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ABSTRACT. The 14C labelling of chlorophylls and carotenoids is Increasingly used to evaluate phytoplanktomc biomass and growth rates in oceanlc systems. Hlgorous testing of the technique in the laboratory, however, is necessary prior to its application in the field. A Mediterranean clone of Prochlorococcus, a photosynthetic prokaryote whlch is a n important component of the aulotrophic b~omass in o l~g o t r o p h~c environments, was subjected to shifts in light intensity. Particulate organlc carbon (POC) was monitored by CHN analysis, pigments by HPLC and frochlorococcus and heterotrophlc bacteria concentrations by flow cytometry. Using a combination of HPLC and on-line rad~oact~vlty detection, I4C labelling kinetics of divinyl-chlorophyll a (DV-chl a) and zeaxanthin were follo\.ved Prochlorococcus changed ~t s DV-chl a content markedly in response to change in light ~ntenslty, but not its zeaxanthin content, which remained nearly constant around 1.07 fg cell-' regdrdless of the ~rradiance. P~g m e n t synthes~s rates were correctly est~rnated from their '"C incorporation rates whatever the light level. From POC measurcmcnts and cell concentrations, the Prochlorococcus carbon content was estimated to be 49 fg C cell-l. bioreover, under both constant and shifted (high to low and vlce versa) light conditlons, Prochlorococcus growth rate (as computed from variations in cell dens~ty) was much better estimated from zeaxanthin than fl-orn DV-chl a labelling rates.

Benedetti et al. Temporal Variability of Plankton in the Iroise Sea copepods (members of the Calanidae) are more abundant in spring than in summer and fall. We propose several hypotheses to explain the observed temporal patterns and we underline their importance for understanding the dynamics of other components of the food-web (such as sardines). Our study is a first step toward the inclusion of the planktonic compartment into the planning of the resources and diversity conservation within the Marine Protected Area.