Xiangxi Bay Research Articles

Improved data assimilation for algal bloom dynamics simulation in the Three Gorges Reservoir using particle filter

Algal blooms have been increasingly prevalent in recent years, especially in lakes and reservoirs; their accurate prediction is essential for preserving water quality. In this study, the observed chlorophyll a (chl-a) levels were assimilated into the Environmental Fluid Dynamics Code (EFDC) of algal bloom dynamics by using a particle filter (PF), and the state variables of water quality and model parameters were simultaneously updated to achieve enhanced algal bloom predictive performance. The developed data assimilation system for algal blooms was applied to Xiangxi Bay (XXB) in the Three Gorges Reservoir (TGR). The results show that the ensemble mean accuracy and reliability of the confidence intervals of the predicted state variables, including chl-a and indirectly updated phosphate (PO4), ammonium (NH4), and nitrate (NO3) levels, were considerably improved after PF assimilation. Thus, PF assimilation is an effective tool for the dynamic correction of parameters to represent their inherent variations. Increased assimilation frequency can effectively suppress the accumulation of model errors; therefore, the use of high-frequency water quality data for assimilation is recommended to ensure more accurate and reliable algal bloom prediction.

Vertical water age and water renewal in a large riverine reservoir

Water quality and hypoxia in lakes and reservoirs are strongly associated with water renewal. Yet vertical water exchange is still not fully understood and challenging to evaluate in highly dynamic systems. Here, we applied a scaling approach using the vertical timescale, vertical water age (VWA), defined as time since a water parcel last touched the water surface. We established a 3D hydrodynamic-based age model to quantify the vertical water renewal in Xiangxi Bay, a tributary bay of the Three Gorges Dam. The integrated effects of hydrodynamic processes like stratification, intruding density currents from the mainstream, and upstream inflow on the vertical renewal were accounted for. Results indicated that the spatial–temporal distribution of VWA in Xiangxi Bay depended on stratification and forms of intruding density currents. Age was large in spring and summer, and small in autumn and winter, reaching a maximum of 25 days in April. The vertical water renewal was faster during bottom intrusions from the mainstream than during middle and surface intrusions. At times, the epilimnion contained old water due to circulations, and the hypolimnion contained young water due to upstream flushing. In contrast to natural lakes, the bottom water was often younger than overlying intermediate waters. This demonstrated that mixed layer depth was insufficient to fully capture the vertical exchange in riverine systems with significant surface/bottom intrusion. The findings suggested VWA as a quantitative measure of vertical water transport in highly dynamic systems and its usability for environmental water management.

Hypoxia and its feedback response to algal blooms and CH4 emissions in subtropical reservoirs

Anthropogenic nutrient input stimulates surface eutrophication and algal blooms and thus exacerbates oxygen depletion and hypoxia in aquatic ecosystems. However, existing studies have paid little attention on the understanding of the feedback relationships among oxygen consumption, algal succession and greenhouse gas effect after river damming, particularly in Three Gorges Reservoir. This field work investigated the thermal stratification, water quality and phytoplankton in four tributaries (Pengxi Bay (PXB), Daning Bay (DNB), Shennong Bay (SNB), and Xiangxi Bay (XXB)) of the Three Gorges Reservoir (TGR) in spring (April 25-28, 2022), and explored the drivers of hypoxia and its feedback response to algal blooms and methane (CH4) emissions. The results showed that during the observation period, DO, thermal stratification, and algal blooms were more severe in PXB. The high Chemical Oxygen Demand (CODMn) (11.27 mg•L-1) and Dissolved Organic Carbon (DOC) content (7.56 mg•L-1) in PXB were the main causes of the progressive hypoxia. Furthermore, stronger density stratification in PXB inhibited the vertical supply of DO and accelerated bottom hypoxia. But the downslope density current replenished bottom DO in the upper reaches of the DNB, SNB, and XXB. The CH4 concentration was generally higher in hypoxia zone, especially in where cyanobacteria are the dominant species (eg., PX03 and PX04). Dam construction led to the succession of algal-dominant species, exerting a significant influence on the river carbon cycle. The expansion of cyanobacterial blooms and the consequence of metabolic substrates increase are gradually increasing oxygen deprivation, even hypoxia, and enhancing CH4 production in rivers. These findings suggest that the succession of algal-dominant species caused by river damming has a pronounced positive feedback effect on reservoir hypoxia processes and greenhouse effect.

