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J. D. Warren
and P. H. Wiebe. 2008. Accounting
for biological and physical sources of acoustic backscatter improves
estimates of zooplankton biomass. Canadian journal of Fisheries
and Aquatic Sciences 65: 1321-1333. doi:10.1139/F08-047
To convert measurements of backscattered acoustic energy to estimates of abundance and taxonomic information about the zooplankton community, all of the scattering processes in the water column need to be identified and their scattering contributions quantified. Zooplankton populations in the eastern edge of Wilkinson Basin in the Gulf of Maine in the Northwest Atlantic were surveyed in October 1997. Net tow samples at different depths, temperature and salinity profiles, and multiple frequency acoustic backscatter measurements from the upper 200 m of the water column were collected. Zooplankton samples were identified, enumerated, and measured. Temperature and salinity profiles were used to estimate the amount of turbulent microstructure in the water column. These data sets were used with theoretical acoustic scattering models to calculate the contributions of both biological and physical scatterers to the overall measured scattering level. The output of these predictions shows that the dominant source of acoustic backscatter varies with depth and acoustic frequency in this region. By quantifying the contributions from multiple scattering sources, acoustic backscatter becomes a better measure of net-collected zooplankton biomass. J. D. Warren
and J. N. Smith. 2007. Density and sound
speed of two gelatinous zooplankton: Ctenophore (Mnemiopsis leidyi)
and lion's mane jellyfish (Cyanea
capillata). Journal
of the Acoustical Society of America 122(1): 574-580.
The density and sound speed of two coastal, gelatinous zooplankton, Mnemiopsis leidyi (a ctenophore) and Cyanea capillata (lion's mane jellyfish), were measured. These parameters are important inputs to acoustic scattering models. Two different methods were used to measure the density of individual animals: one used a balance and graduated cylinder to determine the mass and displacement volume of the animal, the other varied the density of the solution the animal was immersed in. When the same animal was measured using both methods, density values were within 1% of each other. A travel-time difference method was used to measure the sound speed within the animals. The densities of both zooplankton slightly decreased as the animals increased in length, mass, and volume. The ratio of animal density and sound speed to the surrounding seawater (g and h, respectively) are reported for both animals. For Mnemiopsis leidyi ranging in length from 1 to 5 cm, the mean value (±standard deviation) of g and h were 1.009 (±0.004) and 1.007 (±0.001). For Cyanea capillata ranging in bell diameter from 2 to 11 cm, the mean value (±standard deviation) of g and single value of h were 1.009 (±0.004) and 1.0004. ©2007 Acoustical Society of America J. D. Warren
and B. J.
Peterson, 2007, Use of a 600-kHz Acoustic Doppler Current
Profiler to
measure estuarine bottom type, relative abundance of submerged aquatic
vegetation, and eelgrass canopy height. Estuarine,
Coastal, and Shelf Science, 72: 53-62.
The acoustic
backscatter intensity signal from a high-frequency
(600 kHz) Acoustic Doppler Current Profiler (ADCP) was used to
categorize four different types of bottom habitat (sand, mud, sparse
and dense vegetation) in a shallow-water estuary (Shinnecock Bay, NY,
USA). A diver survey of the bay measured sediment and bottom vegetation
characteristics at 85 sites within the bay. These data were used to
groundtruth the acoustic data. Acoustic data were collected at four
sites with known bottom types and used to develop an algorithm that
could categorize the bottom type. The slope of the echo intensity
profile close to the bottom was used to determine the bottom type and
the relative numerical density (sparse or dense) of Submerged Aquatic
Vegetation (SAV). In areas where eelgrass (Zostera marina)
was
the dominant SAV species, the intensity profile data were analyzed to
measure the height of the vegetation canopy. An acoustic survey which
categorized the bottom type of the bay was conducted from a small
vessel. The percentage of sampled sites categorized as each bottom
habitat type from the acoustic survey was similar to those obtained by
the diver survey. These methods may provide a means to rapidly survey
estuarine habitats and measure spatial and temporal variations in SAV
populations, as well as changes in the height of the eelgrass canopy.
