Seasonal and spatial distributions of euphotic zone and long-term variations in water transparency in a clear oligotrophic Lake Fuxian, China

Analysis of dissolved organic matter characteristics in pharmaceutical wastewater via spectroscopy combined with Fourier-transform ion cyclotron resonance mass spectrometry

Studying the changes in organic matter and characteristic pollutants during the treatment of penicillin-containing pharmaceutical wastewater, which can be reflected by changes in dissolved organic matter (DOM), is crucial for improving the effectiveness of wastewater treatment units and systems. Herein, water quality indicators, spectroscopic methods, and Fourier-transform ion cyclotron resonance mass spectrometry were utilized to characterize the general molecular compositions and specific molecular changes in DOM during the treatment of typical penicillin-containing pharmaceutical wastewater, including in each of the influent, physicochemical treatment, biological treatment, oxidation treatment, and effluent stages. The influent exhibited a high organic matter content (concentration of dissolved organic carbon >10,000 mg·L-1), its DOM mainly contained protein- and lignin-like substances composed of CHON and CHONS molecules, and the relative intensity (RI) of penicillin was extremely high (RI = 0.220). Compared with the influent, the abundance of CHON and CHONS molecules detected after physicochemical treatment decreased by 70.3 % and 62.5 %, respectively, and the RI of penicillin decreased by 85.5 %. Biological treatment caused substantial changes in DOM components through oxidation, dealkylation, and denitrification reactions, accounting for 36.8 %, 28.9 %, and 14.8 % of the total identified reactions, respectively. Additionally, lignin-like substances were generated in large quantities, the overall humification level significantly increased, and the RI value increased for the penicillin intermediate, 6-aminopenicillanic acid (6-APA). Oxidation treatment effectively removed phosphorus-containing substances and some lignin-like substances produced by biological treatment; however, it was not effective in removing characteristic pollutants such as 6-APA. Such characteristic substances continued to be present in the effluent, and the DOM mainly contained protein- and humus-like substances, accounting for 30.8 % and 47.3 %, respectively. The study findings reveal the changes in organic matter and characteristic pollutants during the treatment of penicillin-containing wastewater from the perspective of the general molecular composition and specific molecular changes in DOM, providing support for further exploration of wastewater treatment mechanisms and improvements in treatment unit efficiency.

Seasonal dynamics of environmental heterogeneity augment microbial interactions by regulating community structure in different trophic lakes

Understanding how environmental heterogeneity drives microbial communities in lakes is essential for developing effective strategies to manage and restore aquatic ecosystems. However, the mechanisms by which environmental heterogeneity influences microbial community structure, network patterns, and interactions remain largely unexplored. To bridge this gap, we collected 84 water samples from four typical lakes in China (Fuxian, Tianmu, Taihu, and Xingyun) representing a range of trophic levels, across wet and dry seasons. We assessed environmental heterogeneity using 14 water quality parameters, analyzed community structure with Jaccard and Bray-Curtis dissimilarity indices, and developed a comprehensive index to elucidate microbial network complexity. Our study reveals three key findings: (1) Environmental heterogeneity was significantly greater in dry season compared to wet season across all lakes (P < 0.05). (2) Increased environmental heterogeneity led to higher bacterioplankton community dissimilarity, with greater β-diversity observed in dry season (P < 0.05). (3) Shifts in community structure due to increased environmental heterogeneity further enhanced microbial interactions, as evidenced by more complex and interconnected co-occurrence networks in the dry season. In summary, our study demonstrates that environmental heterogeneity significantly impacts bacterioplankton community structure and subsequently enhances microbial interactions. These findings underscore the importance of considering environmental heterogeneity in lake ecosystem management, as it plays a crucial role in regulating microbial community dynamics and interactions.