Vertical mixing and horizontal transport unravel phytoplankton blooms in a large riverine reservoir

A precise understanding of the mechanisms causing phytoplankton blooms in reservoirs is still lacking, especially in large riverine reservoirs. To better understand these blooms, the role of the complex hydrodynamics caused by dam operation must be quantified. Here we examine how synergistic hydrodynamic processes, rather than individual metrics, trigger blooms in Xiangxi Bay, a typical tributary bay of the Three Gorges Reservoir, China. We used a 3D ecological-hydrodynamic model, which integrated hydrodynamics with the abiotic factors that limit phytoplankton growth to simulate one whole year (2010). By implementing a scaling criterion, we quantified the contribution of local phytoplankton growth and hydrodynamic processes, including advection transport and vertical mixing, on bloom dynamics. Results indicated vertical mixing was the main process inhibiting blooms in colder months (from October to February) but horizontal advection, which flushed and diluted blooms, was dominant in warmer months (from May to July) when stratification was intense and nutrients were replete. Accordingly, blooms occurred when both vertical mixing and horizontal advection were low. We suggested a potential dam operation strategy to mitigate blooms during stratification, which involves withdrawing the warm surface water from upstream reservoirs to increase horizontal flows in the surface layer. Extending the application of critical turbulence model, our study shows how vertical mixing and horizontal advection rate interact with phytoplankton growth rate to drive blooms in highly dynamic riverine systems.

Optimal reservoir operation for transport timescales using an integrated methodology

Dam construction hinders the transport process of water constituents, resulting in various water quality issues in reservoir areas that impede the sustainable development of hydropower. Conventional reservoir operation optimizations to address these issues face challenges in mathematizing multiple water quality objectives and solving high-dimensional computational problems. Taking a comprehensive perspective, we propose a methodology that incorporates the concept of transport timescales into optimal reservoir operation. Firstly, a specific transport timescale is estimated through numerical tracer experiments using a 3D hydrodynamic model. Subsequently, a surrogate model is developed to approximate the hydrodynamic model for computationally efficient estimation. Finally, we employ a non-dominated ranking genetic algorithm, combined with the surrogate model, to search for a Pareto-optimal solution for multiple objectives. As a case study, we selected flushing time as the representation of transport timescales and applied it to Xiangxi Bay (XXB) in the Three Gorges Reservoir, which has experienced serious water quality problems since dam construction. Our results show that under the optimal operation scheme, the average flushing time for the entire XXB is 23.991 d, which represents a 10.9% reduction compared to the practical operation scheme. The reduction rate of flushing time along XXB shows a monotonically increasing trend towards the reservoir mainstream, with a maximum reduction of 90.9%. The proposed methodology provides a heuristic tool that links optimal reservoir operation and the transport process of holistic water constituents for comprehensive water quality management in reservoirs.

Variable drivers of surface turbulence under condition of stratified density currents in a tributary bay of Three Gorges Reservoir

