Abstract:Lake evaporation is one of the key links between the hydrological cycle and energy balance, and plays a crucial role in understanding the mechanism of lake-atmosphere interactions. However, due to different morphological characteristics, geographical location and climatic background, the evaporation characteristics of each lake are different on different time scales. There are complex physical driving processes, time-scale dependent feedbacks and spatial heterogeneity of lake evaporation. Therefore, the accurate measurement and quantification of lake evaporation is still an important task in limnology, hydrology and meteorology. Here, we first review the main observational methods of lake evaporation and its variability characteristics on diurnal, seasonal, interannual and interdecadal time scales. Then, the effects of meteorological factors on different time scales, lake morphometry (e.g. surface area, water depth, etc.) and geographical factors (e.g. latitude, altitude, etc.) on lake evaporation are discussed. Finally, the progress of lake evaporation research since the application of high-precision instruments such as the eddy covariance system was summarised.

Abstract:Algal bloom, caused by abnormal proliferation of algae, is one of the major ecological disasters threatening the safety of many drinking water resource reservoirs in China. Investigating the driving factors of algal proliferation in drinking water resource reservoirs is a prerequisite for preventing and controlling algal blooms and water quality problems. To investigate the driving factors of algal blooms in reservoirs, the cell density and biomass of phytoplankton and related environmental factors were analysed during 12 months from Fushi Reservoir and Laoshikan Reservoir in the Taihu Basin. The results showed that reservoirs in the subtropical monsoon region were at risk of proliferation of cyanobacteria and diatoms. A severe diatom bloom dominated by Achnanthes occurred in Laoshikan Reservoir in May, and the corresponding cell density of diatom reached 6232×10⁴ cells/L. Cyanobacterial blooms occurred in June and September in Fushi Reservoir. The result of redundancy analysis indicated that total phosphorus, water temperature and water level were key factors influencing the variation of phytoplankton community structure in reservoirs. A total phosphorus concentration higher than 0.024 mg/L could promote algal proliferation. The risk of cyanobacteria and diatom blooms increased significantly when the water temperature was higher than 20℃ and 16℃ respectively, while the best water temperature for cyanobacteria and diatom blooms was 30℃ and 22℃ respectively. Low or drastic fluctuations in water level could be beneficial for algal blooms. Our results indicate that the abnormal proliferation of phytoplankton in mesotrophic drinking water reservoirs is influenced by nutrient, hydrological and meteorological factors. External nutrient loading should be reduced to prevent and control algal blooms in such mesotrophic reservoirs, and extreme weather events, hydrological processes and food chain regulation should also be considered.

Abstract:From 2008 to 2020, we measured chlorophyll-a (Chl.a) and major environmental factors at five monitoring sites in Xiaojiang River, Three Gorges Reservoir, on a quarterly basis, analysed the spatial and temporal variation characteristics of Chl.a, explored the correlation between Chl.a and environmental factors in water bodies, and analysed the major environmental factors affecting the change of Chl.a in Xiaojiang River. The results showed that the annual average concentration of Chl.a was (20.69±9.41) μg/L with a variation of 5.96-33.90 μg/L. The Chl.a concentration varied seasonally, and the overall characteristics showed that the Chl.a concentration was significantly higher in spring, summer and autumn than in winter, and peaked at 34.48 μg/L in spring. The spatial distribution of Chl.a concentration showed an inverted “N” type distribution pattern, with the highest in the Huangshi section (29.5 μg/L) and the lowest in the Yanglu section (15.23 μg/L). Pearson correlation analysis showed that Chl.a concentration in Xiaojiang was significantly and positively correlated with COD_Mn, conductivity, pH and DO, and significantly and negatively correlated with NO^-₃-N, PO^3-₄-P and Secchi depth. Stepwise regression analysis was used to screen the five most important environmental factors affecting Chl.a. Temporally, the most important environmental factors affecting Chl.a in different seasons were turbidity in spring, DO in summer, NH₃-N in autumn, water temperature in winter. Spatially, the most important factors influencing Chl.a from upstream to downstream in different reaches were conductivity in Qukou, COD_Mn in Yanglu, PO^3-₄-P in Goyang, conductivity in Huangshi, DO in Shuangjiang. DO and COD_Mn were the most important environmental factors affecting Chl.a in Xiaojiang River, where DO had the largest direct effect on Chl.a concentration with a direct path coefficient of 0.451, and COD_Mn had the largest indirect effect on Chl.a concentration with an indirect path coefficient of 0.204. Fitting analysis showed that Chl.a and DO, Chl.a and COD_Mn were best fitted with an exponential function, followed by a power function and finally a linear function. This study is important for the in-depth understanding of the spatial and temporal variation patterns and the main influencing factors of chlorophyll-a, and for the management of eutrophication in Xiaojiang River and other tributaries in the Three Gorges Reservoir area.

Abstract:The water level in Three Gorges Reservoir exhibited drastic change (30 m) within a year, i.e. fall period of water level decrease (April to May), flood period of low water level (June to August), storage period of water level increase (September to October) and high-water period (from November to next March). To study the response of phytoplankton functional groups to changes in water level in Xiangxi Bay (XXB), Three Gorges Reservoir, the characteristics of phytoplankton and physicochemical factors were monitored monthly in 2017. The results showed that phytoplankton species were detected as 7 phyla, 65 genera, 79 species. According to the functional group classification method, the phytoplankton in XXB could be divided into 21 functional groups, among which C, D, F, G, J, Lo, M, P, X2 and Y were the dominant functional groups. C and P functional groups, which are adapted to high nutrient conditions, dominated throughout the year, indicating that XXB was a moderately eutrophic water body. Functional groups (F, G, X2, Y) preferring less disturbed waters and moderately eutrophic waters during the low flow period became dominant taxa; functional groups (G, J, M) preferring stable eutrophication, tolerating high light intensity and being sensitive to scouring during the high flow period became dominant taxa; functional groups (C, P, D) preferring mixed moderately eutrophic waters, tolerating low light intensity and being sensitive to water stratification during the high flow period became dominant taxa. The compositional structure of phytoplankton functional groups changed significantly with water level fluctuations. The comprehensive trophic state index indicated that XXB was mesotrophic during the storage and flood periods, and eutrophic during the other two periods (39.84≤TSI_M≤60.9). The ecological status index values indicated that the water quality of XXB was “good”. Both indicated a deterioration in water quality with decreasing water levels. RDA analysis showed that water level not only directly affected the phytoplankton functional group, but also significantly affected the dominant phytoplankton functional group and its biomass through other factors such as total nitrogen, transparency, COD_Mn, conductivity and pH. The study confirmed that water level fluctuations had a significant effect on the phytoplankton functional group in Xiangxi Bay and demonstrated the possibility of phytoplankton regulation by reservoir operation.

