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To quantitatively calculate the spatiotemporal differences in the effect of the water supply from the Three Gorges Reservoir, three typical falling modes were proposed: falling in advance, uniform ...

Algal blooms have been reported in some tributary bays since the initial impoundment of Three Gorges Reservoir, which has seriously affected the surrounding area’s water ecology and drinking water safety. Hydrodynamics plays a crucial role in algae growth. The recently numerical model of hydrodynamics and algal blooms is an effective method to identify the effects of hydrodynamics on phytoplankton and find potential strategies for controlling algal blooms. In this study, the CE-QUAL-W2 model was developed to simulate the hydrodynamics and algal blooms in Xiangxi Bay (XXB) of the Three Gorges Reservoir. The model performed well in simulating flow patterns, water temperature profiles, and algal blooms. The results indicated that the hydrodynamics showed significant spatial and temporal differences in the XXB. In the mouth area, the intensity and plunge depth of density current were dynamic and characterized by a typical seasonal pattern. The changes in density currents will provide more opportunities for vertical mixing, resulting in un-stratification and low risk of algal blooms. But in the middle and upper areas, strong stratification and low velocity at upstream provide enough favorable conditions for the growth of algae and increase the risk of algal blooms. The simulation results revealed the variation of mixing depth could be good for clarifying the spatial and temporal differences of Chl.a. It played a vital role in seasonal stratification and the dynamics of phytoplankton succession in XXB. To improve the understanding of mixing regimes would be helpful to understand the physical mechanism of algal blooms elimination.

This paper studies the influence of the impounding process during flood recession in the TGR on the distribution and rerelease of sediment phosphorus forms in the Xiangxi Bay in 2016. Influenced by the main stream of the Three Gorges Reservoir (TGR) flowing backward during impoundment, the water environmental conditions, sediment grain size and sediment-forming environment of the tributary bays change correspondingly. In this study, it is found that the water temperature (WT), turbidity and oxidation–reduction potential (ORP) of the overlying water in the Xiangxi Bay decrease, while the pH increases after impoundment in the TGR. The backward flow of the main stream containing a large amount of sediments increases the sediment content of the tributary bay, and the difference in sediment particle size between the main stream and tributaries narrows after resedimentation. The phosphorus adsorbed on the sediments increases the total phosphorus (TP) in the sediments of the Xiangxi River to 1.3 times its value before water impoundment. NaOH-P and HCl-P in the Xiangxi River sediments account for 42% and 38% of the TP, respectively, and the contents of the various phosphorus forms are ranked in the following order: NaOH-P > HCl-P > organic phosphorus (OP). After impoundment, HCl-P accounts for 67% of the TP, and the relative contents of the phosphorus forms are ranked as: HCl-P > OP > NaOH-P. Orthophosphate (PO43−-P) in the Xiangxi sediments before and after impoundment acts as a source, and the PO43−-P diffusion flux increases from approximately − 0.0029 to 0.0059 mg (m2 d)−1 before impoundment to 0.0067–0.1071 mg (m2 d)−1 after impoundment, which is mainly due to the increased difference in concentration of PO43−-P [ρ(PO43−-P)] between the sediment interstitial water and overlying water after impoundment. After nutrient recharge of the main stream of the Yangtze River, ρ(PO43−-P) of the overlying water is 3.7 times its value before impoundment, and after the release of NaOH-P in the sediments, ρ(PO43−-P) of the interstitial water is 8.3 times its value before water impoundment. Hence, the risk of nutrient release from the sediments in the Xiangxi Bay increases.

The increasing freshwater ecosystem nutrient budget is a critical anthropogenic factor promoting freshwater eutrophication and episodic bloom of harmful algae which threaten water quality and public health. To understand how the eutrophic freshwater ecosystem responds in term of phytoplankton community structure dynamics to a sudden rise in nutrient concentrations, a microcosm study by nutrient addition bioassay was implemented in Xiangxi Bay (XXB) of Three Gorges Reservoir, China. Our results showed that dissolved trace elements supply adequately altered the phytoplankton community structure creating a regime shift from cyanobacteria-dominated to essentially Chlorophytes-dominated system, relative abundance (>70%). Combined N, P, and Si led to maximum growth stimulation accompanied by the highest chlorophyll yield (82.7 ± 14.01 μgL−1) and growth rate (1.098 ± 0.12 μgL−1d−1). N separate additions resulted in growth responses which did not differ while P -addition differed significantly (p∠0.05) with the control justifying a P limited system. Si enrichment stimulated diatom growth, relative abundance (20.62%) and maximum utility rate (USi = 83.37 ± 0.33%). This study also reveals that increasing nutrient loading from anthropogenic sources adequately decrease the ecological diversity (H < 1) and community overlap (CC ≤ 0.5) intensifying competition and succession which then select the fast-growing taxa to dominate and expand. Result points to the need for multiple nutrient control of N, P and Si loading into XXB through a prudent nutrient management protocol for lasting bloom mitigation in the tributary bay.

