Photosynthesis-irradiance relationships of antarctic phytoplankton during austral winter

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Abstract
Results from photosynthesis-irradiance (P-I) experiments were used to examine photosynthetic adaptations of phytoplankton populations from the Bransfield Strait region in Antarctica during austral winter. Chlorophyll a concentrations during this period were low, ranging from 0.04 to 0.33 .mu.g l-1 in the mixed layer. Average photosynthetic efficiency (.alpha.) for surface populations was 0.021 mg C (mg chl)-1 h-1 (.mu.E m-1 s-1)-1 (.+-. 0.01), and mean maximum photosynthetic rates (PmB) were 1.19 mg C (mg chl)-1 h-1 (.+-. 0.6). No apparent differences were found between photosynthetic parameters for open ocean regions or within the Bransfield Strait, and there was little evidence for adaptation within the water column. Integrated production within the euphotic zone was low, 1 to 7 mg C m-2 d-1, when estimated either from in situ measurements or from P-I relationships. These low primary productivity values support the belief that the waters within the Antarctic remain a region of extremely low productivity during the winter, despite conditions such as water column stratification and saturating surface irradiance levels which could potentially support phytoplankton growth.