Abstract:Dabokou is located in the south of Caohai, Lake Dianchi, with an area of 0.52 km² and an average water depth of about 2 m. As a typical severely eutrophic lake, Dabokou has undergone two phases of ecological restoration in 2015 and 2019. In recent years, the effects of the restoration have begun to be seen. In order to analyse and explore the reasons for the improvement of water quality and ecosystem in the successful restoration waters, this study selected the continuous monitoring data from February 2015 to December 2021 and divided Dabokou into four parts from A1 to A4 according to the engineering situation and water ecological situation. Firstly, the change trends and correlations of suspended solids (SS), chemical oxygen demand (COD_Cr), total phosphorus (TP), total nitrogen (TN) and chlorophyll-a (Chl.a) in the four areas were analysed. Secondly, the restoration effects of different ecological projects were investigated. Finally, the restoration effects of Dabokou were analysed by comparing with Caohai and Waihai. The results showed that after treatment, COD_Cr, TP and Chl.a decreased stably from A1 to A4 area. COD_Cr decreased by 18.65, 27.96, 25.26, 40.92 mg/L; TP decreased by 0.11, 0.10, 0.11, 0.14 mg/L; Chl.a decreased by 0.037, 0.068, 0.067, 0.106 mg/L, respectively. SS showed strong seasonal variations and TN showed a continuous decreasing trend only in area A4, the south-eastern part of Dabokou, with a decrease of 0.68 mg/L. Correlation analysis showed that COD_Cr and TP had the strongest correlation with Chl.a, which could be the key factors affecting algal growth. The water diversion and exchange project reduced nutrient concentrations and accelerated water circulation, and the microfilter cleaning and algae removal project reduced algal biomass and the risk of algal pollution. And through the purification functions of submerged macrophytes, the plant restoration project provided conditions for the formation of the macrophyte-dominated state lake. It is considered that the plant restoration project is the most direct, economical and long-term measure among the above projects. Comparing Dabokou with Caohai and Waihai, it can be concluded that under the same geographical and climatic conditions, the ecological restoration of the south-eastern water of Dabokou has achieved some success. Overall, the Dabokou area, as a case of successful restoration of the severely eutrophic, algae-stable state lake, its experience has important reference value for the further ecological environment restoration of Dabokou itself, Caohai and even the whole of Lake Dianchi.

Abstract:Submerged macrophyte restoration is the key to ecological restoration of eutrophic lakes. Sediment type and species abundance play an important role in the growth and community stability of submerged macrophytes. This study aimed to explore the effects of different sediments types and species abundance on the growth and root development of submerged macrophytes, and to provide technical parameters for sediment selection and species configuration for the study of submerged macrophyte restoration techniques in eutrophic lakes. The study selected Vallisneria natans (Lour.) Hara, Potamogeton maackianus A. Bennett, Myriophyllum spicatum L., Elodea nuttallii (Planch.) H.St.John), four submerged macrophytes, 11 species combinations were configured according to species abundance (1, 2, 3, 4 species), and four sediments types were selected: without lax layer (a1, a2) and with lax layer (b1, b2), for a total number of 44 treatments (n=11×4). Fourteen growth and root functional traits were measured: above ground biomass, below ground biomass, total biomass, shoot height, maximum root length, total root length, total root surface area, total root volume, average root diameter, total root tip number, number of root branch, number of root cross, root to crown ratio, and root specific surface area. The results showed that (1)the total biomass,below ground biomass, shoot height, maximum root length, total root length, total root surface area, total root volume, average root diameter, total root tip number, root branch number, and root cross number decreased significantly with the increase of submerged macrophyte abundance. (2) total root surface area and total root tip number,were significantly different between sediments types, and most of these traits were significantly higher in the sediment treatment with a lax layer than in the sediment treatment without a lax layer. (3) Among the seven mixed species combinations, M. spicatum competitive ability is stronger in mixed planting than in monoculture at V. natans+M. spicatum, V. natans+M. spicatum+E. nuttallii and V. natans+P. maackianus+M. spicatum+E. nuttallii three mixed combinations, and the competitive ability of V. natans was stronger monocultures than in mixed planting at V. natans+P. maackianus, V. natans+M. spicatum, V. natans+E. nuttallii, V. natans+P. maackianus+M. spicatum, V. natans+P. maackianus+E. nuttallii, V. natans+M. spicatum+E. nuttallii, V. natans+P. maackianus+M. spicatum+E. nuttallii seven mixed combinations, and P. maackianus was stronger monocultures than mixed planting in three mixed combinations, including V. natans+P. maackianus+M. spicatum, V. natans+P. maackianus+E. nuttallii and V. natans+P. maackianus+M. spicatum+E. nuttallii, no significant difference in E. nuttallii. Our study suggests that in the development of submerged macrophyte restoration technology for eutrophic lakes, monoculture submered macrophyte are more appropriate in the early stage of the restoration project, and highlights that lax substrate and the presence of V. natans and M. spicatum are beneficial to the growth and root development of submerged macrophyte communities.

Abstract:Cyanophage can proliferate rapidly in the late stages of cyanobacterial blooms. Cyanophage lysis of cyanobacteria has been considered an important pathway that can induce cyanobacterial apoptosis. However, few reports have focused on the use of cyanophages. In this study, the infection of Anabaena sp. PCC 7120 by cyanophage A-4(L) was used as an example to investigate the influence of illumination, temperature and multiplicity of infection on cyanophage lysis, which can help us to determine the best timing and concentration of cyanophage addition. Our results showed that illumination was a key factor affecting cell lysis. A-4(L) lysed Anabaena sp. PCC 7120 after 8 hours at a MOI of 0.01 during continuous illumination, whereas no lysis occurred during dark incubation. In addition, prolonged illumination resulted in more rapid infection. Further studies showed that adsorption was not dependent on illumination. However, intracellular replication of cyanophage DNA was inhibited under dark conditions. It is speculated that the replication of A-4(L) DNA may be related to the photosynthesis of the host. Within the range of 15-25℃, higher temperature improved the lysis rate. The extracellular A-4(L) titer also increased with higher temperature. In the range of 10^-6-1, A-4(L) lysed cells 4 h earlier when the MOI increased by two orders of magnitude. Taken together, we proposed the addition of cyanophage at a MOI of 0.01 between 7:00 and 12:00 for the purpose of bloom treatment. Further field experiments showed that the addition of cyanophage at a MOI of 0.01 at 7:00 am did indeed significantly reduce the biomass of PCC7120 algal cells by 76% within 24 hours. This study provides a basis for using cyanophages to control the abundance of cyanobacteria during cyanobacterial blooms.

Abstract:The highly efficient CO₂ concentration mechanism (CCM) gives cyanobacteria a competitive advantage under low CO₂ conditions. However, with the rapid increase in atmospheric CO₂ concentration, little information has been presented on how the CO₂ concentration mechanism of cyanobacteria responds. Therefore, this study takes a common blooming cyanobacterium, Microcystis, to investigate in the present by monitoring the dynamic changes and different CCM genotypes of Microcystis in Lake Dianchi. At the same time, the microevolutionary characteristics and dynamic changes of CCM genotypes in Microcystis were also discussed. Furthermore, the competing effects of different CCM genotypes in Microcystis at high (0.08%), medium (0.04%) and low (0.02%) CO₂ concentrations (V/V) were compared. The results showed that the concentration of inorganic carbon (IC) in Lake Dianchi was spatially variable at four sampling sites, but all sites showed a trend of first decreasing and then increasing IC concentrations, and HCO^-₃ was the major form of IC. At the four sampling sites of DDH, GYS, LLH and STS, the sbtA genotype Microcystis was dominant and its relative abundance was much higher than that of the bicA genotype Microcystis. In different bloom periods, the bicA and sbtA genotypes of Microcystis showed opposite trends, i.e. the relative abundance of the sbtA genotype Microcystis gradually increased from early to mid- bloom, while the bicA genotype Microcystis increased only in late bloom. The competition experiments showed that the bicA genotype had an apparent competitive advantage in a high CO₂ environment and showed a trend of transition from the bicA genotype to the sbtA genotype with decreasing CO₂ concentration. These indicate that the IC concentration was relatively low in the middle of the bloom period, resulting in the sbtA genotype showing a competitive advantage in a carbon-limited environment, while the bicA genotype, a sensitive type to changing CO₂ concentration, gradually increased with increasing IC concentration. In addition, the genotypes of Microcystis showed a bicA→sbtA→bicA transition during different Microcystis bloom periods in Lake Dianchi due to the effects of total alkalinity, pH and Chl.a. These results suggest that Microcystis can maintain its competitive advantage and sustain the bloom by regulating the algal strains with different CCM genotypes in response to the IC concentration in the water.

