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Periodic algal blooms in certain tributary bays of the Three Gorges Reservoir (TGR) have become a serious environmental problem. A three-year observational study of the water velocity, temperature, nutrients and chlorophyll a (Chl.a) in Xiangxi Bay (XXB), a which is tributary of the TGR, was undertaken to identify the mechanisms underlying algal blooms. The results showed that, in the Xiangxi Bay, bottom-layer intrusive current (BLIC) happened in winter, middle-layer intrusive current (MLIC) occurred in spring and summer, and surface-layer intrusive current (SLIC) happened in autumn. The MLIC could cause a extraordinary thermal stratification. In addition, these density currents carried large amounts of nutrients into XXB, thus inducing eutrophication. The Critical Depth Hypothesis (CDH) implied that algal blooms in XXB would occur continuously when the MLIC happened. As the hydrodynamics in the TGR could be affected singnificantly by the change of the water level, water level operations may represent a method to control algal blooms.

Water level fluctuation plays an important role in algal blooms within the side arms. Field observations of the impacts of fluctuations of water level in Xiangxi Bay (XXB), one of the tributaries of the TGR, were analyzed from 2008 to 2010. Water level rise can result in increases as well as decreases in Chl. a, which depends on the water circulation patterns in the XXB. Those effective/ineffective water circulation patterns in preventing algal blooms in the XXB were based on the tributary inflow but also from the intrusion flow from the TGR. Flow velocity, thermal structure, hydrodynamic characteristics including water age, the turbulent eddy viscosity, the buoyancy frequency, the mixing depth, and the ratio of the euphotic depth and the mixed layer depth were evaluated for these scenarios. It is showed that the effective water circulation patterns in the XXB occurred when there was a converging of the inflow from the upstream with an intrusive overflow, lower-interflow, underflow or the combination from the TGR mainstream. Larger daily WL fluctuation and a longer duration of WL rise would enhance algal bloom prevention in the XXB. These results can provide assistance in managing the operation of water levels in TGR.

Nutrients are material basis of eutrophication and algal blooms in the water body. Xiangxi Bay (XXB), the largest tributary bay in the lower reach of the Three Gorges Reservoir (TGR), is subject to such eutrophication and algal blooms. In order to investigate the supply patterns of nutrients in XXB, stable isotopes of hydrogen (δD) and oxygen (δ 18 O), nutrients, and flow velocity were observed from September 2012 to December 2013. A two-compartment linear mixing model based on the tracer principle of δD and δ 18 O was employed to calculate the water and nutrients contributions to XXB from the mainstream of TGR and the upper tributaries of the XXB. The average nutrients concentrations of XXB were estimated and compared to the observed values in the field. The results suggested that there was obvious layered and bidirectional density current in XXB, which had a significant impact on nutrient supply pattern and spatial and temporal distribution of the isotopes and nutrients in XXB. The water contributions of the mainstream of TGR and the upper tributaries for XXB were 84% and 16%, respectively. Moreover, the nitrogen and phosphorus nutrients imported into XXB through the mainstream of TGR were over 80% and 60%, respectively, higher than those of the upper tributaries. The calibration of nutrients concentration in XXB showed that the error between the calculated and the observed was about within 20%, which confirmed further the reliability that the nutrients in XXB come mainly from the mainstream of TGR. The results address the attention that the pollution control should be implemented not only in the tributary watershed, but also the whole upstream region of the TGR for prevention of its tributaries algal blooms.