Most aquatic systems show characteristic seasonal fluctuations in the total nutrient pool supporting primary productivity. The nutrient dynamics essentially exacerbate critical demand for the counterpart micronutrients towards achieving ecosystem equilibrium. Herein, the phytoplankton demand for iron (Fe) uptake under high concentration of nitrate-nitrogen during spring in Xiangxi Bay, China, was studied. Our result confirmed that significant Fe concentrations (P = 0.01) in both autumn (0.62 ± 0.02 mgL−1) and winter (0.06 ± 0.03 mgL−1) relative to spring (0.004 ± 0.01 mgL−1) are linked to the low NO3−N paradigms during autumn and winter. As NO3−N showed a sharp increase in spring, a dramatic reduction in the Fe pool was observed in the entire tributary, driving the system to a critical Fe limited condition. Bioassay study involving Fe additions both alone and in combinations led to maximum growth stimulation with biomass as chla (16.44 ± 0.82 μgL−1) and phytoplankton cell density (6.75 × 106 cellsL−1) which differed significantly (P = 0.03) with the control. Further, the study demonstrated that Fe additions triggered biomass productions which increased linearly with cell densities. The P alone addition caused biomass production (15.26 ± 2.51 μgL−1) greater than both NO3−N (9.15 ± 0.66 μgL−1) and NH4+N (13.65 ± 1.68 μgL−1) separate additions but reported a low aggregate cell density (3.18 × 106 cellsL−1). This indicates that nutrient and taxonomic characteristics e.g., high cell pigment contents rather than just the cell bio-volume also determine biomass. The Bacilliarophyta, Chlorophyta, and Cryptophyta with the total extinction of Cyanophyta characterized the bloom in spring. The anthropogenic NO3−N input into XXB would have driven to higher NO3−N than NH4+N situation, and incapacitated the Cyanophyta that preferentially utilize NH4+N. Our study provides a useful report for incorporation into the monitoring programs for prudent management of phytoplankton bloom and pollution across the eutrophic systems.

It is essential to understand the mechanism of algal bloom and develop effect measures to control the hazard in aquatic environment, such as large reservoirs. In this study, a series of experiments, along with field observation from 2007 to 2016, were carried out to identify the hydrodynamic parameters that drive the algal bloom in the Three Gorges Reservoir (TGR), China, and their threshold values were determined. The results show that algae concentration was markedly diluted with a short retention time, and the threshold value of the retention time to avoid algal bloom was approximately less than 3 days. With strong stratification, the algae concentration was able to approach to the level of algal bloom in 10 days, even when the water temperature is lower than 12 °C. The ratio of mixing depth to euphotic depth (Zm/Ze) had significant negative correlations with both algae concentration and algae specific growth rate (SGR). The field monitoring data indicated that Zm/Ze is an important hydrodynamic parameter which sensitively affects algae growth and concentration. This study made the first attempt to determine Zm/Ze >2.8 to restrain algal bloom in the TGR. Our findings shed light on the influence of critical depth on the algal bloom in the TGR, and the results can serve to control algal bloom in reservoirs through discharge operation.

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.

