The individual and combined effects of coastline changes and riverine input on water quality: A multi-scenario simulation perspective.

The Bohai Sea Rim is densely populated and highly industrialized, facing threats from large-scale land reclamation, riverine pollutant discharge, and other anthropogenic activities. Aiming to improve seawater quality, the optimal allocation of land-based total nitrogen (TN) loads among four major rivers around the Bohai Sea has been studied before. However, the individual and combined impacts of coastline changes and riverine input were not previously quantified and analyzed. Therefore, a coupled hydrodynamics-water quality model was used to simulate the variation of dissolved inorganic nitrogen (DIN) and TN caused by reclamation and sedimentation. The results show that coastline changes significantly influence tidal dynamics, tidal prisms, residual currents, and the degree of openness, thereby altering the diffusion pathway of pollutants. Specifically, land reclamation activities and Yellow River mouth movements have led to increased concentrations of DIN and TN in estuarine areas (e.g., Haihe River Estuary, Ziyaxin River Estuary, and Wei River Estuary). Additionally, the combined impacts of coastline changes and TN input variations were found to amplify or diminish their individual impacts on water quality. Considering the individual factors, the impact of coastline change (15-year basis) was much greater than that of land-based TN input (a 10 % and 20 % random error, respectively). This study provides valuable insights into the complex interactions between coastline dynamics and marine water quality, emphasizing the importance of considering both natural and anthropogenic factors in coastal environmental management.