Abstract:Assessments of the ecological condition of Chinese waterbodies have increased substantially in recent years; however, the field is still in an early developmental stage, facing challenges such as an incomplete theoretical framework, imprecise methods, and limited comparability across studies. To advance scientific progress in China by leveraging mature international experience, this review synthesizes the role of ecological theory in guiding aquatic ecological assessments and provides recommendations for its application. The theoretical foundation of these assessments lies in biota-environment relationships, with niche and community-assembly (metacommunity) theories being particularly influential in shaping assessment paradigms, optimizing indices, and improving assessment performance (i.e., the effectiveness in indicating anthropogenic disturbance). Methodologically, assessment approaches have evolved from early physico-chemistry-based evaluations to a contemporary paradigm centered on biological assessment, supported by physico-chemical measures. Diagnostic frameworks that integrate quantitative condition assessment with stressor identification now represent the prevailing approach. In developing assessment indices, there is increasing emphasis on biological functional traits as sensitive metrics, leading to the widespread adoption of multimetric indices that integrate disturbance-tolerant taxa, biodiversity, and functional traits. The performance of these methods is typically evaluated across several dimensions: precision, bias, responsiveness, sensitivity, and consistency. To enhance performance, standardized survey and analytical procedures are essential, alongside the use of anthropogenic disturbance gradients to define reference conditions and predictive models to account for natural variability. Despite these advances, the ecological foundations for setting reference conditions and ecological class criteria, as well as for stressor diagnosis, require further development. International experience underscores that robust aquatic ecological assessment must be grounded in ecological theories. For China, future efforts should prioritize accounting for natural variability, empirically selecting metrics from survey data, and implementing diagnostic frameworks that explicitly link ecological condition to causal stressors. Advancing these elements will consolidate the scientific foundation of aquatic assessment, promoting a transition toward greater precision, standardization, and automation, thereby providing stronger support for aquatic ecological management and international environmental commitments.

Abstract:As emerging environmental contaminants, ensuring the security of water supplies requires a critical understanding of the occurrence, transmission mechanisms and risk control of antibiotic resistance genes (ARGs) in basin water environments. This paper provides a systematic review of the characteristics of the occurrence, pollution sources and spatiotemporal distribution patterns of ARGs in China’s seven major river basins: the Yangtze, Yellow, Huaihe, Pearl, Haihe, Liaohe and Songhua rivers. The paper also analyses the migration and diffusion mechanisms of ARGs in combination with the effects of combined pollution from eutrophication, heavy metals, and emerging contaminants (e.g., antibiotics, microplastics, endocrine-disrupting chemicals, and persistent organic pollutants). The study reveals that sulfonamides, tetracyclines, and aminoglycosides dominate ARGs in China’s basin water environments, primarily originating from non-point source pollution (e.g. agricultural planting and livestock and poultry farming) and point source emissions (e.g., wastewater treatment plant effluents and medical wastewater). In terms of occurrence levels, the absolute abundances of ARGs in sediments (10⁶-10¹⁰ copies/g) are generally three orders of magnitude higher than in water bodies (10³-10⁷ copies/mL), although their relative abundances (copies/16S rRNA) are similar. Spatiotemporally, ARGs are driven by microbial communities, environmental physicochemical factors, human activities and mobile genetic elements (MGEs), with microbial communities exerting the most significant influence. In terms of the effects of pollution, water eutrophication promotes the proliferation of bacterial communities that host ARGs, such as nitrate-reducing bacteria. This results in a significant positive correlation between ARG abundances and total nitrogen and total phosphorus loads. Heavy metals (e.g., copper, zinc and nickel) enhance the efficiency of ARG conjugative transfer through co-selection effects. Emerging contaminants such as antibiotics exert selective pressure on ARGs, while microplastic biofilms can increase ARG transformation frequencies by up to 1000-fold compared to natural substrates. Endocrine-disrupting chemicals (e.g., bisphenol A) and persistent organic pollutants (e.g. perfluorooctanoic acid) promote ARG horizontal transfer by inducing oxidative stress or upregulating plasmid expression. ARG transmission mechanisms primarily include shaping of the bacterial community (e.g., selective enrichment ofFirmicutes andProteobacteria), conjugative transfer (dependent on MGEs and ATP energy metabolism), induced transformation (extracellular DNA adsorption onto suspended particulates) and phage-mediated transfer (preferential packaging of ARG fragments). Notably, fluvial sediment dynamics processes such as suspended sediment transport and resuspension significantly influence ARG transmission fluxes by regulating pollutant partitioning across phases. Future research should investigate the coupled processes of “sediment-water dynamics-contaminants-ARGs”, analyse the cross-scale regulatory mechanisms of ARGs in multi-phase water environments and explore the potential application of sediment-water ecological regulation in ARG risk management.