After the impoundment of the Three Gorges Reservoir (TGR), algal blooms in the sidearm tributaries have resulted from increasing nutrient loads along the major tributaries. Field sampling and in situ nutrient addition bioassay were implemented to examine the nutrient limitation of phytoplankton growth and bloom initiation during autumn in Xiangxi Bay of the TGR. Result shows that P is the primary limiting nutrient for algal growth and bloom in Xiangxi Bay during autumn. The treatment involving the combination of N, P and Si had a significant (p .05) additional effect on the growth of phytoplankton. The N, P, Si combined treatment increased growth by 10-50% relative to the N and P treatments from day 1 to day 4, respectively. Trace metal additions involving Fe, Zn, Mn, and Cu and/or in combination with N, P, and Si initially resulted in an extremely low growth rate which later increased significantly (p .05) towards the end of the study. The present study provides an insight into the responses of different phytoplankton taxa in autumn under nutrient conditions in the tributary bay. The nutrient limitation study is recognized as the first step to mitigating the bloom while proposing an effective nutrient control strategy. The outcome of which can provide the basis for formulating sustainable watershed management. Multiple nutrients reductions with P as primary concern are required for a lasting management solution to the risk of bloom in the TGR.

We analyzed the alteration of discharge and water temperature caused by two newly established reservoirs in the lower reaches of the Jinsha River. In comparison to longer-term observations from the pre-impoundment period, the seasonal flow variability was significantly affected by the larger, upstream-located Xiluodu reservoir, with higher discharge in spring and reduced discharge in summer. The smaller, downstream located Xiangjiaba reservoir did not contribute significantly to the total hydrological alteration caused by the reservoir cascade. Thermal stratification occurred in spring and summer in the Xiluodu reservoir, but was not observed in the Xiangjiaba reservoir. The vertical structure and seasonal dynamics of thermal stratification were mainly governed by the water temperature of the inflow and the depth of the water outlet. Despite the different thermal structure, both reservoirs reduced the amplitude of annual temperature variations and delayed the seasonal temperature cycle in the downstream river water. In contrast to discharge variation, thermal effects were cumulative along the cascading reservoirs. Homogenization and delay effects can be expected to increase further with the completion of ongoing reservoir construction upstream of the two studied reservoirs. Based on our findings, we discuss the larger-scale impacts of cascading hydropower developments and emphasize the need for taking water temperature and its variation into account when developing optimized operation or mitigation strategies for these systems.

To analyze the hydrodynamics in an impounded river in response to discharge regulation at upstream and downstream dams, we developed a one-dimensional analytical model. The model provides analytical solutions for discharge and water level fluctuations along the river for periodic discharge boundary conditions at both dams. As a case study, the model was applied to a 38 km long section of the Yangtze River located between the Three Gorges Dam and Gezhouba Dam. The outflow discharge at both dams fluctuated diurnally due to varying demand of hydropower electricity during day and night. In good agreement with observational data, the model results showed significant diurnal variation of water level and discharge along the river. Using the model, we analyzed the sensitivity of the amplitude of diurnal fluctuation of water level and discharge to the parameters describing diurnal discharge regulation. Rapid discharge changes at the dams induced large amplitudes of diurnal variation of water level and discharge in the river. The amplitude of the diurnal wave in the river depends strongly on the lag time between discharge changes at both dams and was lowest if the lag time corresponded to the propagation time of a gravity wave along the river (0.6 h in the case study). In addition to the diurnal wave, high frequency waves were generated, with a period corresponding to the eigenperiod of the impounded river system (1.2 h in our case study). The standing wave is generated by rapid discharge changes at both dams and its amplitude is strongly dependent on the lag time between these changes. The high frequency eigenwave has little influence on water level variation but relative strong influence on discharge variation in the river.

Among the major consequences of dam construction and operation are the deterioration of water quality and the increasing frequency of occurrence of harmful algae blooms in reservoirs and their tributaries. Former studies at Three Gorges Reservoir demonstrated that the Yangtze River main stream is the main source of nutrients and pollutants to connected tributary bays. Eutrophication and other water quality problems reported for the tributaries along Three Gorges Reservoir are likely a consequence of density-driven exchange flows. Past work has focused mainly on the influence of seasonal and daily flow regulation on exchange flows, less attention has been paid to hydrodynamic processes resulting from sub-daily discharge dynamics. High-frequency measurements of flow velocity and water level in a eutrophic tributary (Xiangxi River) of Three Gorges Reservoir revealed the persistent nature of bidirectional density currents within the bay. Superimposed on this mean flow, we observed ubiquitous flow oscillations with a period of approximately 2 h. The flow variations were associated with periodic water level fluctuations with increasing amplitude for increasing distance from the river mouth (up to ±0.1 m at a distance of 27.4 km from the river mouth). They were caused by a standing wave in the tributary bay, which was generated by rapid increase or decrease in discharge following peak-shaving operation modes at Three Gorges Dam. The high-frequency wave made up the largest contribution to the temporal variance of flow velocity in the tributary bay and represents a so far overlooked hydrodynamic feature of tributaries bays in large reservoirs.

