『Abstract
　 The temporal variation in the stable carbon and nitrogen isotope ratios of particulate organic matter (POM) in the Rhone River was investigated on a monthly basis during a 2-year period (2004-2005). In spite of high month-to-month variation, interannually consistent seasonal trends appeared, with significantly lower δ¹³C (＜-28.2‰) in spring than in the other seasons. In contrast, δ¹⁵N did not exhibit significant temporal variation. In spring and summer, high chlorophyll a and b concentrations were associated with low C/N values (＜8) and a high percentage of organic carbon (％C) and organic nitrogen (％N), testifying to high development of autochthonous riverine phytoplankton (mainly diatoms and chlorophytes). In fall and winter, higher δ¹³C (＞-27.2‰) and C/N (＞8) values, and lower ％C, ％N, and chlorophylls concentrations indicated the predominance of allochthonous terrestrial detritus material in the river POM. The lower δ¹³C values recorded in spring-summer, when the phytoplankton biomass was high, were related to the lower carbon isotopic signatures of freshwater diatoms and chlorophytes compared to those of terrestrial plants. Overall, Rhone River POM was mainly composed of terrestrially derived material (90％), with autochthonous phytoplankton representing only 10％ as a mean, in spite of a higher mean contribution of phytoplankton (27％) to river POM in summer.


Keywords: Chlorophyll; C/N; Freshwater organic matter; Gulf of Lions; Stable isotopes』

Introduction
Materials and methods
　Studied site and sampling procedure
　Stable isotope analysis
　HPLC pigment analysis
　Data analysis
Results
　Seasonal variations
　Quality of the organic matter
Discussion
　Seasonal patterns
　Phytoplankton importance and composition
　River POM export to the sea
Conclusion
Acknowledgements
References