Acoustical study of the spatial distribution of plankton on Georges Bank and the relationship between volume backscattering strength and the taxonomic composition of the plankton

High frequency (420 kHz) sound was used to study the volume backscattering from plankton and micronekton over Georges Bank as part of a study designed to determine the correlation length scales of plankton spatial patterns in relation to physical structure and to inter-compare different kinds of sampling and remote-sensing instrumentation. Two physically distinct areas were studied: a well-mixed area in a shallow portion of the Bank and a stratified area on the deeper southern flank of the Bank. A submersible echo sounder with a down-looking transducer was mounted in a towed V-fin. Volume backscattering data were collected from near the sea surface to the bottom (40-80 m). Vertical and horizontal volume backscattering structure in the stratified region differed from that in the well-mixed area in both mean and variance, providing evidence that physical forcing of the pattern varied significantly between the two areas. Internal waves appeared to modulate the depth of dense mid-depth volume scattering layers in the stratified sites. In the mixed area, there was little horizontal layering or coarse-scale horizontal structure. However, fine-scale vertical lineations were evident with horizontal length scales on the order of the depth of the water column. One hypothesis to explain these vertical lineations in the well-mixed areas involves the development of secondary vertical circulation cells associated with the tidal flows over a rough bottom. Although volume backscattering at the stratified sites was 4-7 times higher than at the mixed site, there was no significant difference in MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) collected biovolumes between these locations. The difference in volume backscattering was due to differences in both the acoustic scattering properties of zooplankton taxa and the taxonomic composition of the plankton between the sites. Correlations between taxon abundance and volume scattering were positive and significant only for pteropods and euphausiid larvae. The abundances of copepods, chaetognaths, fish larvae, and amphipods were not significantly correlated with volume scattering. When taxon-specific model predictions of acoustic backscattering cross-section, developed by Stanton et al. (ICES Journal of Marine Science, 51 (1994) 505-512), were used with field collected individual size and abundance data to predict measured volume backscattering data, good agreement was found between observed and predicted volume backscattering strengths.