Abstract:Aquatic ecological monitoring and assessment in freshwater ecosystems centers on the effective management, protection, and sustainable utilization of freshwater resources. In the past several decades, the management policies in developed countries such as the United States and those in Europe had emphasized the ecological integrity of freshwater ecosystems. Accordingly, a series of research plans and programs had been successively launched for systematically monitoring and evaluating status of freshwater ecosystems using biological communities as core indicators. However, the monitoring and assessment systems in China are mainly based on traditional physical and chemical indices, thus lacking biological indicators to illustrate changes in ecological quality of freshwater ecosystems. Therefore, the progress of water quality improvement cannot meet the long-term goals and overall requirements of the "14th Five-Year Plan" for water environment management-a conceptual change from water pollution prevention and control to a unified framework of "water resource, water ecology, and water environment (i.e., three waters)". In this review, we systematically discussed the experience of monitoring and assessment philosophy and operational techniques and applications of watershed management in developed countries/regions, such as the United States, Australia, and European Union. In addition, we further summarized the existing works of aquatic ecological monitoring and assessment in China, analyzed key issues that restrict the current water ecological monitoring and assessment progress, and finally proposed relevant suggestions for systematically developing aquatic ecological monitoring and assessment systems in China, including protection target, management mode, monitoring network, scientific research, public participation. It would provide decision support and crucial reference for accurately assessing quality of the nationwide freshwater ecosystems and further evaluating the effectiveness of protection and restoration in China.

Abstract:The biological characteristics and competitive predominance of cyanobacteria, as well as the formation and maintenance mechanism of cyanobacteria blooms, have aroused continuous interests among researchers. As an important nutrient for cyanobacterial growth, the assimilation of nitrogen is intimately coupled with the photosynthesis. However, nitrogen exists in various chemical forms in water, which affect the cyanobacterial growth and the formation of blooms in distinct ways. To delve into the formation mechanism of cyanobacterial blooms, it is important to understand the cyanobacterial photosynthetic feature as well as the mechanism and strategy by which the nitrogen of different forms are assimilated into cyanobacteria. This paper reviewed the processes and characteristics of photosynthesis in cyanobacteria, such as light energy capture and excitation energy transfer, primary photosynthetic reaction, CO₂ concentrating mechanism and assimilation. Then, it illustrated the transport and assimilation of different nitrogen forms (NO₃^-, NO₂^-, NH₄⁺, urea and N₂), the regulation of these processes, as well as the mechanisms by which cells coordinate and regulate the carbon/nitrogen metabolism. The paper also summarized the latest research advances in cyanobacterial blooms and suggested on the content and direction of further research.

Abstract:Cyanobacterial blooms pose a huge harm to lake aquatic ecosystems health and drinking water safety, and the high phosphorus concentration of lake water bodies is an important inducement for the frequent outbreak of cyanobacterial blooms in lakes. The efficient operation of cyanobacterial intracellular inorganic phosphate transport system and the phosphate special transport system and polyphosphate (polyP) synthesis in cyanobacteria ensures the absorption of phosphate and its survival in phosphorus fluctuating environment. Three forms of polyP including granular, colloidal and soluble polyP, appear in cyanbacterial cells. The subcellular locations of different polyP forms are varied. The size, quantity and the growth period of polyP synthesized are different in different cyanobacterial species. The synthesis and decomposition of polyP are catalyzed by polyphosphate kinase, polyphosphate exonuclease, polyphosphate endonuclease and P-AMP-phosphate transferase, respectively. PolyP is also a chelating agent of divalent cations and a reservoir of phosphate and high-energy phosphate bonds, providing cations, phosphate and energy for cell survival. Cyanobacterial synthesizing polyP is a response to environmental stresses such as high temperature, peroxidation, and nutrient salt deficiency, which improves the viability of cyanobacteria. Therefore, the synthesis of polyP is required for physiological and biochemical activities in the life activities of cyanobacteria. In addition, the synthesis of polyP plays an important ecological function in the continuous outbreak of cyanobacterial blooms and the maintenance of the homeostasis of algal eco-environments. Therefore, it should strengthen the research on the molecular biological mechanism and the ecological function of cyanobacterial polyP synthesis. How the presence of polyphosphorus changes the phosphorus distribution in lake water-sediment and phosphorus transfer in food webs should also be elucidated. The techniques to control large-scale outbreaks of cyanobacterial blooms by controlling the synthesis of polyP in cyanobacteria may provide new methods to reconstruct benign ecosystems in lake waters.

Abstract:Rivers, as a conduit linking the carbon pools between land and ocean, are considered to be the most important carriers of carbon emissions from inland water bodies and play a crucial role in the global carbon cycle. The study of the significant temporal heterogeneity of carbon dioxide (CO₂) degassing across the global river water-gas interface not only contributes to a deeper understanding of its carbon cycle processes and mechanisms, but also provides scientific support for the accurate assessment of carbon fluxes and the refinement of carbon cycle models. Here, we reviewed systematically the relevant researches in China and abroad and summarized the current dynamics of CO₂ degassing fluxes in rivers at diurnal, seasonal and multi-year scales and its influencing factors. Then, a certain periodicity of diurnal and seasonal variations was demonstrated. In addition, we discussed the temporal differences of CO₂ degassing fluxes at different spatial scales. We believed that the lack of high resolution and long-term continuous direct measurements of river CO₂ partial pressure and CO₂ degassing coefficients limited the periodicity of river CO₂ degassing and its relationship with each other, making it still difficult to quantify and predict the impact of climate change and human activities on the temporal dynamics of river CO₂ degassing. Finally, we prospected the emphasis of future research according to the current problems, which provided new ideas and directions for the study of global river water-air interface carbon cycle processes, mechanisms and modelling, as well as more accurate assessment and prediction of future trends in river carbon emissions.

Abstract:Schizothoracine fishes are distributed in the Qinghai-Tibet Plateau and its surrounding areas. With the uplift of Qinghai-Tibet Plateau, the primitive Barbinae fishes gradually evolved into schizothoracine fishes, adapting to severe environment such as cold, high altitude and fast flow. Schizothoracine fishes share the morphological trait that specialized scales (anal scales) distribute on each side of the anal and anal fin bases. Schizothoracine fishes characterize with slow growth, late sexual maturity, long life-span and low fecundity. Schizothoracine fishes originate from tetraploid organism, and polyploidy events are the key factors driving their evolution. The special geographical distribution, morphological characteristics, genetic structure and life-history strategy of schizothoracine fishes have made them important materials for the study of polyploid and plateau adaptation mechanism, and have received extensive attentions from scientists. Combining with taxonomy, phylogeny and biogeography, genetics and biology, this paper focused on interpreting the systematic evolution of schizothoracine fishes from the perspective of modern archaeology and molecular biology, revealing the molecular mechanism of plateau adaptation in schizothoracine fishes from the transcriptome study. The aim of this study is to provide reference for the systematic evolution of polyploid fishes and the mechanism of extreme environmental adaptation in plateau.