Abstract:Phosphorus is the main indicator of eutrophication in Lake Taihu. In order to understand the trends and characteristics of the internal and external total phosphorus (TP) load in Lake Taihu, the main sources of TP in the lake and the reasons for the high fluctuation of TP concentration were analysed from the dynamic equilibrium of TP pollution load. The mass balance of TP load in different ways into and out of the lake was estimated and analysed based on a long series of water quality monitoring data. The results showed that the input TP load of rivers was about 1835-2799 t during 2007-2020 as the main source, accounting for 55%-73% of the TP load in Lake Taihu. The TP load transported by dry and wet deposition was 353-1380t, which accounted for about 12%-38% and was the second largest input pathway of TP load in Lake Taihu. Meanwhile, 8%-15% of the TP load was retained in the water of Lake Taihu. However, the TP discharged from the outlet river was about 516-906 t, accounting for about 13%-30% of the TP load in Lake Taihu; the TP removal by harvesting aquatic animals and plants was 115-312 t, accounting for 4%-12% of the TP load in Lake Taihu; and the percentage of TP output by drinking water extraction was about 2%-3%. Therefore, about 41%-74% of the TP load remained in the lake, which is an important endogenous factor influencing the epilimnetic TP concentration in Lake Taihu. In addition, the increase in temperature and water flow velocity in Lake Taihu had both accelerated the release of TP from sediment into water, resulting in sediment being one of the limiting factors for TP control.

Abstract:Ferric iron [Fe(Ⅲ)] reduction coupled with anaerobic ammonium oxidation (Feammox), a novel nitrogen removal pathway discovered in recent years, may play an important role in nitrogen self-purification in aquatic ecosystems. However, current research on this process is mainly focused on wastewater treatment or agricultural soils, with limited studies in natural lakes. In this study, sediments collected from different areas of Lake Taihu were anaerobically cultured and the bacteria involved in the iron and nitrogen cycles, the factors influencing Feammox and the potential rates in the sediments were investigated using high-throughput sequencing and isotope technology. During anaerobic culture, changes in the concentrations of iron [Fe(Ⅱ)] and various forms of nitrogen, as well as the morphology and colour of the sediments, indicated the presence of Feammox in the sediments, with nitrate (NO^-₃) being an important product of ammonia (NH⁺₄) oxidation. The addition of NH⁺₄ can promote the reduction of Fe(Ⅲ), but excessive NH⁺₄ can have an inhibitory effect. In the treatments with different Fe(Ⅲ) additions, there were no significant changes in NH⁺₄-N (P>0.05, ANOVA). However, the NO^-₃-N concentration in the treatment with the highest Fe(Ⅲ) addition was significantly higher than in the other treatments (P<0.01, ANOVA), indicating that Fe(Ⅲ) addition could promote the production of NO^-₃-N. The concentration of NO^-₃-N increased sharply and then decreased rapidly during anaerobic culture, accompanied by a significant increase in the relative abundance of genera associated with denitrification, indicating that the coupling of Feammox and denitrification occurred in the system. The results of the isotope enrichment experiment showed that the potential rates of Feammox were relatively high in different regions of Lake Taihu (0.17-0.51 mg N/(kg·d)), indicating that Feammox may play an important role in the nitrogen self-purification of Lake Taihu.

Abstract:Deep reservoirs with stable stratification often ‘turn over’ in winter, leading to a decline in overall water quality and affecting the safety of drinking water. To explore the mixing characteristics of water bodies during the thermal stratification period in southern reservoirs, long-term in-situ monitoring of physical and chemical indices of Tianbao Reservoir in Nanning was conducted from November 2019 to February 2020 during the thermal stratification fading in winter. Combined with the vertical diffusion coefficient of water, the variation characteristics of water temperature (T) and dissolved oxygen (DO) in the whole process of thermal stratification of the reservoir were described by stages (thermal stratification, weakening, complete mixing and formation), and the influences of meteorological factors (air temperature, solar radiation, wind speed, etc.) and hydrodynamic factors (vertical diffusion coefficient K_z ) on the physical and chemical indices of water were discussed. The results showed that the temperature stratification and the dissolved oxygen stratification of the reservoirs were out of sync during the extinction of the thermal stratification, and the destruction of the dissolved oxygen stratification lags behind that of the water temperature stratification, and its structural instability was the result of the combined action of air temperature and solar radiation. During this period, the vertical diffusion coefficient of the water body varied from 0.16 to 380.36 m²/d, with an average value of 30.46 m²/d. The main controlling factor of surface K_z change was air temperature change, and surface K_z was more sensitive to air temperature change than T and DO (R_Kz =-0.44, R_T=0.25, R_DO=0.25, P<0.01). The change in bottom K_z was consistent with the change in water mixing, which could more accurately characterize the vertical mixing of water. The vertical distribution of T, DO and lg K_z in the water body was temporally heterogeneous. The lg K_z was positively correlated with T and DO during thermal stratification and semi-mixing (R_T =0.60, R_DO =0.76, P<0.01), while it was negatively correlated with T (R_T=-0.19, P<0.01) and had no correlation with DO (R_DO =0.04, P>0.05) after complete mixing. Thermal stratification extinction provides environmental conditions for water quality deterioration, so we should pay attention to the change of meteorological factors in winter and take precautions against it in reservoir monitoring and management.

Abstract:The phenomenon of sudden water quality degradation caused by thermal stratification extinction in reservoirs has attracted widespread attention. The reservoirs are mostly warm lakes in southern China that are mixed once a year, resulting in cyclical water quality degradation. However, research on high-frequency monitoring of thermal stratification decline is relatively limited. To investigate the spatial and temporal variation patterns and driving factors of water mixing processes during the thermal stratification extinction period of reservoirs in southern China, taking Tianbao Reservoir in Nanning City, Guangxi as an example. The physical and chemical indices of multi-point water bodies in the reservoir were monitored in situ during the winter thermal stratification extinction period (from November 2019 to February 2020), and meteorological and hydrological data were collected using self-designed weather stations. The results show: (1) During thermal stratification extinction, the transition zone can reach a full mixing state in a short time and the anoxic zone disappears synchronously. The transition zone of the reservoir reaches the full mixing state about 40 days earlier than that of the lake. The depth of the mixed layer increased from 6.85 m to 13.65 m, the anoxic zone gradually decreased and the anoxic index (AI) decreased from 0.40 to 0.07 until the anoxic zone disappeared after the water was completely mixed. (2) Meteorological factors are the main drivers of changes in the thermal stratification structure. Temperature (T) and radiation (R) show a significant negative correlation with mixed layer depth (MLD) (R_T=0.93, R_R=0.93, P<0.01). Wind speed shows a significant positive correlation with MLD (R_WS=0.73, P<0.01). Temperature change is the key factor for the failure of the thermal stratification structure. (3) The physical and chemical indicators of the water body during the period of thermal stratification extinction were significantly influenced by the degree of mixing. Water temperature shows a significant negative correlation with MLD (R=0.87, P<0.01). DO, pH and ORP show a significant positive correlation with MLD (R_DO=0.75, R_pH=0.76, R_ORP=0.75, P<0.01). Thermal stratification extinction provides the environmental conditions for sudden water quality deterioration. It is necessary to pay close attention to the influence of changes in physical and chemical indices on the water quality of the whole reservoir.

