Thermal Stratification And Its Effects On Phytoplankton Blooms In Tributary Bays Of Three Gorges Reservoir

Since the initial filling of Three Gorges Reservoir (TGR), serious phytoplankton blooms have occurred in its tributary bays. Cyanobacteria blooms have been observed in a number of tributary bays and threaten the drinking water security in the TGR region. Extensive field observations were made to better define the physical characteristics of TGR bays, identify the controlling factors of phytoplankton blooms in the bays and an effective management strategy. A field monitoring program was carried out from June2008to May2009in TGR and its major six tributary bays to investigate their hydrodynamic characteristics; another program was performed in2009to study the spatiotemporal distributions of water temperature of these waters; a high-frequency field monitoring of water quality was conducted in Xiangxi Bay (XXB) of TGR from November2009to October2010.The results show that weak thermal stratification (the maximum temperature difference between surface and bottom layer not exceeds2℃) was observed at certain reaches of TGR during winter and spring; there was a significant stagnant effect of water temperature in TGR-a great temperature longitudinal gradient (the maximum temperature difference between upper and lower reaches exceeds3℃), while it was absent in its tributary bays; significant vertical thermal stratification was observed in large tributary bays such as XXB throughout the year with the strongest in summer and the weakest in winter, while no significant thermal stratification was recorded in winter in small tributary bays such as Jiuwan Stream; the vertical thermal structure present quite unique shapes due to the lack of typical thermocline.Significant flow reversals were observed in all investigated reservoir bays; plunging depth of the intrusions was found to depend primarily on density differences produced by temperature differences; low vertical temperature gradients in TGR and well developed temperature stratification in its bays produce the temperature differences. Plunging depth varied by season according to stratification but, during flood season, turbidity determined plunging depth; the thermal structure of the corresponding layer was modified when an intrusion occurred and the existing thermal stratification was disrupted when an overflow occurred.A rapid decrease in mixing depth is associated with the spring bloom, fading of the fall bloom occurs with the rapid increase in mixing depth, and an extremely shallow mixing depth is associated with cyanobacteria blooms that predominate during the summer. Development of thermal stratification in XXB is the major cause of seasonal variation in mixing depth and density current intrusion from TGR is the major cause of short-term variation in mixing depth. The seasonal thermal stratification of XXB is disrupted by sufficiently large water level fluctuations in TGR. The density current is lifted from mid-depths to the surface and chlorophyll a concentrations rapidly decrease in response. Based on these findings, a conceptual model is proposed as a management strategy to control phytoplankton blooms in tributary bays via controlled releases from TGR.