Abstract:Phytoplankton are widely distributed in marine and freshwater ecosystems. Their growth and development show considerable spatial and temporal variation, and their response to climate change is complex. Algal phenology describes the cyclical changes in phytoplankton under long-term adaptation to factors such as climate, water quality and human disturbance, establishing a growth rhythm tuned to environmental conditions. It primarily includes characteristics such as the timing of algal appearance, peak growth and decline or disappearance. Remote sensing technology continuously provides high spatio-temporal resolution data on chlorophyll-a concentrations (an indicator of phytoplankton biomass), allowing long-term monitoring of algal phenology. This paper provided a detailed review of recent advances in remote sensing methods for monitoring and extracting algal phenology, identified current issues and limitations, and looked ahead to future trends. First, it reviewed how existing satellite remote sensing provided comprehensive spatio-temporally continuous information on algal growth. Secondly, it summarized the monitoring of phytoplankton phenological stages and methods for estimating specific algal phenological phases. It also presented common data processing methods used to estimate algal phenology from remote-sensed time series and discussed changing trends in phytoplankton phenological characteristics. Finally, it examined the factors and mechanisms that may influence changes in algal phenology. Based on this analysis, future research on remote sensing of algal phenology should focus on: (1) developing and validating general algorithms suitable for different aquatic environments, integrating machine learning and other intelligent algorithms to improve phenological models and increase the accuracy of phenological monitoring and operational application, (2) combining numerical models with ecosystem dynamics models to investigate the driving mechanisms behind phytoplankton phenology.

Abstract:Lake Taihu is one of China's critical lakes and a pivotal focus for national lake management endeavors. Since the water crisis of 2007, an intensive investment (over 262 billion yuan) has been invested in its restoration, rendering it the most attention in lake restoration globally. In Lake Taihu, phosphorus levels had a resurgence since 2016, and have been better controlled since 2020, resulting in a decline in cyanobacteria blooms in 2023. There was a clear relationship between cyanobacteria blooms and phosphorus levels. A review of previous publications on phosphorus in Lake Taihu showed diverse contributions of internal sediment phosphorus release to water total phosphorus. Such disparities significantly influenced the quantification of the phosphorus load of the lake, and the design of controlling strategies for internal sediment phosphorus release. This study reviewed both domestic and international studies concerning the monitoring, experimentation, and computation of phosphorus release from lake sediments, with a focus on the source, sink, absorption, and release patterns. Through comparative analysis, this study established fundamental insights into the patterns and extents of phosphorus release from Lake Taihu sediments. This study introduced the concepts of “total source”, “total sink”, “net source” and “net sink”, and analyzed the conditions under which five processes may manifest as “net sources”. Fick diffusion release is deemed a continuing process, whereas phosphorus reduction release, occurring at the condition of DO＜2 mg/L, emerges as a notably significant process under specific conditions warranting special consideration. The determination of a “net source” should be conducted through dynamic release experiments under environmental conditions for the lake. Despite Lake Taihu currently acts as a “total sink”, its “net source” remains large. It is urgently needed to identify the hot spots and moments of internal sediment phosphorus release, which can support its control.

Abstract:During winter, the decline of thermal stratification and the mixing of water in deep lakes can alter the distribution pattern of nutrients, and potentially affect the succession of phytoplankton communities in lake ecosystems. In this study, high-frequency observations of physicochemical environmental factors and phytoplankton communities were conducted along vertical profiles during stratification decline, mixing, and stratification formation periods in Lake Fuxian, a deep plateau lake, from November 2022 to April 2023. The spatiotemporal characteristics of phytoplankton communities in response to the thermodynamic mixing process were investigated. The results showed that the mixing process caused significant spatiotemporal changes in physicochemical environmental parameters. Compared with the stratification-declining period and stratification-formation period, the concentrations of total phosphorus (TP), dissolved organic phosphorus (DOP), phosphate (PO^3-₄-P), and chlorophyll-a (Chl.a) in the water were significantly increased during the mixing period, while the concentrations of total nitrogen (TN), dissolved nitrogen (DN), particulate nitrogen (PN), ammonium nitrogen (NH₃-N), and nitrate nitrogen (NO^-₃-N) were significantly decreased by mixing. Among the phytoplankton community, the green algae Mougeotia and Chlorella had higher densities than other species throughout the observation period, while the cyanobacteria Dolichospermum and Cylindrospermopsis had the highest densities during the stratification-declining period, and the diatom Melosira and the cryptomonad Cryptomonas had the highest densities during the mixing period, and the green algae Scenedesmus had the highest densities during the stratification formation period. Redundancy analysis showed that the dynamic changes in NH₃-N, PO^3-₄-P, and alkaline phosphatase activities significantly influenced the composition of the phytoplankton community. Moreover, the sensitivity of phytoplankton to nitrogen and phosphorus nutrients varied among different species. Dolichospermum and Mougeotia showed a positive correlation with N nutrients and a negative correlation with nutrients, while the densities of other phytoplankton species exhibited an opposite pattern, showing a negative correlation with nitrogen and a positive correlation with phosphorus. Structural equation modeling was used to analyze the driving mechanisms of phytoplankton community dynamics during the water mixing period. On one hand, water temperature directly impacted the structure of the phytoplankton community. On the other hand, water temperature indirectly drove changes in the structure of the phytoplankton community by regulating the concentrations of different forms of nitrogen (NO^-₃-N, NH₃-N) and phosphorus (PO^3-₄-P and dissolved organic phosphorus). Ultimately, these direct and indirect effects collectively contribute to the changed in the concentration of Chl.a. This study contributed to our understanding of the effects of thermal stratification changes on the structure and function of lake ecosystems in deep lakes. In the context of global changes, strengthening research on mechanisms and ecological environmental effects of thermal stratification in lakes and reservoirs can support the protection and management of lake ecosystems.

