Decomposition-Induced Change in Stranded Megafauna: Decoupling Postmortem Impacts from the Isotopic Niche

Diet throughout an animal's lifetime can be reconstructed using stable isotope analyses of tissues with different turnover times, yet postmortem changes affecting these measurements remain understudied. Here, we investigate bulk δ13C, δ15N, and δ34S, and amino acid δ13C and δ15N values from multiple tissue types sampled from stranded harbor porpoises across a range of initial decomposition states. Decomposition caused a consistent decline in δ13C values across tissues and amino acid classifications, indicating anaerobic microbial fermentation as a dominant metabolism driving changes in early decomposition. δ15N and δ34S values were largely stable, suggesting limited microbial transformation of nitrogen and sulfur pools prior to more advanced decomposition. Tissue-specific isotopic differences were larger for both bulk and amino acids values, likely driven by metabolic differences between tissue types as well as differences in elemental turnover from diet. Our findings highlight predictable isotopic carbon shifts during early decomposition and call for caution when interpreting stable isotope data from potentially degraded samples. Moderately decomposed marine mammal tissues reflect diet if tissue-specific discrimination is applied, and amino acid selection impacts both nitrogen and carbon interpretations due to varying differences between metabolic classifications.