Three-dimensional acoustic visualization of zooplankton patchiness

Abstract

Acoustic data were collected and visualized to characterize the 3-dimensional patchiness of zooplankton at a thermally stratified site on Georges Bank. The work was carried out as part of a field study conducted to examine the effects of springtime water-column stratification on the distributions of zooplankton and larval fish on the Bank. The acoustic data were acquired as the ship steamed a survey grid relative to the track of a surface drifter with a subsurface drogue. Although quite irregular in geographical coordinate space, the ship’s track relative to the moving water closely matched the intended grid pattern once the drifter’s movement in the tidal flow was taken into account. After changing coordinate systems to compensate for tidal advection, the acoustic data set was transformed from its curtain-like distribution in 3-dimensional space to a volumetric distribution. Two-dimensional point kriging was performed on the irregularly spaced data from each 2-m-thick depth stratum to produce a series of 2-dimensional, regularly spaced data grids. These data grids were then stacked to construct the 3-dimensional data grid required for volumetric visualization. A similar procedure was followed with the error variance values produced at each grid point through kriging to construct a 3-dimensional, volumetric distribution of the error variance. To examine zooplankton patchiness within the surveyed volume of water, isosurfaces corresponding to specific levels of acoustic backscatter were highlighted in the visualization. The 3-dimensional distribution of error variance was used to control the opacity of the isosurfaces to provide an objective, visual approach for displaying the statistical confidence one can have in the patches detected. In this survey, the ship steamed directly over a large, southwest- to northeast-oriented patch of zooplankton on at least three different passes. It also steamed over several smaller patches. The vertically compressed nature of the patches and their high degree of spatial heterogeneity in the horizontal plane are characteristic of the zooplankton distributions found in the deeper, seasonally stratified portions of Georges Bank.