Stable isotopes and fatty acids used as biomarkers to distinguish among Antarctic amphipods trophic guilds

Peracarid crustaceans and amphipods in particular are an important group in the Southern Ocean and one of the most diverse in the macrozoobenthos (Jazdzewski et al., 1991).
As a part of a multidisciplinary study of the amphipods ecological roles in Antarctic benthic systems (De Broyer et al., 2001, Nyssen et al., 2002), about 150 specimens belonging to 25 species of 10 of the most common amphipod families occurring in the Southern Ocean have been involved in this study of amphipod trophic patterns. Beside “classical” stomach content analysis or field observations, the use of naturally occurring stable isotopes of carbon (δ13C) and nitrogen (δ15N) has recently provided new insights into food web ecology (Hobson & Welch, 1992, Lepoint et al., 2000). This method is based on the direct relationship established between the isotopic signature of an organism and that of its preys (DeNiro & Epstein, 1978, 1981, Peterson & Fry, 1987). Nitrogen-15 typically shows a step-wise increase with trophic level within a food chain (Cabana & Rasmussen, 1994). Closer to the value of the diet, carbon-13 is preferentially used to assess the relative proportion of potential primary sources in a trophic web (ex.: pelagic vs benthic contribution to food intake) (Dauby et al., 1998, Hobson et al., 1995). Furthermore, for several species, the lipid signature – which has already been used successfully to help understand marine trophodynamics (Graeve et al., 2001, Nelson et al., 2001, Phleger et al. 1998) – and more particularly the fatty acid composition has been investigated as trophic biomarkers to reveal more precisely to which trophic guild they belong to.