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

Abstract

During austral summer 1997, satellite imagery revealed enhanced chlorophyll associated with the Antarctic Polar Front at 170°W. 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 ∼200 to <100 m from late October to early November. Plankton assemblages on the southern side of the frontal jet (∼0°C waters) differed from those on the northern side (∼2°C) in enhanced relative importance of larger (>20 μm) 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 0.24 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.