Abstract:Biological controls of phytoplankton are divided into traditional biomanipulation using zooplankton and nontraditional biomanipulation using filter-feeding fishes, and the former theory developed by western scholars are suitable for controlling small algae, while the latter one developed by Chinese scholars are effective for controlling colony-forming cyanobacteria (especially Microcystis). Lake Donghu is a eutrophic lake, located in Wuhan City, China, with a surface area of ca. 32 km². Before 2021, cyanobacterial blooms had been absent from the main part of the lake (Guozhenghu area, 12.8 km²) for more than 30 years. However, in the summer of 2021, Microcystis blooms suddenly occurred in the lake, with a maximum coverage of approximately 87%. After examining the long-term monitoring data of the Donghu Experimental Station of Lake Ecosystems, we found that current nutrient (nitrogen and phosphorus) levels are still high enough for the outburst of cyanobacterial blooms in this lake. As it is impossible to efficiently decrease the nutrients within a short period of time (e.g. a couple of years), non-traditional biomanipulation (using the native filter-feeding silver and bighead carps with an appropriate abundance) was applied to counteract the occurrence of cyanobacterial blooms, and as a result, no blooms were present in 2022 in spite of an extremely hot and dry summer, i.e., the Lake Donghu ecosystems was shifted quickly from a bloom steady state to a non-bloom one, demonstrating a new regime shift, and achieving a success that the traditional biomanipulation had never had before. Conclusively, the non-traditional biomanipulation theory was tested and validated using a whole lake experiment, providing an environmentally friendly, extremely low-cost and rather effective way to control colony-forming cyanobacterial blooms in eutrophic lakes.

Abstract:Environmental DNA (eDNA) is DNA extracted from any type of environmental sample (e.g. water, soil, sediment, air, mixture, etc.), which is a DNA mixture originated from different species and individuals, distinguish from a pure DNA sample extracted from a particular organism. eDNA monitoring refers the processes that 1) extracting DNA sample from environmental sample, 2) using definite species-specific primers or meta-barcoding primers to amplify and sequence eDNA sample, 3) clustering the operational taxonomic units (OTUs) and identifying their taxa against reference databases, 4) calculating the relative abundance of each OTUs/ species and other biodiversity indexes, 5) analyzing the corresponding ecosystem structure, processes or function. According to eDNA monitoring, a definite species (or other taxonomic units) in the sampling site could be identified, and the biological information about species composition, community structure, ecosystem processes, ecological function of the research area could be collected. eDNA monitoring has been applied in monitoring and early warning definite species, investigating and assessing biodiversity, detecting and analyzing community structure and function, studying and quantifying ecosystem processes and so on. eDNA monitoring could work in any type of environmental scene where there is unidentified DNA trace, such as in terrestrial environment, aquatic environment, air environment, body surface, organism (inner) surface and so on. As an emerging tool for documenting species presence without direct observation, allowing for sensitive and efficient detection, easy-to-standardize sampling and analyzing approach, comprehensive taxonomic groups coverage, less reliant on taxonomic expertise and auditable by third-party researchers, eDNA monitoring would be a prospective general method for species monitoring, community function predicting and ecosystem processes analyzing in future. Moreover, the objective scope of eDNA monitoring covers all environmental conditions and all biological taxonomies. However, to realize the prospective application vision of eDNA monitoring, there are ten crucial links that need to be standardized at both general level and definite level. 1) Design of duplicated samples for a region with definite environment conditions. The number of duplicated samples could be generally identified just using species accumulation curves. 2) Design of sampling time for a region with definite environment conditions. The interval of sampling time could be generally identified by quantifying the degradation ratio or the retention time of the eDNA from different taxonomic organisms in definite environment conditions. 3) Design of sampling sites for a region with definite environment conditions. The distance of sampling sites could be generally identified by quantifying the effective transportation distance or the spatial heterogeneity of the eDNA from different taxonomic organisms in definite environment conditions. 4) Design of sampling method. For different study areas, objects and aims, there are different optimal sample types (water, soil, sediment or other samples). Don't combine different duplicated samples, or some rare species would be omitted because of their too weak signals. 5) Pretreatment of samples. Pretreatment of samples mainly refers filtration of water samples. It's suggested that filtering water samples should use finer millipore glass fiber filter. Don't remove large particles by prefiltering water sample, or some species signals could be removed. 6) Storage of samples. It's suggested that samples could be stored at -20 or -80 centigrade, except water samples. Water samples should be kept cool in ice bath and be filtered as soon as possible. 7) Choosing of primers. The primers of metabarcoding of the 16S rRNA gene are widely used for detecting bacteria and archaea. The primers of metabarcoding of the ITS and 18S rRNA genes are widely used for detecting fungi. The primers of metabarcoding of the mitochondrial COI, 12S rRNA and Cyt b genes are widely used for detecting metazoan. Metagenome is another choice for identifying species. 8) Experiment processes of DNA extraction, amplifying, sequencing and analyzing. As the experiment processes are more and more tending to be processed by commercial biolabs, a set of general experimental parameters is needed. 9) Taxonomic identification of OTUs. Good reference databases, either comprehensive reference databases or local customizable reference databases, are required. 10) Post-evaluation of results. Post-evaluation of results mainly pays attention on whether the number of duplicated samples is sufficient, whether the frequency of sampling is suitable, whether the spatial distance between sampling sites is suitable, whether the taxonomic identification of OTUs is accurate. Until now, there is no theoretical difficulty in standardizing these ten crucial links. Now, the mainly work is the accumulation of datasets and knowledge. Some studies on supporting the standardization have been processed. Parts of standardizing works have been organized both at home and abroad. We expect that the accumulation of crucial datasets and knowledge on eDNA monitoring in hot regions could finish in future several years, and then the eDNA monitoring could be a general work, even a long term basic work in hot regions. As the eDNA monitoring could produce comprehensive and standard datasets, along with the long term basic work of eDNA monitoring realizing, the long time series datasets could be used to detect the biodiversity (especially hiddenbiodiversity) variations and study the dynamic and evolution of ecosystem structure, processes, function and health. Moreover, we expect a series of datasets with high quality, rigour, availability and transparency in future to support the open science and the data-intensive scientific discovery and ecosystem management.

Abstract:Suspended particles (SPS) are ubiquitous in natural water bodies (ponds, rivers, lakes, etc.) and usually hot spots for denitrification, which tend to adsorb more microorganisms than overlying water and obtain more nitrates than sediments. SPS undergo a series of physical and chemical processes in water bodies, such as collision, flocculation, dissolution, ion exchange, adsorption, and desorption, which would result in changes in particle size and nutrient contents. The sedimentation and resuspension processes of SPS would cause material exchanges between overlying water and sediments, affect nitrogen transformation in the process of aerobic-anoxic transition, and correspondingly affect the denitrification rates of the whole water body directly and/or indirectly. This paper reviews and summarizes (the overseas and domestic) research advances and progresses in SPS' effects on water denitrification with focuses on the factors including concentrations, particle sizes, compositions, and types of SPS. The mechanism of SPS to denitrification through different environmental factors, including dissolved oxygen, functional microorganisms, inorganic nitrogen and organic carbon, has been analyzed. The measurement methods of denitrification with SPS have been compared. Based on the current progress of the research, we suggest that future research should extend into the field of understanding of denitrification with SPS through emerging suspended particles of pollutants, mechanism models, and measurement methods.

