Studies on the total nitrogen, total phosphorus and chlorophyll a variations in the mainstream and main tributaries of the Yangtze River before and after the impoundment in the Three Gorges Project area

Abstract

The effects of the construction of the Three Gorges Dam on the water ecological environment attract lots of actions. The Three Gorges Project on the Yangtze River is so far one of the world’s largest hydropower projects. This paper is about a decade research on the 82 monitoring sections in the mainstream and main tributaries of the Yangtze River during the construction period and trial impoundment period of the Three Gorges Project between 2006 and 2015. The findings suggest that the water quality of the mainstream of Yangtze River remained stable, and that of the tributaries in the project area has been improved to a certain extent. The year-specific SMK statistical analysis shows that the total nitrogen (TN) and total phosphorus (TP) concentrations were both on an ascending trend in general, and that there were more sections that recorded with rising TN concentrations than with elevated TP concentrations. The sections with rising TN concentrations were distributed in a way that was very different from that of the sections with elevated TP concentrations. It was common for the sections to observe notably higher TN readings in the head and central regions of the project area, and this trend extended to more sections in the tail region. The sections with rising TP readings were distributed randomly in the mainstream and tributaries of the Yangtze River. The number of the sections with notably higher TP concentrations was much more than that of the sections with evidently higher TN concentrations in Chongqing and Changshou in the tail region, and much less in Zigui and Yiling of the head region. The TP readings of the backwaters surpassed those of non-backwaters, and the TN readings varied little between the two. The TN and TP concentrations went on a rising trend in the mainstream and tributaries of the Yangtze River. However, only about 20% of the sections marked a notably rising trend of chlorophyll a and those sections were distributed randomly. This paper describes a decade-long, large-scale research on the waters of the mainstream and tributaries of the Yangtze River in the Three Gorges Project area. It provides invaluable information on the effects of the Three Gorges Dam on the water environment, and describes important theories on the water quality management in the project area.