[1] There is growing concern about the transfer of methane originating from water bodies to the atmosphere. Methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport. This work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere. The bubble model predicts the evolving bubble size, gas composition, and rise distance and is suitable for almost all aquatic environments. The model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep, oxic, saline water with excellent results. Methane bubbles from within the hydrate stability zone (typically below $500 m water depth in the ocean) are believed to form an outer hydrate rim. To explain the subsequent slow dissolution, a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone. The calibrated model explains the impressively tall flares (>1300 m) observed in the hydrate stability zone of the Black Sea. This study suggests that only a small amount of methane reaches the surface at active seep sites in the Black Sea, and this only from very shallow water areas (<100 m). Clearly, the Black Sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere, although substantial amounts of methane may reach the surface in shallow lakes and reservoirs.

Active expression of putative ammonia monooxygenase gene subunit A (amoA) of marine group I Crenarchaeota has been detected in the Black Sea water column. It reached its maximum, as quantified by reverse-transcription quantitative PCR, exactly at the nitrate maximum or the nitrification zone modeled in the lower oxic zone. Crenarchaeal amoA expression could explain 74.5% of the nitrite variations in the lower oxic zone. In comparison, amoA expression by ␥-proteobacterial ammonia-oxidizing bacteria (AOB) showed two distinct maxima, one in the modeled nitrification zone and one in the suboxic zone. Neither the amoA expression by crenarchaea nor that by ␤-proteobacterial AOB was significantly elevated in this latter zone. Nitrification in the suboxic zone, most likely microaerobic in nature, was verified by 15 ammonia-oxidizing bacteria ͉ amoA gene expression ͉ marine group ͉ Crenarchaeota ͉ marine nitrogen loss N itrification, the stepwise oxidation of ammonium to nitrite and then nitrate, is a key process in marine nitrogen cycling. It is responsible for the formation of the large deep-sea nitrate reservoir. It connects the recycling of organic nitrogen to the ultimate nitrogen loss from the oceans, because its products are substrates for denitrification and anaerobic ammonium oxidation (anammox), the only two presently known nitrogen loss processes. In productive waters such as upwelling regions, high fluxes of organic matter and thus remineralization create strong subsurface oxygen minima, enabling denitrification (1-4) or anammox (5-8) to occur. Nitrogen losses from these oxygen minimum zones (OMZs) are estimated to account for 30-50% of total nitrogen loss from the oceans (9, 10). Because remineralization also releases large amounts of ammonium, high nitrification rates are often associated with these OMZs (11), implying that nitrification may play an important role in promoting marine nitrogen loss.The Black Sea is the largest marine anoxic basin in the world. A 20-to 40-m-thick suboxic transitional zone, characterized by low oxygen (Ͻ5 M) and undetectable sulfide, persists throughout the basin between the surface oxic layer and the sulfidic anoxic deep water (Ն100 m) (12, 13). The exact depth zonation varies according to the location within the basin because of circulation and gyre formation, but similar concentrations of chemical species can be traced along isopycnals or density ( t ) surfaces throughout the basin (12). Therefore, the Black Sea provides an ideal model system to study nitrogen cycling processes along oxygen gradients. Nitrification has been reported in the lower oxic zone (14) and so has nitrogen loss via anammox in the suboxic zone (15). Nevertheless, the identity and abundance of the responsible nitrifiers, or any coupling between nitrification and nitrogen losses, remain poorly documented.The first and rate-limiting step of nitrification is aerobic ammonia oxidation. It is a microbially mediated reaction. For decades, only specific groups of ␤-and ␥-proteobacteria have been found to exh...

Methane emission pathways and their importance were quantified during a yearlong survey of a temperate hydropower reservoir. Measurements using gas traps indicated very high ebullition rates, but due to the stochastic nature of ebullition a mass balance approach was crucial to deduce system-wide methane sources and losses. Methane diffusion from the sediment was generally low and seasonally stable and did not account for the high concentration of dissolved methane measured in the reservoir discharge. A strong positive correlation between water temperature and the observed dissolved methane concentration enabled us to quantify the dissolved methane addition from bubble dissolution using a system-wide mass balance. Finally, knowing the contribution due to bubble dissolution, we used a bubble model to estimate bubble emission directly to the atmosphere. Our results indicated that the total methane emission from Lake Wohlen was on average >150 mg CH(4) m(-2) d(-1), which is the highest ever documented for a midlatitude reservoir. The substantial temperature-dependent methane emissions discovered in this 90-year-old reservoir indicate that temperate water bodies can be an important but overlooked methane source.

