Acoustic evidence for an intense plankton bloom primarily in the

northern sector of the Southern Ocean GLOBEC broad-scale survey grid in

the austral fall of 2002

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

BIOMAPER-II was deployed from the stern of the RVIB N.B. Palmer and towyo'd between most of the broad-scale survey stations on the 2002 fall broad-scale survey. Multi-frequency acoustic data (43, 120, 200, 420, and 1000 kHz) were collected between the surface and near bottom while video images and environmental data were collected between the surface and bottom of towyo's (max 250 to 300 m) along the towyo path in much of the survey area.

On NBP02-02, at a number of locations along the mid- and outer shelf areas, and offshore waters, the 1 MHz transducers had high backscattering levels in the 0-60 m depth interval that correlated with very high algal mats, chain forming diatom and radiolarian concentrations that were observed on the Video Plankton Recorder (see Ashjian et al. abstract). A diatom bloom of significant proportions was occurring in the northern and central portion of the SO GLOBEC survey grid and this was most evident in the 1 MHz echograms, but also in the 420 kHz echograms. This high backscattering was not observed in Marguerite Bay nor was it very evident on the southern portion of the grid. Although chlorophyll concentrations were not particularly elevated, surface (0-50 m) net tows in the area of high 1 MHz backscatter often came up dominated by a “green goo” in which it was hard to find many larger zooplankton. Survey line 2 was sampled twice during the cruise between station 8 and 10 and on the second pass, the intense backscattering was not present, providing an indication of the time frame for the end of the bloom.

A similar fall bloom was reported by Uli Bathmann during the German survey work on the RVIB Polarstern (13 April to 7 May) in the GLOBEC region shortly before the RVIB N. B. Palmer arrived for work on cruise NBP01-03 in April 2001. It apparently had disappeared by the time of our arrival, although highest fluorescence levels on that cruise were present in the northern portion of the survey grid.