Since 1900, N export by the Yangtze river increased 17-fold and that of P 7-fold. This dramatic increase in coastal nutrient loading could lead to declining oxygen concentrations waters, and the increase in molar N:P ratio could increase the risk of harmful algal blooms.
Increasing nitrogen and phosphorus export
This study reveals that the basin-wide nutrient budget, delivery to surface water, and in-stream retention increased during the period 1900-2010. The Three Gorges Reservoir constructed in the Yangtze river contributes 5% and 7% of N and P basin-wide retention, respectively. However, the increase in the nutrient delivery has been much larger than the increase in retention and, as a result has riverine export to the East China Sea and Yellow Sea (ECSYS ) increased rapidly from 337 Gg N yr-1 and 58 Gg P yr-1 to 5896 Gg N yr-1 and 381 Gg P yr-1. This implies that the nutrient loading has increased by a factor of 17 for N and a factor of 7 for P, and that the molar ratio has increased from 13 to 35.
Agriculture in the midstream and upstream subbasins dominate the N and P exports to the ECSYS, respectively, while urban wastewater dominates the N and P loading in the downstream part of the Yangtze river. Policy strategies to reduce nutrient loading of ECSYS should therefore focus on improving the agricultural fertilizer and manure use efficiency in the upstream and midstream and better urban wastewater management in the downstream subbasin.