Sample acidification effects on carbon and nitrogen stable isotope ratios of macrofauna from a <i>Zostera noltii</i> bed

Acidification treatment has been used in most stable isotope studies to eliminate inorganic non-dietary carbon, allowing d13C measurement of organic carbon needed for trophic studies. Because only limited information about the acidification effects on isotopic ratios exists in the literature, we provide an assessment of such effects for a wide range of benthic macroinvertebrates from estuarine intertidal sediments with sparse Zostera noltii Hornem. vegetation. Our results revealed only few significant d13C shifts for macrobenthos; however, d13C decrease after acidification was substantial for several species. We conclude that acidification is not necessary for removing calcium carbonate in a majority of macrobenthos, but we also suggest preliminary testing before deciding it can be omitted. The magnitude of the d15N shifts was substantial for some species and variability in d15N values of replicate acidified samples exceeded that of non-acidified samples. Therefore, sample acidification should be avoided for d15N determination. Moreover, acidification effects vary considerably among species belonging to a single class, rendering generalisations from single-species information potentially spurious