Impacts of Typhoon‐Induced Heavy Rainfall on the Dynamic of Particulate Organic Matter in a Semi Enclosed Bay With Multiple River Inputs

Abstract Large river‐influenced coastal seas serve as critical interfaces for particulate organic matter (POM) dynamics within the global carbon cycle, characterized by intricate hydrodynamics and diverse sources. However, the response patterns of these POM processes to typhoon events remain poorly understood. This study conducted two field cruises in Laizhou Bay (LZB), a semiclosed bay with multiple river inputs, to compare the impacts of riverine pulses on POM dynamics during posttyphoon Lekima and a nontyphoon period. Typhoon‐induced heavy rainfall significantly altered POM composition and fluxes. Posttyphoon, particulate organic carbon (POC) and particulate nitrogen fluxes increased to 4.9 × 10 10 mmol d −1 and 4.8 × 10 9 mmol d −1 , respectively, compared to 1.5 × 10 10 mmol d −1 and 2.3 × 10 9 mmol d −1 during the nontyphoon period. Monte Carlo (MC) simulations quantitatively revealed riverine organic carbon (OC riverine ) dominated POC posttyphoon (44.8%), followed by marine OC (OC marine ) (35.0%), while deltaic organic carbon (OC deltaic ) was least (20.3%) in LZB. Under nontyphoon conditions, OC deltaic dominated (46.0%) with OC riverine being minimal (25.0%). Both δ 13 C and C/N ratio analyses showed that posttyphoon POM sources included freshwater algae, soil organic carbon, C3 plants, and marine POC, while nontyphoon POM primarily derived from freshwater algae and marine POC. Molecular indicators suggested that typhoon‐driven terrestrial inputs of labile organic matter enhanced POM degradation via the priming effect. Consequently, POM degradation was more extensive posttyphoon, transforming the coastal region from a carbon sink to a carbon source. These disturbances likely reduce carbon burial in marine sediments during extreme weather events.