Effects of Acid Washing on Stable Isotope Ratios of C and N in Penaeid Shrimp and Seagrass: Implications for Food-Web Studies Using Multiple Stable Isotopes

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 $\delta^13 C$ ratios for juvenile Metapenaeus moyebi and resulted in only an ecologically insignificant change (0.3%.) in mean $\delta^13 C$ ratios for larger Metapenaeus bennettae. In contrast, acid washing increased the mean $\delta^15 N$ signatures of shrimp tissue $(\sim 3%.)$ and decreased that of seagrass $(\sim 1.8%.)$ 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 $\delta^15 N and \delta^13 C$, which would lead to a loss of statifical power for testing differences between species, sites, or seasons.