Résumé
The structure, dynamics and productivity of marine ecosystems depend mainly on their underlying food webs. In temperate marine ecosystems, specifically in the English Channel, strong seasonality, habitat diversity and variability in the sources of available organic matters are controlling factors affecting the trophodynamics of these networks. Exploitation through fisheries may also alter these food webs. During the last decade, the study of trophic ecology has greatly benefited from the use of stable isotope analysis (SIA) that integrates diet information over long time periods and complements gut content analysis. Applied in complex ecosystems, such as the English Channel, SIA may help to detect nutrient pathways and trophic links. Stable isotopic analysis can also be used to determine whether trophic level varies with fish size. However, only few studies focused on the Eastern English Channel food web and so far, none was based on the use of stable isotope analysis. The objective of this study is to determine how trophic level depends on individual size within different functional groups and habitats of the English Channel. A sampling survey was conducted in October 2009 along four habitats differing in terms of biotic and abiotic conditions. Fish species and benthic fauna were sampled using a Grande Ouverture Verticale (GOV) demersal trawl. Six fish species, representing several functional guilds were studied to determine size-dependence of trophic level by means of stable nitrogen isotopes. The nitrogen stable isotope ratios of the base of the food web, as measured from primary consumers (here the phytoplankton-feeding bivalve mollusc Aequipecten opercularis), were accounted for in the estimates of fish trophic levels.