Acoustically-Inferred Zooplankton Distribution in Relation to Hydrography

West of the Antarctic Peninsula

Gareth L. Lawson, Peter H. Wiebe, Carin J. Ashjian, Scott M. Gallager,

Cabell S. Davis, and Joseph D. Warren

The relationship between the distribution of zooplankton, especially Antarctic krill (Euphausia superba), and hydrographic regimes of the Western Antarctic continental shelf in and around Marguerite Bay was studied as part of the Southern Ocean GLOBEC program. Surveys were conducted from the RVIB N.B. Palmer in austral fall (April-June) and winter (July-August) of 2001. 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. In order to ground-truth acoustic observations, MOCNESS tows were conducted at selected locations. In fall, acoustic backscattering at 120 kHz, used here as an index of zooplankton abundance, was greatest in the southern reaches of the survey area and inside Marguerite Bay. An increasing gradient in scattering intensity was observed along-shelf (northeast to southwest). Highest backscattering was in the 150 to 450 m depth range and was associated with modified Upper Circumpolar Deep Water (UCDW). The two deep canyons that run diagonally across the continental shelf and intersect the shelf break were characterized by reduced backscattering, similar to levels observed off-shelf and indicative of low zooplankton biomass in recent intrusions of UCDW onto the continental shelf. By winter, scattering had decreased by approximately an order of magnitude (10 dB) in the upper and middle reaches of the water column in most areas, and high backscattering levels were found primarily in a deep (>300 m) scattering layer present close to the bottom in association with mUCDW and UCDW. During the fall survey, the observed distribution of backscattering was consistent with predicted geostrophic circulation, and suggests both along- and across-shelf transport of zooplankton. Such advection of zooplankton, in conjunction with active horizontal movements of certain larger taxa and mortality, likely explains the decrease in backscattering between the two seasons. Predictions of expected backscattering levels based on MOCNESS samples suggest that large krill can account for most of the backscattering in certain localized portions of the water column, particularly in the upper 100 m of high-scattering coastal regions, but that copepods, siphonophores, and pteropods are more important at greater depths and in other regions of the study area.

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