Abstract:Research on the spatio-temporal dynamics characteristics of phytoplankton community in the Reservoir during the high-water storage period can provide a scientific basis for water quality protection, ecosystem structure optimization and health management in Danjiangkou Reservoir. Based on the monitoring data of phytoplankton and physical and chemical parameters of reservoir water from 2018 to 2020, this study analyzed the variation characteristics of phytoplankton in different years, seasons and spaces, together with the main environmental factors affecting the structure of phytoplankton in Danjiangkou Reservoir. The results showed that there were seven phyla in this region, among which the Cyanophyta, Bacillariophyta, Chlorophyta and Cryptophyta had relatively high abundance. The annual average density of phytoplankton showed an increasing trend from 1.96×10⁶ cells/L in 2018, and 2.55×10⁶ cells/L in 2019, to 5.07×10⁶ cells/L in 2020. The seasonal density of phytoplankton was autumn > summer > spring > winter, and the Bacillariophyta had great advantages in the four seasons. In summer and autumn, Cyanophyta and Chlorophyta were dominant, while in spring and winter, Bacillariophyta and Cryptophyta were dominant. Spatially, the density of phytoplankton in the reservoir area was significantly higher than that in the tributaries, and in Hanjiang Reservoir was significantly higher than that in Danjiang Reservoir, in which Cyanophyta and Chlorophyta mainly appeared in the reservoir area, while the tributaries were dominated by Bacillariophyta and Cryptophyta. The RDA analysis showed that the main environmental factor affecting phytoplankton in Danjiangkou Reservoir was water temperature, followed by chemical oxygen demand, ammonia nitrogen and total phosphorus, which respectively played major roles in different seasons, for example total nitrogen in spring, total phosphorus in autumn, permanganate index in winter and many environmental factors together in summer. Spatially, the phytoplankton in Danjiang and Hanjiang Reservoirs were both affected by water temperature, pH, dissolved oxygen, that in Hanjiang Reservoir was also affected by nutrient elements such as total nitrogen and total phosphorus.

Abstract:In order to explore the spatiotemporal variation and driving factors of phytoplankton primary productivity in eutrophic lakes in the middle and lower reaches of Yangtze River, field work and bottling experiments were conducted in April 2020 (spring), August 2020 (summer), October 2020 (autumn) and January 2021 (winter) in Lake Changhu, Hubei Province, while VGPM model was used to estimate the phytoplankton primary productivity (PP_eu). The main environmental factors driving the seasonal change of phytoplankton primary productivity were also explored. According to the results, 194 species of phytoplankton were identified and phyla Chlorophyta (95 species, 49%) and Bacillariophyta (40 species, 21%) were dominant. The total productivity (P_t) value of water column changed as follows:summer ((1841.24±345.93) mg C/(m²·d)) > autumn ((1324.14±208.34) mg C/(m²·d)) > spring ((847.50±247.72) mg C/(m²·d)) >winter ((711.43±133.52) mg C/(m²·d)). The highest value of which was (2424.66 mg C/(m²·d)) at site M2 in summer, while the value of total productivity (GPP) and net productivity (NPP) of phytoplankton gradually decreased with the increase of water depth. The seasonal distribution of PP_eu estimated by VGPM model was autumn ((3713.18±900.30) mg C/(m²·d))>summer ((2643.62±1062.48) mg C/(m²·d))>spring ((2477.13±669.75) mg C/(m²·d))>winter ((708.07±390.4) mg C/(m²·d)), and the trend characteristics of spatial distribution showed Haizihu district>Mahongtai district>Yuanxinhu district. The Spearman correlation between primary productivity measured by bottling experiments and VGPM model (P_t and PP_eu) was significant with a coefficient of 88.2%. They both were highly correlated with phytoplankton density and chlorophyll-a (Chl.a). Multiple stepwise regression analysis showed that PP_eu was mainly influenced by Chl.a, water temperature (WT), pH, total suspended solid and nitrite while Pt can be predicted by conductivity, WT and dissolved oxygen. These data would lay a foundation for the assessment of lake eutrophication and fishery production potential in the middle and lower reaches of the Yangtze River.

Abstract:In order to understand the seasonal changes of niche and community structure stability of dominant species of phytoplankton under the coexistence of multiple water sources (local reservoir water, Yangtze River water, and Yellow River water) in the Xiashan Reservoir, the strategic water source of the Shandong Province, this study conducted the quarterly sampling of phytoplankton at 10 representative sampling sites of Xiashan Reservoir in 2019. Niche breadth, niche overlap, variance ratio method, Chi-square test and association coefficient were used to analyze the niche and interspecific associations of dominant species. The results showed that a total of 7 phyla, 78 genera and 95 species of phytoplankton were identified in Xiashan Reservoir, of which 4 phyla, 12 genera and 12 species were the dominant species. Phytoplankton in all seasons were dominated by Cyanobacteria except for spring. Limnothrix redekei, Planktothrix agardhii and Pseudanabaena moniliformis were the dominant species throughout the year. The diversity index showed a more consistent trend, with higher phytoplankton diversity in spring and the lowest diversity in summer. The niche breadths of dominant species varied greatly, ranging from 0.526 to 2.224, with the maximum and minimum values in Pseudanabaena moniliformis in autumn and Chromulina sp. in spring, respectively. Pseudanabaena moniliformis had the largest niche breadth and strong ability to adapt to diverse habitats in many seasons. Niche overlap analysis showed that Pseudanabaena moniliformis and other dominant cyanobacteria had the largest niche overlap values in different seasons, and it was the only dominant cyanobacteria that declined in spring, summer and autumn but developed rapidly in winter. The overall correlation difference of dominant species in four quarter was more obvious, and the community structure was unstable. The Chi-square test showed that 76% of the species pairs had significant positive associations, forming a stable succession pattern that community structure was dominated by Pseudanabaena moniliformis, sharing the same resources and space with other dominant cyanobacteria. In addition to the possible algal blooms in summer and autumn, reservoir management and water transfer department should also pay close attention to the abnormal proliferation of some algae in low temperature seasons, especially the Pseudanabaena moniliformis in winter. This research can provide theoretical basis and data support for algal bloom early warning and multi-source dispatching of related reservoirs in Jiaodong Water Transfer Project.