Total nitrogen and community turnover determine phosphorus use efficiency of phytoplankton along nutrient gradients in plateau lakes

Numerous studies support that biodiversity predict most to ecosystem functioning, but whether other factors display a more significant direct impact on ecosystem functioning than biodiversity remains to be studied. We investigated 398 samples of the phytoplankton phosphorus resource use efficiency (RUE_P = chlorophyll-a concentration/dissolved phosphate) across two seasons in nine plateau lakes in Yunnan Province, China. We identified the main contributors to phytoplankton RUE_P and analyzed their potential influences on RUE_P at different lake trophic states. The results showed that total nitrogen (TN) contributed the most to RUE_P among the nine lakes, whereas community turnover (measured as community dissimilarity) explained the most to RUE_P variation across the two seasons. Moreover, TN also influenced RUE_P by affecting biodiversity. Species richness (SR), functional attribute diversity (FAD2), and dendrogram-based functional diversity (FDc) were positively correlated with RUE_P in both seasons, while evenness was negatively correlated with RUE_P at the end of the rainy season. We also found that the effects of biodiversity and turnover on RUE_P depended on the lake trophic states. SR and FAD2 were positively correlated with RUE_P in all three trophic states. Evenness showed a negative correlation with RUE_P at the eutrophic and oligotrophic levels, but a positive correlation at the mesotrophic level. Turnover had a negative influence on RUE_P at the eutrophic level, but a positive influence at the mesotrophic and oligotrophic levels. Overall, our results suggested that multiple factors and nutrient states need to be considered when the ecosystem functioning predictors and the biodiversity-ecosystem functioning relationships are investigated.

Patterns of thermocline structure and the deep chlorophyll maximum feature in multiple stratified lakes related to environmental drivers

Thermal stratification and the deep chlorophyll maximum (DCM), two commonly related phenomena in stratified lakes, play fundamental roles in eco-environmental processes. However, the progressive linkages among multi-dimensional environmental factors, thermal stratification and DCM were poorly explored, which greatly constrains our understanding of cross-level governance in deep lakes. In this study, the thermocline structure (i.e., thermocline depth, thickness and strength) and DCM feature (depth and thickness) and their driving factors were investigated at regional scale using data from 18 stratified lakes differing in limnological characteristics, Southwest China. Our study showed that (1) DCM occurred close to the thermocline in most lakes (represented by their depth and thickness), (2) the depths of the thermocline and DCM were both shallower than the euphotic depth, and (3) spatial heterogeneity occurred the thermocline structure and the DCM feature, reflecting different environmental factors. Specifically, water depth and light penetration depths were both positively correlated with thermocline depth and thickness and the DCM feature, and ultraviolet radiation (UVR) was more important than photosynthetically active radiation (PAR) for thermocline depth, but PAR was more important for thermocline thickness; moreover, PAR played a more prominent role than UVR for the DCM feature. As there were interactions between some environmental factors, we built a cascading path using a partial least squares path modelling for the DCM feature: lake morphometry directly impacted the thermocline structure and surface water quality; the water quality further affected light penetration depths as well as the thermocline structure; light penetration depth and thermocline structure combined directly determined the DCM feature, where the importance of light was larger. Our findings provide information on the cascading drivers of the thermocline structure and DCM feature in deep lakes and also constitute a valuable reference for deep lake management under the dual pressure of climate change and eutrophication.

Influence of cyanobacterial bloom accumulation and dissipation on underwater light attenuation in a large and shallow lake

Cyanobacterial bloom accumulation and dissipation frequently occur in Lake Taihu, a typically shallow, eutrophic lake due to wind wave disturbance. However, knowledge of the driving mechanisms of cyanobacterial blooms on underwater light attenuation is still limited. In this study, we collected a high-frequency in situ monitoring of the wind field, underwater light environment, and surface water quality to elucidate how cyanobacterial bloom accumulation and dissipation affect the variations in underwater light attenuation in the littoral zone of Lake Taihu. Results showed that cyanobacterial blooms significantly increased the diffuse attenuation coefficient of ultraviolet-B (K_d(313)), ultraviolet-A (K_d(340)), and photosynthetically active radiation (K_d(PAR)); the scattering of total suspended matter (b_bp(λ)); and the absorption of phytoplankton (a_ph(λ)) and chromophoric dissolved organic matter (CDOM, a_g(λ)) (p < 0.01). The K_d(PAR) decreased quickly during the processes of bloom dissipation, but the decrease of K_d(313) and K_d(340) lagged 0.5 day. Our results suggested that cyanobacterial blooms could increase particle matters and elevated the production of autochthonous CDOM, resulting in underwater light attenuation increase. Ultraviolet radiation (UVR) and PAR attenuation both have significant responses to cyanobacterial blooms, but the response processes were distinct due to the different changes of particle and dissolved organic matters. Our study unravels the driving mechanisms of cyanobacterial blooms on underwater light attenuation, improving lake ecosystem management and protection.

