A systematic 15 N ‐amino‐acid tracer framework to identify sources of pelagic marine organic matter

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Abstract Accurately tracing organic matter sources in marine systems remains challenging, particularly where natural and anthropogenic inputs coexist. Compound‐specific stable isotope analysis of amino acids (CSIA‐AA) offers strong potential for organic matter source discrimination, but its application is limited by the small number of validated δ 15 N‐AA tracers and the lack of tracer selection tools. Here, we evaluate and expand δ 15 N‐AA tracer use by combining controlled laboratory mixtures with a model‐independent tracer selection framework. Artificial mixtures of known composition, including phytoplankton, zooplankton, and fecal pellets from dominant offshore wind farm fouling species ( Mytilus edulis and Metridium senile ), were used to directly assess tracer performance. Application of the “CI/CR/CTS” framework (Conservativeness Index, CI; Consensus Ranking, CR; and Consistency‐Based Tracer Selection, CTS) enabled systematic pre‐unmixing evaluation of δ 15 N‐AA tracers. Trophic (Glu, Ile, Leu, Pro), intermediate (Ser), and metabolic (Thr) AA consistently showed strong source discrimination and reliable mixing behavior. Using this selected AA‐tracer set in a frequentist unmixing model (FingerPro) resulted in improved model fit, stability, and interpretability. These results demonstrate that artificial mixtures provide an effective platform for validating δ 15 N‐AA tracer selection. The “CI/CR/CTS” framework offers a flexible, model‐independent approach that enhances δ 15 N‐AA selection in complex, multi‐endmember marine systems and supports more robust assessments of organic matter source apportionment.

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