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In this paper we provide an overview of new knowledge on oxygen depletion (hypoxia) and related phenomena in aquatic systems resulting from the EU-FP7 project HYPOX ("In situ monitoring of oxygen depletion in hypoxic ecosystems of coastal and open seas, and landlocked water bodies", http://www.hypox.net). In view of the anticipated oxygen loss in aquatic systems due to eutrophication and climate change, HYPOX was set up to improve capacities to monitor hypoxia as well as to understand its causes and consequences. Temporal dynamics and spatial patterns of hypoxia were analyzed in field studies in various aquatic environments, including the Baltic Sea, the Black Sea, Scottish and Scandinavian fjords, Ionian Sea lagoons and embayments, and Swiss lakes. Examples of episodic and rapid (hours) occurrences of hypoxia, as well as seasonal changes in bottom-water oxygenation in stratified systems, are discussed. Geologically driven hypoxia caused by gas seepage is demonstrated. Using novel technologies, temporal and spatial patterns of water-column oxygenation, from basin-scale seasonal patterns to meter-scale sub-micromolar oxygen distributions, were resolved. Existing multidecadal monitoring data were used to demonstrate the imprint of climate change and eutrophication on long-term oxygen distributions. Organic and inorganic proxies were used to extend investigations on past oxygen conditions to centennial and even longer timescales that cannot be resolved by monitoring. The effects of hypoxia on faunal communities and biogeochemical processes were also addressed in the project. An investigation of benthic fauna is presented as an example of hypoxia-devastated benthic communities that slowly recover upon a reduction in eutrophication in a system where naturally occurring hypoxia overlaps with anthropogenic hypoxia. Biogeochemical investigations reveal that oxygen intrusions have a strong effect on the microbially mediated redox cycling of elements. Observations and modeling studies of the sediments demonstrate the effect of seasonally changing oxygen conditions on benthic mineralization pathways and fluxes. Data quality and access are crucial in hypoxia research. Technical issues are therefore also addressed, including the availability of suitable sensor technology to resolve the gradual changes in bottom-water oxygen in marine systems that can be expected as a result of climate change. Using cabled observatories as examples, we show how the benefit of continuous oxygen monitoring can be maximized by adopting proper quality control. Finally, we discuss strategies for state-of-the-art data archiving and dissemination in compliance with global standards, and how ocean observations can contribute to global earth observation attempts., Biogeosciences, 11 (4), ISSN:1726-4170

Accurate assessment of anthropogenic carbon dioxide (CO₂) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere – the “global carbon budget” – is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO₂ emissions (EFF) are based on energy statistics and cement production data, while emissions from land use and land-use change (ELUC), mainly deforestation, are based on land use and land-use change data and bookkeeping models. Atmospheric CO₂ concentration is measured directly and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO₂ sink (SOCEAN) and terrestrial CO₂ sink (SLAND) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ±1σ. For the last decade available (2008–2017), EFF was 9.4 ± 0.5 GtC yr⁻¹, ELUC 1.5 ± 0.7 GtC yr⁻¹ , GATM 4.7 ± 0.02 GtC yr⁻¹, SOCEAN 2.4 ± 0.5 GtC yr⁻¹, and SLAND 3.2 ± 0.8 GtC yr⁻¹ , with a budget imbalance BIM of 0.5 GtC yr⁻¹ indicating overestimated emissions and/or underestimated sinks. For the year 2017 alone, the growth in EFF was about 1.6 % and emissions increased to 9.9 ± 0.5 GtC yr⁻¹. Also for 2017, ELUC was 1.4 ± 0.7 GtC yr⁻¹ , GATM was 4.6 ± 0.2 GtC yr⁻¹, SOCEAN was 2.5 ± 0.5 GtC yr⁻¹, and SLAND was 3.8 ± 0.8 GtC yr⁻¹, with a BIM of 0.3 GtC. The global atmospheric CO₂ concentration reached 405.0±0.1 ppm averaged over 2017. For 2018, preliminary data for the first 6–9 months