Surface turbulence has a direct impact on thermal stratification and important ecological process. The dissipation rate of turbulent kinetic energy (ε), a key parameter to quantify turbulence, also significantly affects the hydrodynamic and biochemical processes. Continuous measurements of high-frequency surface (0.50 m depth) flow velocity, meteorological data, water level and water temperature conducted in Xiangxi Bay (XXB) of the Three Gorges Reservoir (TGR) during summer (2019/6/17–2019/6/24) and winter (2019/12/29–2020/1/4) observational period, have been used to explore the variability of surface turbulence and its possible driving factors. The results show that a two-layered flow structure was clearly visible in summer. The surface water flowed from downstream to upstream (surface intrusion currents), with an average longitudinal velocity of 0.05 m/s and 0.02 m/s during summer and winter observational periods, respectively. Meanwhile, the longitudinal velocity also showed periodic fluctuations at three different time scales, with periods of 1.80 h, 20 min and 1–3 s, respectively. The 1.80 h fluctuation was caused by the flow oscillation driven by daily discharge regulation of TGR. In addition, the surface turbulence dissipation rates varied from 10−8 W/kg to 10−4 W/kg in summer, and from 10−9 W/kg to 10−4 W/kg in winter. In XXB, the movement of surface intrusion currents is an important factor that dominates and maintains the surface turbulence, while the contribution of atmospheric forcing (wind shear and convective cooling) accounts for 27.17 % in summer but 58.95 % in winter. The diurnal water level regulation of reservoir and wind variations are co-driving forces for the diurnal differences of surface turbulence during the observational periods. Therefore, the atmospheric forces and reservoir regulation together determine the variability of surface turbulence. This study will help to deepen the understanding of the hydrodynamic process of large regulated riverine reservoirs.

Viruses may facilitate the cyanobacterial blooming during summer bloom succession in Xiangxi Bay of Three Gorges Reservoir, China.

The occurrence of cyanobacterial blooms in summer are frequently accompanied by the succession of phytoplankton communities in freshwater. However, little is known regarding the roles of viruses in the succession, such as in huge reservoirs. Here, we investigated the viral infection characteristics of phytoplankton and bacterioplankton during the summer bloom succession in Xiangxi Bay of Three Gorges Reservoir, China. The results indicated that three distinct bloom stages and two successions were observed. From cyanobacteria and diatom codominance to cyanobacteria dominance, the first succession involved different phyla and led to a Microcystis bloom. From Microcystis dominance to Microcystis and Anabaena codominance, the second succession was different Cyanophyta genera and resulted in the persistence of cyanobacterial bloom. The structural equation model (SEM) showed that the virus had positive influence on the phytoplankton community. Through the Spearman's correlation and redundancy analysis (RDA), we speculated that both the increase of viral lysis in the eukaryotic community and the increase of lysogeny in cyanobacteria may contributed to the first succession and Microcystis blooms. In addition, the nutrients supplied by the lysis of bacterioplankton might benefit the second succession of different cyanobacterial genera and sustain the dominance of cyanobacteria. Based on hierarchical partitioning method, the viral variables still have a marked effect on the dynamics of phytoplankton community, although the environmental attributes were the major factors. Our findings suggested that viruses played multiple potential roles in summer bloom succession and may help the blooms success of cyanobacteria in Xiangxi Bay. Under the background of increasingly serious cyanobacterial blooms worldwide, our study may have great ecological and environmental significance for understanding the population succession in phytoplankton and controlling the cyanobacterial blooms.

Influences of water level fluctuation on water exchange and nutrient distribution in a bay: Evidence from the Xiangxi Bay, Three Gorges Reservoir

Following the Three Gorges Reservoir (TGR) impoundment, many tributaries were turned into bays; hydrodynamic conditions of TGR profoundly changed the residence time, temperature, and nutrient distributions of bays, and nutrient enrichment occurred in these bays. However, little research has been done on the effects of water level qqfluctuations (WLFs) of TGR on the bay. In this study, Xiangxi Bay (XXB), one of the tributaries of TGR, was selected as the delegate to construct and calibrate a two-dimensional hydrodynamic–temperature–tracer–water quality model based on the CE-QUAL-W2. The results were the following: 1) In spring, as total nitrogen (TN) in the TGR tended to be higher than that in the XXB, the downward WLF increased water exchange, TGR-XXB nutrient flux and TN in the epilimnion of the XXB, and decreased the water exchange and TN in the hypolimnion of the XXB. The upward WLF did the opposite. The situation would be reversed in autumn. 2) Under a larger magnitude or a shorter period of WLF, its corresponding effects on the water exchange and TN increased. 2) Both the downward and upward modes of WLF helped to decrease the thermal stratification of XXB. 4) The upward/downward WLF could be used to decrease the epilimnetic TN of XXB in spring/autumn, and was suggested to reduce the local algal bloom. The WLFs by the TGR regulation could profoundly change the water exchange and nutrient distribution in the bay, which helped to control nutrient concentrations and prevent algal blooms.

