Initiation of the spring phytoplankton increase in the Antarctic Polar Front Zone at 170°W

During austral summer 1997, satellite imagery revealed enhanced chlorophyll associated with the Antarctic Polar Front at 170 degreesW. Phytoplankton growth conditions during the early stages of the spring increase were investigated on the Antarctic Environment and Southern Ocean Process Study Survey I cruise using flow cytometry (FCM) and microscopy to characterize community biomass, composition and biological stratification and dilution experiments to estimate growth and grazing rates, Physical and biological measures showed a general shoaling of mixed layer depth from similar to 200 to < 100 m from late October to early November. Plankton assemblages on the southern side of the frontal jet (similar to0 degreesC waters) differed from those on the northern side (similar to2 degreesC) in enhanced relative importance of larger (> 20 mum) cells, greater contributions of diatoms and ciliates, and a twofold higher ratio of protistan grazers to photoautotrophs. Phytoplankton community growth rates from incubations at 10 and 23% of surface incident light showed good agreement between high performance liquid chromatography estimates of chlorophyll a (Chl a) (0.20 d(-1)) and FCM cell-based (0.21 d(-1))results. Fucoxanthin-based estimates for diatoms were 0.2-1 d(-1). Mean estimates of microzooplankton grazing from the three phytoplankton measures were 0.16, 0.12, and 0.11 d(-1), respectively. Heterotrophs typically consumed 40-100% of their body carbon per day and thus presumably grew at rates similar to phytoplankton. The low net rates of Chl a increase in shipboard bottle incubations (0.04 d(-1)) were consistent with the slow downstream accumulation of phytoplankton biomass (0.03 d(-1)) as measured with instrumented Lagrangian drifters through the month of November. Both were slightly less than the net rate estimates from SeaSoar surveys (0.05 d(-1)) because of the effects of pigment photoadaptation (bleaching) during this time of increasing light level and water column stratification.