Abstract:In this study, sediment cores were collected in Meiliang Bay, Lake Taihu. The surface 50 mm sediment were sliced at 2 mm section using the self-made device to analyze the vertical distribution of bacterial communities in sediment with high-throughput sequencing technology. High-resolution dialysis and diffusive gradients in thin films technology (DGT) with resolution of millimeter were used to analyze the vertical distribution of dissolved and DGT-labile ammonium nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), Fe and P. Results indicated that the composition of bacterial community, concentrations of dissolved and DGT-labile NH₄⁺-N, NO₃^--N, Fe and P changed significantly across the sediment depth. Bacterial nitrate reduction process mainly occurred at the sediment depth from -16 mm to 0 mm, which should cause an obvious decrease in the concentrations of dissolved and DGT-labile NO₃^--N in this sediment depth. Bacterial iron reduction process mainly distributed in the sediment depth from -32 mm to -18 mm, while bacterial sulfate reduction process mainly dominated in the sediment depth from -50 mm to -34 mm. Bacterial sulfate reduction was the main cause for the significant increase of dissolved and DGT-labile Fe and P concentrations in the sediment depth from -32 mm. This study has deepened the understanding of the bacterial influence on the vertical migration and transformation of nitrogen and phosphorus in eutrophic lake sediments.

Abstract:The analysis of temporal and spatial characteristics is of great significance for comprehensively understanding water quality variation. However, the existing temporal and spatial water quality analysis methods still have some shortcomings, such as indistinguishable order of water quality variation, the unclear extreme value of water quality variation, and unclear water quality evaluation characteristic values. To analyze the temporal and spatial characteristics information of water quality more clearly, this study took Qinhuai River as a research object and referred to the empirical frequency method of engineering hydrology. A "hydrological frequency-water quality" fitting curve was established, which was used to explore the difference between high/low activity areas in different periods and wet/dry seasons in different reaches. The "hydrological frequency-water quality" fitting curve method was compared with the traditional boxplot method. The results showed that compared with the boxplot, the "hydrological frequency-water quality" fitting curve could quantify key water quality evaluation points and characteristic value information, making the temporal and spatial variation process of water quality clearer. The best form of the "hydrological frequency-water quality" fitting curve in time was the line number curve, and the water quality concentration generally did not change abruptly. The best form of the "hydrological frequency-water quality" fitting curve in space was an exponential curve, and the water quality concentration had sudden increases were more likely. The concentration of nitrogen and phosphorus in the high activity area was higher than that in the low activity area during each period, and the concentration of nitrogen and phosphorus in the wet season of each river section was lower than that in the dry season. The analysis process of this method was simple and convenient, the results were intuitive and orderly, and the water quality information could be automatically reflected by statistical analysis. It could be used as an optimal method to study the temporal and spatial characteristics of river water quality when the sampling points, sampling time and sampling frequency are typical.

Abstract:To systematically explore the spatial-temporal distribution and main controlling factors of dissolved oxygen (DO) in deep lakes, we take Lake Xiannv, a large subtropical reservoir in south China, as the research object. By collecting the historical data of DO at four state-controlled points in Lake Xiannv from 2008 to 2021 to analyze its interannual changes. We also conducted intensive monthly monitoring of Lake Xiannv from May 2014 to April 2015 and from January 2018 to December 2018, which were before and after the water pollution accident in 2016. The factors influencing DO in Lake Xiannv were quantitatively analyzed by the structural equation model (SEM). Results showed that from 2008 to 2021, the DO concentration of Lake Xiannv first decreased and then increased, with a range of 5.1-18.7 mg/L, and the seasonal means were spring>winter>autumn>summer. Before the water pollution accident, the DO concentration in the central area and the outlet of Lake Wulong was higher and water temperature, chlorophyll-a and turbidity were the main driving factors for the change of DO. After the accident, the DO concentration in Lake Qianyang and the bays of Lake Wulong was higher and chlorophyll-a and water nutrition were the main driving factors for the change of DO. There are common changes between pH and DO. The vertical monitoring results of the deepest point (Jiangkou) showed that the vertical changes of water temperature and DO in Lake Xiannv were summer>autumn>spring>winter. We found that there were low DO (DO<5 mg/L) zones below 5 m of the lake in summer and autumn, thermocline and metalimnetic oxygen minimum (MOM) were observed in summer. In general, the water pollution accident broke the nutrient balance of Lake Xiannv, and the large increase in nutrient concentration led to an increase of phytoplankton, which was the main reason for the increase in DO concentration on the surface of Lake Xiannv, and the changing of deep-water DO concentration is closely related to water temperature and water depth.

Abstract:In order to understand the temporal-spatial distributions and the pollution sources of nutrients in Lake Dalian wetland, the data of water quality from 2008 to 2022, were integrated systematically. Water samples were collected and analyzed from six different land use types in wet and dry seasons from 2021 to 2022, while the diurnal variation (24 h) monitoring was conducted in normal water period of 2021. The interannual results showed that total nitrogen (TN) was in the class Ⅳ-Ⅴ water limit of the Environmental Quality Standard for Surface Water (GB3838-2002) while the maximum value was at dry season in 2009 (2.97 mg/L). The ammonia nitrogen (NH₃-N) could meet the class III water quality standard in recent years while the maximum value of total phosphorus (TP) observed at wet season in 2021 is 0.79 mg/L which exceeded the class V water quality standard 0.4 mg/L. The water quality of Lake Dalian wetland seemed to be improved after ecological restoration, however, the nutrient was still high. This result was consistent with the trend of nutrient contents in Lake Dianshan, implying that the inflow water could be one of the reasons for high nutrients in Lake Dalian wetland. Seasonal data revealed that each indicator presented seasonal differences, and the dry season was slightly worse than the wet season. The pollution brought from the other rivers also could be the reason for the worse water quality at Lake Dalian wetland. The nutrients and organic matters prominently exceeded in fish pond and lotus pond, in particular, the TN content was 2-4 times compared to the class V water standard limit (2.0 mg/L) in fish ponds. The 24 h monitoring data indicated that pH, water temperature(WT), dissolved oxygen (DO) and chemical oxygen demand (COD_Cr) reached the maximum values at 14:30 for most of the sampling sites where pH, WT, DO and COD_Cr were 9.1, 33.3℃, 35.6 mg/L and 90 mg/L, respectively. Nutrients seemed no obvious diurnal variation which might be due to the diversity and complexity of nutrient sources. The results of water quality index (WQI) demonstrated that water quality was in the medium level at most of the sampling sites; the results of comprehensive indicator for eutrophication evaluation showed that all the sampling sites reached eutrophication grade except the B1 in the wet season. Principal component analysis (PCA) illustrated that nutrients and organic matter were the main components of pollutants in Lake Dalian wetland. The main sources of exogenous pollution were the effluent discharge from paddy fields and aquaculture ponds, dosage of fish pond feed and water bamboo field fertilizer, sewage discharge from tourists and local residents. The main sources of endogenous pollution were the decomposing accumulation of dead animals and plants, and the release of organic matters from the sediments. The results obtained here can not only serve as the theoretical basis and technical support for ecological management of Lake Dalian wetland, but also provide scientific basis for pollution source control, pollution risk assessment and management strategy implementation.

