Stable isotope food web studies: a case for standardized sample treatment

Enrichment of the stable isotopes 13 C and 15 N across trophic levels is a commonly used tool in studies on organic matter flow and food webs. However, there is still no accepted standard for pre-analysis sample preparation. Thus, potential methodological bias in single studies may hamper comparability and scalability of data from different sources. Sample CaCO 3 content introduces a positive bias in 13 C measurements and a negative bias in 15 N measurements. The acidification of samples to remove inorganic carbonate significantly reduces both 13 C and 15 N. As a standard procedure we recommend (1) acidifying samples with as little hydrochloric acid (HCl) as possible using the drop-by-drop technique, and (2) restraining from rinsing after HCl application.