PHOTOSYNTHETIC PICOPLANKTON IN FRENCH-POLYNESIAN ATOLL LAGOONS - ESTIMATION OF TAXA CONTRIBUTION TO BIOMASS AND PRODUCTION BY FLOW-CYTOMETRY

Picophytoplankton was studied by now cytometry in 11 Tuamotu (French P olynesia) atoll lagoons and in the surrounding ocean. The respective c ontribution of Prochlorococcus, Synechococcus and picoeukaryotes on bi omass and primary production was evaluated. Red fluorescence was demon strated to be a proxy for chlorophyll. The relative sizes of the 3 pic oplankton groups were estimated using forward light scattering measure d in Takapoto lagoon on living cells. The average diameters for Prochl orococcus, Synechococcus and picoeukaryotes were estimated as 0.62 +/- 0.08 (SD), 0.89 +/- 0.09 and 3.11 +/- 0.22 mu m, respectively. The lo west values occurred before sunrise and the highest in the afternoon. Cellular carbon content was estimated using C/biovolume ratios from th e literature. The average biomass for Prochlorococcus, Synechococcus a nd picoeukaryotes was calculated as 60 +/- 20, 178 +/- 52 and 4695 +/- 834 fg C cell(-1) respectively. Synechococcus formed the predominant group in terms of abundance and carbon biomass and had the highest pla nktonic primary production in most lagoons. As it is generally scarce in deep water with Limited light availability, its biomass contributio n was reduced in deep lagoons. Average lagoonal picoplankton abundance varied by a factor of 200 for the different populations and was affec ted by the geomorphology of the atolls. In very shallow lagoons, no ge neral trend could be observed, as the dominant group appeared to depen d on the water renewal rate within the lagoon. In the surface layer of the surrounding ocean the community structure was dominated by Prochl orococcus. However, the observed percentage of Synechococcus (>10%) is usual for the coastal zone. In the upper 120 m of ocean waters surrou nding Takapoto, the integrated picoplankton biomass (1242 mg C m(-2)) consisted of 65% Prochlorococcus, 1% Synechococcus and 34% picoeukaryo tes.