E. Peacock , R. Nelson, A. Solow, J. Warren, J. Baker and C. Reddy, 2005, The West Falmouth Oil Spill: ~100 Kg of Oil Found to Persist Decades Later , Environmental Forensics, 6(3): 273-281. In order to investigate the long-term fate of petroleum hydrocarbons in salt marsh sediments in Wild Harbor (West Falmouth, MA) impacted by the Florida spill of 1969, 26 sediment cores were collected and analyzed for total petroleum hydrocarbons (TPH). The results from this effort indicate that the distribution of petroleum hydrocarbons is spatially heterogeneous, oil compounds are generally located at sediment depths of 4 to 20 cm in areas closest to the banks of the marsh, and ~ 100 kg of petroleum residues can be found to persist in intertidal sediments that were originally the most impacted. G. L. Lawson, P. H. Wiebe, C. J. Ashjian, S. M. Gallager, C. S. Davis, and J. D. Warren, 2004, Acoustically-inferred zooplankton distribution in relation to hydrography west of the Antarctic Peninsula , Deep-Sea Research. Part II, 51(17-19): 2041-2072. The relationship between the distribution of zooplankton, especially euphausiids (Euphausia and Thysanoessa spp.), and hydrographic regimes of the Western Antarctic Peninsula continental shelf in and around Marguerite Bay was studied as part of the Southern Ocean GLOBEC program. Surveys were conducted from the RVIB N. B. Palmer in austral fall (April-June) and winter (July- August) of 2001. Acoustic, video, and environmental data were collected along 13 transect lines running across the shelf and perpendicular to the Western Antarctic Peninsula coastline, between 65°S and 70°S. Depth-stratified net tows conducted at selected locations provided ground-truthing for acoustic observations. In fall, acoustic volume backscattering strength at 120 kHz was greatest in the southern reaches of the survey area and inside Marguerite Bay, suggestive of high zooplankton and micronekton biomass in these regions. Vertically, highest backscattering was in the 150-450 m depth range, associated with modified Circumpolar Deep Water (CDW). The two deep troughs that intersect the shelf break were characterized by reduced backscattering, similar to levels observed off-shelf and indicative of lower zooplankton biomass in recent intrusions of CDW onto the continental shelf. Estimates of dynamic height suggested that geostrophic circulation likely caused both along- and across-shelf transport of zooplankton. By winter, scattering had decreased by an order of magnitude (10 dB) in the upper 300 m of the water column in most areas, and high backscattering levels were found primarily in a deep (›300 m) scattering layer present close to the bottom. The seasonal decrease is potentially explained by advection of zooplankton, vertical and horizontal movements, and mortality. Predictions of expected backscattering levels based on net samples suggested that large euphausiids were the dominant source of backscattering only at very particular locations and depths, and that copepods, siphonophores, and pteropods were more important in many locations. J. D. Warren , D. A. Demer, D. E. McGehee, R. Di Mento and J. F. Borsani, 2004, Zooplankton in the Ligurian Sea: Part II. Exploration of their physical and biological forcing functions during summer 2000, Journal of Plankton Research, 26(12): 1409-1418. A survey of the biological and physical oceanography of the Ligurian Sea was conducted in the late summer of 2000. Forty-one stations were sampled for nutrients, oxygen, fluorescence and hydrographic information. Acoustic backscatter measurements were used to estimate abundance of small (<5 mm) zooplankton biovolume versus depth and the distribution of northern krill, Meganyctiphanes norvegica. Net-tow and underwater video data were collected to identify the zooplankton present. These data were used to analyze the Ligurian Sea ecosystem for physical and biological linkages that control zooplankton abundance and distribution. Results are compared with those from a similar study conducted in 1999. Hydrographic sampling showed a dome of dense water in the southwestern middle of the basin. The highest chlorophyll a (Chl a) concentrations were measured in this area, while small zooplankton biovolume was evenly distributed throughout the survey. Integrated values of Chl a and small zooplankton biovolume in 2000 were greater than in 1999. Meganyctiphanes norvegica, siphonophores