Abstract:Nitrogen fixation is a vital process in biogeochemical cycles within ecosystems. While current research on nitrogen fixation in aquatic ecosystems has mainly focused on marine environments, studies on waters, such as lakes, have only recently begun. To understand current research hotspots and development trends regarding nitrogen-fixing microorganisms in freshwater lakes, this study examined literature on the topic from the Web of Science database. CiteSpace and VOSviewer were used to construct and analyze knowledge maps, revealing current research hotspots and future research trends in this field. Based on these findings, we conducted a literature integration analysis to outline the effects of nitrogen and phosphorus nutrients on nitrogen fixation rates in water and their underlying mechanisms. The results show that: (1) From 1992 to 2024, the number of publications and citations in the field of nitrogen-fixing microorganisms in global freshwater lakes has steadily increased. (2) National, author and institutional collaboration network analyses indicate that research on nitrogen-fixing microorganisms in freshwater lakes is an interdisciplinary field involving cooperation among multiple countries and institutions. (3)Cluster analysis reveals that research hotspots mainly focus on: nutrient control strategies under phosphorus limitation and their impact on cyanobacterial community succession; the analysis of nitrogen-fixing microbial diversity based on thenifH gene; and the functional characterization of this gene in the nitrogen cycle. The analysis also reveals long-term dynamic changes in phytoplankton community composition driven by environmental factors. (4) The results of the integration analysis indicate that: the geographical distribution of nitrogen fixation quantification studies is imbalanced. North America has established a comprehensive indicator system covering various water types. In contrast, Asia and South America focus on describing cyanobacterial biomass, while Europe primarily focuses on the dynamic changes of nitrogen and phosphorus, as well as the coupling relationship of the nitrogen fixation process. Research on nitrogen-fixing species mainly centres on the phylum Cyanobacteria (e.g.,Nostoc andDolichospermum), while other phyla such as Proteobacteria and Archaea are relatively scarce. Total phosphorus shows a significant positive correlation with nitrogen fixation rates, whereas total dissolved nitrogen, nitrate nitrogen and ammonium nitrogen show significant negative correlations. Non-linear segmented model fitting revealed a critical value of 25μg/L for total phosphorus in regulating nitrogen fixation rates in freshwater lakes. Future research on nitrogen fixation processes urgently requires standardised measurement methods (e.g., acetylene reduction and isotope tracing) and metric units (e.g., area/volume) to improve the comparability of research results. This study summarises changes in the research hotspots and frontiers of nitrogen-fixing microorganisms in freshwater lakes over the past 30 years, suggesting that our understanding of nitrogen fixation processes and their ecological contributions should continue to expand by incorporating diverse analytical indicators (e.g., enzyme activity and transcriptomics), standardized analytical procedures and multi-indicator fusion evaluation methods.

Abstract:The remote sensing inversion of riverine algal blooms is frequently compromised by boundary effects from riparian wetlands, and the accuracy of traditional methods remains limited in narrow, elongated water bodies. As a typical sensitive water area, the terminal reach of the Ganjiang River still lacks a clear understanding of its algal bloom outbreak mechanisms. Utilizing Sentinel-2 and Landsat series satellite data, this study developed an improved method that integrates inward-masking technology with the floating algae index (FAI) and Otsu’s threshold to effectively suppress nearshore interference and accurately extract bloom extents in elongated rivers. Applying this method, we reconstructed the algal bloom outbreak processes in the Ganjiang River from 2019 to 2024. The results indicated that blooms exhibited pronounced seasonality, occurringprimarily in late summer and early autumn (August-September). Spatially, they were significantly aggregated in nearshore slow-flowing zones along the windward banks of the southern and middle branches, with intensity gradually decreasing from the littoral zone toward the thalweg. Driver analysis using a Random Forest regression model revealed that daily maximum temperature was extremely significantly correlated with bloom area, contributing 47.1%—far exceeding the contributions of nutrients (29.9%), mean daily wind speed (9.3%), mean daily discharge (7.3%), and daily rainfall (6.4%). Furthermore, by analyzing bloom dynamics and environmental conditions during typical heatwave events (≥35 ℃ for three consecutive days), this study demonstrates that under meso-eutrophic conditions, low discharge and water retention during summer and autumn form the basis for bloom accumulation, while extreme heatwaves act as dominant drivers. Their coupling with low discharge and nutrient availability significantly amplifies the scale of bloom outbreaks. This study provides valuable insights into the mechanisms governing riverine algal bloom outbreaks.

