DISTRIBUTION OF VEGETATION IN THE WATER-LEVEL-FLUCTUATION ZONE OF THE THREE GORGES RESERVOIR AND ITS EFFECTS ON PHOSPHORUS TRANSFER AND FISHERY PRODUCTION

The water-level-fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) has the characteristics of large water level drop, wide area and high vegetation coverage. In order to explore the distribution law of vegetation in the WLFZ of the TGR and its potential contribution to the ecological environment and fishery in the reservoir area, the plant community structural characteristics, plant nutritional components, section slope and soil physical and chemical characteristics of 30 typical sections of the WLFZ in Zigui, Yunyang and Zhongxian Reservoir areas were investigated and analyzed. The results showed that: (1) 209 species belonging to 61 genera and 54 families were found, of which Cynodon dactylon and Xanthium sibiricum were the main dominant species, with an average coverage of 29.73% and 26.87%, respectively. With the decrease of water level elevation, the coverage of C. dactylon, Alternanthera philoxeroides and Abutilon theophrasti gradually increased, the coverage of X. sibiricum, Melilotus officinalis and Bidens tripartite gradually decreased, while the coverage of Humulus scandens, Setaria viridis and Polygonum lapathifolium first increased and then decreased. The distribution of vegetation was also affected by the slope of the sampling sections. The coverage of C. dactylon was significantly negatively correlated with the slope (P<0.001), while that of X. sibiricum showed a significant single peak distribution (P<0.001). Higher soil moisture content promoted the formation of common dominant communities dominated by C. dactylon, and with the decrease of soil moisture content, vegetation types gradually succeed to common dominant communities dominated by X. sibiricum; (2) The surface soil in the WLFZ at low water level elevation (150—160 m) showed an obvious phosphorus absorption trend, while the soil at high water level elevation (160—175 m) showed an obvious phosphorus release trend, which was closely related to the vegetation types and the soil microbial community structure characteristics of the sampling plots; (3) The total ground fresh weights of C. dactylon and X. sibiricum in the whole WLFZ of TGR was 2.51×10⁸ and 2.48×10⁸ kg, respectively, their total amount and protein content are much higher than those of other plants, and their potential contribution to the fishery productivity in the reservoir area are 1.25×10⁸ and 1.24×10⁸ kg, respectively. To sum up, the dominant species of vegetation in the WLFZ of the TGR are C. dactylon and X. sibiricum. The distribution of vegetation is mainly affected by water level elevation, section slope and soil moisture content. The vegetation in the WLFZ of the TGR has high ecological service value in terms of terrestrial phosphorus interception, phosphorus regulation and fishery contribution. This study can provide an important reference for evaluating the ecological fishery function of the reservoir WLFZ and developing the vegetation restoration technology for the WLFZ.