Rainfall impact on the disappearance of algal blooms in Xiangxi River of Three Gorges Reservoir under stratified density current

为探究在三峡水库特殊分层异重流背景下降雨对水华消退的影响,以香溪河为例,对库湾降雨前后水动力、生态环境因子开展连续三维立体跟踪监测。结果表明:降雨对水华的消退作用显著,降雨后香溪河库湾叶绿素a(Chl.a)浓度明显下降。热分层稳定指数(RWCS/H)变化不大,库湾近河口处分层较弱、中上游分层较强的特性并未随此次降雨发生较大变动。受降雨影响,藻类在表层水体聚集程度降低,藻类聚集度指数(MI)、微藻群体平均深度(MRD)下降。库湾流态随降雨发生而变得复杂,库湾水体浊度明显增加,异重流倒灌形式由近表层倒灌向中下层倒灌转变,雨后又逐渐转变为中层倒灌,长江干流水体倒灌进入库湾的影响范围、潜入深度增加。水体水平输移增强,分散下沉的藻类易随水体环流流出库湾,水华消退。雨后库湾入库流量增加,大部分上游来流依旧由上层流向河口,与中层倒灌异重流形成逆时针环流,藻类无法在表层水体稳定生长,库湾Chl.a浓度能在较长时间内保持较低水平,不会再次迅速暴发水华。;In order to explore the impact of rainfall on the disappearance of algal blooms under the background of special stratified density current in the Three Gorges Reservoir, taking Xiangxi River as an example, this study conducted continuous three-dimensional tracking monitoring of hydrodynamic and ecological environmental factors before and after rainfall in the Xiangxi Bay (XXB). Results showed that rainfall had an obvious effect on the disappearance of algal blooms. The concentration of chlorophyll-a decreased significantly after rainfall in the XXB. The stability index of thermal stratification (RWCS/H) changed slightly. Meanwhile the phenomenon which the stratification was weak near the estuary area but strong in the middle and upper reaches of the XXB did not change significantly under the rainfall. Affected by rainfall, the degree of algal aggregation in surface water, the morisita's depth (MI) and mean residence depth (MRD) all decreased. The flow pattern of the XXB became complex with rainfall, with a significant rise in the turbidity of the water body. The pattern of backflow density current changed from near-surface inversion to middle-lower inversion, and gradually changed into middle inversion after rain. The influence range and depth of the backflow of the Yangtze River mainstream into the XXB increased. Besides, the horizontal transport of water body enhanced so that the algae that dispersed and sank were easy to flow out of the XXB with the water circulation, and algal blooms disappeared. After the rain, the inflow of the XXB increased, and most of the upstream inflow still flowed from the upper layer to the estuary, forming a counterclockwise circulation with the middle inverted density current, which made it impossible for algae to grow stably in the surface water. The concentration of chlorophyll-a could be in low for a long time, and there will be no more rapid blooms again.

Responses of phytoplankton functional groups to water level fluctuations in the Xiangxi Bay, Three Gorges Reservoir

Responses of phytoplankton functional groups to water level fluctuations in the Xiangxi Bay, Three Gorges Reservoir

A method for continuous monitoring of the ebullition process and application to methane flux variations in Xiangxi Bay, Three Gorges Reservoir

A method for continuous monitoring of the ebullition process and application to methane flux variations in Xiangxi Bay, Three Gorges Reservoir

The air-water heat exchange and water vertical stability in the Xiangxi Bay of Three Gorges Reservoir

