meltwater at these latitudes. These changes strengthen the growing evidence of an acceleration of the hydrological cycle and a melting cryosphere resulting from human-induced climate change, which affect Southern Ocean water mass characteristics.

Anthropogenic activities are key drivers of eutrophication and deoxygenation in coastal marine ecosystems. This stimulates the anaerobic degradation of organic matter and the release of reduced products, such as ammonium, methane, and hydrogen sulfide, which may, in turn, exacerbate eutrophication and deoxygenation. In this study, using a combination of chemical and microbial analyses, we assess the nitrogen dynamics in the water column of a eutrophic coastal system (Stockholm Archipelago) at three sites with contrasting redox conditions (oxic to long-term euxinic). At the oxic site, counter gradients of ammonium and oxygen in the water column, low nitrate δ15N values in bottom waters, and the 16S rRNA gene-based presence of nitrifiers indicate nitrification near the sediment-water interface. At the seasonally and long-term euxinic sites, nitrification, as inferred from the water column oxygen and nutrient profiles and the relative abundance of nitrifiers, primarily occurred near the oxycline. At these two sites, nitrate was removed below the oxycline through denitrification linked to sulfide oxidation by Sulfurimonas . Nitrous oxide emissions from surface waters in the archipelago reached up to 40 µmol m-2 d-1 and were not directly related to water column redox conditions, indicating that multiple factors control coastal emissions of this greenhouse gas to the atmosphere. The relative abundance of 16S rRNA genes and of N-cycle genes in metagenomes was highest at the seasonally euxinic site. Importantly, nitrifiers were significantly less abundant at the long-term euxinic site. Our results highlight that prolonged euxinia promotes recycling of ammonium over its removal, likely due to sulfide inhibition of nitrification, which sustains eutrophication and deoxygenation of coastal systems. ### Competing Interest Statement The authors have declared no competing interest.

Ocean acidification induced by the absorption of anthropogenic CO2 and its consequences pose a potential threat to marine ecosystems around the globe. The Arctic Ocean, particularly vulnerable to acidification, provides an ideal region to investigate the progression and impacts of acidification before they manifest globally. Recent documentation of undersaturated surface waters in carbonate minerals in the Sherard Osborn fjord in northwest Greenland, a region visited for the first time in summer 2019, reveals inherent variability in biogeochemical processes. Associated with highly acidic surface waters, the partial pressure of CO2 (pCO2) was undersaturated relative to the atmosphere, indicating this study area as a CO2 sink. To comprehend variations in pCO2 in the northwest Greenland fjords and identify its drivers, we conducted a comparative study between two fjords in the region (Petermann and Sherard Osborn fjords) and used carbonate system data from the temperature minimum layer to examine the winter-to-summer evolution of pCO2 and influencing factors. Additionally, we evaluated pCO2 variations (δpCO2) concerning temperature, freshwater inputs, biological activity, and air-sea CO2 uptake to quantitatively assess the seasonal influencing factors on surface ocean pCO2. In the Sherard Osborn fjord, despite a substantial increase in surface temperature from winter to summer potentially increasing pCO2 and causing CO2 supersaturation relative to the atmosphere, freshwater inflow and biological activity reduced pCO2, resulting in CO2 undersaturation relative to the atmosphere. In the Petermann fjord, pCO2 remained lower than atmospheric levels due to a slight seasonal variation in surface temperature and significant biological activity, reducing pCO2 in surface water.

