Seasonal variation in stable C and N isotope ratios of the Rhone River inputs to the Mediterranean Sea (2004-2005)
Résumé
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 delta(13)C (<-28.2aEuro degrees) in spring than in the other seasons. In contrast, delta(15)N 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 delta(13)C (>-27.2aEuro degrees) 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 delta(13)C 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.