Integrated Land–Sea Management for Coastal Water Quality: Cost-Effective Optimization of Land-Based Pollution Mitigation and Managed Realignment

Integrated land–sea management is essential for addressing coastal water quality degradation. However, existing approaches focus primarily on land-based pollution reduction, while giving limited attention to coastline rehabilitation measures such as managed realignment. In this study, an Integrated Land–Sea Management (ILSM) framework is developed, which couples land-based pollution reduction with coastline restoration through a simulation–optimization approach. The framework captures the influence of reclamation and managed realignment on water quality and incorporates a dynamic checkpoint identification technique that adapts to changing coastlines. It was applied to Jiaozhou Bay, a typical semi-enclosed bay in China characterized by severe pollution, intensive reclamation, and rapid coastal development. In this application, dissolved inorganic nitrogen (DIN) was selected as the primary water quality indicator due to its severe exceedance in this region. An optimal natural coastline proportion of 32% was identified, which can achieve water quality targets with 37% less reduction burden than land-only optimization and is 4.3 times more cost-effective than the current equal-percent reduction policy. This study provides a cost-effective decision-support tool that embeds coastline rehabilitation into land–sea management. For Jiaozhou Bay, this framework offers a quantifiable pathway toward sustainable coastal water management.