Reducing future river export of nutrients to coastal waters of China in optimistic scenarios.

Coastal waters of China are rich in nitrogen (N) and phosphorus (P) and thus often eutrophied. This is because rivers export increasing amounts of nutrients to coastal seas. Animal production and urbanization are important sources of nutrients in Chinese rivers. In this study we explored the future from an optimistic perspective. We present two optimistic scenarios for 2050 (OPT-1 and OPT-2) for China. Maximized recycling of manure on land in OPT-1 and OPT-2, and strict sewage control in OPT-2 (e.g., all sewage is collected and treated efficiently) are essential nutrient strategies in these scenarios. We also analyzed the effect of the current policy plans aiming at "Zero Growth in Synthetic Fertilizers after 2020" (the CP scenario). We used the MARINA (a Model to Assess River Inputs of Nutrients to seAs) model to quantify dissolved N and P export by Chinese rivers to the Bohai Gulf, Yellow Sea and South China Sea and the associated coastal eutrophication potential (ICEP). The Global Orchestration (GO) scenario of the Millennium Ecosystem Assessment was used as a basis. GO projects increases in river export of dissolved N and P (up to 90%) between 2000 and 2050 and thus a high potential for coastal eutrophication (ICEP>0). In contrast, the potential for coastal eutrophication is low in optimistic scenarios (ICEP<0). This is because in 2050 loads of most dissolved N and P in Chinese seas are around their levels of 1970. Maximizing manure recycling can reduce nutrient pollution of Chinese seas considerably. Sewage control is effective in reducing P export by rivers from urbanized areas. The CP scenario, on the other hand, shows that current policy plans may not be sufficient to avoid coastal eutrophication in the future. Our study may help policy makers in formulating strategies to ensure clean coastal waters in China in the future.