[Effect of Nutrient Cycles in Tributaries on the Transport of Nutrient in the Three Gorge Reservoirs:A Case Study of Caotang River].

Backwater regions have been formed in the downstream of tributaries since the Three Gorge Reservoir (TGR) was impounded, as the most influential area to the aquatic environment within the reservoir. In order to study the effects and significance of the backwater regions on the nutrient cycling and substance fluxes, dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), dissolved silicon (DSi) were monitored in situ in Caotang tributary backwater region, the adjacent mainstream and Caotang tributaries' upstream every month from August 2012 to July 2013. The results showed that the concentrations of DIN, DIP and DSi in Caotang tributary backwater region were similar to those in the mainstream, and the monthly variation tendency was very consistent with the mainstream Changjiang. DIN, DIP and DSi in the Caotang tributary were mainly originated from the mainstream. The influence of the mainstream water could extend to the upmost of the backwater region due to the good connection and rapid, sufficient water exchange between mainstream and tributary. Nutrients from the mainstream were retained and consumed by primary producers in the tributary during the growing seasons in spring and summer, while released back to the water during late autumn and winter. Nutrient cycling in the tributary could change the seasonal pattern of nutrient transportation. The mainstream nutrient fluxes could receive significant seasonal modification from the large numbers (more than 40) of tributaries within the TGR domain.