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In aquatic ecosystems with low nutrient levels, organic aggregates (OAs) act as nutrient hotspots, hosting a diverse range of microbial species compared to those in the water column. Lake eutrophication, marked by intensified and prolonged cyanobacterial blooms, significantly impacts material and energy cycling processes, potentially altering the ecological traits of both free-living (FL) and particle-attached (PA) bacteria. However, the extent to which observed patterns of FL and PA bacterial diversity, community assembly, and stability extend to hypereutrophic lakes remains understudied. To address this gap, we investigated bacterial diversity, composition, assembly processes, and stability within hypereutrophic Lake Xingyun. Our results revealed that FL bacterial communities exhibited higher α-diversity than PA counterparts, coupled with discernible taxonomic compositions. Both bacterial communities showed distinct seasonality, influenced by cyanobacterial bloom intensity. Environmental factors accounted for 71.1% and 54.2% of the variation among FL and PA bacteria, respectively. The assembly of the PA bacterial community was predominantly stochastic, while FL assembly was more deterministic. The FL network demonstrated greater stability, complexity, and negative interactions, indicative of competitive relationships, while the PA network showed a prevalence of positive correlations, suggesting mutualistic interactions. Importantly, these findings differ from observations in oligotrophic, mesotrophic, and eutrophic lakes. Overall, this research provides valuable insights into the interplay among bacterial fractions, enhancing our understanding of nutrient status and cyanobacterial blooms in shaping bacterial communities., (© 2024. The Author(s).)

Oligotrophic deep-water lakes are unique and sensitive ecosystems with limited nutrient availability. Understanding bacterial communities within these lakes is crucial for assessing ecosystem health, biogeochemical cycling, and responses to environmental changes. In this study, we investigated the seasonal and vertical dynamics of both free-living (FL) and particle-attached (PA) bacteria in Lake Fuxian, a typical oligotrophic deep freshwater lake in southeast China. Our findings revealed distinct seasonal and vertical dynamics of FL and PA bacterial communities, driven by similar physiochemical environmental factors. PA bacteria exhibited higher α- and β-diversity and were enriched with Proteobacteria, Cyanobacteria, Firmicutes, Patescibacteria, Planctomycetota, and Verrucomicrobiota, while FL bacteria were enriched with Actinobacteria and Bacteroidota. FL bacteria showed enrichment in putative functions related to chemoheterotrophy and aerobic anoxygenic photosynthesis, whereas the PA fraction was enriched with intracellular parasites (mainly contributed by Rickettsiales, Chlamydiales, and Legionellales) and nitrogen metabolism functions. Deterministic processes predominantly shaped the assembly of both FL and PA bacterial communities, with stochastic processes playing a greater role in the FL fraction. Network analysis revealed extensive species interactions, with a higher proportion of positively correlated edges in the PA network, indicating mutualistic or cooperative interactions. Cyanobium , Comamonadaceae, and Roseomonas were identified as keystone taxa in the PA network, underscoring potential cooperation between autotrophic and heterotrophic bacteria in organic particle microhabitats. Overall, the disparities in bacterial diversity, community composition, putative function, and network characteristics between FL and PA fractions highlight their adaptation to distinct ecological niches within these unique lake ecosystems.IMPORTANCEUnderstanding the diversity of microbial communities, their assembly mechanisms, and their responses to environmental changes is fundamental to the study of aquatic microbial ecology. Oligotrophic deep-water lakes are fragile ecosystems with limited nutrient resources, rendering them highly susceptible to environmental fluctuations. Examining different bacterial types within these lakes offers valuable insights into the intricate mechanisms governing community dynamics and adaptation strategies across various scales. In our investigation of oligotrophic deep freshwater Lake Fuxian in China, we explored the seasonal and vertical dynamics of two bacterial types: free-living (FL) and particle-attached (PA). Our findings unveiled distinct patterns in the diversity, composition, and putative functions of these bacteria, all shaped by environmental factors. Understanding these subtleties provides insight into bacterial interactions, thereby influencing the overall ecosystem functioning. Ultimately, our research illuminates the adaptation and roles of FL and PA bacteria within these unique lake environments, contributing significantly to our broader comprehension of ecosystem stability and health., Competing Interests: The authors declare no conflict of interest.

