Particulate Organic Matter Dynamics and Its Isotopic Signatures in the Northern South China Sea During the Decaying Stage of a Super El Niño Event

Abstract Particulate organic matter (POM) plays a fundamental role in the biogeochemical cycling of carbon and nutrients in shelf and coastal marine systems; however, its dynamics and sensitivity to climate‐driven perturbations remain poorly constrained. Using stable isotopic signatures (δ 13 C and δ 15 N), this study investigates the dynamics of POM along the northern shelf of South China Sea (NSCS) during the decay stage of the 2015–2016 super El Niño, a period characterized by complex hydrodynamics driven by an intensified river plume and pronounced coastal warming. A clear offshore gradient was observed in POM δ 13 C, which increased from estuarine waters to the river plume before decreasing toward the outer shelf, while δ 15 N displayed a contrasting spatial pattern. Pronounced isotopic hotspots, defined by enriched δ 13 C (>−20‰) and depleted δ 15 N (reaching 2.8‰), were identified within the river plume, especially in June. Mechanistically, the enrichment in POM δ 13 C was primarily attributable to the combined effects of increased runoff‐derived nutrient inputs and sea‐surface temperature (SST)‐accelerated phytoplankton growth during El Niño. The spatial variation in POM isotopic signatures could be reproduced by a three end‐member mixing model. These findings underscore the combined influence of El Niño‐enhanced river discharge and SST anomalies on the dynamics and source‐to‐sink processes of POM across the coastal‐shelf continuum.