Stable isotope ratios of six size fractions of zooplankton (80 to >2000 mm) were analyzed seasonally in 2009-2010 at the SOMLIT site in the Bay of Marseille. Isotopic ratios generally increased with zooplankton size. The highest delta N-15 values were observed in the 1000-2000 mm fraction. The largest size class (>2000 mu m), dominated by gelatinous plankton, had lower delta N-15 values due to the low isotopic signatures of most of these organisms. In the larger size fractions (>1000 mu m), isotopic ratios were measured at the taxon level. Brachyuran, stomatopod, teleost and cephalopod larvae showed the highest delta N-15 values, and salps and pteropods the lowest ones. Lower values of both delta C-13 and delta N-15 were recorded in winter and spring than in summer and autumn for all fractions. Seasonal variations were consistent with the fluctuations of environmental parameters (temperature, nutrients, Chl a concentration) and were related to phytoplankton and seawater particulate organic matter (POM) composition. Stable isotope and flow cytometry analysis of water POM indicated that sewage wastewater particles were mixed with marine phytoplankton at the SOMLIT site and transferred up into the zooplanktonic food web.
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