| As part of the Southern Ocean GLOBEC study to understand the physical and biological factors that contribute to krill over-wintering survival, four broad-scale surveys were conducted in austral fall (April-June) and winter (July-August) of 2001 and 2002 from the RVIB N.B. Palmer. The study site was the Western Antarctic continental shelf in and around Marguerite Bay. To examine the relationship between the distribution of zooplankton, especially Antarctic krill (Euphausia superba), and the hydrographic regimes, acoustic, video, and environmental data were collected with the BIo-Optical Multi-frequency Acoustical and Physical Environmental Recorder (BIOMAPER-II) along 13 transect lines running across the shelf and perpendicular to the Western Antarctic Peninsula coastline, between -65 and -70 S. MOCNESS tows were conducted at selected locations for ground-truthing acoustic observations. In fall of both years, acoustic backscattering at 120 kHz was greatest in regions of abrupt topography close to shore and an increasing gradient in scattering intensity was observed along-shelf (northeast to southwest). Highest backscattering was in the 150 to 350 m depth range and was associated with modified Upper Circumpolar Deep Water (UCDW). Un-modified UCDW generally was characterized by low levels of scattering, indicative of low zooplankton biomass in offshore waters and recent intrusions of UCDW onto the continental shelf. By winter, scattering had decreased substantially, nearly an order of magnitude, throughout the survey area and highest concentrations were distributed deeper in the water column (>350 m). The observed distributions of backscattering were consistent with predicted geostrophic circulation, and suggest both along- and across-shelf transport of zooplankton. Taxonomic data from MOCNESS samples together with taxon-specific models of target strength revealed that krill can be a dominant scatterer in the upper 100 m of the water column in high-scattering coastal regions, but that copepods, siphonophores, and pteropods are more important at greater depths and in other regions of the study area. The patterns of zooplankton distribution, especially krill, were related to marine mammal and sea bird distributions. |
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