and salps were the dominant components of the macrozooplankton population in the upper 200 m. In the sampled depth strata, siphonophore abundance did not change during the day, while M. norvegica were only caught at night. Acoustic backscatter data show that higher densities of M. norvegica occurred in deeper water and in the western and southwestern areas of the Ligurian Sea. D. E. McGehee, D. A. Demer, and J. D. Warren, 2004, Zooplankton in the Ligurian Sea: Part I. Characterization of their dispersion, relative abundance and environment during summer 1999, Journal of Plankton Research, 26(12): 1409-1418. The distributions of temperature, salinity, chlorophyll and zooplankton were measured in the Ligurian Sea, north of Corsica, in August 1999. To characterize the physical environment, hydrographic and fluorometric profiles were collected. A net and two acoustic systems were used to measure the distribution of small (<5 mm) and large (>5 mm) zooplankton. Highest chlorophyll values were strongly associated with a dome of dense water in the center of the Ligurian Basin. Small zooplankton (copepods and smaller), in contrast, appeared to be associated with the periphery of the basin and were negatively correlated with chlorophyll. Large zooplankton were not correlated with either chlorophyll or small zooplankton. Large zooplankton migrated vertically hundreds of meters every night, while small zooplankton did not appear to migrate much. The physical observations were consistent with (i) a well-documented geostrophically driven cyclonic coastal current (the Ligurian Current) fed by sources in the Algerian Basin and Tyrrhenian Sea and (ii) upwelling in the central Ligurian Basin. Large zooplankton, being strong vertical migrators, were potentially insulated from the effects of the currents and therefore stayed resident. J. D. Warren, T. K. Stanton, P. H. Wiebe, and H. E. Seim, 2003, Inference of biological and physical parameters in an internal wave using multiple-frequency, acoustic-scattering data, ICES Journal of Marine Science, 60(5): 1033-1046. High-frequency sound (>10 kHz) is scattered in the ocean by many different processes. In the water column, marine organisms are often assumed to be the primary source of acoustic backscatter. Recent field experiments and theoretical work suggest that the temperature and salinity microstructure in some oceanic regions could cause acoustic scattering at levels comparable to that caused by marine life. Theoretical acoustic-scattering models predict that the scattering spectra for microstructure and organisms are distinguishable from each other over certain frequency ranges. A method that uses multiple-frequency acoustic data to exploit these differences has been developed, making it possible to discriminate between biological and physical sources of scattering under some conditions. This method has been applied to data collected in an internal wave in the Gulf of Maine. For regions of the internal wave in which the dominant source of scattering is either biological or physical in origin, it is possible to combine the acoustic-scattering data and temperature and salinity profiles with acoustic-scattering models to perform a least-squares inversion. Using this approach, it is possible to estimate the dissipation rate of turbulent kinetic energy for some regions of the internal wave, and the length and numerical abundance of the dominant biological scatterer, euphausiids, in others. P. H. Wiebe, T. K. Stanton, C. H. Greene, M. C. Benfield, H. M. Sosik, T. C. Austin, J. D. Warren, and T. Hammar, 2002, BIOMAPER-II: An integrated instrument platform for coupled biological and physical measurements in coastal and oceanic regimes , IEEE Journal of Oceanic Engineering, 27(3): 700-716. The BIo-Optical Multi-frequency Acoustical and Physical Environmental Recorder or BIOMAPER-II is a newly developed towed system capable of conducting quantitative surveys of the spatial distribution of coastal and oceanic plankton/nekton, near surface bubble fields, and turbulence, as well as field verification studies of theoretical plankton reverberation models. The system consists of a multi-frequency sonar (up-looking and down-looking pairs of transducers operating at five frequencies: 43, 120, 200, 420, and 1000 kHz), a video plankton recorder system (VPR), an environmental sensor system (CTD, fluorometer, transmissometer), and