The stabilization and preservation of soil organic matter have been attributed to the strong reactive sites of mineral surfaces that cause physical isolation and chemical stabilization due to the organic-mineral interface. However, much of the micro-scale knowledge about interactions between organic ligands and minerals largely remains at the qualitative level, and neglects the heterogeneity of functional groups of organic matter. Here, we report the use of molecular-scale technologies of two-dimensional FTIR Correlation Spectroscopy (2D-FTIR-CoS) and X-ray Photoelectron Spectroscopy (XPS) to directly measure the binding processes of humic acid (JGHA) groups onto kaolinite surface. The spectroscopy results showed that the carboxylate groups, aliphatic OH and aromatic structure participate in the binding of JGHA on kaolinite surface. The carboxylic and phenolic hydroxyl interact with kaolinite surface through the interfacial C O Al/Si bonds. Kaolinite prefers to adsorb C-groups at pH 4.0 and O-groups at pH 8.0. The interaction of COO– group at 1566 cm−1 of JGHA leads to the formation of inner-sphere complex first and then outer-sphere complex with increasing contact time. The interaction of COOH group at 1261 cm−1 with the AlOH2+ of kaolinite was could be ascribed to ligand exchange and/or electrostatic attraction, whose contribution was evaluated to be 13.90%, 7.65% and 0% at pH 4.0, 6.0 and 8.0, respectively. These results of molecular binding provide quantitative mechanistic insights into organic-mineral interactions and expound the effect of functional groups of HA on binding mechanisms, and thus bring important clues for better understanding the mobility and transformation of land‑carbon including mineral-bound carbon.

The Lancang River and Nu River are parallel rivers located in southwestern China; while the Lancang River has several dams, the Nu River has none. To explore the impacts of cascade reservoirs on the grain size and mineral composition of fine sediments, 9 fine sediment samples from the Nu River and 29 samples from the Lancang River were collected and tested. The results indicate that in the free-flowing reaches (FFRs) of the Lancang River, the sediment grain sizes fined systematically and the mineral compositions varied slightly, and these results were similar to those of the Nu River without dams along the whole mainstream. In the regulated reaches (RRs) of the Lancang River, the sediment grain sizes and mineral compositions indicated discontinuous fluctuations due to the dams trapping. In the reservoir head areas, the sediments were composed of fine silt or medium silt with 50% clay minerals, but the sediments immediately downstream of the dams were fine sand or medium sand with 71% quartz and feldspar. The relative abundance of each mineral was altered differently by passing through the RRs: quartz, chlorite, and illite increased by 13%, 33% and 20%, respectively, while feldspar and calcite decreased by 26% and 80%, respectively, and dolomite and pyrite were completely trapped in the cascade reservoirs. All these common minerals may be replenished except pyrite, which originates from mining areas in the Lancang subbasin.

Algal blooms occurred in some sections of Shennong bay after impounding of Three Gorges Reservoir.Related environmental and hydrodynamic factors were monitored during the period of algal blooming season in 2014 (March 20, April 13, May 23) in Shennong bay, Three Gorges Reservoir. To study succession of planktonic algae, water stable coefficient, euphotic depth and mixed layer depth were used to analyze stratification and hydrodynamic characteristics.We identified 6 phyla, 38 species (genera) planktonic algae. The sensitive area of algal bloom was at SN05 (677.677×105 cells/L) and SN06 (716.761×105 cells/L), and the planktonic algae biomass during this period was significantly different (ANOVA, p0.05). Moderate water temperature, adequate nutrients, weak stratification and poor mixing promoted the rapid growth and breakout of the diatom bloom with Cyclotella spp. as the dominant species in March. Further increase of water temperature, stronger stratification and decrease of dissolved silicate and mixing layer restricted the diatom growth. Chlorella spp. and Chlamydomonas spp. grew better in shallow mixed layer with rich nutrients and evident stratification. Then Chlorophycean bloom broke out with Chlorella spp. as the dominant species and Chlamydomonas spp. the next-dominant species. High biomass maintained in April. In May, algal bloom gradually vanished due to sharp fluctuation of water level and increase of velocity. Monitored maximum water velocity was 0.1141 m/s at 2 m depth, exceeded an optimal flow rate perfect for growth of planktonic algae.Stratification and hydrodynamic characteristics had important effect on planktonic algae under the condition of adequate nutrients. Velocity became the main factor that inhibited the growth of algae in Shennong bay in pre-flood falling stage of the Three Gorges Reservoir.