Abstract:Over the years, integrated remediation efforts in Lake Dianchi have yielded significant interim success in curbing cyanobacterial blooms. From 2018 onward, both the frequency and spatial extent of these blooms showed a consistent downward trend. However, a marked resurgence was observed in 2023, prompting renewed scientific and public concern. This study systematically investigates the variability and outbreak mechanisms of cyanobacterial blooms in Lake Dianchi from 2018 to 2023, leveraging MODIS satellite imagery alongside integrated water quality and meteorological monitoring data. Analysis of the monitoring data reveals that the annual bloom frequency followed a “V”-shaped trajectory over the six-year period, with 2023 registering a notably high frequency of 87.0%, significantly exceeding the six-year average of 69.3%. In contrast, the average bloom area from 2022 to 2023 was substantially lower than that from 2018 to 2021. Specifically, the average bloom area in 2023 (15.86 km²) was 65.3% below the six-year mean, although it represented an 11.4% increase from 2022. Spearman correlation analysis demonstrated that both monthly bloom frequency and monthly average bloom area were significantly positively correlated with monthly average temperature and monthly precipitation, while showing a significant negative correlation with monthly average wind speed. Additionally, the monthly average cyanobacterial density exhibited a strong positive correlation with the monthly average total phosphorus concentration. Multivariate linear regression analysis highlighted air temperature and wind speed as the dominant meteorological drivers of bloom dynamics in Lake Dianchi. However, total phosphorus concentration had limited explanatory power regarding variations in algal density. Throughout the 2018-2023 period, cyanobacterial density consistently surpassed the mild bloom threshold (1.0×107 cells/L), suggesting that the 2023 resurgence was primarily driven by synergistic meteorological influences. During the non-bloom season (January-May and December), an increased proportion of temperatures between 13-20 ℃ accelerated cyanobacterial resurgence. During the bloom season (June-November), the increase in the proportion of low wind speeds (＜2 m/s) promoted cyanobacterial surfacing and aggregation. A reduced proportion of temperatures below 13 ℃ favored cyanobacterial growth. A significant rise in the proportion of 20-25 ℃ temperatures during the bloom season likely enhanced cyanobacterial buoyancy, contributing to the broader bloom coverage observed in 2023. These findings offer valuable theoretical support for the daily prevention, prediction, and early warning systems for cyanobacterial blooms in Lake Dianchi. Furthermore, they provide a reference for bloom management in other plateau lakes across Yunnan Province.

Abstract:To investigate the spatiotemporal distribution patterns of phytoplankton communities and their driving factors within the Shichuan River Basin, four systematic aquatic ecological surveys were conducted across the main stream of the Shichuan River, three tributaries, and three reservoirs during February (winter) and May (spring) of 2022, and August (summer) and November (autumn) of 2023. A total of 211 phytoplankton species were identified, spanning 78 genera and 7 phyla. Bacillariophyta constituted the highest proportion at 44.08%, followed by Chlorophyta at 31.75%. Dominant species across all four seasons includedCyclotella meneghiniana,Fragilaria acus,Navicula simples,Nitzschia palea, andChlorella ellipsoidea(Y＞0.1). Phytoplankton cell density ranged from 49.90×10⁴ to 631.67×10⁴ cells/L, with an average of 196.57×10⁴ cells/L. Biomass varied between 1.38 and 18.02 mg/L, averaging 4.84 mg/L. The Shannon-Wiener diversity index and Margalef richness index peaked in spring. Significant differences in diversity indices were observed among different water bodies. Average phytoplankton diversity indices in the main and tributary streams exceeded those in reservoirs across all seasons. Redundancy analysis revealed that nitrogen and phosphorus nutrients, along with dissolved oxygen, were the primary environmental factors influencing dominant species distribution in the main and tributary streams. In contrast, water depth was the dominant factor in reservoirs. Structural equation modelingindicated that land use types within the 1000 m riparian buffer zone—specifically cultivated land, forest land, and construction land—indirectly influence chlorophyll-a concentrations by modulating nitrogen and phosphorus levels in the water. Compared to reservoirs, chemical factors in river systems exhibited a more pronounced effect on chlorophyll-a concentrations.

Abstract:Algae are crucial primary producers in lake ecosystems, and their biomass and community structure can reflect water pollution and ecological status. As an important component of algae, the species composition characteristics of diatoms are also sensitive indicators for evaluating lake environmental quality. However, there is still a lack of systematic analysis regarding whether there are significant differences in the variation patterns and response mechanisms of algal biomass and diatom communities in lakes with different pollution levels. This study conducted seasonal investigations and comparative analyses in Lake Datun and Lake Yangzong, which have relatively high arsenic (As) pollution levels, and Lake Yilong, which is not affected by arsenic pollution, in Yunnan Province. The aim was to identify the main characteristics, driving factors, and key processes of algal changes 〖JP2〗under different arsenic pollution levels. Among the 55 surface water samples collected in this study, the concentration of water chlorophyll-a (Chl.a), which indicates algal biomass, was the highest in the eutrophic Lake Datun. Diatom compositions differed among the lakes and showed obvious seasonal fluctuations. Correlation analysis revealed a significant positive correlation between arsenic and Chl.aconcentrations in the lakes. This outcome indicates that phytoplankton are sensitive to arsenic stress in these lakes, and that the low arsenic levels exert a notable stimulating effect on algal growth. Meanwhile, arsenic was also significantly correlated with the main direction of diatom community changes. The results of variance decomposition analysis on algal data and environmental factors from the three lakes showed that the driving effect of the main environmental gradients on the succession of diatom community structure (PC1 index) was more significant than the fluctuation of Chl.a concentration (with average explained variances of 64.1% and 39.8%, respectively). The study results also revealed that the seasonal variation of water arsenic pollution is closely related to water temperature and lake depth type: increased water temperature promotes the chemical activity of arsenic and increases water arsenic concentration in shallow lakes, while in deep lakes, it limits the diffusion of sediment arsenic and the vertical migration of water arsenic through thermal stratification. In conclusion, the interactions between arsenic and factors such as water temperature, nutrients, and water depth under different pollution levels are significant, exerting important influences on the seasonal changes of algae and leading to large differences in the response intensities of different algal indicators. Therefore, for effective ecological assessment and restoration of lake arsenic pollution, it is necessary to comprehensively consider the combined effects of temperature and nutrient levels and conduct comparative analyses using multiple indicators such as algal biomass and diatom communities.