Abstract:Algal blooms caused by excessive growth and reproduction of phytoplankton are becoming frequently in lakes, threatening their health and safety of ecological systems. In recent years, the coverage area of cyanobacterial blooms in Lake Hulun has been expanding. In 2022, the most severe cyanobacterial blooms almost covered the surface of Lake Hulun. A sampling program was conducted in the spring, summer and autumn of 2022, concerning the variables of phytoplankton species, cell density, biomass and several water quality indicators in the surface, middle and bottom water of the lake. A total of 193 species of phytoplankton were identified belonging to 113 genera and 8 phyla, with 14 dominant species. The species composition was of the green algae-diatom-cyanobacteria type. The number of phytoplankton species in spring and autumn was basically the same, that was twice of the minimum level in summer. The cell density in summer was the highest, which was one order of magnitude higher than that in spring and autumn. The phytoplankton biomass in spring, summer and autumn was 686, 510 and 166 mg/L, respectively. The Shannon-Wiener diversity index was the highest in spring. The Margalef richness index was the highest in summer. The Pielou evenness index was the highest in spring. The characteristics and seasonal differences of phytoplankton community were mainly caused by lake water environment conditions and meteorological factors. The correlation analysis revealed that the main influencing factors were water temperature, total nitrogen, total phosphorus, pH, dissolved oxygen, air temperature, evaporation and wind speed.

Abstract:Based on the community change of overwintering cyanobacteria, this study aimed to elucidate ecological changes in different lakes. A sampling program was conducted at 10 sampling sites in Lake Daihai, Lake Wuliangsuhai, and Lake Nanhai in January and July 2022, with samples from the upper water and sediment. These samples were used to investigate the community structure, abundance variations, and diversity of cyanobacteria in various types of lakes in the Inner Mongolia lake region during both the ice-covered and non-ice-covered periods. The results showed large variation in the abundance and diversity of cyanobacteria between the ice-covered and non-ice-covered periods of lakes, providing a good explanation for the ecological changes. During the non-ice-covered period, the abundance of cyanobacteria in Lake Wuliangsuhai and Lake Nanhai was significantly higher than that during the ice-covered period. Conversely, in Lake Daihai, the abundance of cyanobacteria in water was significantly higher during the ice-covered period than that during the non-ice-covered period, attributed to the unique water quality of the lake, and the abundance of cyanobacteria in the sediment during the non-ice-covered period was significantly higher than that during the ice-covered period. Additionally, the ecological niche widths of the dominant genera in different lakes varied across periods. Specifically, the dominant cyanobacteria in Lake Daihai and Lake Nanhai were generally able to utilize a diverse range of resources during both the non-ice-covered and ice-covered periods, whereas the dominant cyanobacteria in Lake Wuliangsuhai had fewer resources readily available to them. Based on neutral modeling and redundancy analysis, it is clear that during the non-ice-covered period, the water bodies and sediments of Lake Wuliangsuhai and Lake Nanhai were significantly influenced by stochastic processes. In contrast, the sediments of Lake Daihai and the water of Lake Nanhai were more constrained by environmental factors. Additionally, there were seasonal variations in the composition and structure of cyanobacterial communities in cold-region lakes, with the main driving factors of water temperature, total nitrogen, ammonia nitrogen and pH. This study can serve as a valuable reference for the protection and management of lake ecology in cold regions.

Abstract:The continuous enrichment and deposition of suspended particulate matter (SPM) aggravate the internal loadings from the sediment of eutrophic lakes. In this study, a monthly field investigation for one year was conducted in the macrophyte-dominated area (MDA) and algae-dominated area (ADA) of Lake Taihu. The nitrogen (N) contents, fractions and sediment exchange characteristics of the sediment-water interface (SWI) in water samples, SPM and sediments were analyzed. The fraction transformation and transport of N across the SWI affected by the SPM deposition in both areas were investigated. The results showed that the annual variation ranges of total nitrogen (TN) in the water of the MDA and ADA were 0.50-1.51 mg/L (average was (0.97±0.23) mg/L) and 1.77-6.12 mg/L (average was (3.67±1.36) mg/L), respectively. The ADA showed higher concentrations and seasonal variations of various N fractions than those of the MDA. The annual average TN concentrations in the SPM of the MDA and ADA were (6998.48±1413.87) and (5162.63±2205.13) mg/kg, and in the sediment of the MDA and ADA were (4219.75±216.62) and (3187.88±103.07) mg/kg, respectively. The MDA showed significantly higher TN concentrations in both the SPM and the sediment than those of the ADA, with significantly higher TN in the SPM than in the sediment of both areas. NH₃-N was the dominant fraction of available N in the SPM and sediment of both areas. The release of NH₃-N from the sediment in the ADA mainly occurred in summer, while that in the MDA mainly occurred from summer to autumn. The continuous deposition of the SPM with high concentrations of available N was believed to be an important reason for the transformation of N concentrations and fractions in the sediment. The proportion of available N in the ADA (5.17%) was significantly higher than that in the MDA (4.34%) during algal bloom seasons. The shift from a macrophyte-dominated system to an algae-dominated system might increase the concentration of SPM in the water, thereby intensify the resuspension of sediment particles. Consequently, the alternated deposition and resuspension processes of SPM would exacerbate the N exchanges across the SWI. Therefore, more attention should be paid to the role of SPM during future management of internal N loadings in lakes.

Abstract:Microcystis bloom is the most common harmful bloom. Microcystis cells in bloom aggregate into colonies, which is crucial for maintaining their ecological advantage. Dispersing Microcystis into the unicellular form can decrease their competitive advantage, and has been used as a strategy to control Microcystis bloom. In this study, five polysaccharide-degrading bacteria (PDB) with the ability to decompose the extracellular polysaccharides of Microcystis were screened by using bacterial polysacchrides xanthan gum and the extracellular polysaccharides of Microcystis flos-aquae as the carbon sources. The effects of the bacteria on the extracellular polysaccharides of unicellular strain M. flos-aquae, and on the colony size of colonial strain M. wesenbergii were observed by co-culture. The results showed that four strains of bacteria were able to significantly reduce the viscosity of the extracellular polysaccharides of M. flos-aquae. All five strains were able to reduce the colonial size and to inhibit the growth of M. wesenbergii. The PDB were identified to the genus Pseudomonas by 16S rDNA. This study showed that the PDB have the potential to control Microcystis bloom.

