Abstract: Dam construction has significantly improved water resource utilization efficiency of rivers. River channelization caused by dam construction is one of the key factors affecting nitrogen and phosphorus transport and water eutrophication. By estimating nitrogen and phosphorus fluxes and assessing water trophic state, we analyzed nitrogen and phosphorus retention characteristics of dammed channelized rivers and their response to water eutrophication during the water stable period in four dammed river segments and one normal river segment of Weiyuan River and Xushui River in the Fuxi River Basin. The results showed that total phosphorus, ammonium, and organic matters were the main pollutants, with their concentrations being higher in small tributaries. The average retention efficiency (R_L) of total nitrogen and phosphorus in the selected river segments was 101.5 and 4.7 mg·s^-1·km^-1, respectively. Nitrogen and phosphorus retention efficiency in the dammed river segments of Xushui River was the highest (310.0 and 12.9 mg·s^-1·km^-1), indicating a net nutrient retention (R_L>0). Conversely, the lower retention efficiency recorded for the dammed river segments of Weiyuan River (-55.1, -0.2 mg·s^-1·km^-1) and the normal river segment (-2.3, -1.9 mg·s^-1·km^-1) indicated a net nutrient output (R_L<0). Flow rate, together with external and internal nitrogen and phosphorus loadings resulted in the lower nutrient retention efficiency in dammed channelized rivers of Fuxi River Basin than that in reservoirs and other river impoundments. The average trophic level index (TLI) of the dammed channelized river segments was 54.1, indicating a mild eutrophic state, while that for the natural river segment was 42.9, indicating a mesotrophic state. As phytoplankton chlorophyll-a was negatively correlated with flow rate and positively correlated with nutrient concentration, the dammed channelized rivers with heavy nitrogen and phosphorus pollution showed higher eutrophication risk. Therefore, further control of pollutants from external and internal sources in the Fuxi River Basin and optimizing river hydraulic conditions are recommended to alleviate eutrophication risk.

Key words: Fuxi River Basin, ship-lock weir, channelized river, retention efficiency, eutrophication.