Anisotropy in high-frequency broadband acoustic backscattering in the presence of turbulent microstructure and zooplankton

High-frequency broadband (120–600 kHz) acoustic backscatteringmeasurements have been made in the vicinity of energetic internal waves. The transducers on the backscatteringsystem could be adjusted so as to insonify the water-column either vertically or horizontally. The broadband capabilities of the system allowed spectral classification of the backscattering. The distribution of spectral shapes is significantly different for scatteringmeasurements made with the transducers oriented horizontally versus vertically, indicating that scatteringanisotropy is present. However, the scatteringanisotropy could not be unequivocally explained by either turbulent microstructure or zooplankton, the two primary sources of scatteringexpected in internal waves. Daytime net samples indicate a predominance of short-aspect-ratio zooplankton. Using zooplanktonacoustic scatteringmodels, a preferential orientation of the observed zooplanktoncannot explain the measured anisotropy. Yet model predictions of scatteringfr...