Abstract:In order to investigate the characteristics, sources and variations in the composition of chromophoric dissolved organic matter (CDOM) during the normal and rainy seasons in the reservoir of the Yellow River Diversion Source Reservoir-Menlou Reservoir, UV-vis and EEMs techniques combined with PARAFAC were used to analyse the changes in CDOM content and components in May 2022 (normal season) and July 2022 (rainy season). The results of the study showed that PARAFAC identified 2 types of fluorescent fractions, C1 (Ex=355 nm, Em=476 nm, humus-like fraction) and C2 (Ex=225 nm, Em=320 nm, protein-like fraction). The fluorescence intensity of the CDOM fraction was significantly higher in wet seasons than in normal seasons. The spectral parameters of CDOM indicated that the water body of Menlou Reservoir was in a mesotrophic state, and the CDOM of the water body was mainly autogenous due to the combined effects of endogenous and exogenous inputs. The reservoir CDOM was characterized by low relative molecular weight and low humification. The levels of eutrophication and the relative concentrations of CDOM in the reservoir were lower in the wet season than in the normal season, and the proportion of hydrophobic components and the degree of aromatization of CDOM were higher than in the normal season. Correlation analysis of water physical and chemical indicators, CDOM components and spectral parameters showed that SUVA₂₆₀ and SUVA₂₈₀ were significantly negatively correlated with DOC, indicating that UV-Vis spectral parameters could be used to estimate the concentration of DOC to some extent; Chl.a concentration, as an indicator characterizing phytoplankton biomass, was significantly and positively correlated with DOC, a₂₅₄. The study revealed the fluorescence characteristics, component source response mechanism and water period variation of CDOM in the water body of Menlou Reservoir, providing basic data for further investigation of the environmental behavior characteristics of CDOM in the Yellow River Diversion Water Source Reservoir, its contribution to water eutrophication and its influence on the environmental behavior of other pollutants.

Abstract:The study of the fractions and distribution characteristics of organic phosphorus in the sediment of the water level fluctuating zone of Lake Nansi is conducive to revealing the transformation of phosphorus in the lake, and has important scientific significance for controlling the eutrophication of Lake Nansi. Based on the sediment of the water level fluctuation zone of Lake Nansi, the improved Hedley continuous grading extraction, ultraviolet-visible spectroscopy and three-dimensional fluorescence spectroscopy were used to characterize the structural characteristics and stability of organic molecules in the sediment, and to reflect the differences in the structure and stability of organophosphates in the water level fluctuating zone; and the Pearson correlation analysis and principal component analysis were used to determine the correlation of various fractions of phosphorus and other physical and chemical indicators. The results showed that the average content of total phosphorus (TP) in the sediment of the water level fluctuation zone of Lake Nansi was 679.90 mg/kg, and the proportion of organic phosphorus (OP) was 20.03%-45.69%. The contents and relative proportions of organic phosphorus in each occurrence form were as follows: Residual organic phosphorus (Residual-Po, 67.58%) > calcium bound organic phosphorus (HCl-Po, 16.61%) > iron/aluminium bound organic phosphorus (NaOH-Po, 7.62%) > potentially active organic phosphorus (NaHCO₃-Po, 5.97%) > weakly adsorbed organic phosphorus (H₂O-Po, 2.22%). The content and fractions of OP in the sediment in the water level fluctuation zone of Lake Nansi were mainly influenced by endogenous sources, mainly from the metabolism of endogenous microorganisms. The risk of sediment phosphorus release in the fluctuating water level zone of the west bank of the lake was higher than that of the east and south banks. Correlation analysis showed that there was a significant negative correlation between sediment pH and phosphorus content in the fluctuating zone, indicating that the increase in sediment alkalinity may lead to a higher risk of phosphorus release to water. There was a significant positive correlation between organic matter and OP (R²=0.719, P<0.01), indicating that organic matter may be an important carrier of OP. Principal component analysis showed that all phosphorus fractions in the sediment were positively correlated, indicating that all phosphorus fractions in the water level fluctuation zone had homology. The results can provide a scientific basis for controlling the release of endogenous phosphorus and the risk of eutrophication in Lake Nansi.

Abstract:Ebullition is one of the main pathways for the release of CH₄ from natural water bodies. It is important to accurately quantify the amount of ebullition in water bodies in order to distinguish between their ‘sink’ and ‘source’ characteristics. However, the discontinuity and uncertainty of ebullition in natural waters make it difficult to monitor the process. To meet the challenge of ebullition monitoring, a method for continuous monitoring of the ebullition process has been proposed by improving the inverted funnel bubble flux monitor. The measurement object of this method is the volume of bubbles released in the monitored water area during a fixed period of time. The theoretical range of this method is 3.6-132 mL/(m²·min) through indoor and outdoor experiments, and the measurement results can better characterize the variation characteristics of ebullitive flux in the slow-flowing water body with the depth of 10-40 m. This method was applied to continuously monitor methane ebullitive flux in Xiangxi Bay, Three Gorges Reservoir, and the influences of various environmental factors on methane ebullitive flux were analyzed. The results showed that during the measurement period, the variation range of methane ebullitive flux was 0.02-8.13 mg/(m²·d), and the ebullitive flux between sampling points showed high spatial-temporal variability. The ebullitive flux was positively correlated with water temperature and pH, and negatively correlated with water depth and conductivity; moreover, water depth might be an important factor in determining whether CH₄ was released by ebullition: after the water depth exceeded 38.35 m, the ebullitive flux was below 0.38 mg/(m²·d). However, whether the threshold value (38.35 m) was applicable to other tributaries of the Three Gorges reservoir requires further experimental verification. This study is useful to reveal the emission process of CH₄ bubbles in typical tributaries of the Three Gorges Reservoir.

Abstract:In order to study the extent of CH₄ release at the sediment-water interface (SWI) of the tributary bay of the Three Gorges Reservoir (TGR), CH₄ production and oxidation at the SWI and its regulators in the Xiangxi Bay were investigated by combining in situ sampling and indoor incubation with sediment characteristics and environmental factors. The results showed that: (1) During the discharge period of the TGR, the potential sediment CH₄ production rate (MPs) (6.35-2029.37 mg/(kg·d)), CH₄ production fluxes (MP, 0.04-0.73 mmol/(m²·d)) and oxidation fluxes (MO, 0.03-0.62 mmol/(m²·d)) at the SWI showed spatial variations among several transections and increased from downstream (XX00) to upstream (XX07). (2) Vertically, the CH₄ production rate decreased with increasing sediment depth, and MPs from the top 20 cm of sediment can be used to estimate sediment MPs in the bay, which accounted for 70% (55%-82%) of the total CH₄ production from the intact sediment core (between 20 cm and 36 cm in length). (3) MP at the SWI was positively and linearly related to sediment total organic carbon, but negatively related to water depth. MP and MO showed a very significant positive correlation with increasing temperature, resulting in higher CH₄ emission corresponding to a linear decrease in methane oxidation rate (MR). Accordingly, the sediment MR in Xiangxi Bay reached 96.62% during the discharge period of the TGR, indicating low CH₄ emission from sediments.