A recently acquired multidisciplinary dataset comprising acoustic surveys (high-resolution sub-bottom profiles, multi-beam bathymetry, and broad band mid-water echo sounder), geochemistry (gas chemical and isotopic composition, porewater chemistry), and sedimentology (core lithology and X-ray CT) in the area of the Landsort deep (450 m of depth), south of Stockholm Archipelago, revealed the existence of an extensive (20 km2) region of the seafloor where massive gas release is occurring in the form of multiple bubble streams. This new discovery represents a major seafloor methane release site in Europe and is comparable in area to other large sites worldwide such as the ones in Svalbard and in the South Atlantic Ocean associated with gas hydrate provinces. The gas is formed mostly by methane of microbial origin. Surprisingly, bubbles rise 100’s of meters above the seafloor and reach surface waters above the halocline/oxycline at around 80 m of depth. Some bubbles appear to reach the sea-air interface and their potential methane contribution to the atmosphere is under investigation. Another surprising observation is the absence of major seafloor features like pockmarks in the gas release area. The reasons for the seafloor methane release in the Landsort deep are still not entirely clear, but our preliminary acoustic and sedimentological data suggest that bottom currents may have acted to facilitate the accumulation of organic-rich sediments in a thick drift deposit during the Holocene and the modern warm period (latest 100 years). Our data further suggest that the high sedimentation rate in the drift deposit continuously supplies fresh organic matter that is quickly buried below a thin sulphate reduction zone, fueling vigorous methanogenesis and abundant methane formation. Similar methane release sites might be discovered in other known large drift deposits in the Baltic Sea.

Planktonic foraminifera are calcifying protists that represent a minor but important part of the pelagic microzooplankton. They are found in all of Earth's ocean basins and are widely studied in sediment records to reconstruct climatic and environmental changes throughout geological time. The Arctic Ocean is currently being transformed in response to modern climate change; however, the effect on planktonic foraminiferal populations is virtually unknown. Here, we provide the first systematic sampling of planktonic foraminifera communities in the “high” Arctic Ocean – defined in this work as areas north of 80° N – specifically in the broad region located between northern Greenland (the Lincoln Sea with its adjoining fjords and the Morris Jesup Rise), the Yermak Plateau, and the North Pole. Stratified depth tows down to 1000 m using a multinet were performed to reveal the species composition and spatial variability in these communities below the summer sea ice. The average abundance in the top 200 m ranged between 15 and 65 individuals m−3 in the central Arctic Ocean and was <0.3 individuals m−3 in the shelf area of the Lincoln Sea. At all stations, except one site at the Yermak Plateau, assemblages consisted solely of the polar specialist Neogloboquadrina pachyderma. It predominated in the top 100 m, where it was likely feeding on phytoplankton below the ice. Near the Yermak Plateau, at the outer edge of the pack ice, rare specimens of Turborotalita quinqueloba occurred that appeared to be associated with the inflowing Atlantic Water layer. Our results would suggest that the anticipated turnover from polar to subpolar planktonic species in the perennially ice-covered part of the central Arctic Ocean has not yet occurred, in agreement with a recent meta-analysis from the Fram Strait which suggested that the increased export of sea ice is blocking the influx of Atlantic-sourced species. The presented data set will be a valuable reference for continued monitoring of the abundance and composition of planktonic foraminifera communities as they respond to the ongoing sea-ice decline and the “Atlantification” of the Arctic Ocean basin. Additionally, the results can be used to assist paleoceanographic interpretations, based on sedimented foraminifera assemblages.

The Amazon River culminates in one a deep-sea fan up to 10 km thick, a dynamic setting in which the rapid deposition of organic-rich sediment drives linked processes of methanogenesis, fluid migration and venting, gas hydrate formation, and large-scale slope instability. Growth of the fan over the last 8 Ma has been accompanied by its gravitational collapse on shale detachments to form extensional and compressional belts across the shelf and upper slope (96%) are