Global warming is leading to extended stratification in deep lakes, which may exacerbate phosphorus (P) limitation in the upper waters. Conversion of labile dissolved organic P (DOP) is a possible adaptive strategy to maintain primary production. To test this, the spatiotemporal distributions of various soluble P fractions and phosphomonesterase (PME)/phosphodiesterase (PDE) activities were investigated in Lake Fuxian during the stratification period and the transition capacity of organic P and its impact on primary productivity were evaluated. The results indicated that the DOP concentration (mean 0.20 ± 0.05 μmol L - 1 ) was significantly higher than that of dissolved inorganic P (DIP) (mean 0.08 ± 0.03 μmol L - 1 ) in the epilimnion and metalimnion, which were predominantly composed of orthophosphate monoester (monoester-P) and orthophosphate diesters (diester-P). The low ratio of diester-P / monoester-P and high activities of PME and PDE indicate DOP mineralization in the epilimnion and metalimnion. We detected a DIP threshold of approximately 0.19 μmol L - 1 , corresponding to the highest total PME activity in the lake. Meta-analysis further demonstrated that DIP thresholds of PME activities were prevalent in oligotrophic (0.19 μmol L - 1 ) and mesotrophic (0.74 μmol L - 1 ) inland waters. In contrast to the phosphate-sensitive phosphatase PME, dissolved PDE was expressed independent of phosphate availability and its activity invariably correlated with chlorophyll a, suggesting the involvement of phytoplankton in DOP utilization. This study provides important field evidence for the DOP transformation processes and the strategy for maintaining primary productivity in P-deficient scenarios, which contributes to the understanding of P cycles and the mechanisms of system adaptation to future long-term P limitations in stratified waters., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Remote sensing monitoring of particulate organic carbon (POC) concentration is essential for understanding phytoplankton productivity, carbon storage, and water quality in global lakes. Some algorithms have been proposed, but only for regional eutrophic lakes. Based on in-situ data (N = 1269) in 49 lakes across China, we developed a blended POC algorithm by distinguishing Type-I and Type-II waters. Compared to Type-I, Type-II waters had higher reflectance peak around 560 nm (>0.0125 sr -1 ) and mean POC (4.65 ± 4.11 vs. 2.66 ± 3.37 mg/L). Furthermore, because POC was highly related to algal production (r = 0.85), a three-band index (R 2  = 0.65) and the phytoplankton fluorescence peak height (R 2  = 0.63) were adopted to estimate POC in Type-I and Type-II waters, respectively. The novel algorithm got a mean absolute percent difference (MAPD) of 35.93 % and outperformed three state-of-the-art formulas with MAPD values of 40.56-76.42 %. Then, the novel algorithm was applied to OLCI/Sentinel-3 imagery, and we first obtained a national map of POC in 450 Chinese lakes (> 20 km 2 ), which presented an apparent spatial pattern of "low in the west and high in the east". In brief, water classification should be considered when remotely monitoring lake POC concentration over a large area. Moreover, a process-oriented method is required when calculating water column POC storage from satellite-derived POC concentrations in type-II waters. Our results contribute substantially to advancing the dynamic observation of the lake carbon cycle using satellite data., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Biological nitrogen fixation plays a critical role in maintaining primary production, particularly in systematic nitrogen deficiency. However, little is known about the dynamics within diazotrophic community facing ongoing nutrient enrichment in freshwater lakes. Here, a consecutive five-year investigation on diazotrophic community was conducted in Lake Fuxian, an oligotrophic deep alpine lake on the trajectory to eutrophic state. Results showed a regime shift from heterotrophic to autotrophic diazotrophs induced by total phosphorus (TP) enrichment. Specifically, heterotrophic diazotrophs dominated the diazotrophic community when TP was lower than 21.8 μg/L, whereas heterotrophic diazotrophs or diazotrophic Cyanobacteria randomly dominated when TP ranged between 21.8 μg/L and 28.8 μg/L. When TP was higher than 28.8 μg/L, diazotrophic Cyanobacteria accounted for 60.4%-97.7% of the total N 2 -fixers, indicating diazotrophic biodiversity significantly declined under TP enrichment scenario. Moreover, the dominance of diazotrophic Cyanobacteria further facilitated phytoplankton growth, which strengthened positive feedback between phytoplankton and phosphorus under nitrogen deficiency conditions. This is the first report on the threshold-like state responses of freshwater diazotrophs to environmental drivers. Our study expands