Abstract:Lake Chenghai, a crucial water body on the Yunnan-Guizhou Plateau, plays a pivotal role in enhancing the ecological environment of its watershed. This is not only essential for Yunnan to accomplish its three major ecological security tasks but also serves as the foundation for sustainable socio-economic development. In recent years, the water environment quality of Lake Chenghai has encountered severe challenges due to both anthropogenic and natural factors. The present study investigates the spatiotemporal evolution characteristics of the trophic state and its key driving factors in Lake Chenghai, a deep, closed lake. To this end, monitoring data from three regions of the lake spanning 2010 to 2023 were analyzed. To examine the spatiotemporal evolution of the lake’s trophic state, elucidate the direct driving effects of various factors on the trophic state, and explore their indirect influence pathways, the Mann-Kendall test, Spearman’s rank correlation analysis, multiple linear regression models, and piecewise structural equation modeling were employed. The objective of this study is to provide both theoretical insights and empirical evidence to support the protection and management of Lake Chenghai’s water environment. The results obtained are as follows: (1) From 2010 to 2023, the trophic state of Lake Chenghai displayed a phased evolutionary pattern, predominantly remaining at the mesotrophic level. It is evident that ecological water replenishment has exerted a beneficial impact on both the trophic state of the lake and its associated trends. (2) Specifically, the comprehensive trophic level index (TLI(Σ)) in the southern and central regions exhibited a significant upward trend before water replenishment (2013-2018), followed by a significant downward trend after replenishment (2019-2023). No significant differences in TLI(Σ) were detected across various regions before and after replenishment, and the trophic state remained mesotrophic. (3)The trophic state of Lake Chenghai was found to be significantly influenced by seasonality, with the TLI(Σ) being higher during the colder spring and winter months compared to the warmer summer and autumn months. This seasonal variation can be attributed to two key mechanisms: concentration effects and thermal stratification within the lake. These observations highlight that physical and chemical factors play a predominant role in driving fluctuations in TLI(Σ). In general, nitrogen and phosphorus inputs—primarily of external origin—exert an indirect influence on TLI(Σ) by modulating other environmental variables. Notably, a series of ecological and environmental protection measures implemented in recent years have effectively reduced the contribution of nitrogen and phosphorus inputs to TLI(Σ). However, despite substantial ecological water replenishment efforts, the lake’s trophic state remains mesotrophic. This finding underscores the critical need to consider both terrestrial pollutant inputs and internal lake pollution when assessing and managing its environmental quality. Consequently, while sustaining ecological water replenishment remains a pivotal priority for future conservation strategies, equal emphasis must be placed on mitigating terrestrial pollutant loads and addressing internal pollutant sources within the lake itself.

Abstract:Under the dual pressures of global climate change and anthropogenic activities, significant shifts have occurred in the succession patterns of lake biological communities and ecosystem structures. Diatoms have emerged as key indicators for assessing lacustrine environmental changes. This study examined Lake Chenghai, a natural closed deep-water lake, investigating seasonal variations in water environmental parameters, diatom community structure, and biodiversity through bimonthly stratified sampling at four open-water stations over one year. The research evaluated the driving effects of key factors including alkalinity, eutrophication status, and water temperature. Hydrochemical analyses indicated that Lake Chenghai was currently a phosphorus-limited system, with water pH consistently exceeding 9.0 annually and reaching maximum values in winter. Chlorophyll-a (Chl.a) and dissolved oxygen (DO) exhibited seasonal dynamics synchronous with pH variations. A total of 122 diatom species from 21 genera were identified. The diatom community demonstrated a seasonal adaptive strategy involving functional group shifts between planktonic and benthic forms, alongside high spatial distribution homogeneity. Seasonal variations in community structure were significant: planktonic species dominated from January to May, benthic/epiphytic species prevailed from July to September, and both groups co-dominated in November. Spatially, dominant species and their relative abundances remained consistent across sampling sites. Redundancy analysisindicated that water temperature (about 31%) was the primary environmental driver of seasonal variation in dominant diatom species, with additional contributions from conductivity (13.7%) and nutrients (6.1%) facilitating seasonal succession. Seasonal variation in thermal stratification intensity may further influence algal growth by regulating vertical mixing. Diatom species richness decreased with increasing water depth and showed significant seasonal variation, with the highest diversity occurring in summer and autumn. Spatially, diatom diversity exhibited no significant differences among sites due to homogeneous aquatic environmental conditions. The spatiotemporal patterns of diatom diversity were significantly influenced by the interactive effects of multiple environmental factors including temperature, water depth, and nutrients. This study provides scientific insights for ecosystem assessment and biodiversity conservation in closed lakes, offering data support for the protection of low-latitude plateau deep-water lakes under future climate change scenarios.