Abstract:The intensification of human activities and the development of the economy and society have led to a continuous increase in the intensity of development in Lake Dianchi. The structure and function of the Lake Dianchi ecosystem have been seriously affected. According to the degree of ecological vulnerability and the main driving factors of Lake Dianchi, the main task is to clarify the current situation and problems of Lake Dianchi ecosystem for its precise management and sustainable development. Based on the "exposure sensitivity adaptation" model (VSD model), 3 criteria, 7 elements and 24 indicators were selected to construct the vulnerability assessment index system of Lake Dianchi ecosystem in this paper. The TOPSIS method was used to compare and select the weighting schemes. The robust test analysis was used to determine the optimal weight calculation scheme. The ecological vulnerability of Lake Dianchi in the three periods of 1980-1989, 1990-2009 and 2010-2020 was analysed, and the main driving factors affecting the Lake Dianchi ecosystem were identified to provide a reference for determining the ecological protection and restoration of Lake Dianchi in the future. The results showed that: (1) From 1980 to 2020, the ecological vulnerability of Lake Dianchi showed a trend of increasing and then decreasing. The highest ecological vulnerability was from 1990 to 2009, which was 0.502, which belongs to moderate vulnerability. (2) The main factor affecting the Lake Dianchi ecosystem was the sensitivity index, followed by the exposure index. In terms of exposure index, the main driving factor affecting the ecosystem had gradually changed from single industrial pollution to combined industrial and agricultural pollution. The industrial effluent discharge from 1980 to 1989 was the main driving factor, the construction land area from 1990 to 2009 was the main stress factor, and the chemical fertiliser application amount (converted into pure) from 2010 to 2020 was the main factor affecting the ecosystem of Lake Dianchi; in terms of sensitivity, the main driving factor affecting the ecosystem had gradually changed from water quality indicators to water ecological indicators. The water restoration capacity was the main driving factor from 1980 to 1989, and the number of native fish species was the main driving factor from 1990 to 2009 and 2010 to 2020. (3) Based on the above analysis, the main countermeasures to improve the health level of Lake Dianchi ecosystem are to improve the habitat conditions in the lake area, restore the population of aquatic organisms, and build an agricultural non-point source pollution control system.

Abstract:Litter decomposition is an important ecological process for maintaining energy flow, material cycling and nutrient balance in wetland ecosystems. From November 2017 to April 2018, a field decomposition experiment was conducted in the Lake Poyang wetland. The decomposition process and δ¹³C, δ¹⁵N differentiation characteristics of cellulose and lignin in the litter of three dominant plants, Phragmites australis, Triarrhena lutarioriparia and Carex cinerascens, were investigated using the decomposition bag method. The results showed that there were significant differences in cellulose and lignin residues, decomposition rate and δ¹³C, δ¹⁵N among the three plant litter types during the 150 d experiment. The main cause of these differences was attributed to the initial chemical properties of the different plant litter types, especially differences in C/N and lignin/N. The decomposition rate of litter cellulose and lignin was fastest in P. australis, followed by C. cinerascens and T. lutarioriparia. The decomposition process of the three plant litters was simulated and predicted using the Olson negative exponential decay model. During the decomposition of the three plant litters, δ¹⁵C fluctuated continuously but showed an overall significant decreasing trend, while δ¹⁵N for all litter types showed a slightly increasing trend in volatility, indicating that it was influenced by both substrate quality and microbial metabolic activities at different stages of decomposition. There was a significant positive correlation between δ¹³C and the decomposition rate of cellulose and lignin in the litter. However, the correlation between δ¹⁵N and the decomposition rate was not significant, indicating that there are other factors besides the migration and transformation of nitrogen elements that affect the change in δ¹⁵N. This study can contribute to a better scientific understanding of the substrate quality control mechanism in the process of litter decomposition, and provide a scientific basis for the protection and restoration of Lake Poyang wetland.

Abstract:As one of the most famous reservoirs with an ecological aquaculture paradigm in China, Lake Qiandaohu(Xin'anjiang Reservoir) has been focused mainly on the stocking of filter-feeding carps such as silver carp and bighead carp while there is still a large information gap on the important piscivorous fish species. In recent years, due to the impacts such as overfishing and habitat fragmentation, piscivorous fish resources declined significantly. However, piscivorous fish are the top predators of aquatic food webs and play an important role in maintaining the structure and function of aquatic ecosystems. Therefore, in this study, we focused on four dominant piscivorous fish species in Lake Qiandaohu: Culter alburnus, Culter mongolicus, Siniperca knerii and Siniperca scherzeri. From March 2021 to December 2022, we collected fish samples monthly in Lake Qiandaohu and the differentiation characteristics of the life history strategies in terms of age, growth, and reproduction were systematically compared. The results indicated that 2-3 years age was the dominant group for all the four populations, and the proportion of senior individuals was relatively low. In terms of the growth patterns, C. alburnus and C. mongolicus belonged to the positive allometric growth pattern, while S. knerii and S. scherzeri showed negative allometric growth, indicated that Culter and Siniperca in Lake Qiandaohu had different growth patterns. It showed that the four piscivorous fishes had different growth strategies. The condition factor was closely related to reproductive activities. And the condition factor for S. knerii was the highest (1.92%±0.03%), followed by S. scherzeri (1.72%±0.02%), C. mongolicus (1.14%±0.01%), while C. alburnus was the lowest (0.97%±0.03%). The difference in condition factor suggested that they might utilize different food resources. The reproductive strategies of the four piscivorous fish species also varied. In terms of the breeding period, the S. knerii and S. scherzeri were relative longer compared to the C. alburnus and C. mongolicus. Regarding fecundity, the relative fecundity of C. alburnus ((254±16) egg/g) and C. mongolicus ((192±10) egg/g) was significantly higher than that of S. knerii ((88±19) egg/g) and S. scherzeri ((88±10) egg/g). Both C. alburnus and C. mongolicus had a unimodal distribution of oocyte diameters, which might be a single-batch spawning type. However, S. knerii and S. scherzeri had a bimodal distribution which suggested that they might be multi-batch spawning types. The variations in reproductive strategies suggest that there might be differentiated. According to the findings, the four piscivorous fishes had been different growth rates, growth patterns, and condition factor as well as the reproductive strategies which might be the key driving factors for the sympatric coexistence of the four piscivorous fish species in Lake Qiandaohu. Our results are of great importance for further understanding the coexistence mechanism of fish species, and will provide guidelines for further conservation and exploitation of the piscivorous fish resource.