the knowledge of the diazotrophic dynamics in freshwater ecosystems and contributes quantitative evidence of ecological thresholds for future environmental policymaking., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Freshwater ecosystems are recognized as one of the important natural methane (CH 4 ) sources, but little is known about the emission hotspots and the effects of algal blooms on CH 4 production in deep lakes. In this study, carried out from the littoral (S1), pelagic (S2-S4), and the deepest site (S5), water samples from different depths and sediment cores were collected along the transect of Lake Fuxian, a deep monomictic lake to investigate the spatial-temporal variations of CH 4 . Dissolved methane concentration observed at the oxic metalimnion was 37.5% and 19.5% higher than that those observed at the epilimnion and at the layer between 80 and 100 m depth, respectively. During the overturn period, the vertical distribution of CH 4 in the water column was uniform, with an average concentration of 0.031 ± 0.007 μM in S2-S5. Statistical analysis indicated that the CH 4 concentration in the water column was significantly higher in S1 than other sites along the transect during both sampling periods. Sediment CH 4 concentration and methane production potential (MPP) were also significantly higher in S1 than in other sites. Along the sediment depth, the maximum MPP was observed at 6-8 cm in S1, but it moved up to the surface layer in S2-S5 in both sampling periods. In addition, stable carbon isotope analysis indicated that the surface sediments in the pelagic zone (S2-S5) mainly comprised autochthonous organic matters. In this zone, MPP had a significantly positive correlation with sediment total organic carbon (TOC) (R 2 = 0.401, p < 0.01). In summary, we described the spatial and temporal distributions of CH 4 in deep Lake Fuxian, littoral zones are CH 4 emission hotspots that can contribute to the CH 4 accumulation in the oxic metalimnion layer during the stratification period. In the pelagic zone, autochthonous organic matter was transported into the surface sediment after a massive algal bloom, representing another hotspot for CH 4 production.

The PAH and OCP concentrations in sediment cores collected from a deep lake were measured and evaluated chronologically. The results indicated that the PAH and OCP concentrations significantly increased from the 1950s to the 1990s and subsequently decreased to recent years. Integrated molecular diagnostic ratios indicated that the predominant petrogenic sources occurred from the 1950s-1980s. Petroleum and fuel combustion dominated the source of contamination more recently as a result of energy structure changes in this region. Additionally, HCHs and DDTs were the dominant OCP compounds, making up a majority of the total OCPs present (>85%). HCHs accounted for a larger ratio of the OCPs between the 1950s and 1980s, suggesting that HCHs were the dominant POPs in this period. DDTs then became dominant in the 1980s and later. High α/γ-HCH ratios suggested that the emission and conversion of local technical HCHs were the predominant HCHs source. The ratios of (DDE + p,p'-DDD)/DDTs and p,p'-DDT/DDTs indicated that the DDTs mainly originated from historical input. In addition, the dramatic decrease since the 1980s may be the result of China's banning of DDTs. However, DDTs were still present in the 1990s, suggesting DDTs were still used in this region and beyond., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

• Subfossil chironomid assemblage exhibited a distinct bathymetric distribution pattern. • The deepest zone showed the least representativeness of entire-lake assemblage. • Chironomid assemblage showed a depth-specific response to environmental change. • Mid-depth zones were recommended as the coring sites for paleolimnological studies in large, deep lakes. In paleolimnology, ecological trajectories of biological communities are typically reconstructed using a sediment core from the deepest part of a lake. However, the representativeness of chironomid assemblages from the profundal zone and its influence on ecological response to environmental change in large, deep lakes remain poorly understood. To address this knowledge gap, we conducted a study on subfossil chironomids in Fuxian Lake, including thirty-five surface sediment samples and two sediment cores from different water depths. Our findings reveal a distinct bathymetric distribution pattern of subfossil chironomid assemblages attributed to limited homogenization in the large, deep lake. The profundal assemblages exhibited the lowest richness representativeness, while nearshore assemblages showed the least similarity to the entire-lake assemblage. Consequently, the response pattern of chironomid assemblage to the environmental change varied with depth. Two notable shifts