indicate a renewed growth in EFF of +2.7 % (range of 1.8 % to 3.7 %) based on national emission projections for China, the US, the EU, and India and projections of gross domestic product corrected for recent changes in the carbon intensity of the economy for the rest of the world. The analysis presented here shows that the mean and trend in the five components of the global carbon budget are consistently estimated over the period of 1959–2017, but discrepancies of up to 1 GtC yr⁻¹ persist for the representation of semi-decadal variability in CO₂ fluxes. A detailed comparison among individual estimates and the introduction of a broad range of observations show (1) no consensus in the mean and trend in land-use change emissions, (2) a persistent low agreement among the different methods on the magnitude of the land CO₂ flux in the northern extra-tropics, and (3) an apparent underestimation of the CO₂ variability by ocean models, originating outside the tropics. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding the global carbon cycle compared with previous publications of this data set (Le Quéré et al., 2018, 2016, 2015a, b, 2014, 2013)

Accurate assessment of anthropogenic carbon dioxide (CO₂) emissions and their redistributionamong the atmosphere, ocean, and terrestrial biosphere – the “global carbon budget” – is important to betterunderstand the global carbon cycle, support the development of climate policies, and project future climatechange. Here we describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO₂ emissions (EFF) are based on energy statistics and cement productiondata, while emissions from land use change (ELUC), mainly deforestation, are based on land use and land usechange data and bookkeeping models. Atmospheric CO₂ concentration is measured directly and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO₂ sink (SOCEAN) and terrestrial CO₂ sink (SLAND) are estimated with global process models constrained by observations. The resulting car-bon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changesin the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as±1σ. For the last decade available (2009–2018), EFF was 9.5±0.5 GtC yr⁻¹, ELUC 1.5±0.7 GtC yr⁻¹, GATM4.9±0.02 GtC yr⁻¹ (2.3±0.01 ppm yr⁻¹), SOCEAN 2.5±0.6 GtC yr⁻¹, and SLAND 3.2±0.6 GtC yr⁻¹, with a budget imbalance BIM of 0.4 GtC yr⁻¹ indicating overestimated emissions and/or underestimated sinks. For the year 2018 alone, the growth in EFFwas about 2.1 %and fossil emissions increased to 10.0±0.5 GtC yr⁻¹, reaching 10 GtC yr⁻¹ for the first time in history, ELUC was 1.5±0.7 GtC yr⁻¹, for total anthropogenic CO emissions of 11.5±0.9 GtC yr⁻¹ (42.5±3.3 Gt CO₂). Alsofor 2018,GATM was 5.1±0.2 GtC yr⁻¹(2.4±0.1 ppm yr⁻¹), SOCEAN was 2.6±0.6 GtC yr⁻¹, and SLAND was 3.5±0.7 GtC yr⁻¹, with a BIM of 0.3 GtC. The global atmospheric CO2 concentration reached 407.38±0.1 ppmaveraged over 2018. For 2019, preliminary data for the first 6–10 months indicate a reduced growth in EFF of +0.6 % (range of −0.2 % to 1.5 %) based on national emissions projections for China, the USA, the EU, andIndia and projections of gross domestic product corrected for recent changes in the carbon intensity of the economy for the rest of the world. Overall, the mean and trend in the five components of the global carbon budgetare consistently estimated over the period 1959–2018, but discrepancies of up to 1 GtC yr⁻¹ persist for the rep-resentation of semi-decadal variability in CO₂ fluxes. A detailed comparison among individual estimates and theintroduction of a broad range of observations shows (1) no consensus in the mean and trend in land use changeemissions over the last decade, (2) a persistent low agreement between the different methods on the magnitudeof the land CO₂ flux in the northern extra-tropics, and (3) an apparent underestimation of the CO₂ variability byocean models outside the tropics. This living data update documents changes in the methods and data sets usedin this new global carbon budget and the progress in understanding of the global carbon cycle compared withprevious publications of this data set (Le Quéré et al., 2018a, b, 2016, 2015a, b, 2014, 2013).