several other bio-optical sensors (down- and upwelling spectral radiometers, spectral attenuation, and backscattering, and absorption meters). The lower four acoustical frequencies utilize split beam technology and are able to make target strength and echo integration measurements. Also included are an electro-optic tow cable, a winch with slip rings, and a van which holds the electronic equipment for real-time data processing and analysis. The vehicle is capable of operating to a depth of 300 m at 4–6 kn, while near the surface it may be towed at speeds up to 10 kn. The system can be operated in a surface-towed down-looking mode, in a vertical oscillatory “tow-yo” mode, or in a subsurface up/down-looking horizontal mode. To enhance the performance and utility of BIOMAPER-II in high sea states, a winch, slack tensioner, and over-boarding J-frame assembly are integrated with the system for deployment and handling. Wire tension records and the power spectra demonstrated the substantial protection that the slack tensioner system provided against excessive shock loading of the cable and the vehicle in sea states that would otherwise prevent work. The scientific capability of the vehicle is illustrated with acoustic, environmental, and bio-optical data sets collected from the Gulf of Maine on cruises in 1997 and 1999. J.D. Warren, T. K. Stanton, D. E. McGehee, and D. Chu, 2002, Effect of animal orientation on acoustic estimates of zooplankton properties, IEEE Journal of Oceanic Engineering, 27(1): 130-138. It is well known that the behavior of zooplankton and, in particular, their orientation distribution dramatically affects the level of backscattered acoustic energy. As a result, interpretation of acoustic survey data in the ocean is subject to error. In order to quantify these effects, laboratory data from two important classes of animals were collected. The data involved broad-band (350-650 kHz) acoustic signals insonifying individual animals whose orientation was varied over the range 0°-360° in 1° increments. The animals were from two major anatomical groups: fluid-like (decapod shrimp; Palaemonetes vulgaris) and elastic-shelled (periwinkles; Littorina littorea). The data were analyzed both in the time domain (with pulse compression processing) and the frequency domain. Averages of the laboratory data over different orientation distributions illustrate the variability in average target strength that can be expected in the ocean environment. The average target strength of the shrimp varied by 3 dB when averaged over orientation distributions centered around broadside and end-on incidence. In addition, size estimates from pulse compression processing of the broad-band echoes were made for various orientation distributions for both the shrimp and periwinkles. These results show the necessity of animal orientation information for the proper interpretation of acoustic backscatter data. J. D. Warren, T. K. Stanton, M. C. Benfield, P. H. Wiebe, D. Chu and M. Sutor, 2001, In situ measurements of acoustic target strengths of gas-bearing siphonophores, ICES Journal of Marine Science 58(4): 740-749. Acoustic target strengths of free-swimming siphonophores were measured in situ at 24 and 120kHz from a remotely operated vehicle equipped with both acoustic transducers and a video camera. The transducers and camera were co-registered by aiming both instruments at the same volume of water and time-stamping the recorded data. The video system allowed us to search for and identify siphonophores, and verified whether individual animals were centered in, or near, the axis of the acoustic beams. A towed, down-looking acoustic survey system (operating at 120kHz) measured the target and volume scattering strengths of scattering layers, presumed to be dominated by siphonophores. Spatial density of the sound scatterers was estimated from survey data. Our results confirm that free-swimming physonect siphonophores have relatively high acoustical target strengths caused by a gas inclusion in the pneumatophore of each animal. A relatively small number of these animals can dominate the backscattering detected by acoustic surveys even though other taxa may dominate the plankton on a numerical or biomass basis. Siphonophore colonies are fragile and cannot be reliably censused with nets. Our estimates of siphonophore target strengths can improve the ability to use acoustics