Methane is an important greenhouse gas, and global methane emission has been estimated separately from the perspective of anthropogenic and natural factors. However, in heavily populated semi-closed bays, methane emissions may be governed by both or even significantly amplified by human activities. One of the main factors mitigating methane emission from marine sediments to seawater and the atmosphere is the anaerobic oxidation of methane (AOM). The sulfate-rich zone acts as a barrier to methane release from the subseafloor because sulfate-dependent AOM removes sulfate and methane dissolved in interstitial water in a 1:1 molar ratio. Due to significant riverine inputs of freshwater and restricted water exchange, the seawater in some of the semi-closed bays is potentially fresher and has lower sulfate concentration, leading to a less effective AOM barrier for methane. Furthermore, the influx of nutrient-rich wastewater to densely populated semi-enclosed bays frequently leads to severe eutrophication, greatly enhancing biological productivity, anoxia, and the accumulation of organic-rich sediments in these systems. The objective of this study is to gain a deeper understanding of how the methane cycle is changed by anthropogenic activities in two case studies, with geochemical datasets collected from Tokyo Bay and the Baltic Sea, both known as heavily populated semi-closed bays. We conducted sediment coring at the entrance of Tokyo Bay and offshore Stockholm in the Baltic Sea. Two cores (2.5 m in length) from Tokyo Bay and six cores (4 to 6 m in length) from the Baltic Sea were recovered, respectively. Organic matter in the surface of 1 m of sediment, which may have been strongly influenced by recent anthropogenic activities, showed 1.5 to 2% and 1.5 to 3.6% of total organic carbon (TOC) in Tokyo Bay and the Baltic Sea, respectively. These results indicate the Baltic Sea has a higher potential to generate more methane than the Tokyo Bay. The sulfate concentration at the seafloor was 27 mM in Tokyo Bay and 4 mM in the Baltic Sea and decreased with depth due to the AOM reaction reaching 0 mM at 2.5 mbsf in both bays. The thickness of the sulfate reduction zone was the same in both bays, even though they have a large difference in sulfate concentration in the bottom seawater. The iodine concentration, which has been used as a tracer for methane due to its close association with organic matter, increased with depth up to 74 µM at 2.5 mbsf in Tokyo Bay and 63 µM at 4.5 mbsf in the Baltic Sea. The iodine flux in Tokyo Bay was two times higher than in the Baltic Sea, indicating the possibility of strong methane flux from deeper sediments, which may not directly derive from Anthropocene organic-rich sediment. We will discuss and compare the details of the geochemical datasets in both Bays in the presentation.

We investigated the relative contributions of various factors that influence seasonal changes in sea surface partial pressure of CO 2 ( pCO 2 , calculated from the measured pH and total alkalinity) in four regions of northwestern Greenland: Nares Strait, Lincoln Sea, Sherard Osborn and Petermann fjords. Using the temperature minimum layer as a proxy for winter conditions, we examined pCO 2 dynamics from the onset of sea-ice melt to summer. Our findings revealed significant spatial variability in pCO 2 , driven by differences in temperature, freshwater inputs, and biological activity. In particular, in Sherard Osborn Fjord substantial freshwater inputs and strong stratification were found to enhance pCO 2 accumulation, while in Petermann Fjord biological CO 2 uptake was the main driver. This study, conducted in summer 2019, underscores the critical role of northwest Greenland's coastal waters as a summer CO 2 sink. It highlights the complex interplay of physical and biogeochemical processes in modulating pCO 2 , suggesting significant regional differences in CO 2 dynamics between two neighboring fjords.

Data presented in the article "Eutrophication and deoxygenation drive high methane emissions from a brackish coastal system ".

Submarine groundwater discharge (SGD) is an important process responsible for transporting terrestrial dissolved chemical substances into the coastal ocean, thereby impacting the marine ecosystem. Despites its significance, there are few studies addressing SGD in the northern Baltic Sea. Here we investigate the potential occurrence of SGD in an area characterized by seafloor terraces formed in varved glacial clay located around Fif & aring;ng Island, Southern Stockholm Archipelago. We analyzed Rn-222 activity and porewater geochemistry in both marine and terrestrial sediment cores retrieved from Fif & aring;ng Island and its surrounding offshore areas. Results from 222Rn mass-balance calculations, water isotopes, salinity, chloride concentration, and dating (including 14C and helium-tritium dating) indicate that modern groundwater flows through varved glacial clay layers and fractured rocks on Fif & aring;ng Island and discharges into Fif & aring;ng Bay. Additionally, the offshore cores reveal a saline groundwater source that, dating of the dissolved inorganic carbon, appears systematically younger than the hosting clay varves dated using the Swedish clay varve chronology. Acoustic blanking in our acquired subbottom profiles may be related to this fluid migration. The occurrence of this saline groundwater seems to be independent from the distance to the submarine terraces. Collectively, our study confirms the occurrence of submarine groundwater in the varved glacial clay close to Fif & aring;ng Island and further offshore. Our findings help establish the significance of submarine groundwater discharge in influencing the past and present coastal environment in the Baltic Sea region.