水库或湖泊的热分层结构是其动力与环境过程的重要研究方面,虽然很多学者针对水体分层结构和演变机理开展了大量研究,但水体通过水-气界面与大气进行热交换的过程,各气象因子的贡献机理等研究成果还很缺乏。本文基于三峡水库香溪河库湾2019年3月-2020年2月期间的水温、水位及气象等监测数据,针对水-气界面热交换过程如何影响水温垂向结构及表层水体湍流混合作用开展研究。结果表明,(1) 香溪河水体年内呈高温期分层、低温期混合的基本特征,高温期混合层深度小于8 m,低温期混合层深度超过30 m。(2) 太阳短波辐射是香溪河水体的主要热源,潜热通量和长波辐射是香溪河水体的主要冷源,感热通量贡献极小。(3) 香溪河平均风速较弱,约为1.6 m/s,主要通过增强潜热和感热通量的方式影响水体垂向稳定性结构特征,其机械扰动作用较弱。(4) 表层水体湍能通量在高温期较低(10^-7 m³/s³量级),此时水体处于分层状态,风应力大概率主导表层水体湍流发育; 低温期表层水体湍能通量较高(10^-6 m³/s³量级),此时潜热通量和长波辐射主导表层水体湍流发育。;The thermal stratification structure of a reservoir or a lake is important to its dynamics and environmental process. Although many scholars have studied this topic, a gap is still existed on the process of heat exchange in the air-water interface and the contribution mechanism of various meteorological factors. In this paper, based on the observation data of water temperature and meteorology from March 2019 to February 2020, in Xiangxi Bay of the Three Gorges Reservoir, we studied how the heat flux and wind stress affect the vertical structure of water temperature and the turbulent mixing. We have conclusions as following. (1) The Xiangxi River is characterized as stratification in high temperature period and mixing in low temperature period. The depth of mixing layers in high temperature period is less than 8 m and that in low temperature period is more than 30 m. (2) Solar short-wave radiation is the main heat source of Xiangxi Bay, while latent heat flux and long wave radiation are the main cold source. The contribution of sensible heat flux is quite small. (3) The wind speed in Xiangxi River region is weak, with an average value about 1.6 m/s, which influence the water vertical stability by enhancing latent heat and sensible heat flux strongly and mechanical disturbance effect weakly. (4) The turbulent energy flux of surface water is much lower in the high-temperature period (10^-7 m³/s³ in magnitude), at which time the wind stress dominates the development of turbulence. The turbulent energy flux is quite high during low-temperature period (10^-6 m³/s³ in magnitude), when the sensible heat flux and long wave radiation dominate the turbulence mixing.

Local flushing time and its relationship to reservoir eutrophication in Xiangxi Bay, China

Zhao Q, Mao J, Cai H, Dai H. 2023. Local flushing time and its relationship to reservoir eutrophication in Xiangxi Bay, China. Lake Reserv Manage. 39:72–87. Reservoir impoundment often accelerates the process of eutrophication. Consequently, algal blooms frequently occur in weakly flushed, eutrophic tributary embayments. As a reliable and straightforward criterion for characterizing the hydrodynamic influence on water quality, local flushing time was investigated in a long, narrow bay formed where a tributary enters the Three Gorges Reservoir in China. We determined the local flushing time using a calibrated 3-dimensional hydrodynamic and transport model. The spatial and temporal variations of the local flushing time were studied under different operational conditions characterized by water level fluctuations. The local flushing time showed responses to both upstream inflow and the Three Gorges Reservoir mainstream backwater. The spatial–temporal distributions of local flushing time are significantly affected by density currents that frequently occur in the bay because of the operation of the Three Gorges Reservoir. Positive correlations were found between local flushing time and total nitrogen, and dissolved silicon concentrations. The spatial heterogeneity of local flushing time demonstrates that there is a specific zone with a longer local flushing time (i.e., low flushing rates) in the upper middle portion of the bay. High-risk areas for algal blooms coincide with the quasi-stagnant flow zone. These findings could help to develop strategies to manage algal blooms in weakly flushed eutrophic bays.