Abstract:This work studied the key indicators of nutritional level of Lake Poyang in recent three years (2018-2020) including temporal and spatial characteristics of Chl.a, total phosphorus (TP) and total nitrogen (TN). The results showed that the water quality of Lake Poyang was mainly Class Ⅳ, and the pollutants exceeding national standard were TP and TN. The average concentrations of Chl.a, TP and TN during 2018-2020 were 7.6 μg/L, 0.070 mg/L and 1.30 mg/L, respectively. Presently, Lake Poyang is under medium-to-rich nutrient level. All the lake area had eutrophication periods in the year, and in all months of the year, there were regions that were eutrophic. The concentrations of TP and TN in dry seasons were higher than those in wet seasons. TP concentrations were the lowest in August and TN concentrations were lowest during August-September. The season with higher Chl.a concentration was mainly from September to November, especially in September when the mean and median values of Chl.a concentrations were 16 μg/L and 12 μg/L, exceeding the mesotrophic limit value of Chl.a, indicating that the whole lake was eutrophic. The reason was that the velocity of water in the lake slowed down due to the retreat of the "Five Rivers" in September and the mainstem of the Yangtze River still in the flood season. The waters with high Chl.a were the tail waters of rivers entering the lake, shallow lake bay and dished lake waters, such as Nanhu village, Jinxizui, Nanjishan, Banghu lake, etc. The ratio of nitrogen and phosphorus in Lake Poyang was 52, and both of TN and TP were excessive for algae production. TP should be controlled as a key point, and TN control should also be paid attention too. The reference of TP of Lake Poyang is in the range of 0.029-0.054 mg/L, and TN is in the range of 0.50-0.81 mg/L. Considering requirement of water pollution prevention and social and economic development, it is suggested that TP control objective of Lake Poyang is set in 0.050-0.100 mg/L, and TN in 1.00-1.50 mg/L. In addition, for the control section of rivers entering the lake, TP control objectives should be more strict than current 0.20 mg/L, and an appropriate TN standard should be properly established.

Abstract:As one of the new environmental pollutants, neonicotinoid insecticides (NEOs) attract close attention due to their large dosage use and significant water solubility. NEOs are easily transferred into the aquatic environment with surface runoff and results in the threat to the water ecosystem and human health. The solid-phase extraction-ultra performance liquid chromatography-tandem mass spectrometry (SPE-HPLC-MS/MS) was used to analyse NEOs in Lake Poyang, aiming to reveal the spatial distribution characteristics of NEOs and analyse its pollution sources and ecological risks. The results showed that nine NEOs were detected to varying degrees in Lake Poyang Basin, among which dinotefuran (DIN), thiacloprid (THI), imidacloprid (IMI) and thiamethoxam (THIA)) with a detection rate of 100%. The concentration of the total NEOs ranged from 8.8 to 88.9 ng/L, with IMI the highest residues and the widest distribution (concentration range was 2.1-42.7 ng/L) which followed by THIA (concentration range is 0.7-18.3 ng/L). The main source of NEOs in Lake Poyang came from the non-point source input of agricultural activities. The ecological risk of NEOs to freshwater aquatic organisms in Lake Poyang Basin was evaluated based on species sensitivity distribution method and joint probability distribution curve method. The results showed that the order of ecological risk of the five insecticides was IMI> clothianidin (CLO)> THI> THIA> acetamiprid (ACE), and insects were the most sensitive species to NEOs. The joint probability distribution curve of NEOs showed that the adverse impact probability of IMI on 5% and 10% species was 28.55% and 0.3%, respectively. The acute risk of NEOs to aquatic organisms was not significant, while the chronic ecological risk of long-term NEOs application needed further attention.

Abstract:C₂₅ highly branched isoprenoids (HBIs) are generally regarded as biomarkers signaling diatoms and have been widely detected in lakes in the lower-middle reaches of the Yangtze River recently. The lakes in Yunnan Provinces are rich and diverse in diatoms, whereas the reports on C₂₅HBIs still very limit. In this study, we first detected C_25:1HBI and C_25:2HBI in sediments of Lake Erhai, and further revealed the potential biogeochemical significance of C₂₅HBIs by comparing the spatial differences of the proxy and other underlying factors such as TOC, TN, water depth, etc. The isomers of both C_25:1HBI and C_25:2HBI were not detected in all surface sediments in Lake Erhai. The concentration of C₂₅HBIs was 11.18 μg/g(TOC), which was much higher than those in eutrophic Lake Taihu and similar with those in Lake Chenghu. The concentration of C_25:1HBI increased from the north to the south in Lake Erhai, while inverse trends for the concentration of C_25:2HBI. C_25:1HBI in Lake Erhai might come from certain predominant diatoms in the southern lake. In sediments with lower concentration of C_25:2HBI (<0.4 μg/g), there existed a weak positive correlation between C_25:1HBI concentration and C_25:2HBI concentration, while in some shallow areas, the concentration of C_25:2HBI was high (>0.4 μg/g) and the concentration of C_25:1HBI was low. Besides, the correlation between C₂₅HBIs concentration and TOC/TN displayed distinct spatial differences. In the northern part of Lake Erhai, C_25:1HBI-sourced diatoms might be inhibited by a large amount of nitrogen and phosphate fertilizer inputs related to agricultural activities. In the central part of Lake Erhai, the concentration of C₂₅HBIs showed a weak positive correlation with TN, but no correlation with TOC, indicating that the underlying control factors of the C₂₅HBIs-sourced diatoms were relatively complex. In the southern part of Lake Erhai, a strong positive correlation could be observed between C_25:1HBI concentration and TN, and between C_25:1HBI concentration and TOC. But the correlations between C_25:2HBI concentration and TN, and between C_25:2HBI concentration and TOC were weak. The presence of C_25:1HBI in the southern part of Lake Erhai could track eutrophication conditions, which was different from those observed in the middle and lower reaches of the Yangtze River. Meanwhile, C_25:2HBI might be contributed from the diatoms also producing C_25:1HBI and other diatoms adapted to live in shallow water, so C_25:2HBI was strongly affected by the hydrological conditions.

Abstract:Based on the environment monitoring data from 1990 to 2020 and the aquatic plant survey data from 2011 to 2020 in Lake Honghu, the evolution mechanism and regime shift crucial threshold of Lake Honghu wetland are studied. The results showed that the evolution of water quality in Lake Honghu from 1990 to 2020 tended to be summarized into five stages and three periods, showing a deteriorating trend. From 2011 to 2020, the Margalef, Shannon-Wiener, Simpson and Pielou index of aquatic plants in Lake Honghu had decreased by 32%, 59%, 60% and 46% respectively, especially after the flood disaster in 2016. The crucial period for regime shift of Lake Honghu wetland ecosystem is 2002 and 2011 and total phosphorus is the key driving factor. The threshold value of total phosphorus from clean water to turbid water is 0.092 mg/L, and that from turbid water to clean water is 0.051 mg/L. This study provides a reference for the estimation of the pollution reduction load into Lake Honghu, and guidance for the priority control strategy of nutrients in the ecological restoration.

