Effects of sample preparation on stable isotope ratios of carbon and nitrogen in marine invertebrates: implications for food web studies using stable isotopes

Abstract

11 páginas, 5 figuras, 3 tablas. ; Trophic ecology has beneWtted from the use of stable isotopes for the last three decades. However, during the last 10 years, there has been a growing awareness of the isotopic biases associated with some pre-analytical procedures that can seriously hamper the interpretation of food webs. We have assessed the extent of such biases by: (1) reviewing the literature on the topic, and (2) compiling C and N isotopic values of marine invertebrates reported in the literature with the associated sample preparation protocols. The factors considered were: acid-washing, distilled water rinsing (DWR), sample type (whole individuals or pieces of soft tissues), lipid content, and gut contents. Twolevel ANOVA revealed overall large and highly signiWcant eVects of acidiWcation for both 13 C values (up to 0.9‰ decrease) and 15 N values (up to 2.1‰ decrease in whole individual samples, and up to 1.1‰ increase in tissue samples). DWR showed a weak overall eVect with 13 C increments of 0.6‰ (for the entire data set) or decrements of 0.7‰ in 15 N values (for tissue samples). Gut contents showed no overall signiWcant eVect, whereas lipid extraction resulted in the greatest biases in both isotopic signatures ( 13 C, up to ¡2.0‰ in whole individuals; 15 N, up to +4.3‰ in tissue samples). The study analyzed separately the eVects of the various factors in diVerent taxonomic groups and revealed a very high diversity in the extent and direction of the eVects. Maxillopoda, Gastropoda, and Polychaeta were the classes that showed the largest isotopic shifts associated with sample preparation. Guidelines for the standardization of sample preparation protocols for isotopic analysis are proposed both for large and small marine invertebrates. Broadly, these guidelines recommend: (1) avoiding both acid washing and DWR, and (2) performing lipid extraction and gut evacuation in most cases. ; This study has been Wnanced by the European Union project WADI (INCO: CE reference 015226) and by the Consejo Superior de Investigaciones CientíWcas (CEAB-CSIC). This work complies with the current laws of Spain. ; Peer reviewed