Variation in trophic shift for stable isotope ratios of carbon, nitrogen, and sulfur

Use of stable isotope ratios to trace pathways of organic matter among consumers requires knowledge of the isotopic shift between diet and consumer. Variation in trophic shift among consumers can be substantial. For data from the published literature and supplementary original data (excluding fluid-feeding consumers), the mean isotopic shift for C was + 0.5 +/- 0.13parts per thousand rather than 0.0parts per thousand, as commonly assumed. The shift for C was higher for consumers analyzed as muscle (+ 1.3 +/- 0.30parts per thousand) than for consumers analyzed whole (+ 0.3 +/- 0.14parts per thousand). Among consumers analyzed whole, the trophic shift for C was lower for consumers acidified prior to analysis (- 0.2 +/- 0.21parts per thousand) than for unacidified samples (+ 0.5 +/- 0.17parts per thousand). For N, trophic shift was lower for consumers raised on invertebrate diets (+ 1.4 +/- 0.21parts per thousand) than for consumers raised on other high-protein diets (+ 3.3 +/- 0.26parts per thousand) and was intermediate for consumers raised on plant and algal diets (+ 2.2 +/- 0.30parts per thousand). The trophic shift for S differed between high-protein (+ 2.0 +/- 0.65parts per thousand) and low-protein diets (45 +/- 0.56parts per thousand). Thus, methods of analysis and dietary differences can affect trophic shift for consumers; the utility of stable isotope methods can be improved if this information is incorporated into studies of trophic relationships. Although few studies of stable isotope ratios have considered variation in the trophic shift, such variation is important because small errors in estimates of trophic shift can result in large errors in estimates of the contribution of sources to consumers or in estimates of trophic position.