Effects of acidification on carbon and nitrogen stable isotopes of benthic macrofauna from a tropical coral reef
Résumé
Stable isotope analyses are widely used to determine trophic levels in ecological studies. We have investigated the effects of carbonate removal via acidification on the stable carbon and nitrogen isotopic composition of 33 species of tropical benthic macrofauna, and we report guidelines for standardizing this procedure for higher taxa in tropical coral reef ecosystems. Many tropical benthic invertebrates are small in size, and therefore body tissue isolation (separating organic carbon from inorganic structures) is difficult and time-consuming. Literature reviews of invertebrate studies show a lack of consistent procedures and guidelines for preparation techniques, especially for carbonate removal via acidification of whole individuals. We find that acidification decreases the δ13C values of samples containing carbonate, with shifts ranging from 0.21 to 3.20‰, which can be related to CaCO3 content (assessed by a carbonate proxy), justifying acid pre-treatment. Carbonate-containing taxa benefiting from acidification included Amphinomida, Terebellida (Annelida), Anomura, Brachyura, Caridea, Amphipoda, Tanaidacea (Arthropoda) and Edwardsiida (Cnidaria). The δ13C shifts of samples containing no carbonate varied up to 0.02 ± 0.20‰. As this induced δ13C shift was lower than the range of an average trophic level shift (0.5 to 1‰), we conclude that acid pre-treatment is unnecessary. Carbonate-free taxa consisted of Eunicida, Phyllodocida (Annelida) and Mollusca. We note minimal impact of acidification on δ15N values except for Brachyura, which showed a shift of 0.83 ± 0.46‰, which is still lower than a single trophic level shift (2.9-3.8‰). We conclude that for trophic level studies, both the δ13C and the δ15N of carbonate-rich macrofauna can be determined from the same acidified sample. x-audience © 2008 John Wiley & Sons, Ltd.