Seasonal dynamics of phytoplankton in the Antarctic Polar Front region at 170°W

Phytoplankton dynamics in the region of 55–70°S, 170°W were investigated using Sea-viewing Wide Field-of-View Sensor satellite imagery, shipboard sampling and experimental rate assessments during austral spring and summer, 1997–1998. We used image-analysis microscopy to characterize community biomass and composition, and dilution experiments to estimate growth and microzooplankton grazing rates. Iron concentrations were determined by flow-injection analysis. The phytoplankton increase began slowly with the onset of stratification at the Polar Front (PF) (60–61°S) in early November. Seasonally enhanced levels of chlorophyll were found as far north as 58°S, but mixed-layer phytoplankton standing stock was highest, approaching 200 mg C m−3, in the region between the receding ice edge and a strong silicate gradient, which migrated from ∼62°S to 65°S during the study period. The most southern stations sampled on four cruises were characterized by small pennate diatoms and Phaeocystis. From the PF to the Southern Antarctic circumpolar current front (∼65°S), this ice margin assemblage was seasonally replaced by a community dominated by large diatoms. The large diatom community developed only in waters where measured iron concentrations were initially high (⩾0.2 nM), and crashed when dissolved silicate was depleted to low levels. Phytoplankton growth rates were highest (0.5–0.6 d−1) between the PF and silicate front (60°S and 63°S) in December. In January, growth rates were lowest (0.1 d−1) near the PF, and the highest rates (0.3–0.4 d−1) were found in experiments between 64.8°S and 67.8°S. Phytoplankton production estimates were highest south of the PF through December and January, averaging 2.2–2.4 mmol C m−3 d−1 and reaching levels of 5 mmol C m−3 d−1 (64.8°S and 67.8°S in January). Microzooplankton grazers consumed 54–95% of production for experiments conducted on four AESOPS cruises. They were less efficient in balancing growth rates during the time of highest phytoplankton growth and increase in December, and most efficient in February–March, after the large diatom bloom had collapsed. The diatom bloom region in the present study is in an upwelling zone for Antarctic circumpolar deep water with high iron content. This may explain why this marginal ice zone differs from others where blooms have not been observed.