Abstract:Dissolved organic matter (DOM) is an important source of energy and nutrients for aquatic ecosystems. Its elemental stoichiometry, such as N/C ratios, can reflect the nutrient sources of lake sediments. Its study can help reveal the mechanisms of the impacts of global warming and eutrophication on the water bodies. In this study, we focused on the N/C ratios of DOM obtained by high resolution mass spectrometry, and examined the global distribution of N/C ratios of several habitats such as lakes, rivers, oceans and peatlands by Meta-analysis. The results showed that the mean values of N/C of habitats such as river waters (0.050±0.062), sediments (0.050±0.014) and wastewaters (0.083±0.084) had significantly higher mean values than those of peatlands (0.026±0.037). The N/C ratios of waters were significantly affected by temperature, pH, ammonia nitrogen and dissolved organic carbon, whereas N/C ratios of terrestrial ecosystems such as peatlands were mainly affected by pH, total dissolved organic nitrogen and dissolved organic carbon. Furthermore, to explore the patterns of all molecules and their two fractions in response to temperature and nutrient enrichment, a microcosm experiment with simulated temperature changes and nutrient enrichment was conducted to quantitatively analyze the influence mechanism based on the elevational gradient of Laojun Mountain in Yunnan Province, and classified all molecules into two categories of active and inactive molecules according to the sediment DOM molecular transformation number. The N/C ratio of DOM was averagely 0.112±0.020 for all molecules. The active molecules (0.119±0.017) were significantly higher than inactive molecules (0.109±0.034). The N/C ratios of all molecules were mainly influenced by nutrients (e.g., nitrite and nitrate) with an explanation of 63.43%. The N/C ratios of active molecules were mainly influenced by nutrients and energy supply (e.g., dissolved organic carbon, total organic carbon and chlorophyll-a), whereas the N/C ratios of inactive molecules were mainly influenced by nutrients. Structural equation modelling (SEM) analyses further showed that nutrient enrichment had a greater effect on the N/C ratios of total molecules than temperature changes, mainly through the indirect effects of energy supply and nutrients. For active molecules, nutrients affected the N/C ratios through the indirect effect of energy supply. For inactive molecules, nutrients mostly showed direct effect. Overall, our study explored for the first time the distribution pattern of N/C ratios in various habitats around the globe, revealed the response patterns and mechanisms of sediment dissolved organic matter N/C ratios to global warming and nutrient enrichment, and provided scientific evidence for a better understanding and prediction of the stability of dissolved organic carbon under a changing world.

Abstract:To clarify the causes of hypoxia in the southern tributary of the Dongjiang River and provide scientific basis for water management, this study measured water quality of the main river sections during spring and summer in 2019. The measurement results showed that the dissolved oxygen in the southern tributary of the Dongjiang River dropped from 6.93 mg/L in the middle reaches at Daojiao Town to 4.08 mg/L at the estuary. Correlation analysis indicated that dissolved oxygen was significantly negatively correlated with water temperature, and positively correlated with salinity, and had a significant negative correlation with nutrients such as ammonium nitrogen and total phosphorus. The increase of nutrients was spatially significantly overlapped with the reduction of dissolved oxygen, especially during the summer. The main cause of hypoxia in the southern tributary of the Dongjiang River was the mixed influence of physical, chemical, and biological factors. The increase in water temperature and nutrients accelerated oxygen consumption in the middle and lower reaches of the water body. The narrowing of the river channel at confluences and poor light penetration in downstream waters led to insufficient replenishment of dissolved oxygen, and thus caused frequent hypoxia at the lower reaches Shatian Sisheng section.

Abstract:The carbon cycling process in eutrophic water bodies has always been a prominent and critical concern in global aquatic ecosystems. This study conducted a year-long in-situ investigation of methane (CH₄) flux at the water-air interface in the campus landscape pond (Lianxin Pond) of China Three Gorges University. This study aims to assess the influence of eutrophication on CH₄ emissions. The findings revealed a CH₄ emission flux rate of 5.06 mg/(m²·h) at the water-air interface, contributing to atmospheric CH₄ levels. The CH₄ flux was influenced by temperature, wind speed, and chlorophyll-a, while showed significant temporal heterogeneity with the following order: summer (8.70 mg/(m²·h))＞winter (4.80 mg/(m²·h))＞spring (3.88 mg/(m²·h))＞autumn (2.87 mg/(m²·h)). Furthermore, the dominant pathway was identified as bubbling (the proportion of bubbling is 88.56%), during the day (0.11 mg/(m²·h)) slightly higher than the night (0.09 mg/(m²·h)). The difference of CH₄ emission flux in different seasons and day and night is related to the change of environmental factors such as temperature, wind speed and chlorophyll-a. With the increase of water eutrophication level, the CH₄ emission flux showed a non-linear increase trend. With the increase of eutrophication level, the release flux of CH₄ release flux showed nonlinear increase. Notably, the mean CH₄ flux in hyper-eutrophic water was 2 times greater than that in moderately eutrophic water. Controlling eutrophication is critical to reducing CH₄ emissions from water bodies, and can serve as a reference for reducing greenhouse gas emissions in other similar aquatic ecosystems.

Abstract:This study aimed to investigate the diverse impacts of land use on water quality of different lake types at different spatial and temporal scales. For this purpose, landscape pattern indices were extracted at six spatial scales (50, 60, 70, 80, 90, 100 m lakeshore buffer zones) from Fushun West Lake (fluvial lake) in Zigong City, Sichuan Province, and Qinghai Lake (volcanic lake) in Tengchong City, Yunnan Province. The data were combined with the water quality data in wet-season (July 2022) and dry-season (April 2023) periods. Redundancy analysis was used to quantitatively investigate the differences in the impacts of land use on water quality of the two types of lakes at different spatial and temporal scales. The results showed that: (1) Fushun West Lake and Qinghai Lake had significant seasonal variations in water quality. Water quality is better in wet season than that in dry season. There was a significant difference in the water quality of these two lakes. The water quality of Qinghai Lake was better than that of Fushun West Lake. Qinghai Lake conformed to the class Ⅱ and Ⅲ standards of water quality of the surface water environment, and Fushun West Lake reached the standards of water quality of class Ⅳ and V. (2) The land use of Fushun West Lake had the strongest impact on water quality during the dry season at the 50-60 m lakeshore buffer zone scale. However, the land use of Lake Qinghai had the highest interpretation rate for water quality during the wet season at the 60 m lakeshore buffer zone scale. (3) The impact of land use indices on the water quality of the two types lakes was different. Patch index, landscape shape index and aggregation index were positively correlated with most of the water quality indicators in the Fushun West Lake watershed. The high degree of aggregation and fragmentation of the “source” landscapes would exacerbate the threat to water quality. The proportion of forest land area and the aggregation index within Qinghai Lake watershed had a positive effect on water quality, which can improve water quality. An increase in patch index and landscape shape index values exacerbated the fragmentation of the patches and led to deterioration of water quality. Based on different spatial and temporal scales, this study explored the relationship between land use and water quality of two types of lakes. It would potentially support the regional planning and management of different types of lakes, and provide a scientific basis for the evaluation of the lake water quality and lake protection, as well as for the development of targeted lake buffer zones and ecological restoration strategies, so as to promote the safety of the water quality of the lakes and the sustainable use of water resources.

