Eutrophication status of the Daning River within the Three Gorges Reservoir and its controlling factors before and after experimental impoundment

We evaluated 4-year-old data set to assess the trophic state and the controlling factors of the TGR during different experimental impounding (January 2005 to December 2005, stage I–stage II; January 2007 to December 2007, stage II–stage III; January 2009 to December 2009, stage III–stage IV) and normal operating (January 2011 to December 2011, stage IV) periods. The trophic state indices (TSI_TP) increased in the study area after TGR impoundment, and the TSI_SD and TSI_CHL did not significantly change after TGR impoundment; the values of TSI_CHL only increased at S1 (upstream sampling sites) after the TGR impoundment. The TSI_SD values in the Daning River (S1, S2, and S3) and the main stream of the TGR (S4) showed similar seasonal variations, with the highest values observed in the flood seasons, followed by the normal seasons and the dry seasons in descending order. The TSI_CHL only values varied substantially among the three seasons in the backwater area of the Daning River (S2), with the highest value occurring in the flood seasons, followed by dry seasons and normal seasons. The differences in the trophic state indices became smaller between the three seasons after TGR impoundment. The values of TSI_TP and TSI_SD showed similar spatial variances, with the highest value observed at S4, followed by S3, S2, and S1 in descending order. The TSI_CHL values were higher at S2 than at S3, S1, and S4. The results showed that the phosphorus and nonalgal turbidity limit trophic status of the mainstream of TGR and Daning River before and after experimental impoundment.