How effective is marine spatial planning in managing transboundary nearshore seawater pollution? Quasi-experimental evidence from China

Transboundary pollution is a main concern in environmental governance, primarily due to “free-riding” behaviors that create boundary effects, which are often poorly mitigated by existing policies. While extensive research has confirmed boundary effects in river and air pollution, their presence in nearshore seawater remains uncertain. Furthermore, whether Marine Spatial Planning (MSP), designed to address the transboundary dimension of marine governance by mapping various sea-use activities, can alleviate the boundary effect remains unclear. To fill these gaps, we examine the third round of China’s MSP, using nearshore seawater pollution data to assess its effectiveness in addressing transboundary environmental issues. An ordinary least squares regression model was employed to identify boundary effects in nearshore seawater pollution in China from 2008 to 2021, with a focus on active phosphate and chemical oxygen demand. A difference-in-differences model was used to evaluate the impact of this round of MSP on these boundary effects. The results showed that the policy did not mitigate the boundary effects; instead, it significantly worsened the levels of active phosphate and chemical oxygen demand. Further analysis using a planning scheme-based evaluation method revealed conflict and intensity values across 51 coastal cities, demonstrating that these values positively moderated the boundary effect. The findings highlight the importance of geographical location in nearshore seawater quality and the necessity of coordination in transboundary marine area planning to mitigate transboundary environmental risks. This study offers valuable insights for China to improve nearshore seawater quality and provides empirical evidence for enhancing MSP, which is applicable to regions or countries with decentralized environmental governance systems.