Large quantities of methane are stored in hydrates and permafrost within shallow marine sediments in the Arctic Ocean. These reservoirs are highly sensitive to climate warming, but the fate of methane released from sediments is uncertain. Here, we review the principal physical and biogeochemical processes that regulate methane fluxes across the seabed, the fate of this methane in the water column, and potential for its release to the atmosphere. We find that, at present, fluxes of dissolved methane are significantly moderated by anaerobic and aerobic oxidation of methane. If methane fluxes increase then a greater proportion of methane will be transported by advection or in the gas phase, which reduces the efficiency of the methanotrophic sink. Higher freshwater discharge to Arctic shelf seas may increase stratification and inhibit transfer of methane gas to surface waters, although there is some evidence that increased stratification may lead to warming of sub-pycnocline waters, increasing the potential for hydrate dissociation. Loss of sea-ice is likely to increase wind speeds and sea-air exchange of methane will consequently increase. Studies of the distribution and cycling of methane beneath and within sea ice are limited, but it seems likely that the sea-air methane flux is higher during melting in seasonally ice-covered regions. Our review reveals that increased observations around especially the anaerobic and aerobic oxidation of methane, bubble transport, and the effects of ice cover, are required to fully understand the linkages and feedback pathways between climate warming and release of methane from marine sediments., Limnology and Oceanography, 61 (S1), ISSN:0024-3590, ISSN:1939-5590

The Okinawa Trough is a natural hydrothermal gas emission field. The flux of CO2 into the water column is dominated by hot hydrothermal CO2-rich vents. The influence of the emitted low-temperatured gas bubbles and droplets (‘condensed phase’) is very likely restricted to the lowermost part of the water column. Despite their proximity the gas composition of the condensed phases varies widely, especially in methane, which is likely caused by diverse phase transformation and thus fractionation below and above the seafloor., Der Okinawa Trog ist ein natürliches hydrothermales Gasaustrittsgebiet. Der CO2-Fluss in die Wassersäule wird durch heiße hydrothermale CO2-reiche Quellen dominiert. Der Einfluss von freigesetzten kalten Gasblasen und Tropfen („kondensierte Phasen“) beschränkt sich sehr wahrscheinlich hauptsächlich auf den unteren Teil der Wassersäule. Trotz ihrer Nähe zueinander variiert die Gaszusammensetzung der einzelnen kondensierten Phasen stark, vor allem im Methangehalt. Diese basieren wahrscheinlich auf verschiedenen Phasenänderungen und damit Fraktionierungen unter- und oberhalb des Meeresbodens.

Methane is the most abundant greenhouse gas after carbon dioxide and accounts for ~25% of atmospheric warming since the onset of industrialization. Large amounts of methane are stored in the ocean seafloor as solid gas hydrates, gaseous reservoirs or dissolved in pore water. At cold seeps, various physical, chemical, and geological processes force subsurface methane to ascend along pathways of structural weakness to the sea floor. Additionally, methane can be produced in situ within organic-rich sediments. Increasing evidence suggests that ocean bottom water warming is leading to enhanced methane fluxes into the water column, for instance by dissociation of gas hydrates or by enhanced methane production in coastal ecosystems. Previous investigations showed that a large portion (~80%) of ascending methane in ocean sediments is utilised by anaerobic and aerobic methanotrophic microbes, but future elevated methane fluxes might not be counterbalanced by this sedimentary methane filter. Today, about 0.02 Gt/yr (3-3.5% of the atmospheric budget) of methane bypasses the benthic filter system on a global scale, and subsequently escapes into ocean bottom waters. In the water column, it can be oxidised aerobically (aerobic oxidation of methane - MOx), or less commonly where the water column is anoxic, anaerobically. Water column MOx is the final sink for methane before its release to the atmosphere. However, little is known on the efficiency of this pelagic microbial filter and its ability to adjust to a (rapidly) changing environment. In order to predict future changes, it is thus crucial to understand the efficiency of current water-column MOx, to identify the key organisms mediating MOx, and - most importantly - to determine environmental parameters