Abstract:Aquatic plants in lakeshore zone play an important role in maintaining the health of lake ecosystem. In order to understand the current status of aquatic plants in the lakeshore zone of Lake Erhai, this study investigated the aquatic plants quarterly in the lakeshore zone from 2020 to 2021. The results showed that there were 206 species of aquatic plants belonging to 56 families and 156 genera in the lakeshore zone of Lake Erhai, including 149 species of hygrophytes, 24 species of emergent plants, 21 species of submerged plants, 7 species of floating plants and 5 species of floating leaf plants. Among all these species, Cynodon dactylon, Zizania latifolia, Vallisneria natans and Trapa bispinosa, etc. were common species, Lonicera japonica, Elymus dahuricus, etc. were occasional species. From the distribution of floristic regions, the species in the lakeshore zone of Lake Erhai were mainly world distribution (83 species) and tropical distribution (55 species), accounting for 40.28% and 26.71% of the total species, respectively. In terms of plant communities, there were 18 main plant community types in the lakeshore zone of Lake Erhai, including 4 types of hyophytes communities, 3 types of emergent plant communities, 9 types of submerged plant communities and 2 types of floating leaf plant communities. Among all the communities, the main dominant communities were Cynodon dactylon communities, Zizania latifolia communities, Vallisneria natans communities and Trapa bispinosa communities. Compared with previous investigations, it was concluded that the diversity of aquatic plants in the lakeshore zone of Lake Erhai had been significantly improved in recent years. However, there are some problems such as the small area of lakeshore zone and the single type of emergent plant community. Therefore, the construction of lakeshore zone should be further strengthened to improve the habitats and increase species diversity.

Abstract:Tian-e-Zhou Oxbow is the first habitats for the ex-situ reserve established for the Yangtze finless porpoise. The rich and abundant forage fish resources are the guarantee for the healthy development of the Yangtze finless porpoise population. Therefore, the change of fish community structure was analyzed with combined of seven fish surveys data in Tian-e-Zhou Oxbow of Yangtze River from October 2017 to April 2021 and historical data before the fishing ban. In view of forage fish of Yangtze finless porpoise, some suggestions were provided for the scientific management of fish resources in Tian-e-Zhou Oxbow. During the investigation, 57 fish species were collected. The biomass of large and medium-sized fish such as Culter mongolicus, silver carp and bighead carp increased significantly while the biomass of small-sized fish such as Hemiculter leucisculus and Pseudobrama Simoni showed a declining trend. The dominant fish species gradually changed from small-sized fish such as Neosalanx taihuensis, Coilia brachygnathus and Hemiculter leucisculus to large and medium-sized fish such as bighead carp, Culter mongolicus and Culter oxycephaloides, in particular, the newly emerged Culter oxycephaloides population developed rapidly. Meanwhile, the body-weight of small-sized fish such as Coilia brachygnathus and Hemiculter leucisculus was decreasing, but the body-weight of large and medium-sized fish was increasing. Because the Yangtze finless porpoise feeds on the small-sized fishes and juveniles of large and medium-sized fishes, the decline of their food resources would threaten the maintenance and growth of the Yangtze finless porpoise population. In order to ensure food supply for the Yangtze finless porpoises in Tian-e-Zhou Oxbow, it suggested that rational capture of silver carp, bighead carp, Culter mongolicus and Culter oxycephaloides should be taken into consideration in order to reduce the food competition pressure of silver carp and bighead carp and the predation pressure of Culter mongolicus and Culter oxycephaloides on small-sized fishes. Besides, another measures to increase the small-sized fish resources like strengthening the connectivity of the oxbow with Yangtze River in the wet season to introduce more small-sized fish resources from Yangtze River and laying artificial fish nests in the oxbow to improve the breeding conditions for the small-sized fish species with adhesive eggs such as sharply should be carried out.

Abstract:It is of great scientific significance to study feature differences of bacterial community structure between sediment and water in the plateau lakes for water ecological management and protection. We selected 19 objective lakes in the central and eastern of the Qinghai-Tibet Plateau, collected the samples of sediment and water, and analyzed differences of bacterial community structures, spatial distributions, co-occurrence network patterns, community assembly processes between sediment and water in lakes based on high-throughput sequencing and statistical analysis. The results showed that alpha diversity indices of bacterial community in sediment, including Chao1, Shannon diversity, Faith's phylogenetic diversity and Pielou's evenness indices, were significantly higher than those in water. Dominant phylum in both sediment and water was Proteobacteria with the relative abundances of 45.33% and 41.77%, respectively, while bacterial community structures in water presented stronger spatial heterogeneity than in sediment. The results of principal coordinate analysis and similarity analysis showed that there were significant differences in bacterial community composition between sediment and water. Altitude and Latitude were the most important environmental drivers in shaping bacterial community compositions in sediment and water. Both sediment and water bacterial community had distance-decay patterns, while bacterial spatial turnover rate in sediment was lower than that in water. The co-occurrence network complexity and stability of bacterial community in sediment were higher than those in water. The mean value of normalized stochasticity ratios in sediment and water bacterial were 0.24 and 0.72, indicating deterministic process played a key role in sediment bacterial community assembly processes, whereas water bacterial community assembly processes was predominant by stochastic process. These findings about bacterial community features, spatial patterns, stability of co-occurrence networks, assembly processes, and their differences of bacterial between lake sediment and water in the Qinghai-Tibet Plateau would provide the scientific support for in-depth understanding mechanisms of bacterial diversity and composition formation in plateau lakes.

Abstract:The construction of Songxin Fishway is one of the important engineering measures to restore the connectivity of Heishui River, an alternative tributary of fish habitat in Baihetan Reservoir area of Jinsha River. In this study, the Songxin Fishway in the Heishui River, monitoring and evaluation of fish passage effect in Songxin Fishway were studied by net catch method and radio frequency identification (RFID) remote sensing technology. The results showed that:(1) A total of 19 species of fish were detected in the downstream of the Songxin dam site of the Heishui River, of which 12 species could reach the upstream river smoothly through the fishway. (2) The fish diversity was influenced by the motivation of fish migration in a seasonal pattern, with the Shannon-Wiener index of fish diversity at the inlet and exit of fish passage being significantly lower in the non-migratory period (December) than in the migratory period (March and May); (3) The import attraction rate of Schizothorax wangchiachii is 31.50% and the passage rate is 44.44%. Using the monitoring data of net, it was found that:when the water temperature exceeded 20℃, the species and quantity of fish decreased with the increase of water temperature; When the discharge in the fishway exceeded 0.50 m³/s, the species and quantity of fish decreased with the increase of discharge in fish passage. In order to identify the key factors affecting the fish passage effect, the S. wangchiachii as a study subject, a Cox proportional hazard regression model was constructed based on multiple factors (fish passage flow, water temperature, circadian rhythm, pH, wet weight, body length etc.). It was found that:fish passage discharge, water temperature and circadian rhythm were the key factors affecting the fish passage effect; When water temperature varied from 14.0 to 20.0℃, the passing rate of S. wangchiachii increased with water temperature. When the discharge in the fishway ranged from 0.01 to 0.50 m³/s, the passing rate of S. wangchiachii showed an increasing trend with the increase of the discharge in the fish channel. The circadian rhythm had a significant effect on the abduction of S. wangchiachii, and S. wangchiachii preferred night upstream migration. The research results are helpful to the fish habitat restoration and the protection of fish resources in Heishui River, and provide a theoretical basis for the construction of fishway in cascade hydropower stations of Heishui River and low head sluice dams of mountainous rivers in southwest China.

