Multi-year monitoring and modeling of benthic environmental improvement by oyster shell capping on coastal sediments

This study evaluated the multi-year efficacy of oyster shell (OS) capping for reducing benthic phosphorus release and improving coastal ecosystems by integrating laboratory experiments, field monitoring, and hybrid biogeochemical modeling. Using ambient pH and dissolved inorganic phosphorus (DIP) concentrations as predictors, the machine learning model successfully predicted the non-linear phosphorus removal rate of OSs (R² = 0.86), which is difficult to reproduce using conventional kinetic models. In Masan Bay, the OS capping layer effectively reduced DIP concentration in the pore waters. Over two years, average DIP concentrations in the 5-cm and 10-cm thick OS plots were 0.16 mg/L (29%) and 0.41 mg/L (58%) lower than the control, respectively. The 10-cm OS capping comprehensively mitigated organic pollution, reducing chemical oxygen demand (COD) by 15.2 g/kg (51%) and improving redox conditions. The phosphorus cycling model confirmed the 10-cm capping drastically reduced DIP release fluxes (from 84 to 34 mgP/day) and maintained stable multi-year performance. Conversely, the 5-cm capping exhibited an earlier decline in its physicochemical buffer capacity, which is likely due to both its thinner thickness and specific local conditions. This hybrid approach provides a preliminary framework for evaluating reactive capping materials within specific environmental boundaries, demonstrating OS recycling as an effective nature-based solution for marine contamination.