Abstract:Microorganisms respond rapidly to environmental changes, rendering them promising candidates as bioindicators for ecological status assessment. However, their application in evaluating aquatic ecosystems remains relatively limited. This study employed high-throughput sequencing to investigate bacterial community structures in both water and sediments across three distinct lake zones within Lake Poyang: the main lake area, dish-shaped lakes, and aquaculture lakes. A machine learning framework was utilized to develop a microbial biotic index (MBI) for assessing the ecological status of these habitats. The results revealed significant disparities in nutrient levels between water and sediments across lake types, which in turn drove spatial variations in bacterial community composition and dominant genera. Although community coalescence between water and sediment bacterial assemblages was limited, elevated trophic state index levels were associated with enhanced cross-habitat connectivity, indicated by a significantly greater proportion of sediment-derived bacteria within the water column. Using quantile regression and machine learning, amplicon sequence variants from both water and sediment were classified into five ecological groups. The resulting MBI demonstrated strong concordance with trophic state indices for both water and sediment compartments. Ecological assessments based on the MBI indicated superior water quality in the main lake and aquaculture lakes compared to dish-shaped lakes. In contrast, sediment ecological conditions were highest in aquaculture lakes, intermediate in dish-shaped lakes, and poorest in the main lake area. Overall, the ecological status of water in Lake Poyang were better than the sediments, suggesting that sediments may act as internal nutrient sources contributing to eutrophication in water body. Therefore, greater emphasis should be placed on the ecological condition of sediments in future lake management strategies. This study proposes a novel microbe-based assessment index, constructed via machine learning, that provides a robust tool for evaluating aquatic ecological status across varying trophic states.

Abstract:This study employedin situ, self-developed two-dimensional miniaturized diffusive gradients in thin films (2D-MDGT) and planar optode (PO) techniques to simultaneously measure labile phosphorus (Labile P), soluble reactive phosphorus (SRP), ammonia nitrogen(NH₃-N), nitrite (NO^-₂-N), and nitrate (NO-3-N) in sediments of Lake Ulansuhai, along with microenvironmental parameters (pH and dissolved oxygen) across the sediment-water interface (SWI) during both freezing and unfreezing periods. The results revealed significant spatiotemporal heterogeneity in the concentrations of these solutes. During the freezing period, mean concentrations were (0.006±0.005) mg/L for Labile P, (0.096±0.070) mg/L for SRP, (2.70±0.50) mg/L for NH3-N, (0.150±0.061) mg/L for NO^-₂-N and (0.48±0.48) mg/L for NO^-₃-N, all significantly lower than those during the unfreezing period ((0.060±0.036）mg/L, (0.15±0.16) mg/L, (24.0±1.7) mg/L, (0.36±0.042) mg/L, and (1.4±1.1) mg/L, respectively). Vertically, Labile P, SRP, and NH3-N concentrations increased with sediment depth, while NO^-₃-N decreased; NO^-₂-N showed no consistent vertical trend. Horizontally, higher nitrogen and phosphorus concentrations were observed in the northern region, due to greater sediment accumulation and intensified reductive conditions. The in situ exchange fluxes across the SWI ranged from -0.32 to 26 μg/(m²·d) for Labile P, -3.9 to 36 μg/(m²·d) for SRP, -18 to 6.9 mg/(m²·d) for NH3-N, -1.5 to 8.0 mg/(m²·d) for NO^-₂-N, and -0.22 to 0.16 mg/(m²·d) for NO^-₃-N. Notably, the unfreezing period exhibited significantly higher release fluxes of nitrogen and phosphorus, substantially increasing the risk of internal nutrient loading and eutrophication in Lake Ulansuhai.

Abstract:A comprehensive study was conducted in the Jiulong-Fenggang alluvial plain on the southern lakeshore of the Lake Poyang polder system to investigate the contamination characteristics and sources of heavy metals in soils. A total of 3799 surface soil samples were collected and the level of pollution was assessed using three methods: the geo-accumulation index, the Nemerow index, and the potential ecological risk index. Principal component analysis (PCA) and positive matrix factorization (PMF) were employed to analyses the sources of the heavy metals in the soil. The results revealed that the average concentrations of arsenic (As), cadmium (Cd), copper (Cu), chromium (Cr), mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn) in the surface soil were 1.06-2.10 times higher than the background soil values in Jiangxi Province. Northern Xinjiang River alluvial deposits showed significant Cd-Pb-Zn co-accumulation and southern bedrock weathering zones exhibited As-Cr-Ni geogenic anomalies. 83.05% of sampling sites were classified as “unpolluted to slightly polluted” (I_geo＜1), and 83.84% exhibited pollution levels below the warning threshold (Pn＜1). Critical ecological risk hotspots were concentrated in the Xinjiang River floodplain and its deltaic inflow zone (RI ＞ 60), covering 10.85% of the total study area. Cd was identified as the sole priority control pollutant in these high-risk sectors. PMF source apportionment identified four primary contributors: secondary enrichment during red soil formation (49.28%), industrial and mining activities upstream (20.19%), weathering of coal-bearing parent rocks (11.91%), and agricultural practices combined with coal combustion (18.62%). This study confirms that Cd contamination primarily originates from upstream mining activities (79.92% via fluvial transport), necessitating targeted source control measures within the Xinjiang River watershed. The integrated methodology provides a replicable framework for ecological risk management in floodplain agricultural systems.

