Acoustic Backscatter from Phytoplankton

Abstract

Methods for the detection and monitoring of phytoplankton are often limited by low temporal and spatial resolution. The use of a high-frequency broadband acoustic system could provide improvements both temporally and spatially, and would be especially advantageous when used in conjunction with methods that provide taxonomic information. With this motivation, a high-frequency broadband active acoustic system has been developed and used in four laboratory trials to observe the backscatter from four morphologically-distinct species of phytoplankton whose sizes range from approximately 10 to 100 μm . The cultures were insonified at frequencies between 0.75 MHz and 6.9 MHz, using three broadband transducers (with center frequencies of 1, 2.25, and 5 MHz). Two of the species were detected with confidence and the concentration detection limits for these two species are proposed. Volume scattering strength spectra for each species are compared with the best-fit scattering models drawn from the zooplankton scattering literature and this is used to suggest probable organism composition and the best options for modelling the scattering from these species. The potential for an acoustic phytoplankton species-detection and monitoring system is discussed.