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 backscatte ring from plankton and micronekton over Georges Bank as part of a stud y designed to determine the correlation length scales of plankton spat ial patterns in relation to physical structure and to intercompare dif ferent kinds of sampling and remote-sensing instrumentation. Two physi cally distinct areas were studied: a well-mixed area in a shallow port ion of the Bank and a stratified area on the deeper southern flank of the Bank. A submersible echo sounder with a down-looking transducer wa s mounted in a towed V-fin. Volume backscattering data were collected from near the sea surface to the bottom (40-80 m). Vertical and horizo ntal 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 be tween the two areas. Internal waves appeared to modulate the depth of dense mid-depth volume scattering layers in the stratified sites. In t he mixed area, there was little horizontal layering or coarse-scale ho rizontal structure. However, fine-scale vertical lineations were evide nt with horizontal length scales on the order of the depth of the wate r column. One hypothesis to explain these vertical lineations in the w ell-mixed areas involves the development of secondary vertical circula tion cells associated with the tidal hows over a rough bottom. Althoug h volume backscattering at the stratified sites was 4-7 times higher t han at the mixed site, there was no significant difference in MOCNESS (Multiple Opening/Closing Net and Environmental Sensing System) collec ted biovolumes between these locations. The difference in volume backs cattering was due to differences in both the acoustic scattering prope rties of zooplankton taxa and the taxonomic composition of the plankto n between the sites. Correlations between taxon abundance and volume s cattering were positive and significant only for pteropods and euphaus iid larvae. The abundances of copepods, chaetognaths, fish larvae, and amphipods were not significantly correlated with volume scattering. W hen 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 a greement was found between observed and predicted volume backscatterin g strengths. Copyright (C) 1996 Elsevier Science Ltd