Abstract: According to the water quality monitoring data obtained from our field survey in the source regions of the Yangtze River and Nujiang River in 2016, we analyzed the physicochemical parameters, nitrogen and phosphorus content and form in water body of rivers and lakes in the source regions, and evaluated the nutrient limitations of nitrogen and phosphorus in rivers and lakes using stoichiometric approach. Results demonstrated that the total nitrogen (TN) content in water body of rivers and lakes in the study area ranged from 0.475 to 0.956 mg/L, ranking between class II and III of surface water quality standard; the total phosphorus (TP) content ranged from 0.006 to 0.017 mg/L, ranking between class Ⅰand II of surface water quality standard. Nitrate nitrogen (NO₃^--N) which accounted for 43.5%-85.5% of TN was the main component of nitrogen in water body of rivers and lakes and contributed more to TN pollution in the source regions of the Yangtze River and Nujiang River. The stoichiometric ratio of nitrogen and phosphorus (m(TN)∶m(TP)) in water body of rivers and lakes of the Yangtze River and Nujiang River was between 33.7 and 79.3, indicating that phosphorus was the limiting nutrient for the growth of phytoplankton in water body of rivers and lakes in the source regions of the Yangtze River and Nujiang River. The research findings offer data support for revealing the eutrophication process and water eco-environment protection in the water body of rivers and lakes in the source regions in the Qinghai-Tibet Plateau.

Key words: water quality, nitrogen and phosphorus content, stoichiometric characteristic;source of the Yangtze River, source of the Nujiang River