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Prochiorococcus sp. would consist of changing the number of photosynthetic units per cell' and, thereby, the minimalturnover time for in vivo electron transport from water to CO2 (t). As a result, the saturation parameter Ek, would tightlycovary with growth irradiance. Also, because the zeaxanthin cellular content in Prochiorococcus sp. is quite stable, theratio of zeaxanthin-to-dv-chl a would also covary with irradiance. A study in the Atlantic Ocean already showed that, tosome extent (i.e. by a factor of 3), the maximum quantum yield of carbon fixation, , isproportional to the ratio of non-photosynthetic pigments (mostly zeaxanthin) to all phytoplankton pigments (hereafter denoted the "non-photosyntheticpigment index; NPP index)2. In turn, the NNP index is linearly correlated with the mean irradiance in the mixed layer.The present study aims at documenting the above mentioned relationships in the equatorial and sub-equatorial Pacific,where prokaryotic photoautrotrophs dominate the phytoplankton community.2. MATERIAL AND METHODSData were collected in the equatorial Pacific during the OLIPAC (JGOFS-France) cruise, in November 1994. Samplingwas conducted along 1500 W, from 16° S to l N. For each seawater sample, the following variables were measured:nanomolar concentration of NO2 and NO3, pigment composition using HPLC, in vivo light absorption by phytoplankton,photosystem 2 energy conversion efficiency (F,/F) measured by FRR fluoremetry, and physiological parameters derivedfrom P vs.E experiments. The maximum quantum yield of carbon fixation was derived from the following expression

Chlorophyll-specific absorption coefficients of phytoplankton (aph) were determined in the upper 200 m along 150 degrees W in the equatorial and subequatorial Pacific, during the OLIPAC-JGOFS FRANCE cruise. A spectral reconstruction technique using the HPLC pigment information was used to partition aph into the contributions of photosynthetic pigments (aps) and non-photosynthetic pigments (anps). The values of aph were observed to decrease from the oligotrophic waters of the sub-equatorial area to the mesotrophic waters of the equatorial area, and from the surface to deep waters. These variations were primarily, but not exclusively, caused by changes in the concentrations of non-photosynthetic pigments. The level of pigment packaging was also variable, both horizontally as a result of changes in populations, and vertically as a result of photoacclimation. In comparison with aph, the chl-specific absorption coefficients of photosynthetic pigments (aps) exhibited a reduced range of variation with depth and from site to site. The variations in aps originating from the package effect were partly compensated by variations in the concentrations of photosynthetic pigments. In agreement with a previous study, we conclude that aps is less dependent on environmental parameters than aph. In addition, our results provide evidence that the variability in aps cannot be neglected. The use of aps instead of aph in light- photosynthesis models might present the advantage of eliminating the variability associated with non- photosynthetic pigments.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Using a highly resolved LTER database collected near Palmer Station, Antarctica, from 1991-1994, the variability in the column photosynthetic cross section ((Psi) , m2, g Chl a-1) was analyzed. For the whole dataset, (Psi) had an average value of 0.0695 m2 g Chl a-1 but extreme values extended over a 50-fold range. A six-fold variation in (Psi) was observed with time of year and was strongly associated with the high seasonality in incident irradiance characteristic of these polar sampling sties. Variability in daily incident irradiance as influenced by cloudiness and variation in chlorophyll content were responsible for an additional two-fold variation in (Psi) . Finally, the taxonomic dependency of (Psi) was demonstrated for the first time. For identical chlorophyll content and surface irradiance, mean (Psi) value of 0.114 +/- 0.051 m2 g Chl a-1 were recorded for diatom blooms and 0.052 +/- 0.011 m2 g Chl a-1 for cryptophyte- dominated populations. Results illustrate the validity of (Psi) -based approaches for estimating primary production for the Southern Ocean but emphasize the need to address taxon- specific photophysiology to better estimate primary production on smaller spatio-temporal scales.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.