Abstract :
[en] Stable isotope analyses (SIA) of carbon (δ13C) and nitrogen (δ15N) are now a common tool to investigate the trophic ecology of organisms and the functioning of food webs. After field sampling, organism tissues are usually preserved frozen or dried before subsequent analysis. However, museum samples, or samples not initially collected for SIA, are often fixed in preservative solutions such as formaldehyde or alcohol. These preservation methods may alter stable isotope ratios in tissues, making them difficult to interpret. Consequently, these effects should be quantified. Here, we investigated long-term effects of common preservation methods on δ13C and δ15N values of sea stars. In particular, we tried to assess the influence of preservation by comparing mean isotopic values and key isotopic niche metrics (standard ellipse area SEAC and SEAB, niche overlap) of fresh and preserved samples. Formaldehyde preservation significantly affected δ13C values, resulting in strong reduction of the overlap between the isotopic niche of the fresh samples and those of the preserved samples. However, after being altered, the mean δ13C value did not change over time. Consequently, a correction factor that can be used for any duration of preservation was designed. δ15N values changed across time for all methods except freezing but this may be the result of a higher intra-individual variability of δ15N values. Bayesian estimations of the standard ellipse areas (SEAB) of preserved and fresh samples were not significantly different, except for the samples preserved 6 months in alcohol. Nevertheless, these changes lead to reduction of the overlap between the isotopic niche of the fresh samples and those of the preserved samples across time. Future results will determine whether this can be generalised to sea stars preserved for a longer time. In particular, they will contribute determine whether museum samples, which have been collected during period when environmental conditions were different from today, may be used to study the past functioning of food webs.
