Mr. Landry et al., Initiation of the spring phytoplankton increase in the Antarctic Polar Front Zone at 170 degrees W, J GEO RES-O, 106(C7), 2001, pp. 13903-13915
During austral summer 1997, satellite imagery revealed enhanced chlorophyll
associated with the Antarctic Polar Front at 170 degreesW. Phytoplankton g
rowth conditions during the early stages of the spring increase were invest
igated on the Antarctic Environment and Southern Ocean Process Study Survey
I cruise using flow cytometry (FCM) and microscopy to characterize communi
ty biomass, composition and biological stratification and dilution experime
nts 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 souther
n side of the frontal jet (similar to0 degreesC waters) differed from those
on the northern side (similar to2 degreesC) in enhanced relative importanc
e of larger (> 20 mum) cells, greater contributions of diatoms and ciliates
, and a twofold higher ratio of protistan grazers to photoautotrophs. Phyto
plankton community growth rates from incubations at 10 and 23% of surface i
ncident light showed good agreement between high performance liquid chromat
ography 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 typic
ally 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 dow
nstream 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.