Phytoplankton responses to solar UVR and its combination with nutrient enrichment in a plateau oligotrophic Lake Fuxian: a mesocosm experiment

Multiple stressors associated with global change are influencing the phytoplankton taxonomic composition and biomass in plateau lakes, such as higher levels of ultraviolet radiation (UVR, 280-400 nm) and the risk of eutrophication. Although the restrictive effects of high UVR on phytoplankton are generally recognized, the effects will be impacted by the UVR levels of seasonal changes and the nutrient status. In this study, in situ phytoplankton productivity was measured seasonally under full solar radiation and only visible light (photosynthetically active radiation, PAR) conditions in the plateau oligotrophic Lake Fuxian, Southwest China. To determine the single effects of UVR and nutrient enrichment and their combined impact on phytoplankton communities, a mesocosm experiment (1 month) was conducted outdoors during late summer (rainy season). The interactive experiment was designed with two radiation treatments (UVR + PAR and PAR) and four nutrient treatments (raw water, nitrogen addition, phosphorus addition, nitrogen and phosphorus addition). Our results suggested that the response of phytoplankton to UVR depended largely on nutrient availability, which was embodied in the no significant responses of phytoplankton productivity and total biomass to UVR in the oligotrophic raw water from Lake Fuxian, but the positive responses of total biomass to UVR in the nutrient enrichment treatment. Furthermore, the response of phytoplankton to UVR was also taxa dependent. Chlorophytes and dinoflagellates showed positive response to UVR, whereas chrysophytes were quite sensitive to UVR; diatoms had no significant response to UVR under all nutrient conditions, while the response of cyanobacteria to UVR was highly dependent on nutrient status. This study highlights that the increase in nutrient concentrations in oligotrophic lake not only directly promotes the growth of phytoplankton but also combines with low UVR during summer to benefit the growth of eutrophic taxa based on the non-negative effect of UVR on total phytoplankton biomass and the positive effect on eutrophic taxa.

Dynamics of euphotic zone depth in the Bohai Sea and Yellow Sea

Euphotic zone depth (Z_eu) plays an important role in studies of marine biogeochemical processes and ecosystems. Remote sensing techniques are ideal tools to investigate Z_eu distributions because of their advanced observation ability with broad spatial coverage and frequent observation intervals. This study aims to develop a new approach that derives Z_eu directly from remote sensing reflectance (R_rs(λ)) values rather than by using other intermediate variables and then reveals the dynamic characteristics of Z_eu in the Bohai Sea (BS) and Yellow Sea (YS). To do this, in situ data collected from various seasons were first used to assess the ability of several spectral indicators of R_rs(λ) for deriving Z_eu and the optimal spectral indicator was determined to build a Z_eu retrieval model. This model was further applied to Geostationary Ocean Color Imager (GOCI) data to study the spatial and temporal variations in Z_eu. The results showed that the new Z_eu retrieval model performed well with R², RMSE and MAPE values of 0.843, 4.42 m and 17.9%, respectively. High Z_eu levels were generally observed during summer for both coastal and offshore waters while the lowest Z_eu values were observed during winter. Changing concentrations of total suspended matter, which are often modulated by sediment resuspension and transportation, are probably the main factor responsible for the spatial and temporal variability of Z_eu. These findings provide crucial information for modeling primary production, carbon flux, and heat transfer, etc., in the BS and YS, as well as contribute a useful alternative approach that will be easily implemented to study Z_eu from satellite data for other water environments.

Occurrence of antibiotics and antibiotic resistance genes in the Fuxian Lake and antibiotic source analysis based on principal component analysis-multiple linear regression model