Abstract:The Yangtze River Basin, covering about 1.8×10⁶ km², is the natural habitat of the Chinese sturgeon (Acipenser sinensis), Chinese paddlefish (Psephurus gladius) and Yangtze sturgeon (Acipenser dabryanus). In recent years, the scale of exotic sturgeon and paddlefish aquaculture in the Yangtze River Basin has been enormous, and escape from aquaculture has become the main way for these exotic sturgeons to enter the Yangtze River Basin, affecting the survival of local sturgeon and paddlefish populations. According to FishBase, of the ten farmed exotic sturgeon and paddlefish species, Acipenser baerii was considered as an invasive species in Germany and Austria, while the paddlefish (Polyodon spathula) was considered as an aquatic invader in Belarus. The Aquatic Species Invasiveness Screening Kit (AS-ISK) was used to assess the invasion risk of exotic sturgeon and paddlefish, and five related fish species (Oncorhynchus mykiss, Ictalurus punctatus, Oreochromis niloticus, Cirrhinus molitorella and Tinca tinca). The suitability regions for exotic sturgeon and paddlefish with the highest risk of invasion were quantified using the maximum entropy algorithm, considering present and future climate variables and altitude. The results showed that paddlefish was classified as having the highest score among ten farmed exotic sturgeon and paddlefish species, indicating the highest invasiveness of the paddlefish in the Yangtze River basin. The results of the maximum entropy algorithm indicated that the distribution of paddlefish in the Yangtze River Basin was mainly influenced by the precipitation of the warmest quarter, the annual precipitation, the maximum temperature of the warmest month, the minimum temperature of the coldest month, the mean diurnal range (maximum monthly temperature minus minimum monthly temperature) and the seasonality of precipitation (coefficient of variation). In the current period, the paddlefish had a wide range of suitability regions in the middle and lower reaches of the Yangtze River Basin, mainly including parts of Hunan, Hubei, Anhui and Jiangxi provinces. And its area of the third-tier region (high suitability region) was about 2996.8 km². Under the 4 scenarios of the global climate model, the number would increase by 1.41 to 1.78 times in the future period, while the centroid of the total suitability region would shift to the northeast by 137.97 km to 163.20 km. This study shows that care should be taken to ensure that the escape of farmed paddlefish does not threaten the survival of native sturgeon in these regions.

Abstract:From 2011 to 2020, Three Gorges Reservoir has conducted a total of 14 ecological operation experiments to promote the natural reproduction of the four major species of Chinese carp in the lower reaches of the reservoir. In order to capture the changes in the response to the natural reproduction of pelagic egg-producing fish since the implementation of ecological operation, and to evaluate the contribution of ecological operation to fish reproduction in the middle reaches of Yangtze River, a survey and study of early fish resources was conducted at a fixed collection point in the Shashi section of the middle reaches of Yangtze River from May to July each year. The results were as follows: In the past ten years, 35 species of drifting fish eggs had been collected, among which 25 species were typical drifting egg producing fishes, mainly including Hemiculter bleekeri, Parabramis pekinensis, Squalidus argentatus, Ctenopharyngodon idella, Hypophthalmichthys molitrix, Culter alburnus and Parabotia fasciata. The diversity index H, evenness index E and stability index ICV of the fish egg community showed the same trend of first decreasing and then increasing, but the dominance index D was the opposite. The inflection point appeared in 2017,indicating that community diversity tended to increase and community structure tended to stabilize since 2017. In general, the breeding size of fishes producing drift eggs first decreased and then increased, with a significant increase in eight fish groups since 2017. The peak frequency of fish breeding gradually increased, and the period of large-scale breeding gradually extended, indicating that the natural breeding situation of fish tended to be stable. Combined with the ratio of the average daily breeding size of fish in the ecological regulation period and the non-ecological regulation period, it was concluded that the implementation of ecological regulation in other years had played a positive role in promoting the reproduction of drifting egg-producing fish, except that there was no large-scale breeding in the ecological regulation period in 2016 and 2020. It is suggested to strengthen the ecological operation of Three Gorges Reservoir in the future, to design more detailed ecological operation plans that meet the needs of different fish, and to further enhance the contribution of ecological operation to the natural reproduction of fish in the middle reaches of Yangtze River.

Abstract:In order to improve the ecological operation effect and increase the success rate of ecological operation practice, a set of ecological operation target quantification methods based on the bio-hydrological response model for stimulating reproduction of four major Chinese carps is proposed. The method includes the definition of upwelling event, the selection of eco-hydrological indicators, the definition of natural fish reproduction event and effective reproduction response, the construction of bio-hydrological response model and multi-indicator discriminative preference. The actual observed data of 47 upwelling events and 24 effective reproduction responses of four fish in the Yichang River section for 6 years after the operation of the Three Gorges Reservoir from 2013 to 2019 (except 2016), were used to construct the model and quantify the ecological planning goals of the reservoir. The results showed that (1) the single-indicator bio-hydrological response model based on the four indicators of initial flow (Q₀), peak flood flow (Q_peak), total flow growth (Q_sum) and daily flow growth rate (dQ) had the ability to discriminate whether the natural reproduction of the four major carp was responsive to hydrological conditions; (2) the optimal thresholds for the above evaluation indicators were 14960 m³/s, 19610 m³/s, 4050 m³/s and 1405 m³/(s·d), respectively; the performance of the single indicator model was ranked as dQ>Q_peak>Q_sum>Q₀; (3) the comprehensive performance of the multi-indicator bio-hydrological response model based on the combination of dQ or Q_peak was the best. The model results are simple and predictable, and can be used to quantify the ecological operating targets, which can provide a reference for discriminating fish reproduction response and formulating reservoir ecological planning in different watersheds and reservoirs.

Abstract:To investigate the patterns and determinants of zooplankton community stability in different regions of Lake Dianchi, seasonal surveys were conducted in Caohai, Dabokou and Waihai in 2020. The results showed that dissolved oxygen and transparency were significantly higher in Dabokou than in Waihai. Meanwhile, the concentrations of total nitrogen, total phosphorus, suspended solids, chlorophyll-a and chemical oxygen demand were significantly lower in Dabokou than in Waihai, and were significantly different from Caohai compared to both Dabokou and Waihai. A total of 41 genera of zooplankton (12 genera of cladocerans, 8 genera of copepods and 21 genera of rotifers) were identified in the three regions. Both species number and density of rotifers were dominant during the surveys. The annual density of zooplankton in Dabokou (7771.3 ind./L) was higher than in Caohai (2901.1 ind./L) and Waihai (634.8 ind./L), while the annual mean biomass of zooplankton in Caohai (3.72 mg/L) was higher than in Dabokou (2.15 mg/L) and Waihai (2.09 mg/L). The results of non-parametric multiple analysis of variance and similarity percentage analysis showed that there were significant differences between the zooplankton communities in the three regions. The community structure of Dabokou (ecological restoration region) was significantly different from that of Caohai and Waihai, and the determinant of the difference was rotifer species. The community structure of cladocerans and rotifers was significantly different between Caohai and Waihai. The stability of the zooplankton community in Waihai was significantly higher than that in Caohai and Dabokou, which was significantly positively correlated with species diversity. Pearson correlation analysis and redundancy analysis showed that the driving environmental factors of zooplankton community stability were dissolved oxygen, pH and chlorophyll-a. The study implied that improving the water environment could reduce the stability of zooplankton community dominated by pollution olerant species.

