On the Effects of Acid Pre-treatment on the Elemental and Isotopic Composition of Lightly- and Heavily-calcified Marine Invertebrates

Carbonate removal using acids is a common practice in ecological studies. The effects, however, of acid pre-treatment on the elemental and isotopic composition of marine invertebrates as well as how these effects vary according to species’ carbonate content is little known. We examined the effects of acid pre-treatment on the elemental (%C, %N, C:N ratio (%C:%N)) and isotopic composition (δ13C, δ15N) of 28 lightly- and heavily-calcified species from Cnidaria, Mollusca, Arthropoda, Bryozoa, Echinodermata and Chordata. The present study showed that acid pre-treatment modified the elemental and isotopic composition of lightly- and heavily-calcified marine invertebrates. The shifts were clearly seen as a decrease in the %C and δ13C of heavily-calcified species while we did not detect a clear pattern for %N and δ15N (in both lightly- and heavily calcified species). Apart from carbonates, acid pre-treatment caused also the loss of organic compounds, thus confounding the interpretation of carbonate proxy (CP) -a widely used proxy for carbonate content. We recommend the use of CP solely with heavily-calcified species. For the first time it was shown that the use of δ15N values from acidified samples can introduce substantial bias in our perception about the number of trophic levels, the distribution of species and distribution of biomass across the trophic levels in a community. We have uncovered and elucidated previously unknown aspects and highlighted the challenge posed when predicting shifts in elemental and isotopic composition of species following acid pre-treatment. The present findings should be considered in future studies using acid pre-treatment as they can contribute to the optimum use of samples while avoiding bias in the interpretation of findings.