Abstract:In August 2021, during the wet season, 45 sampling sites were established across the Lake Poyang area. Concentrations of Manganese, Copper, Zinc, Cadmium, Lead, Chromium, Arsenic and Mercury in the lake water were analyzed using inductively coupled plasma mass spectrometry and atomic fluorescence spectrometry. A human health risk assessment for heavy metals was conducted based on parameters specific to the Chinese population for drinking water and dermal exposure. The spatial distribution characteristics of the heavy metals and their associated health risks were analyzed using the Kriging interpolation method. (1) Key results indicate that the concentration of heavy metals in the study area ranged from 1.89 μg/L to 183.25 μg/L, with Mn exhibiting the highest average concentration and Cr the lowest. Elevated concentrations of heavy metals were observed in the river-lake ecotone in southern Lake Poyang, where Mn levels exceeded safety standards at the confluence of Lake Poyang with the Ganjiang, Xinjiang and Fuhe Rivers. (2) As and Cr poseed significant risks to human health in Lake Poyang. In 2021, carcinogenic risk dominated heavy metal risks in lake water, with As presenting a pronounced carcinogenic threat. Some sampling sites showed low non-carcinogenic risks for Cr. (3) High non-carcinogenic risk areas were primarily found in the southern part of the lake, while high carcinogenic risk areas were concentrated in the central part of the lake. Mn, As and Cr are the heavy metals that need to be strictly controlled in Lake Poyang. Water pollution caused by heavy metals in the southern and central parts of the lake should be prioritized for prevention and control.

Abstract:Matrix-bound phosphine (MBP) represents a substantial yet frequently underestimated pool of biologically available phosphorus in lacustrine sediments. This study presents the first comprehensive investigation into the composition, spatial distribution, and potential environmental implications of MBP within two contrasting plateau lakes: the deep oligotrophic Lake Fuxian and the shallow eutrophic Lake Xingyun. A pre-column cold trap enrichment coupled with gas chromatography was employed for the quantification of MBP. Results indicated that free gaseous phosphine (FGP) concentrations in Lake Fuxian and Lake Xingyun were (5.39±1.43) μg/L and (8.81±2.81) μg/L, respectively, accounting for 31.87% to 73.75% of the soluble reactive phosphorus (SRP) in the overlying water column. These findings underscore the significance of phosphine as a constituent of the phosphorus cycle in both lake systems. The total MBP (TMBP), porewater-soluble MBP (PMBP), and labile-bound MBP (LMBP) concentrations in sediments were determined as follows: Lake Fuxian ((3.59±0.89) ng/kg, (0.98±0.24) ng/kg, and (0.68±0.24) ng/kg, respectively) and Lake Xingyun ((122.84±26.90)ng/kg, (0.83±0.20) ng/kg, and (0.60±0.18) ng/kg, respectively). Notably, TMBP content in Lake Xingyun were significantly higher than those in Lake Fuxian. Both lakes exhibited pronounced spatial heterogeneity in MBP distribution, with elevated content observed in the northern regions compared to the southern and central zones, which function as primary inflow areas. In Lake Fuxian, persistent anoxic conditions in the northern region appear to facilitate MBP generation. Conversely, the northern region of Lake Xingyun receives substantial inputs of agricultural and domestic wastewater, supplying essential precursor substrates for MBP synthesis. These findings provide critical insights into the biogeochemical cycling of phosphorus in plateau lacustrine ecosystems.

Abstract:The biodegradable dissolved organic carbon (%BDOC) has garnered significant attention in recent years due to its critical role in biogeochemical processes and the environmental fate of pollutants. In particular, %BDOC levels influence the toxicity and bioavailability of heavy metals and organic contaminants in aquatic ecosystems. Lake Taihu, located in the economically developed Yangtze River Delta region of China, has experienced severe pollution from industrial and domestic wastewater discharges over recent decades, threatening the safety of drinking water supplies for local communities. Understanding the spatial and temporal dynamics of %BDOC in Lake Taihu is essential for elucidating carbon cycling mechanisms and safeguarding regional water resources. To this end, long-term bio-incubation experiments were conducted from August 2018 to May 2021 to investigate variations in %BDOC and identify key influencing factors. The results revealed distinct seasonal patterns, with higher %BDOC levels observed in February and May compared to August and November. Spatially, %BDOC exhibited a decreasing trend from the northwest to the southeast of the lake, with peak concentrations recorded in Zhushan Bay. Fluorescence spectroscopy analysis indicated that tryptophan-like C2 and red-shifted tyrosine-like C3 components underwent significant degradation after 28 days of bioincubation. These protein-like fluorescent compounds were found to be highly bioavailable, exerting a direct influence on overall %BDOC levels in the lake. To comprehensively assess the relationships between %BDOC and DOM optical indices as well as water quality-related parameters were analyzed using random forest modeling and partial least squares (PLS) regression. The findings demonstrated that %BDOC was significantly and negatively correlated with SUVA254 (an indicator of aromaticity), while showing positive correlations with dissolved organic carbon, chemical oxygen demand, total nitrogen, and total phosphorus. Notably, dissolved oxygen emerged as a key driver of %BDOC variability in Lake Taihu. These insights underscore the importance of intensified research and monitoring of %BDOC in lake systems. Such efforts are vital for understanding organic matter dynamics, evaluating carbon emission potential, and ultimately protecting water supply security.

