Increasing dissolved nitrogen and phosphorus export by the Pearl River (Zhujiang): a modeling approach at the sub-basin scale to assess effective nutrient management

The Pearl River (Zhujiang in Chinese) has been exporting excess of nitrogen (N) and phosphorus (P), causing eutrophication in the coastal waters of southern China for decades. However, sources of these nutrients and their locations are not well studied for the Pearl River basin. As a consequence, it is difficult to formulate effective management options to reduce these nutrients in the river and to prevent further eutrophication. We developed a sub-basin model based onto the Global NEWS-2 (Nutrient Export from WaterSheds) model for the period of 1970-2050 to analyze trends in dissolved inorganic N and P (DIN and DIP) and to identify the main sources of these nutrients and their locations. We validated our model by comparing modeled nutrient fluxes with observed. Future analyses are based on Millennium Ecosystem Assessment scenario that assumes a globalized world with a reactive environmental management. DIN and DIP inputs to the coastal waters are calculated to increase by a factor of 2-2.5 between 1970 and 2050. Over two-thirds of the DIN and DIP inputs to the coastal waters stem from two down-stream basins (Zhujiang delta and Dongjiang), where agriculture and sewage are important drivers of this increase. Agriculture accounts for over 40 % of DIN inputs to coastal waters. Sewage and agriculture account for over 90 % of DIP inputs. Thus nutrient management in agriculture and sewage in down-stream areas is more effective in reducing coastal eutrophication than nutrient management in up- and middle-stream areas of the Pearl River basin.