Ephemerality of discrete methane vents in lake sediments

Methane is a potent greenhouse gas whose emission from sediments in inland waters and shallow oceans may both contribute to global warming and be exacerbated by it. The fraction of methane emitted by sediments that bypasses dissolution in the water column and reaches the atmosphere as bubbles depends on the mode and spatiotemporal characteristics of venting from the sediments. Earlier studies have concluded that hot spots—persistent, high-flux vents—dominate the regional ebullitive flux from submerged sediments. Here the spatial structure, persistence, and variability in the intensity of methane venting are analyzed using a high-resolution multibeam sonar record acquired at the bottom of a lake during multiple deployments over a 9 month period. We confirm that ebullition is strongly episodic, with distinct regimes of high flux and low flux largely controlled by changes in hydrostatic pressure. Our analysis shows that the spatial pattern of ebullition becomes homogeneous at the sonar's resolution over time scales of hours (for high-flux periods) or days (for low-flux periods), demonstrating that vents are ephemeral rather than persistent, and suggesting that long-term, lake-wide ebullition dynamics may be modeled without resolving the fine-scale spatial structure of venting.National Science Foundation (U.S.) (1045193)United States. Department of Energy (DE-FE001399

Processing of multibeam water column image data for automated bubble/seep detection and repeated mapping

Water Column Imaging Multibeam Echosounder Systems (MBES) are effective and sensitive tools for investigating free gas (bubble) release and its rise through the water column. The main advantages of MBES are the detection range and lateral coverage in the water column and at the seafloor; furthermore, they are becoming increasingly available on research vessels worldwide. However, high noise levels and systematic artefacts due to side-lobe induced signal interference degrade MBES Water Column Images (WCIs) and hampered automated bubble detection and related gas seepage investigations. We present a new technique advancing automated detection of bubble streams and moving toward a quantitative gas-release assessment. It is shown that bubble streams can be detected reliably by their spatio-temporal behavior even when they are discontinuous in WCI data. Using assumptions about the bubble rising trajectories, bubble release spots at the seafloor can be traced even if the source location is obscured by acoustic noise or unwanted acoustic targets. A map with acoustic response and source locations of bubbles being released can be produced and serves as a starting point for more detailed quantitative analyses. The efficiency of the method has been assessed at a methane seep site in the Dutch North Sea. Multiple survey lines are merged to a detailed acoustic map of the area. Processed results are in good agreement with manual investigations of the WCI data as well as ROV-based video analysis

Rotation motion of small particles in sound field and possible mechanism of sound perception

It is known several mechanisms of perception of sound by hydrobionts. However for simple organisms like small crayfish it is not clear how they can detect sound. One of possibilities is to detect oscillation water motion by microhairs. In this paper it is analyzed a physical mechanism of rotation oscillations of small particles in acoustic field and it application to a problem of sound perception by some hydrobionts

Evidence for long range chemoreceptive tracking of food odour in deep sea scavengers by scanning sonar data

AbstractThere is much speculation about chemoreception being involved in food finding strategies of deep sea scavengers in the literature (Dahl, 1979; Meador, 1981; Busdosh et al.; 1982, Sainte-Marie, 1992). Most of these ideas have emerged from analysing time-lapse photographs and video recordings of bait deployments in the deep sea (Thurston, 1979; Lampitt et al., 1983; Hargrave, 1985; Priede et al., 1990). However, optical instruments have considerable restrictions in spatial coverage, thus all past efforts in determining any directionality in the appearance of scavengers have been limited from 0.9 to 4 m2 only (Smith, 1985; Wilson and Smith 1984). Here we present data obtained by using a scanning sonar system (SSS) which allows detection of single objects larger than 2 cm at a maximum distance of 50 m in a horizontal plane. Together with the SSS a baited time-lapse camera attached to a free falling lander was used in the Arctic deep sea at two locations in the Fram Strait at about 2500 m water depths. We would like to point out that this combination of optical and acoustical measurements allowed for the first time the long range detection of approaching scavenging amphipods in the deep sea. Eurythenes gryllus, a cosmopolitan deep sea scavenging amphipod, was recorded to attend our bait experiments with a maximum of 618 individuals with first arrival 12 min. after deployment which is one of the fastest arrivals ever observed. We found a significant temporal correlation between integrated backscatter energy (IBE) based on measurements of the SSS and amphipod individuals counted on photographs