Determining dominant scatterers of sound in mixed zooplankton populations

Abstract

High-frequency acoustic scattering techniques have been used to investigate dominant scatterers in mixed zooplankton populations. Volume backscattering was measured in the Gulf of Maine at 43, 120, 200, and 420 kHz. Zooplankton composition and size were determined using net and video sampling techniques, and water properties were determined using conductivity, temperature, and depth sensors. Dominant scatterers have been identified using recently developed scattering models for zooplankton and microstructure. Microstructure generally did not contribute to the scattering. At certain locations, gas-bearing zooplankton, that account for a small fraction of the total abundance and biomass, dominated the scattering at all frequencies. At these locations, acoustically inferred size agreed well with size determined from the net samples. Significant differences between the acoustic, net, and video estimates of abundance for these zooplankton are most likely due to limitations of the net and video techniques. No other type of biological scatterer ever dominated the scattering at all frequencies. Copepods, fluid-like zooplankton that account for most of the abundance and biomass, dominated at select locations only at the highest frequencies. At these locations, acoustically inferred abundance agreed well with net and video estimates. A general approach for the difficult problem of interpreting high-frequency acoustic scattering in mixed zooplankton populations is described.

Acoustic insights into the zooplankton dynamics of the eastern Weddell Sea

“…Organisms that contain for instance hard shells or gaseous enclosures in general scatter sound much stronger than gelatinous creatures. Concise overviews of the backscatter characteristics of certain zooplankton groups and their frequency-dependency can be found in Lavery et al (2007) and in Warren and Wiebe (2008).…”

Observations of Broadband Acoustic Backscattering From Nonlinear Internal Waves: Assessing the Contribution From Microstructure

“…3) Scattering Models: The acoustic scattering models used to predict scattering from zooplankton and turbulent oceanic microstructure are described in detail in [20]. The necessary parameters for making predictions using these scattering models can be obtained from the direct microstructure measurements and zooplankton net tows.…”

“…Many of these scattering sources are intermittent, and can occur simultaneously [16], [17]. Understanding the scattering from any one of these scattering sources is independently challenging as the scattering depends on many parameters [18]- [20], a number of which are difficult to quantify at relevant scales by any sampling technique, particularly in situ. For example, for zooplankton, the scattering characteristics depend in a complicated way on the material properties (sound speed and density), shape and size, orientation of the organisms relative to the incident acoustic wave, and on the acoustic frequency.…”

“…Elastic-shelled pteropods and weakly scattering fluid-like copepods and amphipods dominated the predicted scattering during SW06. The scattering models used for these predictions are described in [20]. The parameters for the zooplankton predictions are based on net tow 4 and the microstructure parameters are based on values obtained for internal wave Mika 1. zooplankton and the diffusive rolloff in the spectrum for scattering from turbulent temperature microstructure for a range of dissipation rates (Fig.…”

“…Further compounding the complications, high-frequency acoustic backscattering measurements are typically performed at a single narrowband frequency, or a small number of narrowband frequencies, limiting the amount of information available for interpretation of the scattering measurements. Generally, an insufficient number of discrete narrowband frequencies are used to accurately map the scattered spectra from the different scattering sources [20].…”