High-frequency acoustic volume scattering from zooplankton and moving oceanic microstructure

Abstract

It is well accepted that high-frequency acoustic scattering techniques can be used to perform rapid, synoptic surveys of fish and zooplankton over relevant spatial and temporal scales. However, the use of these remote sensing techniques to probe small scale physical processes, such as oceanic microstructure, has not been fully accepted, or exploited. Yet there is a growing body of evidence in the form of both laboratory and field measurements suggesting that their use is feasible. Discrimination of scattering from microstructure versus zooplankton, which span similar spatial scales, is typically a limiting factor. Currently, acoustic discrimination of turbulence from zooplankton relies on either source of scattering being dominant as well as on the availability and accuracy of scattering models. A model for scattering from oceanic microstructure that includes fluctuations in the density and sound speed is presented. The effects on scattering from a layer of microstructure with a mean fluid velocity are discussed. Backscattering predictions are made based on data collected in Hudson Canyon with a tethered free-falling high-resolution vertical microstructure profiler, and compared to two-frequency acoustic data (120 and 420 kHz). The contribution to scattering from zooplankton is also estimated from nearby depth-resolved net tows together with zooplankton scattering models.