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.

The initial filling of the Three Gorges Reservoir (TGR), China, has caused serious phytoplankton blooms in its tributary bays. A two-dimensional, laterally averaged, hydrodynamic and water quality CE-QUAL-W2 model was used to simulate the hydrodynamics and water temperature of a typical tributary (Xiangxi Bay XXB) of the TGR to study the relationship between phytoplankton and density currents. The CE-QUAL-W2 model was calibrated to data collected in XXB (depth profiles of velocity and water temperature at three locations) from January 2008 through December 2008. The model performed well in simulating (1) flow velocity profiles, (2) unusual water temperature profiles, (3) the plunge point location of the intrusion layer, (4) propagation speed, and (5) the travel distance of the density currents. The model was then used to examine potential factors that affect the density currents, such as water level fluctuations in the mainstem of the TGR, water temperature differences between the mainstem and XXB, and inflow rates to XXB. Therefore, a better understanding of the hydrodynamics of tributaries in the TGR can be used to understand the aquatic ecosystem dynamics of these tributaries.

Eutrophication has become a severe environmental problem in some tributaries of the Three Gorges Reservoir (TGR) in China. A two-year field investigation of nutrients, oxygen stable isotopes (δ18O), and hydrogen stable isotopes (δD) was performed from January 2010 to December 2011 to determine the sources of nutrients in Xiangxi Bay (XXB). The results showed that nitrogen, phosphorus and silicon varied seasonally depending on hydrodynamic changes. The bottom-layer intrusive density current decreased nitrogen and silicon concentrations and increase phosphorus concentrations in XXB, while the middle-layer intrusive density current increased nitrogen and silicon concentrations and decrease phosphorus concentrations. The differences in δ18O and δD among the Yangtze River (YR), XXB and the region upstream of XXB were significant, and according to the tracer method, the estimated contribution ratios of nitrogen, phosphorus and silicon from the YR to XXB were much larger than those from the region upstream of XXB. These findings suggest that water quality in the TGR can be improved by reducing the pollution load throughout the upstream basin of the YR but not through decentralized efforts in only one or two tributary basins.

A comprehensive monitoring program was conducted to investigate the nutrient spatial pattern in the mainstream of the Yangtze River from the Baihetan Dam down to the Three Gorges Dam located at the upper region of the Yangtze River in China. Samples were taken from 33 different sites from July 30 to August 19, 2011. The nutrient patterns of the three representative tributaries of the Three Gorges Reservoir (TGR)--the Modao, the Daning, and the Xiangxi Rivers--were also investigated. The results show that the mainstream of the TGR has a higher concentration of nitrogen and a lower concentration of phosphorus than that of the upper mainstream before the TGR. Moreover, it was found that nitrate-nitrogen (NO₃-N) is the main nitrogen component, while particulate phosphorus predominates the total phosphorus (TP). It was found that the three representative tributaries of the TGR have lower total nitrogen (TN) concentrations compared to the corresponding sections of the mainstream TGR. Based on the nutrient spatial pattern, the nutrient flux was calculated. The total fluxes of TN, NO₃-N, TP, and orthophosphate (PO₄-P) from the upstream reach into the TGR are 2,155.06, 1,674.97, 212.98, and 83.42 t day(-1), respectively. The amount of nutrients imported from the TGR into its tributaries is more than the amount exported. It was determined that the Xiangxi River has the largest net rate of imported nitrogen at 7.66 t day(-1), whereas the Daning River has the largest net rate of imported phosphorus at 1.75 t day(-1). In addition, compared with the nutrients imported from the TGR into its tributaries, the nutrient flux from the upstream reach into the TGR contributes approximately less than 3 %.

