The effects of acidification on the stable isotope signatures of marine algae and molluscs

Carbon (13C) and nitrogen (15N) stable isotope analysis has become increasingly important in the study of energy flow and tropho-dynamics in many ecosystems. Prior to analysis, samples are often pre-treated with acids to remove inorganic carbonates which may bias the results. The effects of pre-analysis acidification on isotopic values are, however, still poorly understood for marine producers (e.g. algae and cyanobacteria), and consumers (e.g. molluscs), which may confound the comparability of different studies. In this study, such effects (untreated vs. decalcified samples) were examined at two different sampling periods (summer and winter). Acidification did not seem to affect the isotopic composition of consumers, but reduced both δ13C and δ15N of producers. This effect was consistent for the two sampling periods, although both producers and consumers had more enriched δ13C and δ15N values in summer. Acidification had the most distinct, negative effect on the isotopic values of samples which had low carbonate contents. It is, therefore, important to be aware of temporal variations in sample isotopic values and especially the effects of sample treatment when attempting to compare different studies. As an attempt to standardize protocols, it is recommended that only acid-washing carbonate-rich samples is adopted as the most appropriate pre-analysis treatment.