controlling MOx. In this dissertation, I investigated the pelagic MOx filter in contrasting ocean environments using a multidisciplinary approach. Systems studied included a deep-sea brine, two gas seep systems, and a shallow, organic-rich coastal environment. The main goals were to determine hot spots of MOx, identify bacteria mediating this process, and to estimate the efficiency of the pelagic methane filter. Furthermore, an important aim was to identify environmental factors controlling the activity and distribution of MOx, which could help in predicting changes of MOx in a future (warmer) ocean. My investigations revealed the following: 1. In the water column above methane gas seeps at the West Spitsbergen margin, MOx rate measurements together with CARD-FISH analysis of the methanotrophic community revealed rapid changes in the abundance of methanotrophs. Simultaneous measurements of physico-chemical water mass properties showed that the change in methanotrophic abundance correlated with changes in the water mass present above the seep system. This water mass exchange was caused by short-term variations in the position (i.e., offshore or nearshore) of the warm-water core of the West Spitsbergen Current: In its offshore mode, methanotroph-rich bottom waters above the methane seeps showed a high MOx capacity. A shift of the warm-water core towards the shelf break during the nearshore mode of the current displaced this cold bottom water with warm surface water containing a much smaller standing stock of methanotrophs, and led to a drop in MOx capacity of ~60%. This water mass exchange, caused by short-term variations of the West Spitsbergen Current, thus constitutes an oceanographic switch severely reducing methanotrophic activity in the water column. Since fluctuating currents are widespread oceanographic features common at many methane seep systems, it follows that the variability of physical water mass transport is a globally important control on the distribution and abundance of methanotrophs and, as a consequence, on the efficiency of methane oxidation above point sources. 2. At a Blowout in the North Sea resulting from an accident during industrial drilling activities, vigorous bubble emanation from the seafloor and strongly elevated methane concentrations in the water column (up to 42 μM) indicated that a substantial fraction of methane bypassed the highly active (up to ~2920 nmol/cm3/d) AOM zone in sediments. In the water column, we measured MOx rates that were among the highest ever measured in a marine environment (up to 498 nM/d) and, under stratified conditions, have the potential to remove a significant part of the released methane prior to evasion to the atmosphere. We speculate, however, that the MOx filter is intermittently inhibited when the water column is fully mixed, so that the Blowout is a source of methane to the atmosphere. An unusual dominance of the water-column methanotrophic community by Type II methanotrophs is partially supported by recruitment of sedimentary methanotrophs, which are entrained together with sediment particles in the methane bubble plume. Hence, our study demonstrates that gas ebullition not only provides ample methane substrate to fuel MOx in the water column, it also serves as an important transport vector for sediment-borne microbial inocula that aid in the establishment/proliferation of a water-column methanotrophic community at high-flux colds seeps. 3. We investigated MOx in the water column above gassy coastal sediments on quarterly basis over a time-period of two years. At the Boknis Eck study site, which is located in the coastal inlet Eckernförde Bay in the southwestern Baltic Sea, the water column is seasonally stratified with bottom waters becoming hypoxic over the course of the stratification period. We found that MOx rates exhibited a seasonal pattern with maximum rates (up to 11.6 nmol/l/d) during the summer months when oxygen concentrations were lowest and bottom water temperatures highest. Overall, MOx consumed between 70 – 95% of methane under stratified conditions, but only 40 – 60% under mixed conditions. Additional laboratory-based experiments with adjusted oxygen concentrations in the range of 0.2 – 220 μmol/l confirmed a sub-micromolar MOx oxygen optimum. In contrast, the percentage of methane-carbon incorporation into biomass was reduced at submicromolar oxygen concentrations, suggesting a different partitioning of catabolic and anabolic processes at saturated and sub-micromolar oxygen concentrations. Additional laboratory experiments verified the above-described mesophilic behaviour of the MOx communities of both surface and bottom waters. Our results highlight the importance of MOx in mitigating methane emission from coastal waters and indicate the existence of an adaptation to hypoxic conditions on the organismic level of the water column methanotrophs. 4. Life in the deep-sea brine basin Kryos in the Eastern Mediterranean Sea faces extreme challenges since the brine is almost saturated in bischofite (MgCl2 - 3.9 mol/kg). Due to the strong density difference between the anoxic brine and the overlying Mediterranean seawater, mixing is impeded and a shallow (