to quantitatively census siphonophores and other taxa possessing comparably-sized gas inclusions. T. K. Stanton, D. Chu, P. H. Wiebe, R. L. Eastwood and J. D. Warren, 2000, Acoustic scattering by benthic and planktonic shelled animals , Journal of the Acoustical Society of America, 108(2): 535-550. Acoustic backscattering measurements and associated scattering modeling were recently conducted on a type of benthic shelled animal that has a spiral form of shell (Littorina littorea). Benthic and planktonic shelled animals with this shape occur on the seafloor and in the water column, respectively, and can be a significant source of acoustic scattering in the ocean. Modeling of the scattering properties allows reverberation predictions to be made for sonar performance predictions as well as for detection and classification of animals for biological and ecological applications. The studies involved measurements over the frequency range 24 kHz to 1 MHz and all angles of orientation in as small as 1° increments. This substantial data set is quite revealing of the physics of the acoustic scattering by these complex shelled bodies and served as a basis for the modeling. Specifically, the resonance structure of the scattering was strongly dependent upon angle of orientation and could be traced to various types of rays (e.g., subsonic Lamb waves and rays entering the opercular opening). The data are analyzed in both the frequency and time domain (compressed pulse processing) so that dominant scattering mechanisms could be identified. Given the complexity of the animal body (irregular elastic shell with discontinuities), approximate scattering models are used with only the dominant scattering properties retained. Two models are applied to the data, both approximating the body as a deformed sphere: (1) an averaged form of the exact modal-series-based solution for the spherical shell, which is used to estimate the backscattering by a deformed shell averaged over all angles of orientation, and produces reasonably accurate predictions over all k1aesr (k1 is the acoustic wave number of the surrounding water and aesr is the equivalent spherical radius of the body), and (2) a ray-based formula which is used to estimate the scattering at fixed angle of orientation, but only for high k1aesr. The ray-based model is an extension of a model recently developed for the shelled zooplankton Limacina retroversa that has a shape similar to that of the Littorina littorea but swims through the water [Stanton et al., J. Acoust. Soc. Am. 103, 236–253 (1998b)]. Applications of remote detection and classification of the seafloor and water column in the presence of shelled animals are discussed. ©2000 Acoustical Society of America. J.F. Lynch, J. D. Irish, T. F. Gross, P. L. Wiberg, A. E. Newhall, P. A. Traykovski and J. D. Warren, 1997, Acoustic measurements of the spatial and temporal structure of near-bottom boundary layer in the 1990-1991 STRESS experiment , Continental Shelf Research, 17: 1271-1295. As part of the 1990–1991 Sediment TRansport Events on Shelves and Slopes (STRESS) experiment, a 5 MHz Acoustic BackScatter System (ABSS) was deployed in 90 m of water to measure vertical profiles of near-bottom suspended sediment concentration. By looking at the vertical profile of concentration from 0 to 50 cm above bottom (cmab) with 1 cm vertical resolution, the ABSS was able to examine the detailed structure of the bottom boundary layer created by combined wave and current stresses. The acoustic profiles clearly showed the wave-current boundary layer, which extends to (order) 10 cmab. The profiles also showed evidence of an "intermediate" boundary layer, also influenced by combined wave and current stresses, just above the wave-current boundary layer. This paper examines the boundary-layer structure by comparing acoustic data obtained by the authors to a 1-D eddy viscosity model formulation. Specifically, these data are compared to a simple extension of the Grant-Glenn-Madsen model formulation. Also of interest is the appearance of apparently 3-D "advective plume" structures in these data. This is an interesting feature in a site which was initially chosen to be a good example of (temporally averaged) 1-D bottom boundary-layer dynamics. Computer modeling and sector-scanning sonar images are presented to justify the plausibility of observing 3-D structure at the STRESS site. |