Pengxi River is the largest tributary in the northern bank of Yangtze River in the Three Gorges Reservoir (TGR) region, and serious algal bloom has been reported since 2003 when the TGR dam started impounding water. On the other hand, Modao River, a tributary in TGR locating in the same county with Pengxi River, was rarely reported with algal bloom in the same time period. In this study, water samples were collected on the same day in both rivers in spring and summer, the most likely blooming seasons in TGR, to compare the water quality parameters and Chlorophyll-a(Chl-a) concentration dynamics in order to understand the mechanism of algal bloom in TGR. The results showed that the maximum value of Chl-a in Pengxi river reached 60.5 μg·L-1 in spring, and was only 7.8 μg·L-1 in summer, while in Modao river the Chl-a content was 2.92 μg·L-1 in spring and 7.48 μg·L-1in summer. Both rivers stratified during samplings, but having layers of thermocline and hypolimnion with the lack of epilimnion, and most of Chl-a content was located in thermocline. Also in both rivers, TN and TP exceeded the international threshold for eutrophication, with the average TN and TP values of 2.305 mg·L-1 and 0.053 mg·L-1in Pengxi River in spring, and 1.673 mg·L-1 and 0.097 mg·L-1 in summer, respectively, while in Modao River the values were 1.875 mg·L-1 and 0.075 mg·L-1 in spring, and 1.79 mg·L-1 and 0.054 mg·L-1 in summer respectively. Nutrients concentrations showed no significant correlation with Chl-a. On the other hand, conductivity value and trend were totally different between the two rivers:in Modao River in spring, the conductivity in upstream was only 75% of that in the main stream of the Yangtze River, and the backwater from the main stream reached to the middle in Modao river, where the highest Chl-a among all the river sampling sites was detected; summer conductivity distribution was similar with that in spring. Different from Modao River, the conductivity in upstream of Pengxi River in spring was 150% of that in the main stream of Yangtze, the backwater from the main stream reached area between sampling sites of PX04 and PX05 (upper than the middle reach); its upstream had significantly high content of Chl-a and conductivity, and these two factors were significantly positively correlated. The study showed that other than N and P, other ions in the Pengxi River played an important role in bursting "bloom", and need to be considered regarding bloom control.
Keywords: Modao River; Pengxi River; algal blooms; chlorophyll-a; conductivity; stratification.