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Carbon and nitrogen stable isotope ratios of particulate organic matter (POM) in surface water and 63-200 μm-sized microphytoplankton collected at the fluorescence maximum were studied in four sites in the Gulf of Lions (NW Mediterranean), a marine area influenced by the Rhone River inputs, in May and November 2004. Some environmental (temperature, salinity) and biological (POM, Chlorophyll a and phaeopigments contents, phytoplankton biomass and composition) parameters were also analysed. Significantly different C and N isotopic signatures between surface water POM and microphytoplankton were recorded in all sites and seasons. Surface water POM presented systematically lower δ 13 C ( 4.2‰) and higher δ 15 N ( 2.8‰) values than those of microphytoplankton, due to a higher content of continental and detrital material. Seasonal variations were observed for all environmental and biological parameters, except salinity. Water temperature was lower in May than in November, the fluorescence maximum was located deeper and the Chlorophyll a content and the phytoplankton biomass were higher, along with low PON/Chl a ratio, corresponding to spring bloom conditions. At all sites and seasons, diatoms dominated the phytoplankton community in abundance, whereas dinoflagellate importance increased in autumn particularly in coastal sites. C and N isotopic signatures of phytoplankton did not vary with season. However, the δ 15 N of surface water POM was significantly higher in November than in May in all sites likely in relation to an increase in 15 N/ 14 N ratio of the Rhone River POM which influenced surface water in the Gulf of Lions. As it is important to determine true baseline values of primary producers for analysing marine food webs, this study demonstrated that C and N isotopic values of surface water POM cannot be used as phytoplankton proxy in coastal areas submitted to high river inputs.

Levels of polychlorinated biphenyls (PCBs) and polybrominated diphenyl ethers (PBDEs) were determined in the muscle and liver of European hakes (Merluccius merluccius, L.) from the Mediterranean and Atlantic coasts (France). The liver was always the most contaminated organ by both groups of compounds. Hakes from the Gulf of Lions (Mediterranean) present concentrations of PCBs and PBDEs 1.6 to 13.5 times higher than hakes of a similar size from the Bay of Biscay (Atlantic). Although PCBs present higher levels compared to PBDEs, their concentrations were significantly correlated. The contaminant patterns in hakes were similar in the two studied areas, and were dominated by higher chlorinated PCBs (CB153, 138, 180 and 170), and lower brominated PBDEs congeners (BDE47, 100, 49, and 99). Concentrations globally increase with hake size, and males appear to be more contaminated than females of the equivalent size. However, the compound levels also depend on the physiological status of fish.

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