Abstract:As a group of newly identified bioavailable phosphorus sources, phosphonates are an important part of the aquatic organophosphorus pool and constitute a key component in the phosphorus redox cycle. In recent years, dozens of cyanobacterial strains have been demonstrated to participate in the utilization and biosynthesis of phosphonates, aiding in the understanding of how these strains are able to adapt to low concentrations of phosphate. The discovery of cyanobacteria phosphonate utilization promotes our knowledge of cyanobacterial phosphorus strategy and water phosphorus cycle. Intensive studies have shown the duel ecological role of phosphonates that serve both as a source of phosphorus and as a biocide for planktonics in aquatic ecosystems. They can damage the aquatic ecosystems by driving a selective decrease in phosphonate-sensitive portions with a corresponding increase in phosphonate-consuming cyanobacteria. On the other hand, cyanobacterial methylphosphonate catabolism is regarded as a promising mechanism of oxic methane production in the water. This paper reviewed the research history of cyanobacterial phosphonates utilization and biosynthesis. A summary of the research progress on the ecological risks and ecological significance of cyanobacterial phosphonates metabolism was also provided. Finally, we proposed comments on the future focus in this field.

Abstract:The temporal changes of harmful algal blooms (HABs) indicate the phenological characteristics of algal blooms to some extent, the study of the phenological variations in HABs can provide a scientific reference for lake management and ecological restoration. Previous studies with MODIS data cannot monitor the situation before 2000, largely limiting the understandings on the spatiotemporal variation in HABs in Lake Chaohu. Here, we employed heritage Landsat missions and MODIS data to extend the time series of HABs based on the floating algae index and a threshold segmentation technique, the spatiotemporal changes and driving factors of HABs in Lake Chaohu from 1987 to 2020 were analyzed on the basis of ensuring the consistency of HABs results between Landsat and MODIS. The results showed that: (1) Comparing to 1987-1999, the area of algal blooms increased significantly after 2000 and a peak of 608.4 km² was found in 2011. (2) The temporal variations in cyanobacterial blooms in Lake Chaohu were divided into three stages in the period of 1987-2020. From 1987 to 2004, the annual start time of HABs was earlier significantly, while the duration of HABs increased significantly. The start time of the outbreak delayed significantly, and the duration of HABs did not change significantly from 2005 to 2010. From 2011 to 2020, the start time, termination time, and duration of HABs showed fluctuating changes with an insignificant decrease. (3) Changes in the timing of HABs in Lake Chaohu were mainly related to climatic factors such as temperature, precipitation and sunshine duration and were influenced by nutrients as well. During 1987-2004, the earlier start of the outbreak was dominated by the mean annual temperature in the previous year. From 2005 to 2010, the delay in the start of the outbreak was due to the combined effect of temperature, sunshine and precipitation in the spring; the earlier termination time and shorter duration of outbreak were associated with annual precipitation. This study established a four decades of long-term series variations of algal blooms in Lake Chaohu which provided insights to the spatio-temporal characteristics and driving forces of cyanobacterial blooms and largely improved the scientific disposal and response of algal blooms under the context of climate changes.

Abstract:Algal blooms occur frequently in the freshwater lakes and reservoirs in China, but the characteristics, hazards, and treatment methods of different bloom-forming algae species are significantly different, consequently, a critical scientific problem that must be solved is how to distinguish the reflectance characteristics of different algal species and identify dominant species in lakes using remote sensing reflectance. Based on laboratory algae culture experiments, the research produced typical cyanobacteria species (including Microcystis aeruginosa, Pseudanabaena sp. and Aphanizomenon sp.) and green algae species (including Chlorella sp. and Scenedesmus quadricauda), which are prevalent in eutrophic lakes. Subsequently, the normalized in-situ remote sensing reflectance characteristics of different algal species were analyzed, the DI (difference index) and ADI (algae distinguish index) were constructed based on spectral differences, and then the algal species identification model was established. Validated by independent verification data set, the overall recognition accuracy and Kappa coefficient were 77.55% and 0.7178, respectively. The results of classification algorithm match well with the measured biomass data when applied to the field measured remote sensing reflectance data set of Lake Taihu. Further model tests based on the OLCI images of Lake Taihu in December 2019 and August 2020 show that the proportion of cyanobacteria in Lake Taihu is higher than that of green algae in summer and winter, during which Microcystis displayed obvious advantages and a stronger dominant position in summer than in winter. Furthermore, in terms of seasonal and geographical distributions, the dominant areas of Microcystis were in the northern and southern bays of Lake Taihu in winter, while Pseudoanabaena was in the central area of the lake, and scattered with a tiny fraction of Aphanizomenon and Scenedesmus quadricauda at the same time; compared to summer, Microcystis was dominant in most areas of Lake Taihu, Pseudoanabaena was mainly distributed in the center and south of the lake, and the other three species only scattered in a small part of the lake. The development of a remote sensing identification model of the main algal species can provide technological assistances to remote sensing monitoring of the lake environment, as well as algal bloom prediction and prevention.

Abstract:With the expansion of the drainage basin development since the middle of the 20^th Century, catchment export and lake-water enrichment of nutrients have led to a continuous process of lake eutrophication in Yunnan, which has significantly impacted the structuring of biological communities and health of the lake ecosystems. On the regional scale, the time span of modern water quality monitoring records is generally short (i.e.<20 years); this has prevented a reliable evaluation of pre-disturbance level of nutrient status for the lake ecological restoration. The application of paleolimnological methods to establish diatom-nutrient quantitative model can provide important information for water quality evaluation and ecological restoration of the polluted lakes. In this study, 45 lakes in Southwest Yunnan were selected for analyzing the surface sediment diatom communities, and in combination with the water monitoring data, the key environmental gradient was then identified in driving diatom assemblages. We further identified that the lake-water TP was a significant environmental factor responsible for the spatial variations of diatom assemblages (a variance of 4.54% explained independently, P<0.01). Furthermore, the transfer function between the surface sediment diatoms and TP in 45 lakes was established through multivariate statistical analyses. Among the transfer function models established by Weighted Average Partial Least Squares regression (WA-PLS), the component 2 has the strongest prediction ability (R²_jack=0.465, RMSEP=0.396). Combining with the sediment diatom sequence of Lake Xingyun, lake-water TP values were reconstructed with this WA-PLS-2 model for the past 100 years. The null model results confirmed that lake-water TP played a significant role in driving diatom variations, and the predicted TPs show a significant correlation with the observed TPs (R²=0.79, P<0.001, n=13). The reconstruction results show that the background concentrations of TP, prior to the period of eutrophication (before 1950 AD), varied in the range of 12.8-29.3 μg/L for Lake Xingyun, which can be used as the restoration target of nutrient levels for lake restoration. This study shows that the quantitative reconstruction method of lake-water nutrient levels based on biological communities can provide reliable technical support for the construction of environmental restoration target for eutrophic lakes in Yunnan.