Environmental factors indirectly reduce phytoplankton community stability via functional diversity

The biodiversity-stability relationship is a fundamental subject of ecological research. Considerable evidence demonstrates that biodiversity can either increase or decrease stability. Most relevant research mainly focuses on grassland and forest ecosystems. The biodiversity-stability relationship in aquatic ecosystems and the underlying mechanisms remain poorly understood. To fill the gap, we conducted a year-long study on the phytoplankton of reservoir ecosystems in the Xiangxi Bay of Three Gorges Reservoir (TGR) to test the following hypotheses: (H1) phytoplankton species richness and functional diversity directly reduce phytoplankton community stability in reservoir ecosystems; (H2) nutrient enrichment and water temperature increasing directly reduce phytoplankton community stability; and (H3) nutrients and water temperature indirectly reduce phytoplankton community stability via biodiversity. The structural equation model (SEM) found that functional diversity (community-weighted means of traits and functional divergence) had significant negative correlations with phytoplankton community stability (p < 0.05), while the species diversity had no significant correlation with phytoplankton community stability (p > 0.05). This finding partially supported the hypothesis H1, which suggested that the functional diversity had a closer tie with stability than the species diversity. SEM did not find any direct effect of environmental factors on phytoplankton community stability, which rejected our hypothesis H2. Instead, SEM found that water temperature and phosphate decreased phytoplankton community stability by increasing the first principal component of the community-weighted means of traits (CWM_PC1), which supported hypothesis H3. Further analysis showed that the increased water temperature and phosphate concentration can promote “r-strategists” species (larger CWM_PC1), which are less resistant to environmental disturbances, therefore decreasing the phytoplankton community stability. Our study highlights the importance of functional diversity in maintaining the relationship between biodiversity and stability in the phytoplankton community, which may provide a mechanistic understanding of the biodiversity-stability relationships in aquatic ecosystems.

Sediment nitrogen contents controlled by microbial community in a eutrophic tributary in Three Gorges Reservoir, China

Nitrogen pollution caused serious environmental problems in reservoir ecosystems. Reducing nitrogen pollution by enhancing nitrogen removal in river sediments deserved intensive research. Distributions of nitrogen contents in sediment-water interface were characterized along the Xiangxi bay (XXB), a eutrophic tributary in Three Gorges Reservoir, China. More than 47% of total Kjeldahl nitrogen (TKN) and 67% of total organic nitrogen (TON) were degraded during burial. Higher TN, TON and NH4+ consuming at downstream sites indicated stronger nitrogen mineralization and release due to higher turbulence of the overlying density currents. Nitrifying bacteria, denitrifying bacteria, anaerobic ammonium oxidizing (anammox) bacteria and nitrite/nitrate-dependent anaerobic methane oxidation (N-DAMO) bacteria were detected in nitrate-ammonium transition zone. Nitrogen contents transitions were responded to microbial stakeholders indicated microbially mediated nitrogen cycling in sediments. The dissolved oxygen and nitrate availabilities were the key limits of denitrification and associated reactions. These results suggested microbial mediated nitrogen cycling processes in sediments were critical for nitrogen removal in aquatic ecosystems, and replenishing dissolved oxygen and nitrate was expected to enhance sediment denitrification and strengthen potential environmental self-purification.

Implications of evolution of circulation patterns on nutrients regime from 2008–2018, in Xiangxi Bay of the Three Gorges Reservoir, China

