Stable carbon and nitrogen isotope quantitative structural assessment of dominant species from the Vaccarès Lagoon trophic web (Camargue Biosphere Reserve, France)

The Vaccare`s Lagoon (Camargue Biosphere Reserve), a complex brackish ecosystem, is the natural habitat for numerous freshwater, marine and euryhaline species forming a particularly intricate food web. The main objectives of this study were to describe its trophic relations and investigate factors influencing its structure and dynamics. The combined stable C and N isotope method was used to establish a quantitative assessment of the trophic status of Vaccare` s organisms. Although the levels of d15N and d13C showed large intraspecific and interspecific variations, the isotopic signatures of the species assessed revealed a general trend of 15N enrichment with trophic level. Distribution of the biota into four trophic compartmentsddepositivore, zooplanktivore, predator, and top-predatordwas corroborated by the general analysis of the relationship between d13C and d15N. Analysis of seasonal and annual isotopic variations showed that natural environmental changes (increase in food abundance, climatic and salinity variations) as well as individual metabolic capacities would affect the relative stability of an organism’s trophic position. No significant correlation was found between isotopic variation and size and growth in fishes except for the eel population where the positive correlation between these two parameters confirmed intra-population trophic variations. Finally, separately examined species exhibited significant, although incoherent, correlation between their isotopic signature and lipid content. This study confirms the efficiency of the isotopic approach in trophic studies and highlights the need for further investigations of anthropic environmental modifications occurring in this protected area and frequently disturbing its food web.