Abstract:The development of cascade small hydropower to some extent destroyed river connectivity and hindered fish migration. Songxin fishway was built at Songxin Dam in the Heishui River Basin to meet the need for upward spawning of fish. However, as a key behavior in the life cycle of migratory species, the downstream migration of fish is equally important. In this study, the fish downstream dam-crossing effect was evaluated in the water-recharge channel of Songxin Hydropower Station on Heishui River, a tributary of Jinsha River, in order to seek a feasible, efficient and low-damage downstream fish-crossing method and to understand the influence of different environmental factors in the river on fish downstream efficiency. A total of 90 fish were released into each of the two cyprinid species (Abbottina rivularis, Schizothorax wangchiachii) and two cobitidid species (Paracobitis potanini, Paracobitis variegatus), and the effects of fish species, transparency, fish release site, flow, water temperature and water level on fish downstream efficiency were analyzed. The results showed that the descending rates of Abbottina rivularis, Schizothorax wangchiachii, Paracobitis potanini, Paracobitis variegatus were 22.22% (N=20), 28.89% (N=26), 8.89% (N=8) and 15.56% (N=14), respectively. The descending rate of juvenile fish was positively correlated with water temperature, and fish release site, fish species, water temperature and flow had a significant impact on the descending rate of fish. The binary Logistic regression model was constructed to identify the key factors affecting the downward efficiency of fish, and the optimal model was selected based on AIC criterion. The optimal model was based on fish release site, fish species, water temperature and flow. This study provided theoretical basis and technical support for river connectivity restoration, reservoir scheduling scheme and optimization design of downstream fish-crossing facilities under the background of cascade small hydropower.

Abstract:As the largest nature reserve in the Lake Poyang Basin, Duchang Migratory Bird Provincial Nature Reserve in Jiangxi Province plays an important role in protecting worldwide and national key rare and endangered wintering migratory birds. Exploring the zooplankton community structure and its main influencing factors in Duchang Migratory Bird Nature Reserve can provide a scientific basis for the habitat protection and dynamic assessment of bird habitats in Duchang Nature Reserve. In January, April, July and October 2019-2021, the density, biomass and water environment factors of planktonic crustaceans in four areas in Duchang Nature Reserve were investigated, and the water quality in Duchang Nature Reserve and the relationship between floating animals and water environment factors and chlorophyll-a were analyzed by using biodiversity index, Pearson correlation and redundancy analysis (RDA). The results showed that 31 species of planktonic crustaceans were identified during the investigation, including 20 species of cladocera and 11 species of copepoda. The annual average density of planktonic crustaceans in Duchang Nature Reserve was 213.81 ind./L, and the annual average biomass was 1.51 mg/L. The community structure of crustacean plankton in Duchang Nature Reserve greatly changed across time and space. In seasons, the biomass of planktonic crustaceans peaked in summer or autumn, and the richness of crustacean plankton were the least in winter; Spatially, the density and biomass of planktonic crustaceans in the general area of Nanji (D4) were larger than the general area of Cuojihu (D3), and there was little difference between the core area of Xiaojishan (D1) and the core area of Sanshan (D2). According to the diversity index and evenness index, the water quality of Duchang Nature Reserve was analyzed. The results showed that the water quality environment of the general area of Lake Zuoji (D3) was the best, followed by the core area of Xiaoji Mountain (D1), and then the core area of Sanshan Mountain (D2) and the general area of Nanji (D4). Bosmina coregoni also reflected the gradual eutrophication of the water body in the Duchang Nature Reserve, which was consistent with the local occurrence of cyanobacteria bloom during the investigation. The temporal and spatial succession of planktonic crustacean community structure in the Duchang Nature Reserve was mainly determined by total nitrogen, chlorophyll-a, water level and water temperature. The dominant factors affecting crustacean plankton were different in seasons, i.e. total nitrogen, total phosphorus and dissolved oxygen in winter, water level and chlorophyll-a in summer, and water temperature, water level, transparency, chlorophyll-a, dissolved oxygen and total phosphorus in autumn, respectively.

Abstract:Lake Taihu is a center of flood distribution, water resources regulation, storage, and a barometer of the water ecological environment. Its water level affects the system functions of flood control, water supply, water ecology, water environment, shipping etc., which makes the overall dispatching problem of Lake Taihu increasingly prominent. In this paper, Lake Taihu is taken as the main research object. Based on the monitoring data, mathematical statistics and hydrodynamic model calculation were used to analyze the changes of rainfall, discharge of lake water, water ecological environment and their feedback relationship with the water level of Lake Taihu. Considering the objectives of flood control, water supply, water ecology, water environment and its spatial and temporal differences in risk tolerance during different dispatch intervals, the dispatching water level of Lake Taihu was optimized. On this basis, the dispatching function zoning map of Lake Taihu was proposed. The results showed that, under the conditions of design flood and water supply, the water level of Lake Taihu could be pre-reduced in the early stage and appropriately raised in the late stage through the adjustment of dispatching water level and the overall regulation of river basin water projects. It could effectively guarantee the safety of flood control, water supply and shipping in the basin, and effectively improve the ecological environment of rivers and lakes. It would be continuously improved in practice and exploration, so as to lay a stable foundation for the construction of beautiful Lake Taihu.

Abstract:Due to the increasing problems of lake ecology, the security state of lake ecosystem has become a hot issue of people's attention. It is particularly important to understand the status of lake ecosystem and carry out precise management, ecological restoration and protection according to its health status. Based on the long-term monitoring data and other data collected on the ecological environment of Lake Poyang and its watershed, this paper constructed an indicator system for lake health assessment from six aspects:physical form, hydrology, aquatic environment, water area ecology, wetland ecology and social services by using comprehensive indicator system method. It covered 26 indexes, such as lake entrance condition, variation degree of runoff from five inflow rivers to the lake, and water quality compliance rate (as to national creteria) of rivers to the lake. According to the set threshold level, the health status of each level of Lake Poyang ecosystem health was evaluated, and the scores of the criteria layer and the target layer were obtained by weighting scores of each index of lake ecosystem. Finally, the objective evaluation of Lake Poyang ecosystem health was carried out. The results showed that the constructed system had strong pertinence, comprehensive science and operability, and could provide a case and method reference for the ecosystem health evaluation of Lake Poyang and similar Yangtze river-connected lakes. The evaluation results showed that the target level score of Lake Poyang health signs was 73.45, and the evaluation result was sub-healthy. The health of Lake Poyang aquatic ecosystem was mainly affected by discharge capacity, hydrological regime change, eutrophication degree and species diversity. Finally, according to the aquatic ecosystem health rating of Lake Poyang, this paper raised some problems that need to be solved in Lake Poyang aquatic ecosystem protection, and puts forward countermeasures and suggestions for its protection.