Methane is the second most important anthropogenic greenhouse gas on Earth and contributes considerably to global radiative forcing. The last IPCC assessment report 2007 assigns geological methane emissions as a significant source. This thesis therefore concentrates on the quantity and atmospheric implications of methane emissions from the seabed of the North Sea. Sampling of marine seepage is challenging compared to readily accessible terrestrial sites; thus marine seepage sites have scarcely been observed or even yet discovered. Moreover, in terms of atmospheric contribution, the fate of methane after ebullition into the water column is usually not considered. Hydroacoustic systems have proven to be very efficient remote sensing tools for gas seepage analysis even in water depth greater than 2000 m. Technical progress led to much higher remote sensing potential by means of modern multibeam applications for gas bubbles detection in the water column. However, to be effective, these novel multibeam systems require new methods for data analysis. This thesis firstly demonstrates the application of multibeam systems as efficient gas bubble remote sensing tools. Therefore an anthropogenic blowout site was mapped using a multibeam sonar. The advantage of multibeam technology compared to singlebeam is increased efficiency due to larger coverage than singlebeam systems, three dimensional plume mapping, and exact localization of gas sources. Moreover the deployment of the multibeam prototype GasQuant is examined, which is an adapted sounder specifically designed for in situ gas bubble detection. GasQuant was deployed for several days within a gas seep field in the Central North Sea (Tommeliten). Aside from minor system adaptations, major effort was spent to handle the non-standard large datasets by means of various data processing and visualization routines. Taking into account the surrounding tidal current flow field, unique data patterns were extracted to unambiguously detect gas bubbles in the water column. Thus, a total of 52 single seep holes were localized and characterized with respect to their tempo-spatial variability. Recently, water column scanning multibeam mapping systems entered the market. Due to their huge amount of data output, manual processing is no longer feasible. Thus, a generic algorithm for the detection of rising gas bubbles in multibeam data was developed that accounts for the current tidal flow field for detection issues (Appendix A). Incorporation of other disciplines such as geochemistry and oceanography allowed for a methane gas source strength estimate of the Tommeliten gas seepage field in the North Sea. Combined acoustic mapping and in situ sampling revealed a source strength of ~0.8-4.8*106 mol/yr – a considerable quantity compared to prominent gas seep sites around the world (e.g. ~1*106 mol/yr at Vodyanitskii mud volcano, Black Sea; 2.19*106 mol/yr at North Hydrate Ridge offshore Oregon). Obviously previous studies have underestimated the area of active venting at Tommeliten. By modeling gas bubble dissolution and geochemical sampling it was found that the majority of bubble-mediated methane at Tommeliten already dissolves in the ‘deep’ water between the 70 m release depth and 40 m. Thus the methane is trapped below the upper-well mixed summer layer, from which it would readily be degassed by air-sea exchange processes. Given the heavy storm activity during winter, research cruises into the North Sea preferentially take place during the summer, where low atmospheric outgassing/emissions from seabed methane is expected due to stratification. However, considering the distinct hydrographic seasonal cycle of the North Sea, quantitative transport of seepage methane into the atmosphere seems likely during winter after fall mixing. This seasonal bias is not only constrained to the study site, but relevant for the entire Central and Northern North Sea as well as many mid-latitude shallow shelf sea waters showing temporal stratification.

