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 (Metapenae us spp.) and sea-grass (Enhalus acoroides). Acid washing did not affec t the mean delta(13)C ratios for juvenile Metapenaeus moyebi and resul ted in only an ecologically insignificant change (0.3 parts per thousa nd) in mean delta(13)C ratios for larger Metapenaeus bennettae. In con trast, acid washing increased the mean delta(15)N signatures of shrimp tissue (similar to 3 parts per thousand) and decreased that of seagra ss (similar to 1.8 parts per thousand) to a degree that may confound t he interpretation of food webs. The increase in %C and %N in both shri mp and seagrass after acid washing suggests that the changes in isotop e ratios are due to loss of molecules comparatively low in C and N. Tr eating samples by acid washing also resulted in an increase in the var iation among individuals for both delta(15)N and delta(13)C, which wou ld lead to a loss of statistical power for testing differences between species, sites, or seasons.