Variation and Reactivity of Organic Matter in the Surface Sediments of the Changjiang Estuary and Its Adjacent East China Sea

The Changjiang Estuary and its adjacent sea make up a highly dynamic ecosystem where the processes influencing the sources and degradation of organic matter (OM) in sediments are still not well understood. In this study, we analyzed the total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (delta C-13), and total hydrolysable amino acids (THAA) to investigate the distribution and degradation of sedimentary organic matter (SOM). TOC and TN contents ranged from 0.13% to 0.93% and 0.03% to 0.15%, respectively, with elevated values occurring in the muddy areas near the estuary. The delta C-13 of SOM became increasingly enriched from inshore (-23.5 parts per thousand) to offshore (-20.5 parts per thousand), and the TOC/TN molar ratios (C/N ratios) decreased from 8.16 to 5.84, indicating increasing contributions of marine OM. As the labile fraction of bulk OM, THAA concentrations ranged from 1.36 to 11.76 mu mol/g and the distribution of THAA was influenced by terrestrial inputs along with the complex hydrography. The delta C-13 values and C/N ratios indicated that the SOM in the study area was mainly from marine and terrestrial origins (soil derived). The relatively low degradation index (DI) of SOM (mean: -0.57 +/- 0.36) indicated that the OM had already undergone extensive degradation. Consistent with previous findings, our results indicated that the degraded OM was adsorbed by fine-grained particles and selectively transported southward along the coast. The fresh SOM in the sandy area was mainly from in situ production. Additionally, diatoms appeared to be the primary source of the fresh OM buried in the sediments of the study area.

Automated summary

This study investigated the distribution and degradation of sedimentary organic matter in the Changjiang Estuary and its adjacent sea, revealing varying levels of organic carbon and nitrogen influenced by marine and terrestrial contributions. Despite extensive degradation, the organic matter underwent selective transportation southward along the coast by fine-grained particles.