Gareth L. Lawson, Peter H. Wiebe, Carin J. Ashjian, Scott M. Gallager, Cabell S. Davis, and Joseph D. Warren
Two surveys were conducted in austral fall (May-June) and winter (July-September) of 2001 to investigate the distribution of zooplankton in relation to hydrographic regimes of the continental shelf in and around Marguerite Bay, as part of the Southern Ocean GLOBEC program. Acoustic and environmental data were collected with the Bio-Optical Multi-frequency Acoustical and Physical Environmental Recorder (BIOMAPER-II) along transect lines running across the shelf and perpendicular to the Western Peninsula coastline, between -65 and -70°S. The BIOMAPER-II was equipped also with a Video Plankton Recorder (VPR) to describe the distributions of planktonic taxa. In fall, acoustic backscattering at 120 kHz was greatest in regions of abrupt topography close to shore, where backscattering was concentrated in dense patches between 50 and 120 m. The middle to outer portion of the shelf was characterized by less dense layers of scattering extending from 100 m depth to the bottom and a general along-shelf, North-South increasing gradient in scattering intensity. A persistent shallow layer was also present across most of the shelf, generally situated near the top of the pycnocline. By winter, in contrast, scattering had decreased substantially in magnitude throughout the survey area. The shallow layer was still present, but those few areas of high scattering were characterized by a dense layer in immediate proximity to the bottom. There were few large and distinct patches of the sort observed in the fall. The observed distributions of backscattering were associated with concomitantly measured water column properties (e.g., temperature, salinity, and topography).
It is often assumed that krill are the dominant contributors to backscattering in Antarctic waters, and indeed, net samples (MOCNESS) and VPR data suggest a high abundance of adult krill in the high-scattering coastal regions observed in the fall survey. In other regions of the shelf, however, direct observations and predictions of expected backscattering based on net catches and taxon-specific models of acoustic target strength suggest that other organisms such as pteropods and copepods can be responsible for the majority of scattering.