At pressures and temperatures prevailing in the ocean at a few hundred meters depth transfer of methane and carbon dioxide into and within the water column are influenced by the thermodynamic and kinetic stability of clathrates formed by these two greenhouse gases. Thus, a better understanding of hydrate formation and stability is essential to assess the contribution of submarine methane seepage to climate change, the hazard potential of submarine gas hydrate deposits and the feasibility and environmental impact of anthropogenic carbon storage in the marine environment. The present survey deals with laboratory-based investigations of hydrate-controlled transfer and transport processes of the two gases in the ocean. The investigation stresses the importance of water column processes on the fate of gases released at the sediment/seawater interface and fills a gap in the data basis needed to model the balance of the vertical transfer of the two most important greenhouse gases in the ocean. Bei Druck und Temperaturbedingungen im Ozean in wenigen hundert m Wassertiefe werden Freisetzung und Wanderung von Methan und Kohlendioxid in bzw. durch die Wassersäule durch die thermodynamische und kinetische Stabilität von Klathraten dieser beiden Treibhausgase beeinflusst. Ein besseres Verständnis der Stabilität von Gashydraten sowie des hydratbeeinflussten Transfers der beiden wichtigsten Treibhausgase in den Ozean ist daher essentiell um den Beitrag submariner Methanquellen auf den Klimawandel, um das Gefahrenpotential submariner Hydratvorkommen, sowie um die Durchführbarkeit und Umweltbeeinflussung durch Einlagerung anthropogenen Kohlendioxids in den marinen Raum besser abschätzen zu können. Die vorliegende Arbeit beschäftigt sich mit der laborgestützten Untersuchung des hydratkontrollierten Transfers beider Gase in den Ozean. Die Untersuchung unterstreicht die Wichtigkeit der Wassersäule für das Schicksal von an der Sediment/Wassergrenzschicht in den Ozean entweichenden Gasen. Die Untersuchung liefert damit Validierungsdaten, die für eine Modellierung des vertikalen Transfers dieser beiden Gase im Ozean notwendig sind.

Methane is a greenhouse gas that exerts a significant influence on the radiation budget and thereby the earth’s climate. Geological sources of methane, especially submarine, are particularly relevant in this regard because they emit significant quantities of methane into the atmosphere. However, it is difficult to estimate the magnitude of the global geological source because the extent of these methane-releasing areas and the processes, which influence the methane cycle, are not yet well understood. The Black Sea is a region characterized by numerous submarine methane sources. Methane emissions in the form of methane-containing fluids or gas bubbles have been documented here at all water depths, from the shelf down to the abyss. This marginal sea features a unique hydrographic structure resulting from the inflow of freshwater from rivers and the salty waters of the Mediterranean Sea over the Bosphorus. These inputs have very different densities, and this leads to a pronounced stratification of the water column that limits vertical mixing. High biological production in the surface waters leads to an intense consumption of oxygen in the upper water column, which can not be balanced by downward ventilation of oxygenated surface waters. The pronounced redox-gradient has in turn an important influence on the biochemical cycles in the water column and leads to anoxic conditions at depths below approximately 100 m. These particular conditions make the Black Sea the largest anoxic marine basin on earth. Chapters 2 and 3 of this work (as well as the articles in the appendix) explore the questions of what influence methane emissions have at different water depths on the methane cycle of the water column and what significance these submarine emissions have as sources of atmospheric methane. To this end, the zones of methane sources along the northwest shelf and continental slope (Paleo Dnepr Area) as well as in the northeastern deep sea region (Sorokin Trough) of the Black Sea were explored throughout 2003 and 2004. The observed depths span from between 60 and 2100 m and encompass the entire biogeochemical spectrum of the water column. The methane concentrations of the water column and the overlaying atmosphere were carefully recorded, as were the carbon isotope signatures of this methane. The physical description of the water column, the microorganisms involved in the methane cycle and their methane consumption rates were additionally documented. The area affected by the emission of methane from submarine sources was ascertained through extensive sampling campaigns. The dataset was used to model the methane flux at the sea/air interface and to evaluate the influence of methane sources, present in different water depth on the marine methane emission. In addition, the exploration of this area throughout two consecutive years provides insight into the seasonal variability of the source strength of these submarine emissions. Chapter 4 serves to combine the results of this investigation with those present in the literature for the purpose of constructing a methane budget that quantifies the significant sources and sinks of methane in the Black Sea. A transport-reaction model investigates the release of methane at discrete water depths and identifies those of particular interest. Various model scenarios were used to determine to what extent sizable methane emission events, for example from mud volcano eruptions or submarine landslides, affect the methane budget of the Black Sea and the associated methane emissions at the ocean surface. The following work cannot completely address the complex methane cycle of the Black Sea and its associated processes. This work serves rather to provide new insights, discuss relevant areas and point out still unanswered questions. The increase in research on methane in the Black Sea during recent years shows that its methane cycle is of great scientific interest.