Abstract:A custom-built large-scale flume was utilized to conduct heterogeneous aggregation experiments between microplastics and sediment under the influence of shear flow, and thus to analyze their morphological characteristics and aggregation mechanisms. Two typical types of microplastics (PMMA and PS) in the upper Yangtze River were selected for our investigation. The sediment sample configuration was determined based on the mineral composition and the grading of the suspended sediment in the Three Gorges Reservoir backwater area. The results showed that under turbulent shear flow, the particle size of flocs formed by the heterogeneous aggregation of microplastics and sediment was larger than that of flocs formed by homogeneous sediment aggregation. The number of microplastics adsorbed on microplastic-sediment flocs per milliliter of sample increased rapidly with the increase in shear rate of the water flow. The results of scanning electron microscopy and energy dispersive spectroscopy analyses showed that heterogeneous aggregation of sediment with the PMMA and PS occurred under hydrodynamic conditions. During aggregation, electron transfer and shifts in chemical states occurred, leading to the formation of unsaturated bonds (C C). The variation in the binding energy of PMMA functional groups was greater than that of PS. This suggests that PMMA lost more electrons in this process, resulting in a higher surface charge and propensity increase to aggregate with sediment.

Abstract:As an important carbon pool around the world, wetlands play an important ecological carbon sink function in mitigating climate change. However, in the context of climate warming, carbon emissions (especially CH₄ emissions) are highly variable and uncertain. In order to explore the characteristics of carbon emission fluxes in Northeast saline-alkali wetlands under different water depth gradients, this study took the Momog Wetland as the study area. Five typical vegetation types (Phragmites australis, Bolboschoenus planiculmis, Schoenoplectus nipponicus, Typha orientalis C. Presl and Suaeda glauca) were selected to simulate habitats with different water depths through the Marsh Organ medium scale experimental ecosystem to reveal the changes of CH₄ and CO₂ emission fluxes and their environmental impact factors. The results showed that in water depth of -10 to 50 cm, the CH₄ emission fluxes of five selected vegetation species in the growing season ranged from 0.07 to 86.74 mg/(m²·h), with an average value of 8.89 mg/(m²·h). The main driving factors were water depth, air temperature and surface 10-cm soil moisture. In the water depth range of the study, the CO₂ emission flux of the vegetation growing season ranged from 10.59 to 1891.08 mg/(m²·h), and the average CO₂ emission flux was 450.12 mg/(m²·h). The main influencing factors were water depth, air temperature and surface 10-cm soil water content. Diverse vegetation had different contribution on carbon emission. Reed and cattails had the highest CH₄ emission, while Scirpus had the highest CO₂ emission. The carbon emission presented a segmented pattern with the water depth change. The CH₄ emission showed a trend of first increasing and then decreasing with the water depth change, while the CO₂ emission showed an opposite pattern, i.e., first decreasing and then increasing with the water depth change. The critical threshold of the water depth was 22 cm. In various water depth ranges, the sensitivity of CO₂ emissions to water depth was greater than that of CH₄. In case of the water depth ＞22 cm, both CH₄ and CO₂ show higher water depth sensitivity. When the water level was less than 22 cm, both CH₄ and CO₂ show higher temperature sensitivity. However, when the water level was greater than 22 cm, both of them were not sensitive to temperature. The results showed that under the background of warming, the key ecosystems in the water-land transition zone of typical saline-alkali wetlands in Northeast China typically show asymmetric response patterns in different water depth ranges, and frequent water level fluctuations will greatly change the wetland carbon emission pattern, thus affecting the stable play of wetland carbon function.

Abstract:Submersed macrophytes are an integral component of shallow lake ecosystems, and their distribution patterns are influenced by environmental factors, particularly water depth. Key factors affecting the growth of submersed macrophyte vary across different water depth gradients, and they interact synergistically with water depth conditions to influence submersed macrophyte communities. Dongshan Bay, a typical macrophyte-dominated lake area in the eastern part of Lake Taihu, was selected as research area in this study. From October 2020 to June 2021, three surveys were conducted on water physicochemical indicators and the species richness and abundance of submersed macrophyte at 31 sampling sites with varying water depths. The relationship between submersed macrophyte and environmental factors at different water depths were analysed using one-way ANOVA, Spearman correlation analysis, and redundancy analysis, to identify key environmental factors influencing submersed plant community composition in different water depth zones. The results indicated: 1) physicochemical parameters of the water exhibited significant spatiotemporal differences. The average chlorophyll-a (Chl.a) content and the ratio of Secchi depth to water depth (SD/WD) in the nearshore waters of shallow areas in the Dongshan Bay of Lake Taihu were significantly higher than those in deeper near-lake center area. Total nitrogen (TN), nitrate nitrogen, total phosphorus (TP), phosphate and suspended solids concentrations were significantly higher in spring than in autumn and summer. 2) This study recorded 12 species of submersed macrophyte in Dongshan Bay, with higher frequency of submersed macrophyte in the middle waters. The submersed macrophyte communities in different water depth areas showed differences in species composition, with species like Potamogeton wrightii/Potamogenton malaianus, Vallisneria natans and Hydrilla verticillata exhibiting higher frequency and abundance in the near-lake center area. 3) Key water environmental factors influencing the submersed macrophyte community in the Dongshan Bay include water depth, TN, TP, SD/WD and Chl.a, and the composition of submersed macrophyte communities in different water depth areas is regulated by different water environmental factors. The key environmental factors affecting submersed macrophyte growth in shallow nearshore areas include water depth and Chl.a; submersed macrophytes in the midwater area are mainly influenced by water TN and TP; while submerged macrophytes in deeper areas near the lake center are significantly correlated with water depth, TN and TP. The results of this study provide important theoretical basis for shallow lake water level management and submersed macrophyte restoration under different water depth environments.

