SOURCES OF VARIABILITY IN THE COLUMN PHOTOSYNTHETIC CROSS-SECTION FORANTARCTIC COASTAL WATERS

Using a highly resolved Long Term Ecological Research (LTER) database collected near Palmer Station, Antarctica, from 1991 to 1994, the vari ability in the column photosynthetic cross section (Psi, m(2) g Chl a (-1)) was analyzed. The relationship between the daily integrated prim ary production rates versus the product of surface irradiance (Q(PAR)( O-2(+)) and the integrated chlorophyll content (down to 0.1% Q(PAR)(0( +)) gave a Psi value of 0.0695 m(2) g Chl a(-1) (r(2) = 0.85, p < 0.0 01, n = 151) which is similar to those determined for temperate and tr opical seas. However, the average value of single Psi estimates is hi gher (0.109 +/- 0.075 m(2) g Chl a(-1)) with extreme values extending over a fiftyfold range (0.009-0.488 m g Chi a(-1)). The possible drive rs of this variability are analyzed in detail, considering variables w hich are presently used in biooptical models (e.g., surface irradiance and chlorophyll content) and those which are not (taxonomic compositi on). A sixfold variation in Psi was observed with time of year and st rongly associated with the high seasonality in incident irradiance cha racteristic of these polar sampling sites. Variability in daily incide nt irradiance as influenced by cloudiness and variation in chlorophyll content were responsible for an additional twofold variation in Psi. Finally, the taxonomic dependency of Psi was demonstrated for the fi rst time. For identical chlorophyll content and surface irradiance, me an Psi values of 0.114 +/- 0.051 m(2) g Chl a(-1) were recorded for d iatom blooms and 0.053 +/- 0.011 m(2) g Chi a(-1) for cryptophyte-domi nated populations. Results illustrate the validity of Psi-based appro aches for estimating primary production for the Southern Ocean but emp hasize the need to address taxon-specific photophysiology to better es timate primary production on smaller spatio-temporal scales.