Abstract:Seasonal temperature changes significantly affect microbial functional transformations and greenhouse gas emissions in ecosystems. Current studies have shown that temperature directly regulates microorganisms in plant-free systems. The lakeshore zone is an important site for the response to climate change and is characterized by periodic water-level fluctuations that subject sediments to different flooding conditions. This has a profound effect on the sediment microenvironment and microbial activity. However, the impact of temperature on functional microorganisms in lakeshore sediments under different flooding conditions has not been well studied. This study investigated the short-term effects of temperature on denitrifying microbes in lakeshore sediments by simulating four flooding conditions: non-flooded (NF), intermittently flooded (IF), flooded with alternating high and low water levels (HLF) and flooded with a constant water level (WF). The results showed that increasing the temperature significantly increased the rate at which N2 was released from the sediment and decreased the rate at which N2O was released under each flooding condition. Regarding microbial metabolism, temperature had a variable effect on carbon metabolic pathways: under NF and IF conditions, 15 ℃ promoted glycolysis and the pentose phosphate cycle, while 30 ℃ inhibited them. However, under HLF and WF conditions, 30 ℃ significantly activated various carbon metabolic activities. Meanwhile, the content of electron donors decreased at 30 ℃ under NF and IF conditions, but increased significantly under HLF and WF conditions. Notably, temperature altered the electron-consuming capacity by modulating the activity of key denitrification enzymes, resulting in an increase in the potential release rate of N2 and a decrease in the potential release rate of N2O. Regarding microbial communities, temperature significantly altered the diversity and compositional characteristics ofnirS-andnosZ-type denitrifying bacteria in lakeshore sediments under different flooding conditions. Further analysis using partial least squares path modelling revealed that thenirS-andnosZ-type denitrifying bacterial community was a key indicator for predicting the potential release rates of N2 and N2O at different temperatures, rather than the metabolic processes of microorganisms. Additionally, estimating the potential emissions of N2 and N2O from sediments in the lakeshore zone revealed that sediments in areas with frequent water level fluctuations have a higher nitrogen removal capacity in high-temperature environments (30 ℃). This is a critical spatial and temporal factor in the conversion of greenhouse gases to the environmentally friendly end-product N2.

Abstract:Denitrification is a crucial process for removing nitrogen from aquatic ecosystems. To evaluate the impact of algae on the denitrification of reservoir water, we selected Xiangxi Bay, a typical tributary of the Three Gorges Reservoir (TGR). Water samples were collected in situ, and experiments were conducted to evaluate the denitrification potential of algae at various concentrations, species and growth conditions. The acetylene inhibition method was employed to measure the denitrification rate. The results showed that, within a certain concentration range, the denitrification rate in the water column increased as the algal concentration rose. However, when the algal concentration (Chl.a) reached 472.1 μg/L, the denitrification rate declined due to limited nitrate. The smaller cell size and number ofMicrocystis aeruginosa cells added under the same Chl.a concentration conditions may account for the slightly lower denitrification rate compared to that ofChlorella pyrenoidosa cells. Decaying algae at suitable concentrations can provide the essential carbon and nitrogen substrates, as well as the favorable dissolved oxygen environment, required for denitrification. Denitrification rates in water containing decaying algae were significantly higher than in the control group. Specifically, the addition of decayingMicrocystis aeruginosa ((42.50±1.26) nmol/(L·h)) promoted denitrification significantly more than the addition of decayingChlorella pyrenoidosa ((29.02±0.10) nmol/(L·h)). This enhancement is related to the characteristics of the cell composition of the algae, asMicrocystis aeruginosa can provide more organic matter and a lower dissolved oxygen environment for denitrification thanChlorella pyrenoidosa can. When managing algal blooms and controlling nutrient levels, it is advisable to create suitable environmental conditions to maximize the positive effects of nitrogen removal. For example, it is needed to determine the optimal concentration and community structure of growing algae for denitrification in TGR tributaries. While controlling algal blooms through methods such as fluctuating water levels, it is also crucial to consider enhancing algal-mediated denitrification. This approach will provide a theoretical basis for accurately assessing the effects of denitrification in reservoirs and optimizing reservoir operations to promote denitrification.