Since the water storage was initiated in 2003, the environment of Three Gorges Reservoir (TGR) has changed significantly. Algal blooms and eutrophication have been a frequent occurrence, with serious eutrophication in the tributary bays. To provide some theoretical evidence for the prevention and control of algal blooms, the goal of this study is to elucidate factors that influence algal blooms at different sections of the Xiangxi Bay (XXB). Using field data from the XXB, the responses of phytoplankton communities to their habitats were investigated from March to May, 2010. The results indicated a significant spatial and temporal heterogeneity in phytoplankton composition, cellular abundance, and habitats in the spring. Fifty-four genera representing 6 phyla were monitored. Redundancy analysis indicated that the variation in water temperature and relative water column stability (RWCS) contributed greatly to the succession of spring phytoplankton. Due to different physiological adaptabilities and mechanisms of competition among the algae species, significant succession of the community structure had been observed. The predominant species appears to have changed from those adapted to low temperatures and strong mixing (dinoflagellates and diatoms) to those adapted to high temperatures and weak mixing (green algae and cyanobacteria). The lack of silicate resulted in the succession from diatoms to green algae. Due to the influence of the Yangtze River, there is a low potential for algal blooms at lower reaches of the bay because of frequent water exchange. In contrast, the potential is high at middle and upper reaches where the water temperature increases gradually. The hierarchical status of the two sections is significantly different. Precipitation would inhibit algal blooms somewhat, and heavy rainfall would eliminate algal blooms throughout the bay. Phytoplankton are sensitive to their changing habitat in XXB. For a bloom to occur, sufficient nutrients, a lower flow velocity, and appropriate temperature and light conditions are necessary. As an artificial regulating reservoir, proper ecological regulation could not only significantly affect the dynamic conditions of the water body tributaries, but it could also change the transfer characteristics of light and heat, abolishing the algae habitats and thereby inhibiting the water bloom.

Harmful algal blooms, which frequently occur in different tributaries, have become a serious water quality problem of the Three Gorges Reservoir (TGR) since its initial filling. A three-year monitoring campaign from 2008 to 2010 was conducted to identify the controlling factors of algal blooms and effective preventive measures. The following results were obtained: 1) Layered and bidirectional currents, called intrusive density currents, dominate the tributary bays of the TGR year round; 2) continuously increasing water level can make the intruding density current shift from the middle to the surface, and surface intrusive water can reduce the surface water temperature and break the stable stratification of the tributary; 3) the appearing or fading away of algal blooms is closely associated with fluctuations in the TGR water level. Thus, the water level fluctuation patterns of different “tide-types” may prevent the occurrence of algal blooms in the spring and summer, and the “stair-type” tides may control autumn blooms during the impounding period. These methods were feasible and effective throughout the actual TGR’s operation in 2009–2011. These patterns formed an eco-environmentally friendly operation (EEFO) to mitigate algal blooms. As an in situ treatment with easier operation and less negative impact and loss, compared with chemical and filter-feeding fish methods, the EEFO is currently considered as the most feasible remedial measure to remit the eco-environmental problems at the Three Gorges Dam (TGD).

The water temperature regimes of the Yangtze River has changed a lot since the impoundment of the Three Gorges Reservoir (TGR), which lead to a significant effect on the water environment in the TGR. This paper focuses on the analysis of characteristics of water temperature regimes of the main stream in the TGR and the lagging effect of water temperature on temperature structure differences in six typical tributaries. The results showed that water temperature gradually decreased along the main stream and stratified position of water temperature was about 5–10 m arisen in estuaries of six tributaries in heating period. However, water temperature gradually increased along the main stream, and water temperature stratification of the six estuaries disappeared in cooling period. Annual vertical water temperature showed little change. After the impoundment of the TGR, the impact of water temperature is mainly reflected in lag characteristics. The monthly mean temperature difference significantly doubled, and the whole process of water temperature had been put off one month. Through further analyses, water temperature profile tends to be stable with the success of the TGR impounding to 175 m and contributes to water temperature differences in tributary, which is the main reason for water temperature structure changed from three layer to two layer gradually.