Abstract:With the improvement of aquatic trophic level, phytoplankton proliferates rapidly, and a large amount of algae debris finally settled on the surface of sediments. Priming effect would occur on the sediment organic matter (SOM) due to the deposition of algae debris, a fresh labile organic matter, which would affect the cycling process of carbon in sediments. In the current study, the sediments in the southern lake center of Lake Fuxian, the second deepest lake in China, was selected as the research object, and the dominant species Aphanizomenon flos-aquae in Lake Fuxian was chosen to carry out the cultivation experiments with different concentrations of algae debris (×1, ×5 and ×10 treatments). The carbon isotope on-line monitor was conducted to explore the priming effect of algae debris on SOM mineralization and to predict the possible environmental influence. The results revealed that: (1) the addition of algae debris increased the concentrations of CDOM, and influenced the compositions and properties of chromophoric dissolved organic matter (CDOM) in both overlying water and porewater of sediments. It significantly increased the content of protein-like components in CDOM at the earlier stage, and the humification degree of CDOM at the later stage. The addition of algae debris increased the release of CO₂ and the activity of proteinase and invertase simultaneously as well; (2) different levels of priming effect were detected in the algae debris addition treatments. The positive priming effect on the SOM mineralization was found in ×1 and ×5 treatments at the early stage, and then turned into negative excitation effect afterwards, with the highest priming effects of (12.18±0.65) and (26.60±9.14) μg/(mL ws), respectively, while the negative priming effect was shown in ×10 treatment during the whole experiment; (3) the addition of algae debris improved the enrichment of Fe²⁺ and S^2- in porewater and inhibited the denitrification rate, of sediments, which helps to enrich NO₃^-. The activity of alkaline phosphatase in sediments increased due to the addition of algae debris, which indicated the release of reactive phosphorus. It suggested that the addition of algae debris would increase the eutrophication and the risk of black bloom in Lake Fuxian. Therefore, the systematically study of the influence of labile organic matter inputs on SOM biogeochemical process could provide theoretical basis for protecting the water quality of Lake Fuxian.

Abstract:In order to explore the ecological niche and interspecific association of dominant phytoplankton community, we selected the middle and lower reaches of Lhasa River and its tributaries as research sites. Seventeen sampling stations were set up in May 2015, August 2015 and November 2016, and water samples were collected in three hydrological periods. A total of 393 phytoplankton species were identified, belonging to 100 genera, 48 families, 8 classes, 24 orders and 6 phyla. The dominance index (Y) of the dominant species was calculated, and the niche width, niche overlap, ecological response rate, total linkage, χ² test and common occurrence percentage were used to analyze the niche and interspecific linkage of the dominant species. The results showed that the whole phytoplankton community was diatom-green algae-cyanobacteria. There were 19 dominant phytoplankton species (Y>0.02), all belonging to diatoms, among which 7 dominant species such as Synedra acus and S. amphicephala were common dominant species in three hydrological periods. The occurrence frequency and dominance degree of dominant species were different in different hydrological periods. The niche width of the dominant species varied from 0.089 to 0.751, and the niche width values of S.amphicephala were all larger than 0.5 in the three hydrological periods, showing strong ecological adaptability and belonging to a wide niche species. The species of Achnathes exigua were highly competitive and their habitats tended to be specialized, which could be used as an indicator of pollution in the middle and lower reaches of Lhasa River. The niche overlap values of the 19 dominant species ranged from 0.001 to 0.966, and Diatoma aulgare occupied similar habitat resources in wet season, showing competitive interference. The niche overlap values were higher in normal seasons than in dry seasons, and the niche overlap degree was not high in general. The commonness of the use of resources is small; The relative resource occupancy and niche width of dominant species in different hydrological periods were basically the same, and most of the algae species with higher resource utilization had stronger development space. According to the analysis of interspecific association, the association of dominant species decreased gradually with the change of seasons, and the interspecific relationships of species in the community changed from close to loose from normal to wet to dry seasons, showing a trend of reverse succession. The associations of most species pairs did not reach the significant level (χ²>3.841), and the interspecific association was not close. The relative dynamic balance between dominant species and environment was not reached.

Abstract:The energy structure in China is facing sustainable transformation and development. Hydropower, as a source of clean energy, is widely built up in the upper reaches of Changjiang River drainage basin where great terrain differences occurred. However, there is a lack of research on the greenhouse gases (GHGs) emissions of reservoirs in the hydropower life cycle in a specific region, and the inconsistent estimation methods lead to poor comparability of the estimation results. Based on Tier1 in the IPCC National Greenhouse Gas Inventories, this study explores the life cycle GHGs emissions of 24 medium and large reservoirs in the upper reaches of Changjiang River, and conducts uncertainty and sensitivity analysis of model parameters through Monte Carlo simulation. The results show that the average carbon emissions of the 24 reservoirs are distributed in the range of 0.0342-140.59 Tg CO_2eq. The total carbon emissions reached 264.05 Tg CO_2eq (CO₂ emissions accounted for 9.12%, CH₄ emissions accounted for 90.88%). The average carbon emission per unit power generation is 3.30 g CO_2eq/(kW·h) (0.01-17.64 g CO_2eq/(kW·h)), and the minimum and maximum values appeared in Jinping Ⅱ Reservoir and Pengshui Reservoir, respectively. In addition, sensitivity analysis found that among the parameters involved in the model, the nutritional adjustment coefficient played a leading role in the uncertainty of the final estimation results, while the ratio of total downstream flux of CH₄ to the flux of CH₄ from a reservoir's surface to the atmosphere had the weakest sensitivity to results. This provides good guidance to select parameters when using this model and to carry out sampling detection in the future.

Abstract:Submerged macrophytes are regarded as key components of aquatic ecosystems and also profoundly influence nutrient cycling through various physical, chemical, and metabolic processes. Vegetated shallows may act as hotspots for nitrogen (N) transformation in eutrophic rivers. Explosive growing of submerged macrophyte Potamogeton crispus may support enhanced nitrogen processing, but little is known about the variability of denitrification in epiphytic biofilms of P. crispus and the contributions of denitrification to nitrogen removal in rivers. Yi River of Linyi City is strongly influenced by rubber dam and human activities, and its water is rich in nutrition. P. crispus have become the dominant species in the water area in spring and they have been seen under explosive growth from April to May every year. P. crispus and epiphytic biofilms were sampled at explosive growing stages every 7 days in Yi River, and denitrification rates of epiphytic biofilms were analyzed using a ¹⁵N enrichment experiment. The results showed that biomass of P. crispus increased rapidly during explosive growing (from early April to early May) with the maximum biomass of P. crispus at sampling sites up to 1817-3334 g/m². Dry weight, organic matter, total organic carbon, total nitrogen and denitrifying bacteria of epiphytic biofilms increased significantly, which provided abundant material and microenvironment for denitrification. Denitrification rates (as N₂) of epiphytic biofilms significantly increased, the greatest denitrification potential at 6 monitoring sites were 424.18-1728.39 μmol/(m²·h), which was 17.29-29.09 times higher than that of sediments. The contribution to N₂ production by denitrification of epiphytic biofilms was high, with 77%-96% to N₂ (denitrification + anammox) production of biofilms, and 32%-96% to N₂ production by denitrification in river ecosystem. An estimate of epiphytic biofilms N₂ production by denitrification in P. crispus beds, based on continuous observations suggests that these epiphytic biofilms are major hotspots for N removal in Yi River during the later spring months. Scientific management and regulation of P. crispus beds through constructing complex submerged plant communities, moderating harvesting, and regulating water levels for urban river ecosystem service restoration efforts have been recommended.