Abstract:Under the dual influence of climate change and human activities, the river runoff in the Xijiang River Basin has changed to varying degrees. It is of great significance to re-understand and master the temporal and spatial evolution law of runoff in the changing environment for the scientific management of the river basin. Based on the daily runoff data of 7 controlled hydrological stations in the main and tributaries of the Xijiang River Basin for nearly 60 years, and comprehensive extreme-point symmetric mode decomposition method (ESMD), Mann-Kendall test, R/S analysis, wavelet analysis and other methods, the temporal and spatial evolution characteristics of runoff were analyzed from multiple time scales of the inter-decadal, inter-annual, seasonal and intra-annual. The results showed that the runoff in the Xijiang River Basin alternated and varied greatly at the inter-decadal scale. The runoff was abundant in the 1970s and 1990s, and dry in the 2010s. Rainfall affected the runoff, and the upper and middle reaches of the Basin were more prone to flood and drought disasters. The oscillation periods of seasonal runoff were 2-7, 15-20, 28-29 a; 3-5, 7-10, 20 a; 2-3, 6-8, 12-15 a and 3-8, 12-15, 20 a, respectively, all of them showed double period characteristics of interannual and interdecade. The variation of runoff was mainly determined by IMF1. Affected by rainfall and reservoir regulation and storage, there are a downward trend in the annual, summer, and autumn runoff, a persistent predicted downward change, and an upward trend in the overall spring and winter runoff. The change trend in the upper reaches of the space is more significant. The annual and seasonal runoff increased abruptly after 1980, especially from 2000 to 2010. Human activities and climate change were the main reasons for the sudden change in the Xijiang River Basin. After 2000, except the tributary Liuzhou station, the annual distribution of runoff tended to be uniform, the variation range of monthly average runoff decreased, the concentration period moved forward, and the reservoir group had a significant effect by storing water during the wet season and replenishing water during the dry season. The results provide reference for the rational development, utilization and effective management of water resources in the Xijiang River basin.

Abstract:In order to investigate the impact of hydrological data assimilation scenarios on streamflow simulations of hydrological models, a data assimilation framework was proposed based on the Xin'anjiang model and the ensemble Kalman filter, in which remotely sensed evapotranspiration products and measured streamflow were used as observation data. Based on this framework, four assimilation scenarios including DA-ET, DA-ET(K), DA-ET-Q, DA-ET-Q(K) and a comparative scenario (i.e., OL) were designed and tested in Ganjiang River Basin, aiming to evaluate the impact of the time resolution of remote sensing evapotranspiration products, whether the time-parameters related to model evapotranspiration and multi-source data assimilation on runoff simulation after hydrological data assimilation. The results showed that assimilation of two evapotranspiration products with different time resolutions could improve the accuracy of model's streamflow simulations in the DA-ET scenario, and model performed better by assimilating the products with higher temporal resolutions. On the basis of DA-ET scenario, adding measured streamflow into assimilation could improve the accuracy of model's streamflow simulations, and the reduction of the relative error obtained by DA-ET(K) and DA-ET-Q(K) scenarios was more than 20%. The results indicated that the accuracy of streamflow simulations was improved through assimilation of ET products, especially when the model evapotranspiration parameter was treated to be time-varying. Compared to the OL scenario, four assimilation scenarios could improve the accuracy of model's the 10% highest streamflow simulations in different degrees, but the other scenarios with no significant difference was better than DA-ET-Q(K) scenarios. Our results could further understand the differences of streamflow simulation in different data assimilation scenarios and provided scientific basis for the efficient utilization and management of water resources.

Abstract:Global evapotranspiration products from GLEAM (Global Land Evapotranspiration Amsterdam Model) and MOD16 (MODIS Global Evapotranspiration Project) have been widely validated and applied. However, due to insufficient observational data, there is still a lack of products validation in plateau areas. This study took Lake Ranwu, Yamzho YumCo, Nam Co, Siling Co and Taro Co basins as study area, and applied basin water balance, correlation coefficient, relative error, root mean square error and Kling-Gupta coefficient to verify and evaluate the accuracy of GLEAM and MOD16 products in the Qinghai-Tibet Plateau. The results demonstrated that GLEAM products were underestimated in Lake Ranwu, Siling Co and Taro Co basins, slightly overestimated in Yamzho YumCo and Nam Co basins, while MOD16 products were slightly underestimated in Siling Co basin and overestimated in other lake basins. GLEAM and MOD16 products were overestimated in dry year, and underestimated in wet year in the five lake basins. GLEAM products showed high accuracy in Lake Ranwu, Yamzho YumCo and Siling Co basins, while MOD16 products had high accuracy in Nam Co and Taro Co basins. In general, the accuracy of GLEAM products was significantly better than that of MOD16 products in the central and southeastern lake basins of the Qinghai-Tibet Plateau on the annual scale and the multi-year average scale. This results provided an important reference for the selection of evapotranspiration products suitable for the Qinghai-Tibet Plateau.

Abstract:The phenomenon of lateral erosion and collapse of bank is widely distributed in major rivers in the world, which is a kind of natural disaster with great harm. Seasonal frozen rivers are affected by the coupling of hydrodynamics and freeze-thaw, and the mechanism of bank collapse is complex. The research on the collapse process is of great significance to river bank protection and comprehensive river management. Taking the downstream part of the main stream of the Songhua River near the dam of Dadingzishan Aviation Power Project as an example, the BSTEM model was used to analyze the bank slope stability in different periods by considering the freezing and thawing effect, and the collapse process of the dual structure bank was simulated. The results showed that the stability of the river bank was higher in the dry and high water periods, and the stability was lower in the flood period and the receding water period, which was the stage of frequent bank collapse. Considering the freezing and thawing effect, the simulated river bank slope shape and collapse width were in good agreement with the measured values, and the amount of erosion and collapse was increased by 7%-41%. The research results can provide some reference for the study of river bank collapse in seasonally frozen areas.

