Mesoscale variability in biological community structure and biomass in the Antarctic Polar Front region at 170°W during austral spring 1997

The influence of the Antarctic Polar Front (PF) on microbial biomass and community structure was investigated during late spring, October - November 1997, as part of the U.S. Joint Global Ocean Flux Study Antarctic Environment and Southern Ocean Process Study. In conjunction with SeaSoar sampling, samples for flow cytometry and epifluorescence image analysis were collected across the PF region along a 170 degreesW transect and in two maps involving repeated crossings of the front. Phytoplankton abundance and size estimates clearly showed the influence of the front, with smaller, more numerous cells to the north and larger, less abundant cells to the south. Autotrophic 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 meander 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 trends 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 meander was biologically distinct and spatially separated from the community sampled at the marginal ice zone. Analysis of phytoplankton biomass increases 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 mechanism for chlorophyll accumulation. The significance of the front on the development of the seasonal phytoplankton increase is evident, yet the spatial heterogeneity of the microbial assemblage indicates a complex physical environment with multiple mesoscale influences.