Abstract:The 30 km² net-pen aquaculture in East Lake Taihu was cleared in 2019, ending its 34 years history of net-pen aquaculture breeding. The temporal-spatial differences and influencing factors of water quality at 14 monitoring sites during the period of 30 years' (1990-2021) net-pen aquaculture were analyzed. The results showed that the variations of nutrient concentrations in East Lake Taihu in the past 30 years varied with the changes of aquaculture structures (from fish to crab) and aquaculture scales (from excessive utilization to net-pen aquaculture rectification and demolition). The water quality of breeding crabs in net is better than that of breeding fish, owing to the effective management of aquatic plants and mixed breeding pattern. However, as the increase in the intensity of net-pen aquaculture, the water quality degraded. The reduction of the scales of net-pen aquaculture could help improve water quality. The concentrations of total nitrogen (TN), total phosphorus (TP), the permanganate index (COD_Mn) and chlorophyll-a (Chl.a) increased from 0.53 mg/L, 0.014 mg/L, 3.58 mg/L and 2.90 μg/L in 1990 to 1.87 mg/L, 0.128 mg/L, 5.72 mg/L and 28.09 μg/L in 2021, respectively. TN was the main pollution factor. The concentrations of TN, TP, phosphate and suspended solid (SS) in the former net-pen areas and lake center area maintained and unchanged in the past three decades owing to higher aquatic vegetation coverage. The transparency (SD) in the former net-pen areas maintained at 0.8 m. The concentrations of TN, TP, Chl.a, COD_Mn and SS at the sites of Taipu River Estuary and Dongjiaozui showed an increasing trend, while the SD at these sites decreased significantly. The average concentrations of Chl.a at the sites of Taipu River Estuary and Dongjiaozui were significantly positively correlated with the water discharges of the Taipu River. The removal of net-pen, the increase of water discharges and the disappearance of aquatic vegetations increased the hydrodynamic effect. The hydrodynamic effect and the release of nutrients from the sediments in the former net-pen areas resulted in the significant increase of nutrient concentrations in the southern parts of East Lake Taihu. The nutrient concentrations in East Lake Taihu were high in summer and autumn and low in winter and spring since all the net-pen aquaculture was closed down. The implications of this study to other lakes are, in order to ensure the restoration of healthy and natural macrophytic ecosystem patterns, the regulations of aquatic plant resources to be carried out as early as possible so as to strengthen water purification, monitoring of the ecological lake environment, establishing ecological archives, and creating new fishery models.

Abstract:Rice-crayfish rotation system is expanding rapidly and becomes the most widely applied crayfish culture modes in the Jianghan Plain, China. However, the wastewater of these operations is directly discharged into the surrounding water system, and the environmental pollution caused by the crayfish culture in the paddy fields is normally neglected and has not been included in the regulatory system of aquaculture regulations. In order to explore the impact of non-point source pollution emission from the rice-crayfish rotation mode on water environment and evaluate rationally its environmental sustainability, the concentrations of total nitrogen, total phosphorus, chemical oxygen demand and ammonia nitrogen in the drainage from the paddy fields after the harvest of crayfish in the Jianghan Plain were monitored. The results showed that the ranges of concentrations of total nitrogen, total phosphorus, chemical oxygen demand and ammonia nitrogen in the effluent were 0.53-5.36, 0.12-0.70, 6.60-78.39 and 0.34-1.75 mg/L, respectively, and the average concentrations of total nitrogen, total phosphorus and chemical oxygen demand all have exceeded the class III water quality standard in the environmental quality standards for surface water (GB 3838-2002). According to the equivalent standard analysis, total nitrogen was the primary pollutant to the environment which showed the highest equivalent standard load ratio. The amounts of discharge of total nitrogen, total phosphorus, chemical oxygen demand and ammonia nitrogen per ton crayfish were 2.994, 0.458, 35.132 and 1.405 kg/t, respectively. The emergy analysis with the integration of environmental pollution showed that the renewable fraction and emergy sustainability index of rice-crayfish rotation system was higher than the rice monoculture system and environmental loading ratio was lower. The results indicated that the non-point source pollution discharge rates of rice-crayfish system were relatively low and rice-crayfish rotation system was more sustainable, as indicated by lower environmental pressure and higher emergy sustainability index. Nevertheless, it is urgent to incorporate wastewater discharged from rice-crayfish farming into the supervision system of aquaculture wastewater discharge regulations and formulate environmental pollution risk control policies to ensure the sustainable development of the rice-crayfish integrated system. The results provided basic data for the further accounting of the pollution contributions of rice-crayfish farming and basis for the non-point source pollution prevention and control, and the sustainable development of rice-crayfish rotation farming.

Abstract:The drainage process during aquaculture pond cleaning would cause non negligible pollution of surrounding water environment, but there haven't been sufficient research on the characteristics of pollutant discharges and the impact degrees of this process. To effectively mitigate the environmental pollution from the drainage of pond cleaning process and promote the green development of aquaculture industry, in this study, we selected a typical intensive fish aquaculture area as the study object. Through high frequency sampling, we analyzed the changes of suspended matters, organic matters and nutrients concentrations in the tailwater of mixed fishponds to understand the discharge characteristics of pollutants. The changes of water quality in different sections of the receiving water were analyzed as well. According to our results, the concentrations of total suspended solids concentration (TSS), COD_Mn, total phosphorus (TP), total nitrogen (TN) and ammonia nitrogen (NH₃-N) all exhibited an increasing trend with the drainage and increased rapidly at the end of the drainage event. The concentration of phosphate (PO^3-₄-P) increased rapidly only at the end of the drainage event, while the concentration of nitrate nitrogen (NO₃^--N) decreased continuously during the whole drainage event. According to the secondary standard of "Freshwater Pond Culture Water Discharge Requirements", the exceedance multiples of TN, TP and TSS at the end of discharge reached 4.70, 6.66 and 206.90, respectively. After the tailwater flow discharged into the external river at a ratio of about 1/200 to the river volume, within 400 m from the outlet, only TSS, COD_Mn, TN and NH₃-N would significantly affect the river water quality, and the influencing distance exhibited a trend of TN>TSS=NH₃-N>COD_Mn. Therefore, prohibiting direct discharge of aquaculture tailwater into the nearby river without treatment, especially the tailwater of the last 20-30 cm water depth of the pond, can effectively limit the output loads of the concentrated pollutants in aquaculture tailwater.

