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Art of electronics

With a current estimate of ~1,000 million tons, mesopelagic fishes likely dominate the world total fishes biomass. However, recent acoustic observations show that mesopelagic fishes biomass could be significantly larger than the current estimate. Here we combine modelling and a sensitivity analysis of the acoustic observations from the Malaspina 2010 Circumnavigation Expedition to show that the previous estimate needs to be revised to at least one order of magnitude higher. We show that there is a close relationship between the open ocean fishes biomass and primary production, and that the energy transfer efficiency from phytoplankton to mesopelagic fishes in the open ocean is higher than what is typically assumed. Our results indicate that the role of mesopelagic fishes in oceanic ecosystems and global ocean biogeochemical cycles needs to be revised as they may be respiring ~10% of the primary production in deep waters. Mesopelagic fishes dominate the global fishes biomass, yet there exist major uncertainties regarding their global biomass. Irigoien et al.analyse acoustic data collected during a circumglobal cruise and show that biomass estimates should be raised by an order of magnitude.

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Large mesopelagic fishes biomass and trophic efficiency in the open ocean

Underwater acoustic channel models provide a tool for predicting the performance of communication systems before deployment, and are thus essential for system design. In this paper, we offer a statistical channel model which incorporates physical laws of acoustic propagation (frequency-dependent attenuation, bottom/surface reflections), as well as the effects of inevitable random local displacements. Specifically, we focus on random displacements on two scales: those that involve distances on the order of a few wavelengths, to which we refer as small-scale effects, and those that involve many wavelengths, to which we refer as large-scale effects. Small-scale effects include scattering and motion-induced Doppler shifting, and are responsible for fast variations of the instantaneous channel response, while large-scale effects describe the location uncertainty and changing environmental conditions, and affect the locally averaged received power. We model each propagation path by a large-scale gain and micromultipath components that cumulatively result in a complex Gaussian distortion. Time- and frequency-correlation properties of the path coefficients are assessed analytically, leading to a computationally efficient model for numerical channel simulation. Random motion of the surface and transmitter/receiver displacements introduce additional variation whose temporal correlation is described by Bessel-type functions. The total energy, or the gain contained in the channel, averaged over small scale, is modeled as log-normally distributed. The models are validated using real data obtained from four experiments. Specifically, experimental data are used to assess the distribution and the autocorrelation functions of the large-scale transmission loss and the short-term path gains. While the former indicates a log-normal distribution with an exponentially decaying autocorrelation, the latter indicates a conditional Ricean distribution with Bessel-type autocorrelation.

Statistical Characterization and Computationally Efficient Modeling of a Class of Underwater Acoustic Communication Channels

River plumes are generated by the flow of buoyant river water into the coastal ocean, where they significantly influence water properties and circulation. They comprise dynamically distinct regions spanning a large range of spatial and temporal scales, each contributing to the dilution and transport of freshwater as it is carried away from the source. River plume structure varies greatly among different plume systems, depending on the forcing and geometry of each system. Individual systems may also exhibit markedly different characteristics under varied forcing conditions. Research over the past decade, including a series of major observational efforts, has significantly improved our understanding of the dynamics and mixing processes in these regions. Although these studies have clarified many individual processes, a holistic description of the interaction and relative importance of different mixing and transport processes in river plumes has not yet been realized.

Mixing and Transport in Coastal River Plumes

In this paper we review on the technologies available to make globally quantitative observations of particles, in general, and plankton, in particular, in the world oceans, and for sizes varying from sub-micron to centimeters. Some of these technologies have been available for years while others have only recently emerged. Use of these technologies is critical to improve understanding of the processes that control abundances, distributions and composition of plankton, provide data necessary to constrain and improve ecosystem and biogeochemical models, and forecast changes in marine ecosystems in light of climate change. In this paper we begin by providing the motivation for plankton observations, quantification and diversity qualification on a global scale. We then expand on the state-of-the-art, detailing a variety of relevant and (mostly) mature technologies and measurements, including bulk measurements of plankton, pigment composition, uses of genomic, optical, acoustical methods and analysis using particles counters, flow cytometers and quantitative imaging devices. We follow by highlighting the requirements necessary for a plankton observing system, the approach to achieve it and associated challenges. We conclude with ranked action-item recommendations for the next ten years to move towards our vision of a holistic ocean-wide plankton observing system. Particularly, we suggest to begin with a demonstration project on a GO-SHIP line and/or a long-term observation site and expand from there ensuring that issues associated with methods, observation tools, data analysis, quality assessment and curation are addressed early in the implementation. Global coordination is key for the success of this vision and will bring new insights on processes associated with nutrient regeneration, ocean production, fisheries, and carbon sequestration.

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Globally consistent quantitative observations of planktonic ecosystems

Blainville’s beaked whales (Mesoplodon densirostris) use broadband, ultrasonic echolocation signals with a −10dB bandwidth from 26to51kHz to search for, localize, and approach prey that generally consist of mid-water and deep-water fishes and squid. Although it is well known that the spectral characteristics of broadband echoes from marine organisms vary as a function of size, shape, orientation, and anatomical group, there is little evidence as to whether or not free-ranging toothed whales use spectral cues in discriminating between prey and nonprey. In order to study the prey-classification process, a stereo acoustic tag was deployed on a Blainville’s beaked whale so that emitted clicks and the corresponding echoes from targets in the water could be recorded. A comparison of echoes from targets apparently selected by the whale and those from a sample of scatterers that were not selected suggests that spectral features of the echoes, target strengths, or both may have been used by the whale to discrimina...