Abstract:The Macrobenthic Integrity Index (B-IBI) was constructed based on the macrobenthic community data in Lake Dongting for more than 30 years (1988-2021) in order to assess the ecological quality of Lake Dongting, capture the status and change trend of aquatic biological integrity in Lake Dongting, and support the assessment of ecological condition in the Yangtze River Basin. Based on the integrity assessment results, the main environmental factors and contribution rates of the B-IBI in Lake Dongting over the past 30 years were analysed using the multiple stepwise regression method. The reference state of the benthic characteristics was set by the data during the low-pressure period (1988-1994). The B-IBI consisted of five core parameters: total taxon number, crustacean and mollusk taxon number, Shannon diversity index, total density and BI. At the same time, the expected values of the five parameters were determined and the criteria for B-IBI scoring were established: B-IBI≥6.34, excellent; 4.75≤B-IBI<6.34, good; 3.17≤B-IBI<4.75, average; 1.58≤B-IBI<3.17, poor; B-IBI<1.58, very poor. Box plot analysis of the B-IBI in the low- and high-pressure groups further verified the discriminatory power and applicability of the B-IBI. According to the B-IBI scoring criteria, the B-IBI decreased from 6.99 (excellent) in 1988 to 2.97 (poor) in 2021, indicating that the integrity of the benthic fauna in Lake Dongting showed a significant decline. The analysis of related environmental factors showed that the comprehensive nutritional status index, total nitrogen and chlorophyll-a in Lake Dongting showed a significant upward trend. The analysis showed that B-IBI was negatively correlated with total nitrogen and chlorophyll-a significantly. Multiple stepwise regression analysis showed that total nitrogen explained 56.1% of the variation in macrobenthic integrity, and 61.3% when combined with chlorophyll-a, indicating that the change in macrobenthic integrity was mainly driven by the increase in nutrients. The B-IBI assessment standard showed good applicability in Lake Dongting, which could provide a valuable scientific basis for long-term monitoring of changes in lake integrity and analysis of changes in ecological status. In the context of the ecological protection of important water bodies in the Yangtze River basin, it is suggested to pay more attention to the influence of nitrogen pollution in the continuous control of phosphorus, to strengthen the control of total nitrogen, and to include total nitrogen in the national and local water ecological environment control systems.

Abstract:Water and sediment are two different but closely related habitats in the lake ecosystem, and the bacteria inhabiting these two habitats play a key role in maintaining the ecosystem balance and driving elemental cycling. To investigate the distribution patterns of bacterial communities in lake water and sediment, this paper investigated the bacterial communities in water and sediment in four regions of Lake Taihu, and analysed the bacterial community composition and diversity levels, distribution characteristics and their drivers in these two types of habitats based on high-throughput sequencing technology and statistical analysis. The results showed that Actinobacteria, Proteobacteria and Cyanobacteria were the main bacteria in the surface water bacterial community, while Proteobacteria dominated in the surface sediment. In the two habitats, bacterial richness and local contribution to beta diversity (LCBD) were relatively higher in the western region of Lake Taihu, and the community structure showed significant regional differences. For surface water bacterial communities, the important factors influencing community richness were water conductivity, pH, sediment porosity and PC1 (principal component of heavy metal elements), while the important factors influencing community uniqueness were PC1, water temperature and pH. In addition, network complexity increased significantly with increasing pH. For surface sediment bacterial communities, total phosphorus and lithium were the most important factors influencing community richness and community uniqueness. Furthermore, network complexity decreased with increasing metallic element content, whereas it increased with increasing total phosphorus, ammonium and phosphate concentrations in the sediment. These results showed that the bacterial communities in the water and sediment were driven by different environmental factors, providing a new perspective for aquatic ecosystem conservation and water resource management in Lake Taihu.

Abstract:Since the 20th century, the area and pattern of Lake Dongting wetland have changed significantly under the combined effect of natural and human activities. Based on a lot of historical materials, related literatures and data, this paper identified 4 evolutionary regimes of Lake Dongting wetland area and landscape pattern in the past 120 years (1900-2020) using the piecewise linear regression method, and analysed the driving factors and their interactions of each regime. The evolution of Lake Dongting wetland area could be divided into the obvious decline period (1900-1949), the rapid decline period (1950-1978), the stable period (1979-1998), and the slight recovery period (1999-present). The evolution of the landscape pattern experienced the relatively stable period (before 1949), water transformation to floodplain stage (1960s-1990s) and floodplain transformation to water stage (2000s-present). Due to differences in socio-economic and productivity levels, the drivers of wetland evolution were different between regimes, resulting in different rates and directions of wetland evolution. Lake reclamation and the return of agricultural land to the lake were the main drivers of wetland change. Meteorological fluctuations, reservoir construction, sand mining and channel regulation affected the wetland pattern by changing the water-sediment situation in the lake. They had also influenced the reclamation of lake embankments and the return of farmland to the lake. In order to meet the demands of economic development, the government's participation in wetland development had gradually increased, and the government's attitude towards lake reclamation had gone through stages, including “encourage-participate-prohibit-return lakes”, which was the core driving factor for the development of Lake Dongting Wetland over the past 120 years. The development process of Lake Dongting Wetland over the past one hundred years is the comprehensive representation of regional socio-economic development, the wrestling process of human and natural forces, and the harmonious development and tendency to gradually reduce the contradiction between man and land.

Abstract:Patterns of river systems and hydrological connectivity in different river basins are changing significantly, mainly due to the effects of climate change and human activities. This may lead to a series of water resource problems such as floods and droughts. In this paper, we took the whole Lake Poyang Basin as our study area, and used graph theory to construct the evaluation systems based on the extracted river data of five periods during 1989-2020 by using Google Earth Engine (GEE). Therefore, we aimed to quantitatively analyse the spatial and temporal evolution of the river system pattern and structural connectivity in the study area. According to the data of catchment topography, land use and normalised difference vegetation index (NDVI), we used the index of connectivity (IC) to evaluate the spatial and temporal changes of functional connectivity, and further discussed the relationships between hydrological connectivity and runoff-sediment discharge. The results showed that the rivers in the Lake Poyang Basin had changed significantly over the past 30 years and tended to be more complicated. The number and length of rivers showed an increasing trend (except for the main rivers of the basin), and the associated changes occured mainly in the northern regions of the basin, especially for third class rivers. The water surface ratio, river development coefficient, drainage density and complexity showed a clear increasing trend. Moreover, the rate of change after 2000 was about twice as high as before. In general, river network connectivity, node connectivity and river network connectivity in the catchment showed increasing trends, and structural connectivity was in good condition with little variation. Functional connectivity analysis showed that IC values were generally high in flat areas close to the main downstream river channel, while IC values were relatively low in densely vegetated areas far from upstream river channels. Over the last 30 years, IC had shown decreasing variation and marked spatial heterogeneity. Statistical results showed that IC had a significant positive correlation with annual runoff and sediment discharge (correlation coefficient is 0.6 and 0.7, respectively). In short, the river structure in the Lake Poyang Basin had changed significantly, and the structural connectivity showed an increasing trend, while the functional connectivity of the discharge and sediment transport process gradually decreased after 2000. From the perspective of basin structure and functional connectivity, the results of this paper can provide scientific reference and support for relevant studies on the development of river system connectivity and water-soil resource management in the Lake Poyang Basin.