Inland waters transport and transform substantial amounts of carbon and account for ∼18% of global methane emissions. Large reservoirs with higher areal methane release rates than natural waters contribute significantly to freshwater emissions. However, there are millions of small dams worldwide that receive and trap high loads of organic carbon and can therefore potentially emit significant amounts of methane to the atmosphere. We evaluated the effect of damming on methane emissions in a central European impounded river. Direct comparison of riverine and reservoir reaches, where sedimentation in the latter is increased due to trapping by dams, revealed that the reservoir reaches are the major source of methane emissions (∼0.23 mmol CH4 m(-2) d(-1) vs ∼19.7 mmol CH4 m(-2) d(-1), respectively) and that areal emission rates far exceed previous estimates for temperate reservoirs or rivers. We show that sediment accumulation correlates with methane production and subsequent ebullitive release rates and may therefore be an excellent proxy for estimating methane emissions from small reservoirs. Our results suggest that sedimentation-driven methane emissions from dammed river hot spot sites can potentially increase global freshwater emissions by up to 7%.

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In lakes and reservoirs with variable water level, gas ebullition can play a substantial role in methane transport in the water column and to the atmosphere. However, measuring methane ebullition from sediment is difficult as releases are highly heterogeneous and intermittent on macro- and micro-scales. In contrast to conventional gas traps and optical methods, hydroacoustic technology allows rapid scanning over large volumes of the water column synoptically to quantify gas bubble abundance. A 120-kHz dual beam downward-looking echosounder was used to measure the size distributions of bubbles that do not resonate at the sonar frequency. Data obtained with this sonar permit accurate calculation and evaluation of ebullition flux from the bottom. A robust relationship was established between gas volumes and backscattering cross-section of individual bubbles in experimental conditions, and rise velocities of bubbles were precisely measured. The volume backscattering coefficient was shown to be a good gauge of the total volume of bubbles per cubic meter of water, allowing the use of a single-beam sonar for measuring volumetric bubble concentrations. Data obtained from hydroacoustic surveys on Lake Kinneret, where gaseous methane is emitted from randomly dispersed sediment sources, indicated that ~90% of bubbles escaping from soft sediments ranged from 1.3 mm to 4.5 mm and ~50% ranged from 2.0 mm to 3.2 mm in equivalent radius. In summer-fall 2001, the gaseous methane fluxes from hypolimnetic sediments was ~10 mmol m-2 d-1, accounting for one-third of the observed methane accumulation in the hypolimnion. This relatively high ebullition rate could be attributed to the gradual decreasing of the mean water level in preceding years

Oxic lake surface waters are frequently oversaturated with methane (CH4). The contribution to the global CH4 cycle is significant, thus leading to an increasing number of studies and stimulating debates. Here we show, using a mass balance, on a temperate, mesotrophic lake, that ~90% of CH4 emissions to the atmosphere is due to CH4 produced within the oxic surface mixed layer (SML) during the stratified period, while the often observed CH4 maximum at the thermocline represents only a physically driven accumulation. Negligible surface CH4 oxidation suggests that the produced 110 ± 60 nmol CH4 L−1 d−1 efficiently escapes to the atmosphere. Stable carbon isotope ratios indicate that CH4 in the SML is distinct from sedimentary CH4 production, suggesting alternative pathways and precursors. Our approach reveals CH4 production in the epilimnion that is currently overlooked, and that research on possible mechanisms behind the methane paradox should additionally focus on the lake surface layer.

The widely reported paradox of methane oversaturation in oxygenated water challenges the prevailing paradigm that microbial methanogenesis only occurs under anoxic conditions. Using a combination of field sampling, incubation experiments, and modeling, we show that the recurring mid-water methane peak in Lake Stechlin, northeast Germany, was not dependent on methane input from the littoral zone or bottom sediment or on the presence of known micro-anoxic zones. The methane peak repeatedly overlapped with oxygen oversaturation in the seasonal thermocline. Incubation experiments and isotope analysis indicated active methane production, which was likely linked to photosynthesis and/or nitrogen fixation within the oxygenated water, whereas lessening of methane oxidation by light allowed accumulation of methane in the oxygen-rich upper layer. Estimated methane efflux from the surface water was up to 5 mmol m 22 d 21 . Mid-water methane oversaturation was also observed in nine other lakes that collectively showed a strongly negative gradient of methane concentration within 0-20% dissolved oxygen (DO) in the bottom water, and a positive gradient within $ 20% DO in the upper water column. Further investigation into the responsible organisms and biochemical pathways will help improve our understanding of the global methane cycle.