Description: Highlights • A new bentho-pelagic transport mechanism of microorganisms is hypothesized • A bubble transport hypothesis was tested using a new gas bubble-collecting device • Bubble-mediated transport rate of methanotrophs was quantified at a gas vent • The Bubble Transport Mechanism may influence the pelagic methane sink Abstract The importance of methanotrophic microorganisms in the sediment and water column for balancing marine methane budgets is well accepted. However, whether methanotrophic populations are distinct for benthic and pelagic environments or are the result of exchange processes between the two, remains an area of active research. We conducted a field pilot study at the Rostocker Seep site (Coal Oil Point seep field, offshore California, USA) to test the hypothesis that bubble-mediated transport of methane-oxidizing microorganisms from the sediment into the water column is quantifiable. Measurements included dissolved methane concentration and showed a strong influence of methane seepage on the water-column methane distribution with strongly elevated sea surface concentrations with respect to atmospheric equilibrium (saturation ratio ~17,000%). Using Catalyzed Reporter Deposition Fluorescence In Situ Hybridization (CARD FISH) analysis, aerobic methane oxidizing bacteria (MOB) were detected in the sediment and the water column, whereas anaerobic methanotrophs (ANME-2) were detected exclusively in the sediment. Critical data for testing the hypothesis were collected using a novel bubble catcher that trapped naturally emanating seep gas bubbles and any attached particles approximately 15 cm above the seafloor. Bubble catcher experiments were carried out directly above a natural bubble seep vent and at a nearby reference site, for which an “engineered” nitrogen bubble vent without sediment contact was created. Our experiments indicate the existence of a “Bubble Transport Mechanism”, which transports MOB from the sediment into the water column. In contrast, ANME-2 were not detected in the bubble catcher. The Bubble Transport Mechanism could have important implications for the connectivity between benthic and pelagic methanotrophic communities at methane seep sites.

Description: Highlights • First study using long-term passive acoustic monitoring of methane seeps at well blowout site 22/4b. • Seep acoustic temporal variations correlated with ocean tides. • Major acoustic transient event recorded on 8 December 2011 with high temporal resolution. Abstract Marine seeps produce underwater sounds as a result of bubble formation and fragmentation upon emission from the seabed. The frequency content and sound levels of these emissions are related to bubble size distribution and emission flux, providing important information on methane release from the seafloor. Long-term passive acoustic monitoring was used to continuously record seep sounds over a 7-month period within the blowout crater at the abandoned well site, 22/4b, in the central North Sea. Also recorded were water column fluid velocities and near-seafloor water conductivity, temperature, and pressure. Acoustic signatures were primarily from ∼1 to 10 kHz. Key features were relatively broad spectral peaks at about 1.0, 1.5, 2.2, 3.1, 3.6 and 5.1 kHz. Temporal variations in spectral levels were apparently associated with tides. The recordings also documented a series of major episodic events including a large and persistent increase (∼10 dB) in overall sound levels and spectral broadening on 8 December 2011. The acoustic temporal pattern of this event was consistent with other recorded large transient events in the literature, and the major event was correlated with dramatic changes in other measurements, including increased water column fluid velocities, increased pressure and decreased salinity, indicating real changes in emission flux. Observed seabed morphology changes reported elsewhere in this special issue, also likely were related to this event. These data demonstrate the dynamic nature of marine seepage systems, show the value of monitoring systems, and provide direct supporting evidence for a violent formation mechanism of many widespread seep-associated seabed features like pockmarks.

Description: The presence of a seasonal thermocline likely plays a key role in restraining methane released from a seabed source in the deeper water column, thereby inhibiting exchange to the atmosphere. The bubble plume itself, however, generates an upward motion of fluid, e.g. upwelling and may thereby be partially responsible for an early breakdown of the seasonal thermocline. Measurements at site 22/4b, located at (57°550N, 1°380E) in the UK Central North Sea, 200 km east of the Scottish mainland, where gas is still being released since a blow out in 1990, have been used to identify the generation of the seasonal thermocline, and thus, the depth of the upper mixed layer and its breakdown in autumn. Data derived from two landers, containing an Acoustic Doppler Current Profiler and a Conductivity Temperature Depth recorder, were used to determine the mixed layer depth and the breakdown of the thermocline. Mixing of upper layer fluid into the lower layer has been inferred from large amplitude variations in the nearbottom temperature. The ADCPs estimate velocity profiles in four beam directions using Doppler shifted frequency from acoustic pings sent out and received by four different transducers in a specific configuration. Besides that, the intensity of the backscattered sound per transducer is also recorded. Bubbles from the nearby plume contaminate the signal during part of the tidal cycle, but in bubble free periods, the mixed layer depth can be estimated using the acoustic backscatter signal as local maxima. Results show that the thermocline broke down between mid-October and early November, several weeks earlier than the breakdown of the thermocline in nearby/comparable areas, likely caused by bubble-induced downwelling at the site. The early breakdown of the thermocline was accompanied by multiple occurrence of a strong jet-like structure, associated with the seasonal tidal mixing front.