Abstract:As an essential water ecosystem in rural areas, the conditions of ponds are closely related to agriculture activities, and daily life and health status of humans. In order to investigate the contents of organic matters and nutrients in the surface sediments of rural ponds around the western part of Lake Poyang, surface sediments of 23 ponds in 4 counties of the region were collected and analyzed from 2018 to 2019, and correlation and difference analyses were undertaken to identify the pollution sources of the sediments. Moreover, comprehensive pollution index method and principal component analysis were carried out for the evaluation of sediment pollution in ponds. Results showed that the contents of organic matter, total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) were 5.81%±2.16%, 2.46%±1.02%, (6.48±2.35) mg/g and (1.89±0.80) mg/g, respectively. The contents of three inorganic nitrogen in sediments followed the sequence of ammonia>nitrate>nitrite. Sediment TP content was greatly affected by washing wastewater. In addition, dredging and circulating significantly reduced the TN and TOC pollution in sediments. Results of comprehensive pollution index showed that the ratio of ponds assessed with heavy pollution and moderate pollution were 95.65% and 4.35%, respectively, and the average pollution level was heavy pollution. And the results of principle component analysis showed that the pollution state of P18 was the worst, while that of P6 was the least. The correlation index of comprehensive pollution index and total score of principle component analysis was 0.92, indicating the results of these two evaluation methods were relatively consistent. The results of this study can provide data and technical support for the ecological restoration of rural ponds around the western part of Lake Poyang by analyzing their sediment nutrient contents, pollution sources and pollution status.

Abstract:As antibacterial active ingredients, biocides are widely used in medicines, personal care products, food, agriculture, and other sectors. Large amounts of biocides enter aquatic environments directly or via effluents of wastewater treatment plants (WWTPs), which might pose negative effects on ecological environments and human health. In this study, the occurrences, distributions, and ecological risks of 13 biocides in the sediments of 61 lakes in the middle and lower reaches of the Chnagjiang River were investigated. Eleven biocides, including 5 fungicides, 1 antifouling agent, 3 parabens, and 2 disinfectants, were detected in these lake sediments, with total concentration levels up to 103 ng/g dry weight (dw). The detection frequencies of carbendazim and methylparaben were both greater than 50%, which were 100% and 96.2%, respectively. The mean concentrations of carbendazim and methylparaben were (1.79±2.76) and (11.4±8.19) ng/g, respectively, while the mean concentrations of other compounds were lower than 1.0 ng/g. Compared with other rivers and lakes abroad, the pollution of biocides in area of this study would be ranked at low-medium levels, with the mean value of total biocide concentration of (16.7±14.5) ng/g. Domestic sewage might be the main source for fluconazole, clotrimazole, triclosan and triclocarban, while carbendazim and thiabendazole might come from non-point sources. The distributions of parabens in the lake sediments were significantly related with total organic carbon of the sediments. The ecological risk assessment of biocides in the lake sediments were performed by using risk quotient. Carbendazim, methylparaben and triclocarban showed high ecological risks at some sampling sites. To reduce the adverse effects of biocides on aquatic organisms and human health, it is necessary to take proper measures to control the pollution of biocides in the lake environments, especially at the source area of drinking water.

Abstract:The sluice construction has a blocking effect on river, which changes the flow pattern, the transport of pollutants and the temporal and spatial distributions of water quality, hence may have an impact on the water environmental capacity. In this study, the urban section of the Lhasa River is taken as the research case, and the influence of sluice construction on hydrodynamic conditions has been studied. On the basis of hydrodynamic and water quality simulations of two-dimensional model, the water environmental capacity of COD before and after sluice construction is calculated and analyzed, and the influence of continuous sluice construction on river water environmental capacity is discussed. The results show that there are two main aspects of the influence of sluice construction on water environmental capacity. First, once sluice stores water, the water body changes from flowing to relatively static state, the flow velocity in front of sluice slows down, the water depth increases; these affect the diffusion of pollutants, reduce the degradation coefficient of pollutants, and then affect the degradation process of pollutants. Second, sluice storage makes pollutants stay in the reservoir area, so that the retention time of pollutants will be increased, which leads to the increase of degradation of pollutants. After the construction of the Lhasa River sluices, due to the influence of sluices water storage, the velocity of pollutants is slowed down, and the diffusion of pollutants to the central part of the river is hindered, which allows pollutants to easily accumulate on the sewage-draining side. Comparing with the natural river, the average flow velocity in the sluice storage area decreased from 0.34 m/s to 0.10 m/s, the average water depth increased from 0.6 m to 2.0 m, and the COD degradation coefficient decreased from 0.12 d^-1 to 0.04 d^-1 (about 67% decrease), and the pollutant retention time increased by about three times. The calculation results show that, one sluice reduces the COD capacity by about 2.2%, two 9.9%, and three 12.3%. Overall, the continuous sluices have certain impacts on the river water environmental capacity. The more sluices, the greater the impacts on the water environmental capacity.

Abstract:Colored dissolved organic matter (CDOM) widely exists in water, accounting for 10%-90% of dissolved organic carbon (DOC). In order to analyze the distributions of DOC concentrations in the reservoirs in the northeastern China, five field campaigns were carried out in Baishan Reservoir and Fengman Reservoir in the Songhua River Basin from 2015 to 2020. Based on the analysis of the absorption characteristics of CDOM coefficient (355 nm), the remote sensing inversion model of CDOM absorption coefficient at 355nm (R²=0.82) is established by band ratio method based on Landsat series satellites and the DOC concentrations of the reservoirs are estimated according to the strong correlations between CDOM and DOC. The results show that: The CDOM concentrations of inland reservoirs in the northeastern China can be well retrieved by using the in-situ sampling data and Landsat datasets; The annual spatial-temporal distributions of CDOM and DOC concentrations in the Baishan Reservoir and the Fengman Reservoir from 2000 to 2020 are with low fluctuations, and the concentration of CDOM and DOC appeared an upward trend after 2010. The Baishan Reservoir and the Fengman Reservoir have lower M values (the ratio of absorption coefficient of CDOM at 250 nm and 365 nm) and S_275-295 (the slope of the absorption spectrum of CDOM at the 275-295 nm), and higher SUVA₂₅₄ (the ratio of CDOM absorption coefficient at 254 nm to DOC concentration), indicating that the humic contents in the CDOM compositions are higher, and tend to be consisted of terrigenous macromolecules. Using multiple regression analysis method to analyze the annual driving factors of CDOM distributions of the reservoirs is mainly affected by rainfalls, fertilizer applications and sewage discharges. Finally, this study enriches the understanding of the variations of CDOM concentrations in typical reservoirs in the northeastern China and provides a reference for the construction of DOC inversion models and contributes to the accuracy estimation of DOC concentrations.