Abstract:The skin effect is defined as the difference between the skin temperature and the bulk temperature of the surface water. Quantifying lake skin effect and analysing its influencing factors can improve our understanding of the response of lake physical, chemical, biological and ecological processes to global warming. Based on the water temperature gradient, four radiation components and microclimate observations from 2011 to 2020 from the Taihu Eddy Flux Network, this paper quantified the skin effect on different time scales and the difference of the skin effect under different weather conditions. The results showed that the warm skin effect in Lake Taihu was strongest at 15:00-16:00, with a value of 1.95℃. The cool skin effect was strongest at 7:00-8:00 with a value of -0.50℃. The intensity of the skin effect was strongest in spring and weakest in summer. Therefore, the surface temperature obtained by satellites passing over Taihu Lake in the afternoon cannot be validated with the surface water bulk temperature. The validation bias caused by the skin effect can reach 2℃, especially in spring. On an interannual scale, the surface water temperature of Lake Taihu warmed at a rate of 0.14℃/a, which was identical to the warming rate of air temperature during the same period. The surface water bulk temperature warmed at a rate of 0.12℃/a. The warming rate of Lake Taihu described by surface temperature was 0.02℃/a higher than that indicated by surface water bulk temperature. The warm skin effect could reach 1.64℃ under clear sky with light wind. Under cloudy conditions with strong winds, the skin effect was only 0.32℃. Compared to solar radiation, wind speed had a greater effect on the skin effect of Lake Taihu. Thus, wind disturbance was the main factor influencing the skin effect of Lake Taihu. A new parameterisation using wind speed for the skin effect of Lake Taihu was proposed here with 10 years of observations.

Abstract:The damming of Three Gorges Reservoir (TGR) has caused the upstream water level to rise and the water surface area to increase significantly. In order to show the changes in water surface area and evaporation loss before and after the construction of TGR dam, this study used Landsat image data to extract the water surface area from Cuntan to the TGR dam during the period 1982-2021. Then, the daily water surface area was calculated by the water level and area relationship established by cubic polynomial. Based on the estimated daily water surface area of the TGR and the corresponding potential evaporation data, the evaporation loss of the reservoir was estimated. The results showed that after full operation of the TGR in 2010, the average water surface area in the reservoir area increased from 372.96 km² before damming to 761.31 km². At the same time, the storage and discharge regulation of TGR had altered the original hydrological rhythm of the river reach in the reservoir area, causing the seasonal variation of the water surface area in the reservoir area to change significantly compared to the pre-storage period. After impoundment, the largest water surface area occurred in winter with an average of 843.81 km², an increase of 1.89 times compared to that before impoundment; followed by areas in autumn and spring with 818.73 km² and 735.28 km², respectively, an increase of 97.17% and 1.28 times compared to that before impoundment; and the smallest water surface area occurred in summer with 653.03 km², an increase of only 39.06% compared to that before impoundment. After full operation of the reservoir, the average annual evaporation loss from the reservoir increased to 565 million m³ from 268 million m³ in the pre-TGR period. The seasonal rhythm of evaporation loss from the water surface in the TGR area had not changed significantly compared to that before impoundment, with an evaporation loss of 126 million m³ in spring, an increase of 1.4 times compared to that before impoundment. Summer was still the season with the highest evaporation loss, with an evaporation loss of 211 million m³, an increase of 61.14% compared to that before damming, but its share in the total annual evaporation loss was reduced from 48.80% before damming to 37.32%. The evaporation loss in autumn was 149 million m³, which was 1.23 times higher than before damming. Although the water surface area in winter was the largest, its evaporation loss was still the smallest at 80 million m³, which was 3.34 times more than before damming, and its proportion of the total annual evaporation loss increased from 6.85% to 14.11% before damming. The above results are useful in showing the impact of the TGR on the regional hydrological cycle.

Abstract:Over the past 70 years, the water diversion at the three outlets of the Jingjiang River has been continuously decreasing. In order to investigate the driving factors at different historical stages, this study used the measured hydrological data of the hydrology and topographic features of the mainstream and tributaries from 1955 to 2021, and identified the contributions of the factors based on the variable control approach. The results showed that the water diversion had gradually decreased, and the decrease had mainly occurred from May to November. The decrease in water diversion was greatly aggravated by the reduced mainstream inflow due to the decrease in precipitation in the upper Yangtze, and the erosion of the mainstream bed and deposition at the three outlets caused by the dam projects in the lower Jingjiang River and the sediment trap in the Gezhouba. However, the Tiaoxiankou dam had little effect on water diversion. Since the damming of the Three Gorges Reservoir (TGR), the mainstream and the three tributaries had experienced synchronous riverbed erosion, resulting in an imperceptible change in water diversion. The reduction in rainfall had been concentrated in the rainy season, which had had a major impact on the reduction in water diversion from May to November. Meanwhile, water diversion was mainly concentrated in the rainy season when the mainstream was at flood level, resulting in a large decrease in water diversion from May to November, while the Lower Jingjiang Cutoff Projects triggered a large lowering of the mainstream riverbed. In addition, the operation of Gezhouba, the initial damming of TGR and the experimental damming of TGR were mainly carried out from May to November, reducing water diversion by 0.4×10⁸ m³, 12.5×10⁸ m³ and 38.6×10⁸ m³, respectively. The enabling effect of the Tiaoxiaokou dam on water diversion was far less than the constraining effect of rainfall reduction, resulting in a negative contribution ratio for the dam. In comparison, the contribution ratio of the lower Jingjiang dam projects reached 85%. In the post-dam period, precipitation dominated water diversion reduction with a contribution ratio of more than 50%, while Gezhouba operation hardly played a role and its downstream riverbed adjustment produced a contribution ratio of 21%. During the initial and experimental damming of the TGR, the contribution ratios of its operation increased to 7% and 23%, respectively, while those of its downstream riverbed adjustment were 10% and 19%. As the precipitation decline in the upper Yangtze will continue at least until the end of this century, the annual water diversion will show a long-term downward trend, while the water diversion at a given mainstream flow will change little due to synchronous riverbed erosion between the mainstream and the three outlets.

Abstract:The nitrogen isotope (δ¹⁵N) of lake sediments plays an important role in indicating climate change in terms of paleoenvironment and palaeoclimate change. However, there is still a lack of in-depth research on the characteristics of climate change indicated by δ¹⁵N records as a proxy for climate change. This paper took the sediment of Jiulong Wetland in Fanjing Mountain, Guizhou Province, as a research object, and analysed and discussed the process and characteristics of dry and wet climate change in the Fanjing Mountain area recorded by δ¹⁵N, combined with the data of total organic carbon (TOC), total nitrogen (TN) and carbon-nitrogen ratio (C/N). Finally, a comparative study was carried out, the results showed that: (1) Because the organic matter in the sediment of Jiulong Wetland was mainly from the watershed, when the climate was wet, the soil moisture increased and terrestrial plants grew well, leading to the increased external input on the organic matter to the lake and the negative δ¹⁵N value of sediment.When the climate was dry, the terrestrial plants and soil moisture decreased, leading to the reduction of the impact of external input on the organic matter in the lake sediments, the δ¹⁵N value of sediments was positive. (2) The δ¹⁵N records of the Jiulong Wetland sediments revealed well the Holocene climatic processes (dry/wet changes) in the Fanjing Mountain area. In the early (11.5-9.2 ka B.P.), the Asian summer monsoon was enhanced, and the climate evolved from dry to wet. In the middle (9.2-3.0 ka B.P.), the Asian summer monsoon was strong and the climate was in an overall wet period. In the late period (3.0-2.2 ka B.P.), the Asian summer monsoon weakened and the climate evolved from wet to dry. This record is regionally consistent with Holocene climate change recorded by other climate proxy indicators in southwest China, their differences in some periods indicating significant regional characteristics. This study shows that the sediment δ¹⁵N record of Jiulong Wetland can be used to reconstruct the characteristics of Holocene climate and environmental changes in the study area, and it is a very useful palaeoclimate proxy record.