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 sea-grass (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 parts per thousand) in mean delta(13)C ratios for larger Metapenaeus bennettae. In contrast, acid washing increased the mean delta(15)N signatures of shrimp tissue (similar to 3 parts per thousand) and decreased that of seagrass (similar to 1.8 parts per thousand) 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 statistical power for testing differences between species, sites, or seasons.