Abstract:Reservoirs are pivotal sources of greenhouse gases (GHGs) to the atmosphere that may influence regional climate change. However, the spatial heterogeneity and the vertical characteristics of dissolved GHGs in reservoirs are still poorly understood. This study aimed to reveal the characteristics of greenhouse gases dissolution and emission fluxes in reservoirs during stratification and mixing periods, and to provide important supports to clarify the key processes of GHGs production and emission in reservoirs. We selected Tanghe Reservoir, a large reservoir in Northeast China, and conducted vertical stratified monitoring of dissolved GHGs at different locations (in front of dam, mid, end transects) from July-September and October 2021 (representing the stratification and mixing periods, respectively). The results showed that the CH₄ fluxes ranged from 0.018-0.174 mmol/(m²·d) and all sites were the source of CH₄, the spatial distribution of CH₄ fluxes was end>mid>in front of dam. The CO₂ fluxes ranged from -4.91-58.77 mmol/(m²·d), and all sites were the source of CO₂ except for the eastern end transect during the stratified period, and the contents of spatial distribution of CO₂ fluxes was in front of dam>mid>end. In terms of temporal differences, CH₄ fluxes in the stratified period ((0.071±0.044) mmol/(m²·d)) were higher than in the mixed period ((0.027±0.008) mmol/(m²·d)), and CO₂ fluxes in the stratified period ((5.12±5.50) mmol/(m²·d)) were lower than the mixed period ((42.76±16.72) mmol/(m²·d)), and both water temperature and runoff affected CH₄ and CO₂ fluxes. The vertical distribution characteristics of dissolved GHGs indicated that the hot zone of reservoir CH₄ generation was mainly in front of dam and end transects. The CH₄ was heavily accumulated in the bottom water of the dam front and mid transects during the stratified period due to the persistent anoxia in the hypolimnion and the limitation of the exchange capacity in the water column. The comparison with other reservoirs GHGs fluxes in China revealed that the different periods (stratification and mixing period), spatial variations (in front of dam, mid, and end transects) and reservoir ages were the key factors for accurate assessment of reservoir GHGs emissions.

Abstract:The total installed capacity of the four cascade hydropower stations in the lower reaches of the Jinsha River is equivalent to two Three Gorges reservoirs. They are the source project of the central region of the "west to East power transmission". The benefits of the project are of great significance to economic and social development. Since 2012, Xiangjiaba, Xiluodu, Wudongde and Baihetan power stations had successively stored water and operated to intercept the sediment of Jinsha River layer by layer. From 2013 to 2020, the average annual sediment discharge from Xiangjiaba decreased to 1.52 million tons, a decrease of more than 99%. A large amount of sediment was deposited in the cascade reservoirs and scoured to the river channel below Xiangjiaba. Based on the observation data since the preparation of the lower Jinsha River project, this paper studied the sediment deposition of cascade reservoirs and the scouring law of the river channel downstream of the dam. The results showed that from the completion and operation of the four cascade hydropower stations in the lower reaches of Jinsha River to the end of 2020, the accumulated sediment deposition was about 598 million m³, of which the sediment deposition in Xiluodu Reservoir accounted for 92.5%. The sediment discharge ratios of Xiluodu and Xiangjiaba reservoirs from 2013 to 2020 were 2.64% and 22.2%, respectively. The reservoir sediment was mainly deposited in the main stream channel of the perennial backwater area, mainly in the dead storage capacity, and the proportion of occupying the effective storage capacity was less than 1.3%. The canyon characteristics of the main stream channel in the reservoir area of the lower reaches of Jinsha River were obvious, and the siltation was mostly the flat siltation of the main channel. The sediment deposited in Xiluodu and Xiangjiaba reservoir areas had obvious sorting characteristics along the way. The closer to the front of the dam, the medium particle size decreasd, the percentage of fine sediment increased, and the very fine sediment would be deposited in a large amount in a certain range of the reservoir area. The downstream riverbed of Xiangjiaba was generally scoured, but the scouring intensity was weakened obviously. The thalweg high convex section of the downstream channel of Xiangjiaba was relatively stable, so that the low water level at the downstream of the dam had not decreased significantly, and the cumulative decline of the low water level at 2000 m³/s was about 0.43 m. The riverbed at the downstream of the Xiangjiaba dam was obviously coarsened.

Abstract:Lake sediments record large amount of environmental information from the watershed over a long time, and has become a high-resolution archives for investigating the lake environmental change at the centennial scale. In this paper, we analyzed the features of radionuclide profiles through the combined dating method of ²¹⁰Pb_ex and ¹³⁷Cs, investigated the temporal and spatial changes of sedimentation rate, and extracted the environmental information indicated by sediment particle size to reveal the patterns of the sedimentary environment during the last century. The results showed that ²¹⁰Pb_ex profiles presented four stages:a multi-stage decay, a mixed-decay-equilibrium, a decay-background and a no-deposition. The sediment depths of 200 cm in the three major lake bodies covered a chronological span of 127-162 years, and a depth-age relationship was constructed for Lake Dongting sediments. The mean mass accumulative rate (MAR) ranged from 1.47 to 3.11 g/(cm²·a), demonstrating an overall spatial characteristic of the highest MARs in the south, and higher in the west than the east. Since 1900, MARs in Lake Dongting increased 3-7 times, showing an upward trend in the chronology. The depositional change of sediment grain size in Lake Dongting was characterized by three stages and two transitional periods, which corresponded well with anthropogenic activities such as deforestation, hydrological construction, and afforestation in the basin in the period. These findings suggested that anthropogenic activities had been an important factor for sediment deposition in Lake Dongting. The foundings in our study can provide a theoretical reference for enhancing the ecological protection of the lake and its watershed.

Abstract:The Indian summer monsoon (ISM) is an important part of the global monsoon system. In southwestern China, the abnormal variations of the ISM not only cause abrupt changes of the climate and the ecological environment, but also bring huge economic losses to the society. The late Holocene is the most recent period in geological history. Therefore, understanding the history and mechanisms of the ISM and its influence on the regional hydrology during the late Holocene is important and helpful for predicting the future monsoon precipitation changes in southwestern China. Lake Gongka (3529 m above sea level) is an alpine lake with limited influence from human activities in the Hengduan Mountains and the climate in the region is mainly affected by the ISM. Lake Gongka is sensitive to climate change, making it an ideal region for paleoclimatic research. A 204.5-cm-long continuous and high-quality sediment core was drilled from the center of Lake Gongka (GK20B) by using a steel-barreled Livingstone square-rod piston corer in October, 2020. Four samples of terrestrial plant materials and four total organic matter samples from the core were dated by using the AMS-¹⁴C dating method. Based on the analysis of climatic and environmental proxies including XRF-based elements content, loss-on-ignition (LOI), total organic carbon and nitrogen (TOC, TN) contents and organic carbon isotope (δ¹³C_org), we reconstructed the hydrological changes of the watershed since 3300 cal a BP. The sedimentary lithology which was the inter-bedding between peat and lacustrine sediments and the corresponding changes of climatic and environmental proxies revealed the huge fluctuations of the lake water level during the late Holocene. The results showed that the water level of Lake Gongka had been continuously decreasing in volatility during the late Holocene, which possibly reflected the decreasing trend of the ISM intensity. This trend most likely be associated with the decreasing summer solar insolation in the Northern Hemisphere. In addition, the lake level variations also revealed that the intensity of the ISM fluctuated on centennial time scales, which might be related to the influence of air-sea interactions within the earth's climate system.