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Classification of broadband echoes from prey of a foraging Blainville’s beaked whale

We have performed measurements of the spatial distribution of current in various alkali and reactive ion beams over the energy range 5–600 eV using a Faraday cup. Ion beam current densities have been extracted from these measurements using a simple deconvolution procedure. Our results reveal that the beams are Gaussian in shape with a constant width, σ, for energies greater than approximately 75 eV and for all ion species investigated. This width is consistent with that determined from the distribution of oxygen on a Cu(001) crystal after an O+ ion beam deposition, measured using Auger electron spectroscopy. Using the measurement technique outlined in this article, together with the linear relationship between current density and Faraday cup current, it is possible to determine the beam current density using a single current measurement.

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A technique for accurate measurements of ion beam current density using a Faraday cup

Laboratory measurements of high-frequency broadband acoustic backscattering (200-600 kHz) from the diffusive regime of double-diffusive microstructure have been performed This type of microstructure, which was characterized using direct microstructure and optical shadowgraph techniques, is identified by sharp density and sound speed interfaces separating well-mixed layers Vertical acoustic backscattering measurements were performed for a range of physical parameters controlling the double-diffusive microstructure The echoes have been analyzed in both the frequency domain, providing information on the spectral response of the scattering, and in the time domain, using pulse compression techniques High levels of variability were observed, associated with interface oscillations and turbulent plumes, with many echoes showing significant spectral structure Acoustic estimates of interface thickness (1-3 cm), obtained for the echoes with exactly two peaks in the compressed pulse output, were in good agreement with estimates based on direct microstructure and optical shadowgraph measurements Predictions based on a one-dimensional weak-scattering model that includes the actual density and sound speed profiles agree reasonably with the measured scattering A remote-sensing tool for mapping oceanic microstructure, such as high-frequency broadband acoustic scattering, could lead to a better understanding of the extent and evolution of double-diffusive layering, and to the importance of double diffusion to oceanic mixing

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Acoustic scattering from double-diffusive microstructure

There is anecdotal evidence that under conditions of moderate to high wind speeds (8-15 m ·s- 1), clouds of bubbles entrained in the near-surface layer by breaking waves can create a benign underwater communications channel through the resonant absorption of forward-scattered sound, reducing reverberation times and the occurrence of high-intensity, Doppler-shifted arrivals. Current models for the effects of bubbles on surface-interacting sound show two effects: refraction of low-frequency sound due to reductions in sound speed near the surface and resonant absorption at higher frequencies. These models include uncertainty in the numbers and sizes of the largest bubbles present in the near-surface layer, and their dependence on wind speed. This uncertainty makes quantitative prediction of bubble effects in the underwater acoustic communications band of workhorse frequencies (10-30 kHz) difficult. The model calculations presented here show that resonant absorption associated with the largest bubbles is strongly frequency and wind-speed dependent. The frequency dependence can be explained by the concept of a bubble escape radius; this being the radius of a bubble for which turbulent fluid velocity fluctuations and bubble terminal velocity in the upper ocean boundary layer balance. Bubbles smaller than the escape radius tend to remain trapped by fluid turbulence while larger bubbles are lost to the surface through buoyant degassing. Calculation of the escape radius provides a means of estimating the lowest frequency at which resonant absorption can be expected for a given wind speed. Initial estimates suggest that resonant absorption at 10 kHz begins at 10-m wind speeds of around 8 ms -1, and significant surface bounce losses at frequencies lower than this are expected in the range of wind speeds 13-20 m·s- 1.

The Suspension of Large Bubbles Near the Sea Surface by Turbulence and Their Role in Absorbing Forward-Scattered Sound

Deterministic structures in sound reflected by gravity waves, such as focused arrivals and Doppler shifts, have implications for underwater acoustics and sonar, and the performance of underwater acoustic communications systems. A stationary phase analysis of the Helmholtz-Kirchhoff scattering integral yields the trajectory of focused arrivals and their relationship to the curvature of the surface wave field. Deterministic effects along paths up to 70 water depths long are observed in shallow water measurements of surface-scattered sound at the Martha's Vineyard Coastal Observatory. The arrival time and amplitude of surface-scattered pulses are reconciled with model calculations using measurements of surface waves made with an upward-looking sonar mounted mid-way along the propagation path. The root mean square difference between the modeled and observed pulse arrival amplitude and delay, respectively, normalized by the maximum range of amplitudes and delays, is found to be 0.2 or less for the observation periods analyzed. Cross-correlation coefficients for modeled and observed pulse arrival delays varied from 0.83 to 0.16 depending on surface conditions. Cross-correlation coefficients for normalized pulse energy for the same conditions were small and varied from 0.16 to 0.06. In contrast, the modeled and observed pulse arrival delay and amplitude statistics were in good agreement.

Andone C. Lavery

Papers

Classification of broadband echoes from prey of a foraging Blainville’s beaked whale

A technique for accurate measurements of ion beam current density using a Faraday cup

Acoustic scattering from double-diffusive microstructure

The Suspension of Large Bubbles Near the Sea Surface by Turbulence and Their Role in Absorbing Forward-Scattered Sound

Deterministic forward scatter from surface gravity waves.