| Summary: | Abstract. The response of phytoplankton to variations in the light regime was studied during the VULCAN and ACDA cruises in the Antarctic. Unenriched batch cultures of 12 -19 days' duration reached chl concentrations of 10-50 pg" 1 and exhibited exponential growth rates, with the maximal rate being 0.41 doubl. day" 1 . Ice edge algae exhibited maximum growth rates at photon flux densities (PFD) of 30-100 /iE m"^" 1 , and the growth rate was reduced by about 30% at 500-1000 ME m^s" 1 . The chl/C ratio ranged between 0.004 and 0.018, with the lowest ratios at PFDs above 500 p£ m~2s~'. Chl/C ratios were also below maximum at PFDs below 40-50 y.E m~2s~'. The C:N:P ratios were close to the Redfield ratios; the Si/C ratio averaged 0.16 (atoms), and the ATP/C ratio averaged from 0.0024 to 0.0050 in different culture series. When thawed after having been frozen for 10 days, shade-adapted cultures were in a much better condition than sun-adapted ones. P versus /data showed that the maximum assimilation number varied from 0.75 to 4.4 /ig C (;ig chO^'h" 1 . It varied inversely with the chl/C ratio; therefore the maximum carbon turnover rate varied little between samples (0.024-0.035 h" 1 ). Low biomass communities exhibited relatively high values for a (the initial slope of P versus / curves), low values for 7^, (160-330 pE m~2s"'), and they were susceptible to photoinhibition. In contrast, communities dominated by Odontella weissflogii exhibited low values for a, a high value for /^ (560 pB m~2s~l), and they tolerated high PFDs. The photoadaptational status of the phytoplankton in natural water samples is discussed relative to the profile of water column stability and mixing processes.
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