Long-term changes in nutrient concentrations of the Changjiang River and principal tributaries

We present long-term nutrient data on the Changjiang River (Yangtze River) at six hydrological stations and eight principal tributaries during the period 1958–1985. Three patterns of temporal changes were observed in nitrate \( ({\text{NO}}^{ - }_{3}) \) and nitrite \(( {\text{NO}}^{ - }_{2}) \): minimal variations in the upper catchment area, rapid increases in the middle watershed towards the end of the 1970s, and a gradual increase in the lower drainage basin. Prior to the 1970s, the level of \( {\text{NO}}^{ - }_{3} \) throughout the Changjiang River system remained fairly constant. In the 1980s, however, this changed, with the lowest values in the upper Changjiang changing rapidly to the highest in the middle reaches and then declining slowly but steadily in the lower courses. Compared to \( {\text{NO}}^{ - }_{3} \) and \( {\text{NO}}^{ - }_{2}, \) ammonium \( ({\text{NH}}^{ + }_{4} )\) and soluble reactive phosphorus (SRP) showed smaller increases or no long-term variations, while dissolved silica (DSi) concentration generally decreased at most stations. These three patterns of \( {\text{NO}}^{ - }_{3} \) and \( {\text{NO}}^{ - }_{2} \) changes in the Changjiang River system were reflective of the difference in chemical fertilizer use and landscape features (e.g., slope, soil type and water body area) of the drainage basins of the primary tributaries. The decreases in DSi were most likely attributed to a reduction in suspended sediment loading due to dam constructions and increasing diatom consumption. The increase in \( {\text{NO}}^{ - }_{3} \) and \( {\text{NO}}^{ - }_{2} \) with a reduction in DSi concentrations in the Changjiang River could have significant effects on the stoichiometric balance of nutrients delivered to the East China Sea and the ecosystem in this dynamic region.