Abstract A 2-D CE-QUAL-W2 hydrodynamic and water quality model was established from 2008 to 2018 in Xiangxi Bay (XXB). The recently built upstream Xiluodu, Wudongde, Xiangjiaba and Beihtan dams have altered the ecohydrology of mainstream and tributaries, so nutrients regime during pre and post dam stages are comparatively investigated here. Although total phosphorus (TP) increased at the lower reach, eutrophication remained TP limited with few exceptions. Maximum supply from main stream, potential of water exchange on surface corresponding to increased overflow intrusion facilitated increased average total nitrogen (TN) at euphotic zone in XXB by 0.1, 0.3 and 0.6 mg/L respectively since 2014. Eutrophication and algal growth in XXB remained TP limited with few exceptions at the upper reach. Average TN increased by 0.1, 0.3 and 0.5 mg/L at the upper, middle and lower reaches respectively in Late March since 2014 compared to those in early years. It increased by 0.2, 0.4 and 0.6 mg/L at the upper, middle and lower reaches in Early April since 2014. It increased by 0.3, 0.6, and 0.9 mg/L at the upper, middle and lower reaches in mid April since 2014. It increased slightly at the upper reach, 0.6 mg/L at the middle reach and 0.9 mg/L at the lower reach during late April. It increased slightly at the upper reach and by 0.3 mg/L at the middle reach and by 0.5 mg/L at the lower reaches corresponding to either of the patterns. During mid May, it remained the same at the upper reach, decreased by 0.2 mg/L at the middle reach and 0.5 mg/L at the lower reach. During late May, it remained the same at the upper reach, decreased by 0.4 mg/L at the middle reach and 0.6 mg/L at the lower reaches. These statistics indicate average an increase in TN in spring except in late May .TN concentration was beyond danger limit sustaining and prolonging eutrophication status in spring. As overflow, interflow and upper interflow patterns are prevalent in random order so nutrients could not be predicted theoretically, however nutrients management from non-point sources and the main supplier Three Gorges Reservoir is advisable.

Meteorological forcing and hydrological connectivity together control the thermal regime of Xiangxi Bay, a tributary of the Three Gorges Reservoir

Thermal conditions, such as stratification and mixing characteristics, are important for water quality and ecology in lakes and reservoirs. The prevailing strong stratification is a major factor contributing to global algal blooms, underscoring the need to understand the thermal regime and driving mechanisms. The thermal conditions of riverine/canyon-type reservoirs and tributaries are distinct from those of other districts, and hydrological connectivity and meteorological forcing play an important role in stratification and mixing. To test this concept and analyse how hydrological and meteorological conditions affect the thermal regime of Xiangxi Bay (XXB), one of the largest tributaries at the head of the Three Gorges Reservoir (TGR) in China, we conducted approximately one year of field monitoring. The results indicate that XXB is a typical subtropical continuous warm polymictic water body. Affected by the regulation of the TGR, the water level of XXB fluctuated up to 30 m. The stratification started in March and reached its maximum during the flood season; with reservoir storage in September, the stratified structure gradually collapsed. The combination of lower water level, increased shortwave radiation (SW) and warmer air temperature (AT) enhanced the stratification intensity (N). Vertical mixing produced by meteorological forcing was limited, turbulent mixing in the near-surface was dominated by the convection velocity (w∗) rather than the frictional velocity (u∗w), and high wind speeds enhanced the convection effect. During the heating period, the peak u∗w lagged behind w∗ by approximately 3 h, and near-surface stratification suppressed wind-induced turbulence. In addition, no temporal heterogeneity was observed during the cooling period. The heat budget was mainly controlled by the intrusive discharge of the mainstream of the TGR, and the annual water level fluctuation was the main factor affecting the heat imbalance (ΔQT-ΔQM). However, in the near-surface of the water column, heat distribution (indicator: mld) was mainly dominated by the water–air heat flux, Qnet (r = -0.833, p < 0.01, n = 323), and the difference in the cooling rate between the mainstream of the TGR and XXB induced a diurnal heat imbalance. The results of this study, the first long-term, muti-parameter field monitoring effort in XXB, provide new insights into the thermal regime of riverine reservoirs and their influencing factors while offering a thermodynamic perspective on eutrophication and algal blooms in reservoir tributaries.