Abstract:Extreme flooding significantly affects the habitat of aquatic plants. Its knock-on effects may be one of the reasons for a massive decline in submerged macrophytes. Increased nutrient loading and rising water level are major factors affecting the reproduction and growth of submerged macrophytes. Therefore, we used H. verticillata, a typical submerged macrophyte in the lake, as the target species. A simulation study was conducted to explore the effects of water-level fluctuations (an increase in water level within a short period of time) and nutrient pulses (an increase in nutrient concentration within a short period of time) caused by extreme flooding events on the submerged macrophyte biomass, growth and reproductive traits, water physicochemical properties and periphyton. Four water level and nutrient loading scenarios were set up, i.e., control (stable water level of 75 cm), gradual increase of water level from 75 cm to 150 cm + N （nitrogen) and P （phosphorus) inputs, sudden increase in water level from 75 cm to 150 cm + N and P inputs, and stable water level of 75 cm + N and P inputs. The latter three treatments had identical total N and P inputs. Phytoplankton, periphyton, and N and P concentrations in the water column were monitored in a 90-d period. The results showed that the sizes of females were greater than those of males in the control, while the relative growth rate (RGR), root biomass and the reproductive organ number in males were higher than those in females in the extreme precipitation treatment (sudden change in water level). These findings demonstrated an evident sexual dimorphism in H. verticillata. Females responded more sensitively than males to water-level fluctuations (sudden and gradual water level change) and nutrient pulses. Both sudden and gradual increase in water level suppressed root biomass, RGR and the main branch number of plants, whereas plant height, aboveground and reproductive biomass remained unchanged. At the same time, a sudden rise in water level inhibited periphyton growth. Nutrient pulses increased the N concentration in the overlying water, promoted phytoplankton and periphyton growth, and significantly inhibited the biomass of shoots, roots and reproductive tissues. Within the designed range of water levels (75-150 cm), increased water level had no significant effect on the aboveground growth and reproduction of plants, while the combined effect of increased N and P inputs and water-level fluctuations had a negative effect on both aboveground biomass and reproductive output. Thus, the nutrient pulses aggravated the effect of water-level fluctuations on H. verticillata. In conclusion, the joint effect of water-level fluctuations superimposed on nutrient pulses caused by extreme flooding events will have a significant inhibitory effect on submerged macrophytes. With the increasing intensity and frequency of precipitation in the future, it would cause considerable impacts on aquatic ecosystems.

Abstract:In recent years, the problem of biological fouling by Limnoperna fortune (Dunker, 1857) has occurred in water resources and hydraulic engineering, affecting the safety of water transmission and production of water conservancy and hydropower projects to varying degrees. The problem has attracted extensive attention from the domestic and foreign communities. This study showed that L. fortunei in the Yangtze River Basin was mainly distributed in the low altitude area of Sichuan and below. No record was found in the high-altitude area above Sichuan. Phylogenetic tree analysis utilizing COI and ITS1 genes revealed two groups of L. fortunei in the Yangtze River Basin. The A+T content of COI sequences was notably higher at 64.7% compared to the C+G content at 35.3%, with intraspecific genetic distances ranging from 0 to 0.123. The total haplotype diversity index for COI sequences was 0.893. The nucleotide diversity index was 0.069. For the ITS1 sequences, the A+T content was 45.3% and the C+G content was 54.7%, with intraspecific genetic distances ranging from 0 to 0.061. The total haplotype diversity index for ITS1 sequences was 0.679. The nucleotide diversity index was 0.028. Genetic differentiation analysis based on the COI gene yielded a coefficient of genetic differentiation of 0.844 and a coefficient of differential gene differentiation of 0.118 between the two related groups. Similarly, analysis based on the ITS1 gene resulted in a coefficient of genetic differentiation of 0.927 and a coefficient of differential gene differentiation of 0.492. The morphological characteristics and habitat analysis showed that there were significant differences in the morphological characteristics and habitats of the two ecotypes of L. fortunei in the Yangtze River basin, which can be clearly divided into two groups, i.e., Group 1 and Group 2 as shown in COI and ITS1 gene analysis. The length-height ratio, length-width ratio and ligament length of the two groups were significantly different. Group 1 mainly lived in the areas with slow flow and pebble-based bottom, showing a slow flow type. Group 2 mainly lived in the areas with fast flow and sand-based bottom, showing a rapid flow type, and had stronger adaptability to the environment. It had a wider distribution range than Group 1. This study improved our understanding of the basic biological information of L. fortunei in the Yangtze River basin, and provided a reference for the comprehensive prevention and control of L. fortunei in the basin.

Abstract:Exploring the distribution and their historical changes of taxonomic and functional diversity in urban lakes improved our understanding on the effects of human disturbances on ecosystems. The Lake Donghu is a medium-sized shallow lake in the middle reaches of the Yangtze River, located in the eastern part of Wuchang District, Wuhan City. It was a river-connected lake formed by natural siltation of the Yangtze River on the basis of the tectonic movement. To understand the historical changes of its zoobenthic taxonomic and functional diversity, this study conducted three seasonal surveys of zoobenthos in Lakes Guozheng, Tangling, Hou, Tuan, Lingjiao, Yujia and Miao during 2020-2021, and collected historical data from previous literatures and books during 1970-2020. We compared the distribution and historical changes of taxonomic, functional and phylogenetic diversity. By obtaining the proportions of different land use types in 1 km buffer zone in 2000, 2010 and 2020, we analyzed the correlations between zoobenthic diversity and environmental factors. The results showed that 97 taxa of zoobenthos were identified with the highest number of taxa in Lake Tuan. The functional diversity of zoobenthos was the highest in Lake Hou, and the functional evenness was the highest and the lowest in Lake Miao and Lake Lingjiao, respectively. For phylogenetic diversity, the taxonomic diversity index was the highest in Lake Tuan, and the taxonomic distinctness index was the highest in Lake Miao. The total number of taxa was negatively correlated with total nitrogen and total phosphorus. The number of pollution-tolerant species was negatively correlated with total phosphorus. The functional evenness was positively correlated with the proportion of cultivated land. The functional richness was negatively correlated with total nitrogen and total phosphorus. The taxonomic diversity index was negatively correlated with total nitrogen and total phosphorus. Combined with historical data from 1970s to 2020s, aquatic insects showed a trend of decreasing first and then increasing. The groups of zoobenthos showed a trend of individual miniaturization in the 2010s, and then recovered.

