China's coastal seawater environment caused by urbanization based on the seawater environmental Kuznets curve

Rapid urbanization has had many negative impacts on China's coastal waters. Based on the seawater EKC, we analyse the evolution of seawater quality in China's mainland coastal waters. We explore the specific urbanization factors affecting seawater quality in coastal waters via multiple panel regression analysis. China's coastal seawater environment is improving. The proportion of Class I and Class II seawater in China's coastal waters fluctuates while inferior Class IV seawater decreases. The proportion of good-quality seawater in China's coastal waters is mainly affected by the population urbanization rate and per capita gross domestic product (GDP). In contrast, the proportion of poor-quality seawater in China's coastal waters is mainly affected by land urbanization, non-agriculturalization, and per capita GDP. Various dimensions of urbanization have different impacts on seawater quality in China's coastal waters at middle and high urbanization levels. The population urbanization rate increases the proportion of poor-quality seawater at the moderate urbanization level and inhibits it at the high urbanization level; the land urbanization rate has reverse effects.