Abstract:With the completion and operation of several world-class high dam reservoirs in the Jinshajiang River watershed, the high sensitivity and vulnerability of the reservoir water level fluctuating zone (WLFZ) ecosystem have become increasingly prominent. How to restore the vegetation in the WLFZ is of great significance to protect the stability and diversity of ecological functions and maintain a critical cycle of the ecosystem. With ecological niche theory and interspecific relationship analysis methods, including ecological niche determination, variance ratio method, χ² test, association coefficient A_C, Ochiai index O_I, Pearson correlation coefficients and Spearman rank correlation coefficients, intrinsic interspecific linkage, ecological adaptability and community succession status of dominant plants in the WLFZ of the Jinshajiang River Reservoir were revealed. The dominant plants were classified into three ecological species groups to provide reference for suitable plants and their scientific allocation for ecological restoration in the WLFZ. The results show that: 1) A total of 37 plant species belonging to 19 families and 36 genera were recorded in the reservoir WLFZ of the Jinshajiang River watershed, and the dominant species at a frequency of ≥20% being Cynodon dactylon, Polygonum lapathifolium, Symphyotrichum subulatum, Eleusine indica, Alternanthera sessilis, Abutilon theophrasti, Chenopodium ambrosioides and Xanthium sibiricum, in which C. dactylon had the greatest importance value and ecological niche width, dominating the absolute competition. 2) The overall similarity and overlapping in ecological niches of dominant plants in the WLFZ are small, and the similarity in resource use is low, indicating high degree of ecological niche differentiation between dominant plant species. 3) The overall connectivity of dominant plants shows non-significant negative associations, and the overall stability of the community is poor. The χ² test, Pearson correlation coefficients and Spearman rank correlation coefficients all showed low significance and weak interspecific linkages. Based on the results of the ecological niche and interspecific relationship studies, eight dominant plant species are recommended as pioneer species, and the three ecological species groups are allocated along the water level gradient according to the delineation, so that the interspecific plant relationships evolve towards harmony and benefit the natural restoration process of the vegetation in the WLFZ.

Abstract:Water temperature has been recognized as one of the important hydrological indicators. However, changes in water temperature are going to threaten the sustainability of aquatic ecosystems owing to the climate change and the increased anthropogenic activities. While some of the world's largest cascade reservoirs have been developed in the upper reaches of the Changjiang River, it remains unclear about the combined effects of these reservoirs and climate changes on the water temperature and aquatic ecosystems. In this study, long-term water temperature and meteorological data (1989-2019) from Yibin-Chongqing segment of the Changjiang River were used to investigate the impacts of meteorological factors and Jinshajiang River cascade reservoirs on downstream water temperature, as well as to reveal the responses of fish spawning time to the water temperature changes in the National Nature Reserve for the Rare and Endemic Fishes located in the upper reaches of the Changjiang River. The major results of this study are: The average annual water temperatures in the study areas increased significantly over the last 30 years, with an increase of 0.5-1.1℃; After the impoundment of Xiangjiaba-Xiluodu cascade reservoir, the annual water temperature processes were delayed about 36, 13 and 7 days at three stations along the reservoir, respectively. The “cooling effect” and “warming effect” in the lower reaches were significant in spring and autumn-winter due to the impoundment of the cascade reservoirs, with the maximum increased and decreased ten-day mean water temperature of 1.8-3.7℃ and 1.3-3.6℃, respectively. Therefore, the “warming effect” was greater than the “cooling effect”; The annual variations of water temperatures were mainly caused by the cascade reservoirs and the inter-annual variations were mainly caused by climate changes, which clearly showed an increasing trend; The time to reach the optimum spawning water temperature for important fish nearby the Xiangjiaba Hydrometric Station was delayed by 30-40 days after the reservoir impoundment, in the order of Ancherythroculter nigrocauda > Leptobotia elongate, Coreius guichenoti, Procypris rabaudi > Jinshaia sinensis > Acipenser Dabryanus > Myxocyprinus asiaticus. Notably, the overlaps of the ecological niches for fish populations were increased. The results of the study may provide guidance to mitigate the impacts of hydropower development on aquatic ecology.

Abstract:In order to investigate the community compositions and distribution patterns of bacterioplankton in Lake Dongting and Lake Poyang, and identify their affecting factors, we studied the compositions of aquatic bacteria community of Lake Dongting and Lake Poyang in 57 sampling sites through high-throughput sequencing technology in September 2017. The physical and chemical parameters of these samples were detected. The results showed a total of 20 phyla, 42 classes, 93 orders, 191 families, and 533 genera. At the phylum level, Actinobacteria, Cyanobacteria and Proteobacteria were the main bacteria in Lake Dongting and Lake Poyang. The results of principal component analysis showed that environmental heterogeneity in Lake Poyang was higher than that in Lake Dongting. Mental tests showed that the main factors affecting the compositions of cyanobacteria and other groups were ammonia-nitrogen, nitrate nitrogen, total nitrogen (TN), phosphate phosphorus (PO^3-₄-P), total phosphorus (TP), water temperature (WT), electric conductivity (EC) and salinity (Sal) in Lake Dongting and PO^3-₄-P, WT, EC and Sal in Lake Poyang. The results of the null model showed that the stochastic process was the main driver of aquatic bacteria community assembly. Overall, Lake Poyang was more affected by stochastic process and less deterministic process under higher environmental heterogeneity, whereas Lake Dongting was affected by both stochastic process and deterministic process. Known the spatial distributions and community assembly mechanisms of aquatic bacteria are of great guiding value to the monitoring and evaluation of Lake Dongting and Lake Poyang.

Abstract:Microorganisms are important organic components of lake ecosystems. Understanding the differences in bacterial community structures, environmental driving factors and their community assembly processes in different periods in the lakes on the Qinghai-Tibet Plateau (QTP) has significant guiding role for water ecological management of the plateau lakes. Water samples were collected from five lakes on the northeastern of the QTP in June and December in 2020 for investigating the differences of community structures, environmental driving factors as well as the contributions of neutral processes, deterministic processes, and stochastic processes of bacterial communities in the plateau lakes between non-freezing and freezing periods using the high throughput sequencing, statistical analysis, and the model analysis. Results show that: (1) The Chao 1 richness index, Simpson, and Shannon diversity indices of bacterial communities were lower in non-freezing periods than those in freezing periods. Spearman correlation analysis revealed that water temperature and altitude were the most important factors in influencing Chao 1 richness index and diversity indices in non-freezing periods, while total nitrogen had a significant relationship with bacterial Chao 1 richness index and diversity indices in freezing periods. (2) The predominant phylum was Proteobacteria both in non-freezing and freezing periods. High average relative abundances of Bacteroidetes, Actinobacteria and Firmicutes were observed in non-freezing periods. Additionally, the results of principal coordinate analysis (PCoA) and similarity analysis showed that there were significant differences in the bacterial community compositions in non-freezing and freezing periods. (3) Results of redundancy analysis indicated that driving factors of bacterial community compositions were also different between non-freezing and freezing periods, which were in the order of total nitrogen>latitude>total dissolved solid>longitude>dissolved organic carbon in non-freezing periods and altitude>dissolved oxygen>dissolved organic carbon>pH in freezing periods, respectively. (4) Variations of bacterial communities were controlled by neutral process which was stronger in freezing periods than that in non-freezing periods and a higher dispersal rate was found in freezing periods than that in non-freezing periods (freezing periods: 0.289; non-freezing periods: 0.130) which reflected that the dispersal ability of bacterial species was higher in freezing periods than that in non-freezing periods. Dispersal limitation of stochastic processes shown to be a dominating process in the assembly of the bacterial communities both in non-freezing (contribution=54.74%) and freezing (contribution=55.56%) periods. Our study has analyzed the characteristics and driving mechanisms of bacterial communities in the plateau lakes in two different periods and will provide guidance for protection and management of the water resources in the plateau and basis for understanding bacterial community assembly processes.