in the mid-depth core, occurring in 1979 CE and 2005 CE, were primarily attributed to eutrophication processes. However, the chironomid assemblage from the deepest zone exhibited only one significant change in 2005 CE, likely indicating exacerbated hypoxia in the profundal zone. Our combined analysis of surface samples and sediment cores underscores that the profundal zone, where bottom water is characterized by low temperature and anoxia, is unsuitable as a coring site in large, deep lakes due to its extremely simplified species composition. Instead, mid-depth zones should be prioritized as candidate coring sites in large, deep lakes, given their relatively high richness representativeness, head capsule concentration, and stable sedimentary conditions. Specifically, we recommended retrieving sedimentary cores around the water depth of 50 m when conducting subfossil chironomid analysis in Fuxian Lake. [ABSTRACT FROM AUTHOR]

The fragile ecosystems of plateau lakes are in face of ecological risks from emerging toxic elements. Beryllium (Be) and thallium (Tl) have been considered priority control metals in recent years owing to their persistence, toxicity, and bioaccumulation. However, the toxic factors of Be and Tl are scarce and ecological risks of them in the aquatic environment were seldom investigated. Hence, this study developed a framework for calculating the potential ecological risk index (PERI) of Be and Tl in aquatic systems and used it to assess the ecological risks of Be and Tl in Lake Fuxian, a plateau lake in China. The toxicity factors of Be and Tl were calculated to be 40 and 5, respectively. In sediments of Lake Fuxian, the concentrations of Be and Tl were between 2.18 and 4.04 mg/kg and 0.72−0.94 mg/kg, respectively. The spatial distribution indicated that Be was more abundant in the eastern and southern regions, and Tl had higher concentrations near the northern and southern banks, consistent with the distribution of anthropogenic activities. The background values were calculated as 3.38 mg/kg and 0.89 mg/kg for Be and Tl, respectively. In comparison with Be, Tl was more enriched in Lake Fuxian. The increasing Tl enrichment has been attributed to anthropogenic activities (e.g., coal burning and non-ferrous metal production), especially since the 1980s. Generally, Be and Tl contamination has decreased over the past several decades, from moderate to low, since the 1980s. The ecological risk of Tl was low, whereas Be might have caused low to moderate ecological risks. In the future, the obtained toxic factors of Be and Tl in this study can be adopted in assessing the ecological risks of them in sediments. Moreover, the framework can be employed for the ecological risk assessment of other newly emerging toxic elements in the aquatic environment. [Display omitted] • The toxic factors of Be and Tl were calculated as 40 and 5, respectively. • RGBs of toxic elements can be identified using the statistical iterative 2σ method. • Tl was more enriched than Be in sediments of Lake Fuxian. • Potential ecological risks of Be and Tl were at low levels in Lake Fuxian. [ABSTRACT FROM AUTHOR]

Plateau lakes in China are faced with heavy metal contamination. The present study investigated the concentrations, sources, and ecological risks of 11 heavy metals in sediments of Lake Fuxian over the last 100 years. The concentrations of V (124–244 mg/kg), Cr (69.3–127 mg/kg), Cu (59.4–105 mg/kg), Ni (36.0–66.3 mg/kg), and Co (15.4–25.5 mg/kg) significantly decreased in the last 100 years while Cd, Sb, Pb, and Zn had contrary trends. In the last 100 years, correlations between the heavy metals changed in reverse due to the combined influences of lithogenic and anthropogenic sources. In the last 40 years, the lithogenic sources mainly contributed to the concentrations of As, Hg, V, Cr, Co, Ni, and Cu, with the rates of 55 %− 85 %. Moreover, Cd, Sb, Pb, and Zn were mainly affected by anthropogenic sources (e.g., mining activities and human settlement change), with the contribution rates of 53 %− 80 %. The change of stable Pb isotope in the sediment cores indicated that galena mining was the main source of Pb. With increasing anthropogenic activities, there was a possibility of 90 % that a moderate ecological risk might appear in the last 40 years. Among the heavy metals, Cd should attract attention because it contributed 96 % to the combined ecological risk. [Display omitted] • Correlations between heavy metals changed in reverse among three different periods. • Anthropogenic sources obviously accelerated the accumulation of Cd, Pb, Sb, and Zn. • Smelting and mining of galena was the main source of Pb in the lake sediments. • A moderate ecological risk of metals might happen, with the possibility of 90 %. • Cd was the most sensitive heavy metal in evaluating the combined ecological risk. [ABSTRACT FROM AUTHOR]