Regulating factors and spatiotemporal patterns of terrestrial dissolved organic carbon adsorption in a high-turbidity estuary

The adsorption of terrestrial dissolved organic carbon (tDOC) onto surfaces of suspended sediment plays a fundamental role in regulating carbon fluxes across the river-estuary-ocean continuum. It is an important process that modulates carbon transport, transformation, and long-term carbon storage, influencing regional and global carbon budgets. However, the role of suspended sediment is frequently neglected in related coastal and estuarine studies due to the complex interplay of physical and biogeochemical processes. To elucidate the relationship between suspended sediment and tDOC and quantify the adsorption process, this study developed a tDOC-adsorption-floc-population model that integrates floc behavior with tDOC adsorption processes. Taking the Changjiang Estuary as an example, the model quantified tDOC removal through adsorption and examined the key mechanisms governing this process. Results indicate that approximately 12.8 $\pm$ 1 % of DOC is removed via adsorption when passing through the turbidity maximum zone (TMZ). The dominant mechanism of tDOC adsorption is governed by floc size, with Brownian motion and differential sedimentation alternating as the primary mechanism, whereas fluid shear exerts a relatively minor influence. The adsorption process is spatially aligned with the TMZ, but its influence, driven by the hydrodynamics, can extend into adjacent areas. These findings highlight the need for incorporating suspended sediment dynamics into regional and global carbon cycle models to enhance predictions of carbon transport and transformation in estuarine and coastal systems.