In recent years, the dramatic increase in antibiotic use has led to the evolution of antibiotic resistant genes (ARGs), posing a potential risk to human and aquatic ecological safety. In this study, source contribution and correlations between twelve antibiotics and their corresponding ARGs were firstly investigated in surface water in the Fuxian Lake. The results showed that sulfamethoxazole (SMX) (0.98-14.32 ng L^-1) and ofloxacin (OFL) (0.77-7.3 ng L^-1) were the dominant antibiotics in surface water, whereas erythromycin-H₂O (EM-H₂O), SMX and OFL posed the medium risk to aquatic organisms. Meanwhile, the mean concentrations of MLs in inflowing rivers were 5.6 times more than those in the lake, which was related to dilution and degradation. Moreover, the facter1 (co-sources L (Living quarters), M (Mining area), A (Agricultural district) and T (tourist area)) contributed 78% of antibiotic concentrations, and the source L was predominant. The results also revealed the prevalence of intL1, sul1 and sul2 in all the sampling sites, and that the abundance of ARGs in the lake was significantly lower (P < 0.01) than that in inflowing rives. Additionally, significant correlations (p < 0.0001) between intL1 and sulfanilamide resistance genes (sul1, sul2) were detected, indicating that intL1 promoted the propagation and they originated from the same anthropogenic sources. Overall, our findings revealed the presence of antibiotics and ARGs and their inconsistent correlations in the Fuxian Lake, which provides a foundation to support further exploration of the occurrence and transmission mechanisms of antibiotics and ARGs.

Radiation dimming and decreasing water clarity fuel underwater darkening in lakes

Long-term decreases in the incident total radiation and water clarity might substantially affect the underwater light environment in aquatic ecosystems. However, the underlying mechanism and relative contributions of radiation dimming and decreasing water clarity to the underwater light environment on a national or global scale remains largely unknown. Here, we present a comprehensive dataset of unprecedented scale in China's lakes to address the combined effects of radiation dimming and decreasing water clarity on underwater darkening. Long-term total radiation and sunshine duration showed 5.8% and 7.9% decreases, respectively, after 2000 compared to 1961-1970, resulting in net radiation dimming. An in situ Secchi disk depth (SDD) dataset in 170 lakes showed that the mean SDD significantly decreased from 1.80 ± 2.19 m before 1995 to 1.28 ± 1.82 m after 2005. SDD remote sensing estimations for 641 lakes with areas ≥ 10 km² showed that SDD markedly decreased from 1.26 ± 0.62 m during 1985-1990 to 1.14 ± 0.66 m during 2005-2010. Radiation dimming and decreasing water clarity jointly caused an approximately 10% decrease in the average available photosynthetically active radiation (PAR) in the euphotic layer. Our results revealed a more important role of decreasing water clarity in underwater darkening than radiation dimming. A meta-analysis of long-term SDD observation data from 61 various waters further elucidated a global extensive underwater darkening. Underwater darkening implies a decrease in water quality for potable water supplies, recession in macrophytes and benthic algae, and decreases in benthic primary production, fishery production, and biodiversity.

Spatial and Historical Occurrence, Sources, and Potential Toxicological Risk of Polycyclic Aromatic Hydrocarbons in Sediments of the Largest Chinese Deep Lake

Lake sediments are important reservoirs for polycyclic aromatic hydrocarbons (PAHs) in catchments. Knowledge of occurrence, sources, and toxicological risk of PAHs is crucial to abate their pollution and risk. We investigated the spatial and temporal occurrence, sources, and potential toxicological risks of 12 PAHs in the surface sediments and one sediment core of the largest deep lake (Lake Fuxian) of China. Our results indicated the average ΣPAH₁₂ in the surface sediments of this lake was 1550.6 ± 231.4 ng g _dw ^-1 , much higher than those of most Chinese shallow lakes. The average ΣPAH₁₂ in the lake area was higher than that in the estuaries. The average ΣPAH₁₂ in the estuaries of influent rivers was higher than that of the outlet river. Coal combustion, gasoline combustion, and diesel combustion were the major sources, which contributed 68.5%, 19.8%, and 11.8% to the ΣPAH₁₂. The average total benzo[a]pyrene toxic equivalent concentration (TEQ^carc) of the six most carcinogenic PAHs was 317.1 ± 86.3 ngTEQ^carc g^-1 in the surface sediments. The ΣPAH₁₂ increased from 301.7 to 1964.4 ng g _dw ^-1 from 1945 to 2011 and significantly increased with the GDP and population of the catchment. The contribution of coal combustion to the concentrations of PAHs increased gradually with time. The total TEQ^carc, and the percentage of ΣPAH_carc to ΣPAH₁₂ in the sediment core increased from 5.0 to 84.6 ngTEQ^carc g^-1 and from 5.7 to 23.3%, respectively. Our study highlights the importance of such deep waters in burying PAHs and the increasing risk of PAHs from human activities.