Microbial community structure and biomass in surface waters during a Polar Front summer bloom along 170°W

As part of the US Joint Global Ocean Flux Study (US JGOFS) Southern Ocean Program, flow cytometry and epifluorescent microscopy were utilized to determine abundance, distribution and size structure of the microbial community in the Polar Front region during the summer biomass maximum. Surface samples were collected approximately every 10 km along 170 degreesW during two N-S transects, separated in time by two weeks. Phytoplankton abundance and size structure varied with distinct latitudinal trends. Autotrophic biomass was lowest north of the Polar Front reflecting the dominance of small cells. The highest biomass (170 mug Cl-1) occurred at 65 degreesS where the composition was strongly influenced by large centric diatoms. Farther south, the diatom community shifted to the dominance of smaller pennate diatoms. Total grazer biomass and size distributions followed similar patterns, ranging from 4 mug Cl-1 in the north to 52 mug Cl-1 in the south where larger (> 20 mum) grazers were more abundant, Heterotrophic bacteria varied over an order of magnitude in abundance across the study site, with size generally increasing from north to south. In the second transect. phytoplankton biomass at 65 degreesS was 50% lower, and grazer biomass and bacterial populations were slightly greater, indicating the decline of the bloom. The changes in biomass and community structure along 170 degreesW and the reduction of phytoplankton standing stock at 65 degreesS over time suggests adjacent, yet different, microbial systems in terms of carbon flux, spanning from primarily recycling to export-dominated. (C) 2001 Elsevier Science Ltd. All rights reserved.