Abstract:In order to explore phytoplankton community diversity and its driving factors in the lower reaches of the Yarlung Zangbo River, phytoplankton samples were collected at 34 sites in May 2022 (spring), July 2023 (summer) and September 2023 (autumn). Phytoplankton species were identified by microscopy. Environmental factors during the studied period were investigated. Based on the measured data, the phytoplankton community structure was characterized. The diverse phytoplankton community in different seasons and its driving factors were explored. The results showed that: 1) a total of 269 species and varieties of phytoplankton were identified, belonging to 8 phyla, 10 classes, 22 orders, 40 families and 87 genera. The order of cell abundance was spring＞summer＞autumn. the order of species number was autumn＞spring＞summer. The phytoplankton community structure as a whole showed diatom-green algae-cyanobacteria type. Twelve dominant species were selected, all of which were diatoms. 2) Shannon diversity index, Pielou evenness index, Simpson diversity index and Margalef richness index were lowest in summer. The principal coordinate analysis results showed that there were seasonal variations in phytoplankton community composition. By analyzing β diversity and component decomposition of these three seasons, we found that this difference was mainly due to turnover, in particularly in spring. 3) The neutral community model and the corrected random rate showed that the random process dominated the dynamics of phytoplankton communities in the lower reaches of the Yarlung Zangbo River in these three seasons. The variance decomposition analysis showed that the explanation rate of geographic factors (10.13%) was greater than that of environmental factors (7.46%). Latitude and altitude were the main geographic factors affecting the dynamics of phytoplankton communities. Water temperature was the main environmental factor affecting the dynamics of phytoplankton community. 4) The results of cooccurrence network analysis showed that phytoplankton interaction was mainly cooperative. The community structure in autumn was more concise and stable than that in spring and summer.

Abstract:With the advancement of gene sequencing technology, researchers have been able to uncover microalgae (e.g., picocyanobacteria) that were previously unknown in the environment. Despite this, previous publications shed limited light on the comprehensive taxonomic analysis of picocyanobacteria in China. This study focused on the isolation and purification of Cyanobium gracile HNU001 from a waterworks, which was the first study reporting this species in detail. The morphology, ultrastructure, and molecular characteristics of the organism were investigated. Morphological observations showed an ellipsoidal or elongated shape of the algal cells that divided symmetrically into shorter cells. Ultrastructural analysis indicated that cell division through dimitosis and the arrangement of thylakoid membranes were parallel to the cell membrane. Analysis of the 16S rRNA gene sequences demonstrated a high similarity of over 99.9% between this strain and Cyanobium, placing it within a distinct molecular cluster. Comparative analysis of the D1-D1′ and Box-B helical structures of Cyanobium gracile HNU001 with Cyanobium gracile PCC 6307 showed high consistency. Based on these findings, the species was formally named Cyanobium gracile, i.e., the first taxonomic characterization in China.

Abstract:Salinity is one of the most important environmental factors affecting bacterial communities in lake sediments. Revealing the impact of salinity on bacterial community changes is of great significance for a deeper understanding of bacterial communities in aquatic ecosystems. This study focused on 22 lakes in the southwest of the Qinghai-Tibet Plateau, explored the bacterial richness, evenness and community composition of 33 sediment samples, analyzed their changes with salinity, and elucidates their potential environmental driving factors. The results showed that Proteobacteria, Bacteroidetes and Actinobacteria were the dominant bacterial phyla in the sediment of lakes, with relative abundances of 46.54%, 12.41% and 7.23%, respectively, totaling 66.18% of the entire bacterial phyla. For bacterial α diversity, species richness, and evenness showed a significant downward trend with increasing salinity. For bacterial β diversity, based on non-metric multidimensional permutation analysis, bacterial community distribution could be divided into three significant independent clusters by salinity, corresponding to freshwater, saline and high salinity environments, respectively. The Mantel test results indicated that compared to geographical distance, the Bray-Curtis dissimilarity of lake sediment bacterial communities varied more significantly with salinity. The results of variance decomposition analysis and random forest analysis indicated that the best predictive factors for changes in bacterial species richness, evenness and community composition were chemical factors such as salinity and phosphate, with overall contribution rates of 72.98%, 59.82% and 60.83%, respectively. Among them, salinity was the main driving factor for changes in richness, evenness and community composition, with relative contribution rates of 45.47%, 79.18% and 79.50%, respectively. Furthermore, relying on structural equation modeling, changes in bacterial richness, evenness and community composition in lake sediment were directly or indirectly affected by salinity. The results of this study fully explored the impact of salinity on bacterial community changes and revealed the bacterial composition and environmental driving factors of sediment in lakes on the Qinghai-Tibet Plateau. This provided scientific support for understanding the response of bacterial communities in lake ecosystems to environmental changes.

