Early-stage phytoremediation and bioturbation effects on natural purification of contaminated sediments in tidal flat systems: A mesocosm study

Early-stage responses of halophytes (Phragmites australis, Spartina alterniflora) and benthic fauna (Helice tridens, Perinereis linea, Cyclina sinensis) to contaminated sediments were investigated using a mesocosm system. This study explicitly focuses on elucidating the transient dynamics and biological adjustments following initial physical disturbance. Results indicated high temporal variability, reflecting active non-equilibrium system responses rather than steady-state conditions. Compared with bare flats (115% residual), biological treatments consistently showed lower sediment metal(loid) residuals (halophytes: 89-93%; macrofauna: 74%). Specifically, P. australis exhibited phytoextraction via shoot translocation, whereas S. alterniflora demonstrated phytostabilization through root retention. Notably, C. sinensis contributed to significant contamination reduction (≥50%), while P. linea and H. tridens served as potential bioindicators. These findings represent preliminary, process-oriented observations of early-phase biogeochemical interactions rather than evidence of sustained remediation efficiency. By characterizing these initial stabilization mechanisms, this study provides a critical foundation for designing future long-term sediment restoration strategies.