Acoustically-Inferred Zooplankton Distribution in Relation to Hydrography

Gareth L. Lawson, Peter H. Wiebe, Carin J. Ashjian, Scott M. Gallager, Cabell S. Davis, and Joseph D. Warren

We have examined the distribution of zooplankton in relation to hydrography in and around Marguerite Bay, using acoustic backscattering intensity (43, 120, 200, 420, and 1000 kHz) measured with the BIOMAPER-II as an index of zooplankton abundance. At present, post-processing of acoustic data has been completed for only the 2001 cruises (NBP01-03, NBP01-04), and analysis of acoustic data collected during 2002 (NBP02-02, NBP02-04) is limited to qualitative descriptions.

At 120 kHz, a persistent dense scattering layer 50 to 200 m high was found in association with the bottom throughout much of the survey area during all four cruises. Midwater scattering was moderate, but a distinct layer of intensified backscatter was typically observed at the top of the pycnocline. Whether this layer stemmed from sound scattered by physical features of the water column itself, or by organisms associated with these features, remains to be determined. Overall, scattering levels decreased substantially between the fall and winter surveys. During both falls and the winter of 2001, large krill-like patches were predominantly associated with nearshore areas of complex bathymetry. In contrast, during the winter of 2002, such patches were more widely distributed across the continental shelf, and often found close to the continental shelf break. The observed distributions of backscatter appear to be consistent with predicted geostrophic circulation, and suggest both along- and across-shelf transport of zooplankton. Analysis of backscatter distribution in relation to hydrographic and bathymetric features is ongoing.

In the future, in addition to completing the post-processing of all 2002 survey data, we will use Video Plankton Recorder observations, MOCNESS catches, and frequency-related acoustic signatures, to ground-truth the taxonomic composition of our acoustic observations. This will allow us to infer the abundance and distribution of individual zooplankton taxa, and of krill in particular. Preliminary analyses suggest that krill account for the majority of backscatter in the upper 100 m of the water column in high-scattering coastal regions, but that copepods and siphonophores are more important at greater depths and in other regions.