Abstract:The Chishui River is an important component of the National Nature Reserve for rare and endemic fish species in the upper Yangtze River. Its ecological connectivity is crucial for protecting endemic fish species in the upper Yangtze River. However, the widespread distribution of small hydropower stations in the Chishui River Basin has severely affected the connectivity of this region. This study collected detailed information on the spatial location, construction year, and installed capacity of small hydropower stations in the Chishui River Basin. Four connectivity indices, namely the dendritic connectivity index (DCI), stream connectivity index (SCI), catchment area fragmentation index (CAFI), and the newly proposed modified catchment area fragmentation index (CAFI_M), were used to assess the spatio-temporal changes in the connectivity of the Chishui River Basin due to small hydropower stations. The results showed that almost every river in the Chishui River Basin has small hydropower stations, mainly distributed in low-order streams. The DCI and SCI indices showed that as the number of stations increased, the connectivity of the Chishui River Basin significantly decreased. The connectivity in the middle and lower reaches was more severely deteriorated than that in the upper reaches. The CAFI and CAFI_M indices revealed similar trends, with the degree of river fragmentation continuously increasing, particularly in the Xishui River, Datong River, and Tongzi River Basins. The proposed CAFI_M index incorporated the factor of installed capacity of small hydropower stations, enabling a more comprehensive and accurate assessment of the impact of these stations. Compared to traditional indicators, the CAFI_M index is more sensitive to changes in installed capacity, and can effectively reflect the impact of large-scale power stations. Based on the assessment results of the CAFI_M and other indices, the study evaluated the priority of obstacles posed by each small hydropower station in the Chishui River Basin. The study demonstrated that priority should be given to rectifying stations with large installed capacities located at high-order streams to maximize the improvement of basin connectivity. The study found that the location and installed capacity of the stations were the key factors determining the priority of their removal. The removal of stations located on high-order streams with large installed capacities (such as Jinyang and Yuanmanguan) had the most significant effect on improving basin connectivity. This study comprehensively evaluated the connectivity status of the Chishui River Basin, proposed strategies for optimizing the removal of small hydropower stations, and provided a decision-making basis for ecological restoration in the basin. It is thus of great significance for protecting endemic fish species in the upper Yangtze River. This research provided theoretical foundations and methodological support for assessing the impact of small hydropower stations on watershed river network connectivity and developing restoration strategies.

Abstract:River young water fraction and residence time are critical parameters for describing streamflow processes, as well as the transport of pollutants and nutrients within a watershed. Determining the sources of water and its residence time within the watershed is critical for us to understand watershed behavior and functionality. To quantify the river young water fraction and residence time for the Lake Poyang Basin, this study conducted high-frequency monitoring of stable isotopes (δ¹⁸O and δD) in precipitation and river water for five major inflow rivers of the basin, i.e., the Gan River, Xiu River, Xin River, Fu River, and Rao River (hereinafter refered to as “Five Rivers”). The study analyzed the variations in δ¹⁸O and δD values for precipitation and river water using a sine fitting method. The study achieved the following findings. (1) The deuterium and oxygen isotopes in the “Five Rivers” water and precipitation exhibited similar characteristics, i.e., low isotopic values during the rainy season (May to July) and high values during the dry season (December to February in the following year). There was a high correlation between river water isotopic variations and precipitation isotopes as well as groundwater isotopes, indicating good hydrological connectivity among different water bodies in the watershed. (2) The young water fraction in the “Five Rivers” catchments, with an age less than 2 months had a range from 3% to 42%. Its mean residence time varied from 123.83 days to 2495.0 days. Among the inflow rivers, the Gan River had the longest mean residence time and the lowest young water fraction, while the Le'an River and the Chang River exhibited the highest young water fraction and the shortest mean residence time. (3) Catchment area and the main stream length per unit area were identified as the primary driving factors for the young water fraction in the “Five Rivers” catchments. Slope was negatively correlated with the young water fraction. However, the negative correlation was overshadowed by the positive correlation between the main stream length per unit area and the young water fraction. (4) Compared with other catchments, the Xiu River and Gan River catchments were more significantly influenced by evaporative fractionation. (5) Lake Poyang water had significantly higher isotopic values than river water. Within Lake Poyang, the Hukou station had lower isotopic values than the Duchang station. This result indicated a combined influence of Yangtze River and evaporative fractionation on Lake Poyang water. The study improved our understanding of hydrological connections and processes within the Lake Poyang Basin, and thus provided a scientific reference for modelling mass transport and learning the underlying mechanisms.

Abstract:This study investigated the impact of coupling the conceptual hydrological model (GR4J) with the long short-term memory model (LSTM) in a physics-informed machine learning (PIML) framework for runoff simulation. Three scenarios (H1, H2 and H3) were designed to examine the effects of the physical model parameter feedback mechanism, the consideration of soil moisture as an intermediate variable, and the former both on the PIML models, respectively. The case study was conducted in the upper Han River Basin, with the Ankang hydrological station as the control station. The main findings were as follows: (1) Compared with the LSTM model, all three PIML models had improved performance on runoff simulation, with a 10.6% increase in average Nash-Sutcliffe efficiency (NSE) during the validation period. Additionally, both the PIML-H1 and PIML-H3 models exhibited better performance than the GR4J model, with a 4.2% increase in average NSE during the validation period. Notably, the PIML-H3 model outperformed other PIML models, indicating that coupling GR4J and LSTM models simultaneously considering intermediate variables and parameter feedback yielded the most significant improvement in the model performance of runoff simulation. (2) For low flows, all three PIML models outperformed the GR4J and LSTM models, and the PIML-H3 model achieved the best performance. For high flows, the performance of all three PIML models was not high, implying that PIML models were suitable in simulating low flows events. (3) The runoff simulations from the three PIML models exhibited significantly seasonal variations during both the training and validation periods. The seasonal variations in the PIML-H2 and PIML-H3 models were more pronounced compared to that in the PIML-H1 model, indicating that the seasonal variations in simulated runoff results of the PIML model were influenced by intermediate variables. This study contributed to a better understanding of the performance differences among various PIML model schemes in runoff simulation, providing technical support for runoff simulation and forecasting in the study area.