Abstract:Lake ecosystem services (LES) is of great importance in guaranteeing regional and global ecological security, which provides a research hotspot in ecological economics at home and abroad. However, there are only few models and methods suitable for LES evaluation quantitatively, under the circumstance of large lake areas, a variety of types, large regional differences, lack of basic data, and so on. Based on final ecosystem services and key characteristic indicators, this paper took Lake Bosten as an example, built a model named lake ecoservice production function (LEPF) to assess lake ecosystem services valuation (LESV) and identify the contributions of key characteristic indicators quantitatively. The results showed that: (1) From 1990 to 2019, the average final services of Lake Bosten reached ￥2226.84 billion yuan, with an overall trend of "rising-falling-rising", and the dominant ecosystem services included climate regulation, flood control and water storage; (2) The LEPF fitting results nicely reflected the contribution degree of the selected key characteristic indicators on the LES, from high to low, as follows: lake water level>evaporation>area>aquatic vegetation area>integrated nutritional status index; (3) The output elasticity of the water level in the five types of the key characteristic indicators was relatively high, indicating that the fluctuation of the water level was closely related to the changes in the ultimate service value of the ecosystem.

Abstract:Lake Erhai, a significant freshwater lake located in the subtropical plateau of southwest China (Yunnan), has experienced water quality degradation and eutrophication under the influence of intensive human activities and recent climate change, especially in the northern zone of the lake. In this study, rotifer communities and environmental factors were seasonally investigated in 2020 at five sites in the northern zone of the lake. The seasonal succession of the rotifer communities and the associated environmental factors were explored by multivariate analyses. A total of 48 rotifer species from 26 genera were identified in this investigation, most of which were cosmopolitan or warm-water species. Ascomorpha, Asplanchna, Collotheca, Euchlanis, Keratella and Polyarthra appeared in all the investigated seasons. Trichocerca was the genus contributing the most species, including nine species. K. cochlearis, P. dolichoptera and A. priodonta were the dominant species. In the investigated seasons, K. cochlearis was the first dominant species in abundance, and A. priodonta was the first dominant species in biomass. Nonmetric multidimensional scaling analysis (NMDS) showed a significant seasonal difference in the community structure of rotifers in the northern zone of the lake. The significant seasonal difference of rotifer communities mainly reflected the direct and indirect effects of temperature, among which the indirect effects mainly included the seasonality of fish predation pressure and food resource. Redundancy analysis (RDA) showed that temperature, transparency and phytoplankton abundance were the important variables explaining the community variation and the seasonal succession of rotifers. Both seasonality and species composition of rotifer communities indicate the sensitivity of abundant species to the change of water quality and water temperature.

Abstract:Groundwater dynamics and variabilities of floodplains play an important role in affecting river and lake water resources, water pollution and eco-environment functions, due to the change of its hydrological regimes. The floodplain wetlands of Lake Poyang are located strategically in the middle and lower reaches of the Changjiang River, while its dynamic behaviors of water resources and water exchanges have many uncertainties. This study focuses on the typical floodplain area of Lake Poyang. We used a two-dimensional groundwater flow model to simulate floodplain surface water-groundwater interactions and the associated exchange fluxes. The results show that floodplain groundwater recharges to lake during the dry and recession periods, while lake recharges to groundwater during the rising and flooding periods. Groundwater levels across the whole floodplain exhibit a similar variation trend to lake water levels. Variation amplitudes of groundwater levels adjacent to the main lake are distinctly higher than those of the floodplain. In general, groundwater flow velocities in the northern area are obviously higher than those in the southern area, and the main lake regions are higher than the lake's floodplain, with velocity values less than about 1-2 m/d. Water budget analysis indicates that floodplain groundwater system mainly receives rainfall (52%) and lake inputs (39%), and losses water by groundwater evaporation (72%) and discharging to lake (24%). Additionally, recharge process of the groundwater system mainly occurs during the spring and summer seasons, and discharge process occurs during the autumn and winter seasons. In conclusion, surface topographical features play a dominant role in controlling groundwater level distributions and flow fields, but the dynamics of lake water levels are regarded as a key force that leads to a seasonal variation of the lake-groundwater exchange. The outcomes of this study can provide scientific references for future water resources management, allocation, and sustainable development of the eco-environment ofthe Lake Poyang floodplain wetlands.

Abstract:The distribution characteristics of the biological communities in the flood plain areas of Lake Poyang are greatly influenced by the hydrological connectivity between a region and the main lake. Due to the water level fluctuations, the hydrological connectivity of the eastern bay of Lake Poyang is sensitive to the water level of the main lake. Taking the eastern bay of Lake Poyang as an example, this study analyzed the water level fluctuations in the main lake as well as its influences. The hydrological connectivity between the eastern bay and the main lake was quantitatively characterized from the aspects of connectivity days and occurrence time. Furthermore, the changes of hydrological connectivity and its potential impacts on the habitats of wetland plants and migratory birds were discussed. The results showed that when the water level was 13-16 m asl, there was a good hydrological connectivity between the eastern bay and the main lake, during which the suitable habitat area for the survival of wetland plants was the largest. The water level of the main lake has generally decreased since 2003, and the durations of low water levels have been prolonged. According to the results of IHA/RVA analysis, the water levels in the main lake had changed to a moderate extent, with an overall change degree of 40.2%. The hydrological connectivity between the eastern bay and the main lake has changed significantly, with annual connection days reduced by 46.2 days, and the variation is 15.9%. The occurrence time of non-connection period is about one month earlier, and the yearly time span is longer. The decreased hydrological connectivity caused the average water level of the eastern bay to drop by 0.6 m from March to October, with a decrease of about 1 m from September to October, while the water surface area corresponding to the suitable water depth for Carex sp. and Vallisneria natans increased in most months, especially from September to October (more than 40%), but the area of submerged macrophytes decreased and the biomass of hygrophytes increased. The weakening of hydrological connectivity led to the early emergence of the beaches, the transfer of hygrophytes such as Carex cinerascens to the center of the saucer-shaped lake, the decrease of the upper limit of submerged macrophytes such as Vallisneria natans by about 1 m, and the migration of migratory birds to the center of the lake or other shoals. In order to effectively protect the Lake Poyang wetland ecosystem, it is recommended that more attention should be payed to the regulating and control roles of the Three Gorges Dam on the Changjiang River in mitigating the impacts of changes in hydrological connectivity on wetland plants growth and migratory bird habitats in the flood areas of the lake.

