Deep-Sea Research II 49 (2002) 1843–1865 Seasonal dynamics of phytoplankton in the Antarctic Polar Front region at 1701W Michael R. Landry a, *, Karen E. Selph a , Susan L. Brown a , Mark R. Abbott b , Christopher I. Measures a , Suzanna Vink a , Colleen B. Allen a , Albert Calbet c , Stephanie Christensen a , Hector Nolla d a Department of Oceanography, University of Hawaii at Manoa, 1000 Pope Rd., Honolulu, HI 96822, USA b College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331-5503, USA c Institut de Ci " encies del Mar, P. Joan de Borb ! o s/n, 08039 Barcelona, Spain d Cancer Research Laboratory, 434 LSA, University of California, Berkeley, CA 94720, USA Abstract Phytoplankton dynamics in the region of 55–701S, 1701W 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– 611S) in early November. Seasonally enhanced levels of chlorophyll were found as far north as 581S, 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 B621S to 651S 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 (B651S), 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 (X0.2 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 (601S and 631S) 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 64.81S and 67.81S. 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 (64.81S and 67.81S 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. r 2002 Published by Elsevier Science Ltd. *Corresponding author. Fax: +1-808-956-9516. E-mail address: landry@soest.hawaii.edu (M.R. Landry). 0967-0645/02/$ - see front matter r 2002 Published by Elsevier Science Ltd. PII:S0967-0645(02)00015-2