The objective of this research is to investigate the relative importance of environmental vs. biological effects on the &13C of POC in the surface layer of the open ocean. The distribution of &13CPOC in the ocean as a function of latitude can not be explained by any single cause, such as temperature, concentration of dissolved carbon dioxide (?CO2!aq) or lipid content. Multicausal explanations will have to be sought to explain this distribution. The effects of the composition of the phytoplankton community on the distribution of &13CPOC in the ocean will be investigated since physiological characteristics of 13C fractionation vary among groups of microalgae and the composition of the phytoplankton community varies as a function of latitude in the ocean. Patterns of &13C fractionation in different groups of microalgae in the laboratory and in the field will be documented as a function of growth rate, cell size and ?CO2!aq. In the field carotenoids will be used as chemosystematic indicators for the composition of the phytoplankton community and measure the &13C of carotenoids to determine the &13C of the different groups of microalgae. The relationship between &13CPOC and the &13C of carotenoids will be established for laboratory cultures. A comparison of the variations of &13CPOC and &13Ccarotenoid within the different microalgal groups along a N-S transect in the North Atlantic will establish the relative effect of phytoplankton community composition on the &13C of plankton in the ocean.