Coastal benthic hypoxia and anoxia develop in thermally stratified coastal waters during warm summer months. They alter the chemical composition, biogeochemical cycling, and ecosystem functioning at the seafloor and can render the benthic habitat uninhabitable for higher life forms. With more and longer heatwaves expected due to global warming, the strength and persistence of stratification is expected to increase leading to longer and more extensive bottom water hypoxia in the coastal ocean. However, on short timescales benthic oxygen availability can be dominated by highly dynamic lateral transport and transient vertical mixing events that can compensate for the sediment oxygen demand through short-term ventilation events. The occurrence, temporal dynamics, and quantitative impacts of these ventilation events have so far been poorly understood. We present results of a two-week summer field campaign at a 38 m deep thermally stratified coastal site in the western Baltic Sea. An autonomously operating benthic lander system equipped with stationary oxygen optodes at fixed depths, a continuously profiling multiparameter probe, a high-frequency downward-looking ADCP was deployed together with an eddy correlation system, within 50 meters distance. The setup enabled the study of the vertically resolved temporal evolution of oxygen in relation to hydrodynamic parameters in the bottom waters at a second- and centimetre-scale resolution for a 280-hour long deployment period together with continuous measurements of the benthic oxygen consumption. At the beginning of the deployment bottom-water free-flow velocities were on average 1.6 cm/s consisting of a translatory and a rotating diurnal oscillatory component. Weakening of the translatory current component gradually turned the system into an almost pure oscillatory state with free-flow velocities of about 0.8 cm/s. Bottom-water oxygen concentrations were constant down to 5 cm above the sediment at an initial normoxic concentration of 170 μmol l-1 that decreased with decreasing flow velocity to hypoxia below 63 µmol l-1 by the end of the measurement series. During purely oscillatory flow the balance between sediment oxygen uptake and vertical transport resulted in a net bottom water oxygen loss of 6.4 μmol l-1d-1 increasing to -14.5 μmol l-1d-1 following a resuspension event. Even at low-flow velocities the bottom water remained well mixed. Bottom water oxygen loss was not continuous and instead varied between +43.5 and -45.2 μmol l-1d-1 corresponding to changes in lateral transport. Temporary changes (<2 hours) up to 30 μmol l-1 were found due to convergence/divergence events of the bottom water during flow reversals. These dynamic bottom water changes would have been undetectable using conventional shipboard tools due to their close proximity to the sea floor. We suggest that areas undergoing frequent hidden hypoxia and re-ventilation are more common than previously thought and have so far unexplored effects for benthic ecosystem functioning.

The vast oxygen-depleted area of the central Baltic Sea is the largest human-induced dead zone in the world with 70,000 km2 or approximately three times the second largest one in the Gulf of Mexico. Methane occurs in high concentrations in bottom waters (3200 nM) and sediments (30 mM), and its dynamics is better constrained for the water column, but still poorly understood on sediments. Here we show that sediment accumulation rate plays a major role in regulating the quantity of organic matter and its residence time in the sulphate reduction and methanogenesis zones and, therefore, affects methane generation, consumption, and diffusive flux in sediments near the seafloor (< 1 m). High fluxes found in high sediment accumulation rate areas and competition for substrate (organoclastic sulphate reduction vs. anaerobic oxidation of methane with sulphate), compromise the ability of the thin microbial filter to consume and prevent methane diffusion through the seafloor.