Synchronous changes in mercury stable isotopes and compound-specific amino acid nitrogen isotopes in organisms through food chains

The relationship between stable isotope of mercury (Hg, Δ¹⁹⁹Hg and δ²⁰²Hg) and compound-specific nitrogen isotope of amino acids (CSIA-AA, δ¹⁵N_Glu and δ¹⁵N_Phe) remains poorly understood. In this study, we investigated bird species and their prey in an abandoned Hg mining area, southern China to elucidate these correlations for a better understanding of Hg sources, biological transfer, accumulation and amplification through food chains. Our findings revealed distinct isotopic patterns: Δ¹⁹⁹Hg showed a positive correlation with δ¹⁵N_Glu, indicating trophic transfer processes, while a negative correlation with δ¹⁵N_Phe suggested differences in Hg sources among birds. The wide ranges of δ¹⁵N_Phe and Δ¹⁹⁹Hg observed in birds appear to reflect mixtures of multiple nitrogen and Hg sources, likely due to their diverse food sources and the large variation in the proportion of MeHg in total Hg (MeHg%). The consistent slope between Δ¹⁹⁹Hg/δ¹⁵N_phe and MeHg%/δ¹⁵N_phe, reflecting both energy and Hg sources, provides new insights into the biotransfer and accumulation of Hg in organisms. Notably, the trophic magnification factor (TMF) of MeHg observed in water birds, such as egrets, reached an exceptionally high value of 97.7 estimated from CSIA of multiple amino acids (i.e., TMF_M), underscoring the significance of investigating Hg sources in birds. Our results demonstrate that the synchronous changes between CSIA-AA and odd Hg isotopes effectively identify Hg sources and transfer across multiple ecological systems.