Urbanization and water quality dynamics and their spatial correlation in coastal margins of mainland China

The urbanization of coastal margins and the resulting deterioration of coastal water environment have been a major concern, due to their substantial threat to the health and sustainability of coastal marine ecosystems. However, the spatiotemporal information about these two aspects and their correlation analysis are still lacking in China. This study quantitatively investigated the urbanization and water quality dynamics from 54 statistical units of coastal margins with a width of 40 km in mainland China, and examined the response characteristics of different water quality indicators to the coastal margin urbanization (CMU). Our results revealed that CMU generally shifted from low to medium/high levels nationwide in 2000–2013, with a 75% increase in the average urbanization index, while the nearshore water quality did not vary significantly over time, maintaining a medium level in the sequence of water quality from good to bad during the study period. The spatial heterogeneity of CMU and water quality distribution was evident. Yangtze River Estuary, Pearl River Estuary, Hangzhou Bay, Bohai Bay and the coastal provinces located here, including Shanghai, Tianjin, Zhejiang, and Guangdong, should receive more attention due to their excessive CMU and the worst coastal water quality. The indicators that characterize the overall state and eutrophication of water quality were found to be sensitive to CMU. Compared with coastal cities, coastal margins were stronger in terms of urbanization intensity and impact on coastal water quality. The findings will provide basic scientific information for decision-making on urbanization control and ecological restoration in coastal margins, and will also promote the understanding of the CMU process and its environmental impact from a continental-scale perspective.