Microbial community composition and growth dynamics in the Antarctic Polar Front and seasonal ice zone during late spring 1997

The microbial community between the Antarctic Polar Front region (APFr) and the seasonal ice edge at 170°W was examined as part of the US JGOFS AESOPS Process 1 Study during late spring (December) 1997. Water-mass identities of the sampled stations were determined based on physical properties, with three regions identified: the APFr, the South ACC region (SACCr), and the southern region of the ACC near the ice edge (SACCr-IE). The APFr (60.1–60.5°S) was dominated by centric diatoms, with mixed-layer communities tending toward single genera (i.e., Chaetoceros and Corethron). Stations in the SACCr (61.4–63.0°S) were also characterized by large centric diatoms, but no single genus dominated. In contrast, the SACCr-IE stations (64.1–64.4°S), positioned near the retreating ice edge, were dominated by colony-forming Phaeocystis. A seasonal succession in phytoplankton is proposed, with the youngest communities near the ice edge and the most “mature” in the polar front region. We found an inverse relationship between chlorophyll a values and phytoplankton growth rates. Phytoplankton growth rates were the highest (up to 1 d−1) in the APFr, despite relatively low chlorophyll a values. Microzooplankton grazing accounted for ∼50% of phytoplankton growth. The SACCr stations had intermediate chlorophyll a concentrations and phytoplankton growth rates (0.5–0.9 d−1), with grazing consuming ∼50–70% of growth. The SACCr-IE stations had the highest chlorophyll a values and the lowest phytoplankton growth rates (0.2 d−1) of the transect, while grazing accounted for ∼70% of growth in the one dilution experiment that gave a significant result. Given these growth and grazing estimates, a higher proportion of production in the northern diatom-dominated regions was available for export than in the Phaeocystis-dominated ice edge stations. However, since microzooplankton produce small fecal debris, the bulk of the consumed production was more likely remineralized in the upper water column.