MEPS 408:195-205 (2010)  -  DOI: https://doi.org/10.3354/meps08518

ABSTRACT: Stable isotope analyses (SIA) of carbon and nitrogen are used routinely in food-web studies to determine diet and trophic position. We tested several common assumptions of SIA by rearing juvenile mummichog Fundulus heteroclitus on 5 isotopically distinct diets under controlled laboratory conditions. We determined the effect of diet type and lipid extraction on δ¹³C and δ¹⁵N fractionation between diet and muscle. We also examined fractionation of δ¹³C between otolith and both diet and muscle. Both ¹³C and ¹⁵N were enriched from diet to fish muscle, but the degree of fractionation differed among diets. Carbon isotope fractionation from diet to fish muscle exceeded assumed values of <1‰ and ranged from 1.2 to 3.9‰, while nitrogen fractionation ranged from 2.7 to 7.8‰. Extracting lipids from fish muscle increased both δ¹³C and δ¹⁵N by approximately 1‰. Lipid extraction also increased variation in treatment means for δ¹⁵N, but not δ¹³C. Otoliths were enriched in ¹³C compared to both diet and fish muscle. Bulk otolith δ¹³C values were strongly correlated with muscle tissue, and reflected the same total change in δ¹³C observed among diet treatments. It was tempting to conclude that otoliths were accurately recording δ¹³C values of the diet. However, more information is required on the effects of diet, metabolic rate, and δ¹³C of ambient dissolved inorganic carbon on otolith δ¹³C before these structures can be used to reconstruct diet histories of individual fish.

KEY WORDS: Diet · Food web · δ¹³C · δ¹⁵N · Carbonate · Calcified structures · Isotope enrichment