Abstract:Denitrification is an important process for reducing the amount of nitrogen in water. Accurate and rapid measurement of the denitrification rate in water bodies is crucial for assessing the nitrogen removal potential of lakes, reservoirs and rivers. This study proposes a novel method for rapidly measuring denitrification rates in situ in water bodies based on nitrogen gas increment to address the issues of existing methods, such as the acetylene inhibition method and the 15N isotope tracer method, which are unable to provide rapid in situ measurements and are costly and time-consuming. This method involves in situ water sampling, purging, incubation and measurement, enabling the rapid detection of total denitrification rates in the field. In May-June 2024, this method was used to conduct point-based continuous monitoring and bay-wide surveys of denitrification rates in the Pengxi River, a tributary of the Three Gorges Reservoir. A comparison with the acetylene inhibition method was performed and the influence of various environmental factors on denitrification rates was analyzed. The results showed that the denitrification rates measured by this method were approximately seven times higher than those obtained using the acetylene inhibition method, and there was a significant correlation between the two methods. During the monitoring period, denitrification rates in the Pengxi River were highest in the surface layer, followed by the bottom and middle layers. Denitrification rates in the surface layer were mainly influenced by algal concentrations, while rates in the bottom layer were primarily affected by turbidity. This study has significant implications for rapidly measuring denitrification rates in water bodies and for accurately understanding and evaluating their nitrogen removal potential.

Abstract:The sources of the Yangtze and Lancang rivers are important water sources and natural barriers for the aquatic ecosystems in the lower reaches of the river basins. In recent years, the ecosystems and habitats of the two source areas have faced significant challenges in terms of protection, driven by the combined effects of climate change and human activities. Chromophoric dissolved organic matter (CDOM) is an important component of aquatic ecosystems and plays a key role in revealing changes and responses within them. In this study, the spectral characteristics of CDOM in the main streams and tributaries of the Yangtze and Lancang rivers during the ice-melting and water-abundance periods were explored using a combination of ultraviolet-visible (UV-Vis) and 3D fluorescence spectroscopy. This study revealed the compositional characteristics, sources and differences of CDOM in the two source areas, aiming to provide a scientific basis for understanding the carbon cycle and the migration and transformation processes of organic matter in the hinterland of the Tibetan Plateau, as well as evaluating the degree of human influence in the source areas. The results showed that: 1) the absorption coefficients (a254) and dissolved organic carbon (DOC) in the Yangtze River and Lancang River sources were higher during the ice ablation period than during the abundant water period. This indicates that meltwater from glaciers and permafrost significantly increased the input of organic matter to these two source areas. Spectral parameters E2/E3 and SUVA254 indicated that the humification degree, aromaticity and relative molecular mass of CDOM were higher overall in the two source regions, all of which showed characteristics of a strong land source of organic matter. Secondly, a total of eight fluorescence fractions were extracted from the two source areas. The CDOM fluorescent components in the Yangtze River source area were composed entirely of terrestrial humic substances, including UVA and UVC. In contrast, the Lancang River source contained tryptophan-like substances, in addition to terrestrial humic substances, indicating a certain autochthonous characteristic highly correlated with human activities. The mean fluorescence index (FI) and autochthonous index (BIX) values for the Yangtze River source were 1.36 and 0.26, respectively. These values were lower than those for the Lancang River source, where the mean FI and BIX values were 1.45 and 0.52). This indicates that the terrestrial humus characteristics of the CDOM in the Yangtze River source are higher than in the Lancang River source.The CDOM in both source areas is mainly derived from organic matter produced by soil erosion or buried in glacial permafrost through atmospheric deposition. However, the CDOM in the Lancang River source is also affected by the input of organic matter from agriculture and livestock in the basin.

Abstract:In-lake dissolved carbon turnover represents a critical yet underexplored component of the global carbon cycle. While previous studies have predominantly focused on the hydro-chemical characteristics of dissolved inorganic carbon (DIC), the internal linkages and mutual transformations between DIC and dissolved organic carbon (DOC) remain poorly understood. This study investigates the seasonal dynamics and interconversion of dissolved carbon components in Lake Lugu, a subtropical alpine lake in southwestern China. We analyzed seasonal variations in DIC species (including DIC, HCO^-₃, CO^2-₃, and CO₂) and DOC optical properties (S_275-295, SUV_A254, SUV_A280, URI, BIX, HIX, and FI). Complementary methods, including fluorescence region integration (FRI), excitation-emission matrix parallel factor analysis (EEM-PARAFAC), measurements of partial pressure of CO₂ (pCO₂), and CO₂ flux (F) were employed to provide a comprehensive characterization of the dissolved carbon pool. The results revealed significant seasonal differences in DIC species, driven primarily by carbonate dissolution dynamics. A marked seasonal shift in the spectral slope (S_275-295) indicated the input of allochthonous, high-molecular-weight DOC during the rainy season. Elevated biological index (BIX) and reduced humification index (HIX) values suggested sustained microbial activity. Soluble microbial by-products constituted the dominant DOC fraction, averaging between 44.53% and 71.97%, with humic-like and tryptophan-like compounds prevailing throughout the lake. During the rainy season, Lake Lugu functioned as a distinct CO₂ source (pCO₂＞470 μatm, F＞0), which was attributed to enhanced microbial mineralization of DOC. Overall, this study elucidates how seasonal microbial metabolism regulates carbon transformation: respiratory processes dominate in the rainy season, converting DIC into CO₂, while photosynthetic uptake prevails during the dry season. These findings provide new insights into carbon fixation mechanisms and sequestration potential in freshwater lakes.