Since its impoundment in 2003, the Three Gorges Reservoir (TGR) has brought far-reaching economic and other benefits to China. However, the resulting negative ecological and environmental impacts such as eutrophication and algal bloom have been problematic. This paper describes a study on nutrient estimation concerning eutrophication and algal bloom based on Huan Jing-1 (HJ-1) satellite imagery of the Xiangxi Bay (XXB), a tributary of the TGR in China. Models of nutrient concentration were established by correlation analysis between field measurement of nutrient concentration and the values obtained from the simultaneous satellite images of the study area. The results showed that total nitrogen (TN) had the best correlation with B1/(B3 + B4), with a maximal Spearman correlation coefficient of 0.885. Both nitrate (NO3 −–N) and dissolved silicon (D-Si) had the best correlation with B1/(B2 + B4), with the maximal Spearman correlation coefficients of 0.818 and 0.756, respectively. All band combinations had little correlation with NH4 +–N, TP, and PO4 3−–P. The single variable quadratic models were established for predicting the concentration of TN and D-Si, while, a single variable cubic model was for the concentration of NH4 +–N. Robustness analysis and validation were undertaken for the feasibility of the models, which showed that the predicted models were acceptable. Through the models, the levels of TN and NO3 −–N were predicted on Jan 12, May 2, July 30 and Oct 8, 2010. The results showed that the water in the XXB was eutrophic mostly. The nitrogen level was higher on Jan 12 than that of May 2, July 30 and Oct 8. Compared with the upstream, the downstream of the XXB had higher nitrogen level on Jan 12 and May 2, whereas, on July 30, it was higher for the upstream. The nitrogen level was lowest on Oct 8, with the uniform distribution in the upstream and downstream of the XXB. The predicted results coincided with the field observation. The results will help to estimate the eutrophication and algal bloom through large-scale remote sensing monitoring and water quality management in the TGR.

The vertical migration patterns and buoyancy regulation of Microcystis in water bodies have important effects on algal bloom formation. To understand the migration mechanism of Microcystis in deep bodies of water, two sets of experiments were carried out to illuminate the complex vertical movements in the Three Gorges Reservoir. Samples were taken during a 46-h period, with detailed monitoring of key parameters such as the solar irradiance, chlorophyll-a concentration, and water temperature. The index of dispersion and mean residence depth were computed to better understand the vertical distribution over time. The Microcystis colonies moved upward from the early morning until noon and sank from the surface in the afternoon and night. The vertical migration of Microcystis was the dominant factor affecting the dynamic changes of Microcystis in the enclosure. The water motion was also important in the diel dynamic changes of the Microcystis distribution. These results enable us to better understand the process of the initiation, development and appearance of blooms and allow us to devise new control strategies of algal bloom.

The water level of the Three Gorges Reservoir (TGR) rose to 172.5 m in two stages from September to November, 2008. The hydrodynamic parameters and water quality parameters (e.g. flow velocity, temperature, turbidity and nutrient content) had been continuously monitored to reveal the influence of the impounding process of the TGR on the water eutrophication in the Xiangxi Bay (XXB). The maximal daily rise of the water level of the TGR was about 2.38 m. The water exchange between the mainstream (the Changjiang River) and the XXB of the TGR was enhanced by density current, and its main characteristics were a density-stratified flow in different directions. The water exchange reduced the differences of the water quality parameters (for example, temperature, turbidity, nutrient level, pH, etc.) between the mainstream and the XXB during the impounding process. Meanwhile, the water stability index in the mainstream remained small but it was lowered in the XXB. The main causes included the dilution by water of lower concentration of the chlorophyll a (Chl. a) entering from the mainstream, the decline of the water stability, and the increase of suspended silt, which induced the Chl.a to decrease in the XXB during the impounding process. The grade of the water eutrophication changed from middle eutrophic level to mesotrophic state in the process. However, as available nutrients were imported into the XXB from the mainstream during the impounding process the risk of water eutrophication increases in the XXB in the future.

Dissolved oxygen characteristics of spring algal bloom in Xiangxi Bay of Three Gorges Reservoir were studied. In surveys, 12 stations have been investigated and 132 samples were collected weekly from February 24 to May 10 in 2008. Chlorophyll a, pH and water temperature could be the significant influence factors to dissolved oxygen in spring algal bloom by using stepwise multiple binomial regression and correlation analysis method. Mean value of dissolved oxygen in Xiangxi Bay during spring algal bloom was 8.555 2.414 mg/L with a minimum and maximum concentration of 4.719 and 19.543 mg/L respectively and dissolved oxygen increased to the peak level as a result of algal bloom occurred in Xiangxi Bay and oxygen released during phytoplankton photosynthesis. Dissolved oxygen decreased from the surface to the bottom in the water column. The spatial change of dissolved oxygen in the bottom was not significant and difference of dissolved oxygen between the surface and the bottom in Xiangxi Bay was more significant than Yangtze River.