Influence of Hydrodynamics on the Composition and Reactivity of Particulate Organic Matter in a Large River Influenced Ocean Margin

Marginal seas influenced by large rivers are characterized by complex hydrodynamic and organic matter cycling processes. However, the impacts of hydrodynamics on the composition and reactivity of particulate organic matter (POM) remain unclear. Here we conducted a comprehensive study on the bulk, molecular and biological properties of suspended POM in the Changjiang Estuary and adjacent area subjected to strong currents, eddies as well as typhoons during spring and autumn. D/L-enantiomers of particulate amino acids (PAA) were analyzed to evaluate the bioreactivity of POM and quantify bacterial-derived organic carbon. We found that POM bioavailability as indicated by carbon-normalized yields of PAA (PAA-C%) reflected the ecosystem productivity. Relatively high PAA-C% values (20-35%) were observed in productive areas influenced by Changjiang River plume, cyclonic eddies and typhoons, likely related to the enhanced nutrient availability arising from hydrodynamic processes. In contrast, the oligotrophic Taiwan Warm Current-influenced regions featured relatively low POM bioavailability (PAA-C% < 10%) despite typhoons facilitating water mixing. The PAA-C% values showed a significant positive correlation with extracellular enzyme activity, indicating that bioavailable POM can rapidly stimulate heterotrophic transformation. Hot spots of elevated bioavailable POM showed high contributions of bacterial organic carbon. A large portion (similar to 2/3) of bacterial organic carbon was present in the form of bacterial detritus, suggesting that patches of these biological hot spots represent important sites of carbon sequestration. Together, our findings indicate that fresh POM production is largely controlled by nutrient supply driven by hydrodynamic processes, with important implications for carbon sequestration in the dynamic ocean margins.