ABSTRACT
Stable isotopes are becoming a standard analytical tool in food web ecology. Differences in carbon and nitrogen isotope ratios between consumers and their diet provide information on energy flows, nutrient sources, and trophic relationships. Typically, carbon provides information on the primary energy source (e.g., benthic vs. pelagic photosynthesis), while nitrogen allows discrimination among trophic levels. Relative enrichment with increasing trophic level often allows a better interpretation of dietary relationships than gut content analysis alone because stable isotopic ratios record material that is actually assimilated (Michener and Schell, 1994). A recent survey has shown an average enrichment of 0.05‰
±
0.63
δ
C and 3.49‰
±
0.23
δ
N in field studies (Vander Zanden and Rasmussen, 2001), and these values are similar to the frequently used average trophic fractionation values of 1‰
δ
C and 3.4‰
δ
N (Minagawa and Wada, 1984; Michener and Schell, 1994). The ease of stable isotope analyses makes them an appealing tool in ecology, but both sampling design and interpretation of the results should be undertaken carefully (Gannes et al., 1997; O’Reilly et al., 2002). An isotopic ratio of an organism represents its diet, but it should be remembered that this isotopic value is also time specific and is an average ratio related to tissue turnover rate and the life of the organism.
