Mesoscale variability in biological community structure and biomass in theAntarctic Polar Front region at 170 degrees W during austral spring 1997

The influence of the Antarctic Polar Front (PF) on microbial biomass and co mmunity structure was investigated during late spring, October - November 1 997, as part of the U.S. Joint Global Ocean Flux Study Antarctic Environmen t and Southern Ocean Process Study. In conjunction with SeaSoar sampling, s amples for flow cytometry and epifluorescence image analysis were collected across the PF region along a 170 degreesW transect and in two maps involvi ng repeated crossings of the front. Phytoplankton abundance and size estima tes clearly showed the influence of the front, with smaller, more numerous cells to the north and larger, less abundant cells to the south. Autotrophi c biomass varied substantially across the region, ranging from 8 to 102 mug C L-1 Biomass accumulation, dominated by Phaeocystis spp. and Chaetoceros spp., was particularly apparent in discrete areas downstream of a frontal m eander feature. Grazer biomass, ranging from 1 to 31 mug C L-1, was usually much less than 50% of phytoplankton biomass and did not show any spatial t rends with regard to the PF. The distribution of heterotrophic bacteria was dearly influenced by the PF, with larger, less abundant cells south of the frontal zone. The developing assemblage of phytoplankton in the frontal me ander was biologically distinct and spatially separated from the community sampled at the marginal ice zone. Analysis of phytoplankton biomass increas es along PF current streamlines yielded net growth rates of similar to0.08 d(-1), pointing to in situ growth, rather than transport, as the primary me chanism for chlorophyll accumulation. The significance of the front on the development of the seasonal phytoplankton increase is evident, yet the spat ial heterogeneity of the microbial assemblage indicates a complex physical environment with multiple mesoscale influences.