Abstract:Surface water temperature is a key factor influencing the lake aquatic ecosystem. Studying its responses and mechanisms to climate change is an important issue for assessing the sustainability of the lake ecological environment. This paper focuses on the issue of long-term trend of water temperature. Based on the observed hydrological and meteorological data, the evolution characteristics of the lake surface water temperature driven by meteorological conditions were examined by means of the Air2water data-driven model to reconstruct the long-term, time-series water temperature in Lake Dongting, which provides a theoretical basis for the monitoring of lake ecological environment, water security and comprehensive management. The main conclusions are: (1) Although the Air2water data-driven model describes the main heat exchange processes of a lake in a simplified form of an ordinary differential equation, it can well reproduce the actual trend of water temperature. The long-term, time-series observed air temperature can be used to reconstruct the daily averaged water temperature time series in Lake Dongting from 1973 to 2020 with high reliability. (2) From 1973 to 2020, the water temperature within a year in Lake Dongting had apparent warming and cooling periods, where the cooling rates were faster than the warming rates. Driven by the global climate warming, the annual mean water temperature showed consistent increasing trends, and the increasing rates were much more significant after the mutation in 1996, in which the increasing rates of annual mean water temperature at the Chenglingji Station and the Nanju Station were 0.20 and 0.16℃/10 a, respectively. The abrupt warming of Lake Dongting in 1996 was mainly driven by the significant warming process during the cold season. (3) The coupling relationships between water temperature and air temperature have been established using the general unit hydrograph theory, where the rising rates of water temperature with air temperature increased to a maximum value and then gradually slowed down. After the mutation of water temperature in 1996, the rates of water temperature variation with air temperature slightly decreased, indicating that the sensitivity of water temperature to air temperature decreased and the coupling relationships between water temperature and air temperature were weakened.

Abstract:Previous studies on Sanmenxia Reservoir mostly have been focused on the interannual degradation and aggradation changes, while there were few studies on the erosion and deposition distributions of the reservoir area during flood/non flood seasons. Identifying the erosion/deposition centers (sub-reach with greatest adjustment rates during flood/non flood seasons), the migration of erosion/deposition centers in the reservoir area and their rates and influences to the changes of the riverbed elevations at Tongguan during flood/non flood seasons were analyzed based on the data of water discharges, sediment loads, and erosion/deposition volumes after the controlled releases of “storing the clear water during the non-flood seasons and discharging the turbid water during flood seasons” in the period of 1974-2018. Results showed that the reservoir was generally degraded during the flood seasons and aggraded during the non flood seasons, and the erosion/deposition centers generally migrated correspondingly. The erosion/deposition centers had migrated downstream with an average rate of about 1-2 km/a at 90 km away from the dam before 2010 and then moved upstream at 60-70 km in front of the dam in 2018 with significantly weakened erosion/deposition rates. There is a strong positive correlation between the river kilometer (RKM) of the erosion center from the dam and the 2-year exponential weighted average value of flow energy during the flood seasons. So did the RKM of the deposition center and the backwater length with 5-year exponential weighted average value. The riverbed elevations of Tongguan are negatively correlated with the bed slope of Tongguan-Tai'an Reach which means the greater the gradient, the lower the riverbed elevation of Tongguan. The channel slope of Tongguan-Tai'an Reach increased easily and the riverbed elevation of Tongguan decreased when the erosion center was located at the downstream of the Tongguan-Tai'an Reach. When Tongguan was out of the backwater area and the erosion/deposition centers were also located below the Tongguan-Tai'an Reach after 1985. During this time, the riverbed elevations of Tongguan had been less affected by the erosion/deposition centers.

Abstract:Riverine carbon dioxide (CO₂) emission is an essential component of the global carbon budget. Here, environmental variables were measured in the Yue River of the upper Han River draining to the Yangtze River in the dry season (December 2016) and the rainy season (June 2017) to explore the dynamics and driving factors of the river pCO₂ in the low order streams. Results reveal that around 87% of samples have pCO₂ levels that are supersaturated. Riverine pCO₂ ranges from 135 μatm to 5960 μatm and shows a significant seasonal difference. The pCO₂ level constantly increases with the stream order increases. Correlation analysis shows that pCO₂ is positively related to construction and farmland proportion and TN, TP, DOC concentrations in the low order streams, while is negatively related to the forest land proportion, DO concentration and slope. The low order stream pCO₂ dynamics are driven by anthropogenic activities and environmental factors, as the influence of the former decreases with the stream order increases. We further develop multiple regression models to fit the different order streams pCO₂ using environmental factors and land use. Our results will provide insight into the accurate estimation of region and global carbon budget in the headwater rivers.

Abstract:Based on 505 scenes of Landsat images, the distributions and variations of the glacial lakes within Chinese territory were investigated by combining automated glacial lake boundary extraction and artificial visual interpretation in 2000 and 2020. Compared with the glacial lake inventory data in 1990, the characteristics and affecting factors of the glacial lake changes in China during the period of 1990-2020 were analyzed. The results showed that the glacial lake area in China increased by (180.1±0.1) km² from 1990 to 2020, with a change of 17.9%. Thereinto, the glacier-fed lake area expanded significantly, with the area expansion of 22.9%, while the non-glacier-fed lake area changed only 4.9%. The glacial lake area showed a rapid growth trend at high altitude during 1990-2020. During this period, the glacial lake area expanded the most at above 5500 m, reaching 30.5%. At the subregional scale, precipitation and evaporation mainly affect the changes of non-glacier-fed lakes, where evaporation played a more significant role. Temperature rise and glacier retreat are the main reasons for rapid expansion of glacier-fed lakes.

Abstract:As one of the major dust source areas of the world, the arid central Asia has recently attracted much attention in environmental changes during the Holocene. However, continuous sedimentary records of the history of aeolian activities are lacking in this region. Here we report a sedimentary record of regional aeolian activities using a sediment core from Lake Tolbo (TB19-A, 332 cm-long) in the NW Mongolia. The core was dated using AMS¹⁴C and the reservoir effect was assessed for the chronology. Lognormal distribution function fitting and grain size-standard deviation methods were employed to extract the aeolian components in the sediments from the grain size data of the sedimentary sequence. The history of aeolian sand activities or dust storms in the NW Mongolia was then reconstructed. The results show that fractions with the median grain size of 15-84 μm and >84 μm in the sediments from Lake Tolbo can be used as indicators of regional dust storms and local aeolian sand activities, respectively. At the late Last Deglacial (13.75-11.6 cal ka BP), there were strong local aeolian sand activities in the basin, which may be related to the YD cold events. Since Holocene, the changes of the local aeolian sand activities in the basin were consistent with the regional dust storms in the NW Mongolia, which can be shown as follows: during the early and middle Holocene, aeolian sand activities were generally weak, and relatively strong regional dust storms occurred during the periods of 9.7-8.6 cal ka BP and 7-6 cal ka BP. After 5 cal ka BP, local aeolian sand activities and regional dust storms intensified significantly, and both were at high levels until 0.7 cal ka BP. Extremely strong regional dust storms occurred in the past 0.3 cal ka BP. The dust storm history in the NW Mongolia is similar to those documented in Lake Gonghai in northern China and Lake Bosten in the NW China, reflecting the influence of the atmospheric circulation associated with the Siberian High on large-scale aeolian activities in arid regions. The increase of winter insolation may have also strengthened the aeolian activities during the late Holocene. The strong regional dust storms in the past 300 years may have been compounded by human activity in addition to the natural factors.