Acid washing effect on elemental and isotopic composition of whole beach arthropods: Implications for food web studies using stable isotopes

Inorganic carbon removal through acidification is a common practice prior to isotopic analysis
of macroinvertebrate samples. We have experimentally tested the effect of acidification
on the elemental and isotopic composition of a range of beach arthropod species.
Acidification resulted in a significant depletion of 7.7% and 1.2% on average for carbon
and nitrogen, respectively, suggesting that acid washing affects body carbon compounds
other than carbonates. With a few exceptions, d13C and d15N showed no changes following
1 N HCl attack. Based on those exceptions, our results show that only those samples with
a high CaCO3 content result in impoverished 13C as a consequence of acidification. Those
suspected to be carbonate-free are not significantly affected. Concerning d15N values,
only high carbonate species were affected when treated with HCl. As a standard protocol,
it is recommended to acidify only carbonate-rich samples prior to d13C analyses. When
possible, muscle tissue samples should be used instead of the entire organism.