Optical and biomarker indicators reveal contrasting saltmarsh and riverine contributions of terrigenous dissolved organic carbon

Dissolved organic carbon (DOC) in the coastal ocean originates from multiple sources that differ in composition and reactivity, influencing their fates in the ocean. The diffuse export of coastal marsh‐derived DOC remains poorly quantified, creating uncertainties in ocean carbon budgets. A major challenge is identifying marsh‐derived DOC within heterogeneous mixtures in coastal waters. Here, we introduce a new multivariate framework to partition terrigenous DOC (tDOC) into saltmarsh‐derived and riverine fractions in nearshore and coastal waters, focusing on the northern Gulf of Mexico. The approach integrates ratios of cinnamyl (C), vanillyl (V), and p ‐hydroxyphenyl (P) dissolved lignin phenols (C/V and P/V) with salinity and other compositional indicators of tDOC (spectral slope coefficient, S 275–295 , and carbon‐normalized yield of lignin phenols carbon, TDLP 9 ‐C) to quantify DOC contributions from riverine, marsh, and marine sources. Implementation of the source framework revealed that nearshore tDOC was dominated by riverine inputs near the Mississippi and Atchafalaya river deltas, whereas saltmarsh‐derived tDOC was prevalent near the marsh‐dominated Terrebonne Bay. Offshore, tDOC comprised a small but non‐negligible fraction (< 20%) of the DOC pool, with ~ 90% derived from rivers, and ~ 10% from saltmarshes on average. This study provides a novel framework for quantifying the contributions of marsh‐derived and riverine DOC in coastal waters, advancing understanding of their roles in coastal carbon budgets.