Distribution and migration of polycyclic aromatic hydrocarbons in sediment and water of the Three Gorges Reservoir

AbstractSediment and water samples were collected from Xiangxi Bay in the Three Gorges Reservoir area of China to study the spatial distribution, migration, and ecological risks of 16 priority polycyclic aromatic hydrocarbons (PAHs) across four periods associated with different water levels from June 2017 to March 2018. Results showed that PAH contents ranged from 178.61 to 250.20 ng L−1 in water and from 5.26 to 1,325.25 ng g−1 in sediments, leading to a classification of moderate pollution in Xiangxi Bay relative to that in other estuarine systems. Polycyclic aromatic hydrocarbon levels varied across the four stages, with the proportion of two‐, three‐, and four‐ringed PAHs increasing in water and five‐ and six‐ringed PAHs decreasing in sediment across these periods. Pearson correlation analysis showed that the amount of five‐ and six‐ringed PAHs in water was significantly and positively correlated with the amount of suspended solids (p &lt; .05) and that three‐ringed PAHs in sediments were significantly and positively correlated with the amount of soil organic matter (p &lt; .05). Based on fugacity fraction estimation, two‐ringed PAHs mainly diffused from sediment to water, whereas five‐ and six‐ringed PAHs mostly migrated from water to sediment across stages. Ecological risk assessment revealed that PAHs present a moderate ecological risk in Xiangxi Bay overall, with low ecological risk in the second and fourth water level stages and moderate ecological risk in the first and third stages in sediment.

Evolutionary mechanisms and shifting trends in water circulation patterns in a tributary of the Three Gorges Reservoir, China

Abstract A 2-D CE-QUAL-W2 hydrodynamic model was established to simulate evolutionary mechanisms and shifting trends of flow patterns per annum and over seasons from 2008 to 2018 in a reflective tributary Xiangxi Bay (XXB) of the Three Gorges Reservoir, China. Reasons behind shifting trends of flow patterns were also investigated. Model performance was validated and simulated data was synchronous to observed data. In general, percentage of Pattern (6) was 14%, 20%, 17%, 12% and 11% per annum and in spring, summer, autumn and winter respectively by 2013. It was increased by 26%, 30%, 22%, 25% and 35% per annum and in spring, summer, autumn and winter respectively since 2014. Increased temperature and flow dynamics (such as 10,000 m3 s−1 in spring) in Three Gorges Resevoir (TGR) since 2014 were underlying shifting trends of density current patterns. Correlation among patterns prior to and after newly built upstream reservoirs was novel and innovative in finding hydrodynamic thresholds to increase effectivity. Particulars elaboated and associated with respective density current patterns indicate increased surface velocity and water exchange with increased overflows. This could help understanding hydrodynamics and ecological variations in TGR and XXB. Thermal establishment and flow dynamics in TGR triggering overflow intrusion in XXB are required to be achieved. A hydrodynamic and water quality model of XXB coupled with TGR mainstream is recommended to correlate additive impacts of advantageous and disadvantageous patterns and to evaluate hydrodynamical thresholds triggering advantageous patterns in XXB.

Contribution of the reservoir backflow to the eutrophication of its tributary: a case study of the Xiangxi River, China

AbstractBased on the routine water quality monitoring data of the Xiangxi River and its main monitoring section from January 2014 to May 2016, the monthly dynamics of nutrient concentrations and fluxes were analyzed, and the relationship between the water exchange between the reservoir and the tributaries and the changes of nutrient fluxes in the river was established. The results showed that the nutrient flux in the Xiangxi bay estuary was negatively correlated with the reservoir reflux intensity, and the overall correlation coefficient was −0.52. Four nutrient indexes, total nitrogen, total phosphorus, ammonia nitrogen (), and soluble phosphorus, all showed the same regularity and characteristics, among which was particularly prominent. It can be concluded that the variation of nutrient salts in the water body of the tributary bay is mainly determined by the water exchange between the reservoir and the tributary. In addition, the change of chemical oxygen demand and chlorophyll a flux in the Xiangxi Bay and water exchange in the reservoir area were analyzed. We found that changes in the hydraulic conditions of the return flow in the reservoir area can destroy the suitable flow environment necessary for the survival of algae, thus preventing the proliferation of harmful algae. Therefore, by controlling the hydrodynamic conditions between the reservoir area and the tributaries and changing the degree of water exchange between them, the objective of improving the water environment in the Xiangxi Bay can be achieved to a large extent.