Abstract:Cascade reservoirs in the lower reaches of Jinsha River play a crucial role in watershed temperature regulation. However, there is still a lack of knowledge on how cascade reservoirs affect downstream temperature. This study quantitatively analysed the cumulative effect of four cascade reservoirs (Wudongde, Baihetan, Xiluodu and Xiangjiaba) in the lower reaches of Jinsha River on downstream temperatures. It examined overall temperature deviation, phase deviation, and extreme amplitude using long-term temperature data from Longjie (Three), Wudongde, Baihetan, Xiluodu, and Xiangjiaba stations. The results indicated that: (1) after the completion of the lower reaches of the Jinsha River cascade reservoirs, the “heat source effect” was significant. The average annual discharge water temperature of Wudongde, Baihetan, Xiluodu and Xiangjiaba increased year by year. The annual water temperature variation decreased by 4.1, 2.4, 1.6 and 3.2 ℃ compared with that of the natural water temperature. The increased rate of the temperature along the reservoir decreased from 0.348 ℃/100 km in the natural state to 0.256 ℃/100 km at the present stage. (2) As the operational period of the cascade reservoirs along the Jinsha River increased, the cumulative effect on water temperature in the downstream reservoirs gradually intensified, with a slowing rate of change. By the year 2022, following the full commissioning of the four hydropower stations, Wudongde, Baihetan, Xiluodu, and Xiangjiaba exhibited temperature lag times of 19, 58, 49 and 65 days, respectively. (3) Among these four reservoirs, Baihetan exerted the greatest effect on temperature. Different types of reservoirs exhibited diverse patterns in temperature transmission and accumulation. A hybrid reservoir showed positive effect when acting as a “head” reservoir. Stable stratification reservoirs showed positive effect. Unstable stratification and transitional reservoirs showed positive or negative effect at different stages. This study provided valuable guidance for optimizing water resource management and enhancing hydropower efficiency, while offering a scientific foundation for maintaining the health of downstream ecosystems.

Abstract:Lake ice is a crucial subject in cryospheric hydrology, and can be a key indicator to represent climate change due to its sensitivity to climate change. Lake ice growth and melt, as a response to climate variations, influence material and energy exchange between lakes and the atmosphere, thereby regulating regional climate and lake ecosystems. The thickness of lake ice is a critical variable in studying its growth and melt processes. Understanding its characteristics is of significant theoretical and practical importance for unraveling lake responses to climate change. In this study, the ice in Lake Nanhu, Inner Mongolia was taken as the study area. Based on the prototype ice measurement data from 2013-2017 and 2022-2023, the ERA5-Land reanalysis data were used as the atmospheric forcing field. The process-based model of SIMSTRAT was used to reconstruct the growth-melt process in Lake Nanhu from 2003 to 2022, and to investigate its change characteristics. The results showed that: 1) The SIMSTRAT model showed a high level of agreement with prototype observations. The mean deviation of the simulated first ice day and final ice day was 3.4 days, while the mean bias for ice thickness was 1.29 cm. The mean absolute error and root mean square error of the ice thickness were 1.29 cm and 1.90 cm, respectively. 2) From 2003 to 2022, the average freezing period of Lake Nanhu lasted 119 days, with the ice growth, balance, and melting periods averaging 64, 34 and 21 days, respectively. The freezing period exhibited an overall shortening trend at a rate of 4.27 days per decade. Among these, the melting period showed the largest interannual variability, with a shortening rate of 3.67 days per decade. 3) Over the past 20 years, the annual average ice thickness of Lake Nanhu ranged from 14 to 30 cm. From 2012 to 2017, interannual fluctuations in ice thickness were pronounced, showing an overall declining trend. Ice thickness increased rapidly during December and January at rates of 0.43 cm/d and 0.55 cm/d respectively, and melted quickly in March at a rate of 0.74 cm/d. 4) The SIMSTRAT model further elucidated the comprehensive impact of meteorological factors, including air temperature, precipitation, and wind speed on lake ice thickness dynamics. Air temperature was identified as the primary factor, with decreased cumulative temperature significantly extending the ice growth and balance periods, concurrently increasing the maximum ice thickness for the year. Precipitation and wind speed also exhibited varying degrees of influence on lake ice thickness dynamics. This study enhanced our understanding of lake ice processes and their response to climate change, providing valuable insights for future studies in related fields.

Abstract:Tibetan Plateau is particularly sensitive to climate change and is a hotspot for studying ecosystem responses to global change. Currently, there is much controversy surrounding studies of aquatic biota response to climate change in this region, requiring further investigation. In our study, the evolution of the lake ecosystem over the last 300 years was investigated by paleolimnological methods in Lake Xingyicuo on the southeastern Tibetan Plateau. The results showed that the lake and surrounding environment had undergone limited changes before 1930. Since 1930, weathering and erosion had increased in the area leading to increased nutrient levels in the lake. Principal component analysis revealed significant ecological changes in rare diatom species after 1930, while the dominant species remained stable. Our study indicated that climate change was the primary cause of increased weathering and erosion in the Lake Xingyicuo watershed, with human grazing activities having a possible exacerbating effect. Although the current environmental changes have not yet caused significant changes in the aquatic ecosystems, our studies found rare species in the lakes responded quickly to climate changes since the 1930s. In the future, the lake ecosystems in this region may face a risk of rapid change if environmental pressures continue to increase.

Abstract:Water clarity is an important indicator for assessing eutrophication in lakes or reservoirs. Traditionally, water clarity is determined in the field using Secchi disk depth (SDD), which is time consuming, labour intensive and not suitable for large scale water clarity assessment. Water clarity is mainly determined by non-algal particulate matter, algal abundance and colored dissolved organic matter (CDOM) in the water column, which can be monitored by optical remote sensing. In this study, eight cruises were conducted over Lake Chagan in 2004-2009 and in situ measured SDD were determined. Correlation analyses between in situ measured SDD and Landsat calibrated top-of-atmosphere (TOA) reflectance were performed to determine the best band or band combinations. Based on the in situ measured SDD and TOA pairs, we developed an algorithm based on the ratio of red and blue bands and the average of red, green and blue, e.g. AV(B3, B1), to estimate the SDD in Lake Chagan. High model calibration accuracy was achieved, with lower RMSE and MAPE for model validation. The algorithm was applied to Landsat images to derive SDD distribution maps from 1984 to 2019. The Landsat-derived results indicate that the SDD in Lake Chagan ranges from 1.0 cm to 63.0 cm with an annual mean of 17.6 cm, with a significant increasing trend during 2002-2019 with an increasing trend of 2.3 cm/10 a. The annual mean values of the SDD variation dynamics in Lake Chagan went through two phases. From 1984 to 2001, the SDD showed no obvious trend, and then it showed a significant increasing trend. Our analysis showed that the SDD in Lake Chagan had some relationship with wind speed, precipitation and the normalized difference vegetation index (NDVI). The changes in water transparency in Lake Chagan were monitored by remote sensing on a long time scale, and the characteristics and rules of the changes were analyzed to provide a scientific basis for ecological management in the Lake Chagan Basin.