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

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  • 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 if diatoms and ciliate, 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 estimated 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 carbon per day and this 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 photoadaption (bleaching) during this time of increasing light level and water column stratification.
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  • Landry, M. R., Brown, S. L., Selph, K. E., Abbott, M. R., Letelier, R. M. , Christensen, S., Bidigare, R. R., and Casciotti, K., 2001, Initiation of the spring phytoplankton increase in the Antarctic Polar Front Zone at 170°W: Jour. of Geophysical Research, v. 106, p. 13,903-13,915.
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