We investigated the effects of acid washing on the carbon and nitrogen composition and stable isotope ratios of C and N in shrimp (Metapenaeus spp.) and seagrass (Enhalus acoroides). Acid washing did not affect the mean δ13C ratios for juvenile Metapenaeus moyebi and resulted in only an ecologically insignificant change (0.3‰) in mean δ13C ratios for larger Metapenaeus bennettae. In contrast, acid washing increased the mean δ15N signatures of shrimp tissue (∼ 3‰) and decreased that of seagrass (∼1.8‰) to a degree that may confound the interpretation of food webs. The increase in %C and %N in both shrimp and seagrass after acid washing suggests that the changes in isotope ratios are due to loss of molecules comparatively low in C and N. Treating samples by acid washing also resulted in an increase in the variation among individuals for both δ15N and δ13C, which would lead to a loss of statistical power for testing differences between species, sites, or seasons.

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