Shift of phytoplankton assemblages in a temperate lake located on the eastern route of the South-to-North Water Diversion Project

Identification of key feature variables and prediction of harmful algal blooms in a water diversion lake based on interpretable machine learning

Harmful algal blooms (HABs) as an increasing environmental problem in lakes, and water diversion has become a common and effective strategy for mitigating HABs. Early and accurate identification of the occurrence of HABs in lakes is essential for scientific guidance of water diversion. Furthermore, the inevitable changes of hydrodynamic and water environment in the receiving area during water diversion make it more challenging to identify the important environmental features of HABs. Therefore, we constructed a machine learning modelling framework suitable for predicting HABs with favorable performance in both non-water diversion and water diversion states. In this study, we collected data from three monitoring sites for the years 2008-2020 (non-water diversion period from 2008 to 2013 and water diversion period from 2014 to 2020) as external validations and six sampling sites for the years 2021-2022 (2021 non-water diversion period and 2022 water diversion period) as internal validation. The CatBoost (AUC = 0.948) model fared best performance was obtained by comparing 10 machine learning models for comprehensive HABs prediction analyses in the external cohorts of Yilong Lake, and the 24 features were reduced to obtain the 8 (Including TP, TN and CODCr, etc.) most important environmental features. In addition, the SHapley Additive explanation (SHAP) method was used to interpret this CatBoost model through a global interpretation that describes the whole features of the model and a local interpretation that details how a certain forecast of HABs is made for a single sample via inputting the individual data. The CatBoost interpretable model also performed well in internal validation and the model has been converted into a convenient application for use by the Bureau of Yilong Lake Administration personnel and researchers. Finally, the results of the PLS-PM explains that water diversion indirectly mitigates HABs mainly through diluting nutrient concentrations. Overall, the final model of this study has a good performance and application benefits in predicting HABs during the non-water diversion period and water diversion period of Yilong Lake, which provides a guideline for water diversion. Furthermore, this study also provides a reference for other similar eutrophic lake water diversion strategies.

Distribution and transport of phosphorus species along with large-scale water diversion project in the Taihu Lake basin of China: Transformation between sediment and water

To date, there remains a lack of comprehensive understanding regarding the impacts of water diversion project on large-scale receiving watersheds. Large-scale water diversions not only introduce external phosphorus but also alter hydrological conditions, leading to the redistribution of phosphorus within the watershed. In this study, the distribution of phosphorus species in sediments and waters was examined in detail through a case study of a water diversion project in the Taihu Lake Basin, China. The concentration of total phosphorus (TP) increased in both surface waters and sediments following the commencement of the project, particularly in surface waters, where it was 1.27 times higher. In terms of spatial distribution, the concentrations of TP in both waters and sediments exhibited a pattern of initial decrease followed by an increase along with the direction of water flow. The highest average TP levels were observed in area A (311.56 μg/L in water and 670.98 mg/kg in sediment, respectively), where water was diverted, due to the introduction of a significant amount of particulate phosphorus (PTP). As the deposition of PTP occurred, the TP concentrations in area B dropping to 222.64 μg/L in water and 523.42 mg/kg in sediment. However, there was an slight increase in colloidal phosphorus (CTP) and dissolved phosphorus (DTP) in area B. Turbidity and conductivity measurements suggested that water transfer may have facilitated the conversion of PTP or slightly disturbed the sediment, leading to the production of CTP and DTP. In area C, where rivers converged, the TP concentrations in water and sediment increased to 313.83 μg/L and 551 mg/kg, respectively. The increased turbidity indicated that the convergence of rivers had a significant impact on the release and redeposition of PTP between sediments and overlying water. This study provided new sights for the dynamic changes of phosphorus species with large-scale water diversion projects.

Effects of artificial mixing on phytoplankton in a warm stratified drinking water reservoir: Characterization, mechanism, and implication

To improve water quality, water-lifting aerators (WLAs) are usually installed in reservoirs for artificial mixing. In this study, using taxonomic groups methods of phytoplankton, the characteristics and mechanisms of their response to artificial mixing were investigated during a two-year monitoring period in Xikeng Reservoir (XKR) in southern China, and compared the differences in response in the context of higher and lower phytoplankton abundance. The results showed that artificial mixing caused a significant decrease in phytoplankton abundance in the surface water column, but a short-term increase followed by a decrease in phytoplankton in the middle and bottom, and ultimately a homogenization of vertical phytoplankton with complete mixing of the water column. The phytoplankton showed a shift from Cyanobacteria to Bacillariophyta in artificial mixing process, while morphological characteristics of phytoplankton shifted from the larger Volume (V) to the smaller V at the same time. Besides, artificial mixing may be more likely to result in structural variation in lower phytoplankton abundance than in higher abundance. The mixing depth (Zmix) and light availability (Zeu/Zmix; the euphotic depth (Zeu)) were the key drivers of phytoplankton succession caused by artificial mixing in XKR, rather than water temperature (WT) and nutrients. This study also provided a successful example of effective control of phytoplankton overgrowth in a reservoir under higher WT and nutrient conditions, which had important implications for ecological managers and researchers in reservoirs or lakes.

Spatiotemporal Variation Assessment and Improved Prediction Of Cyanobacteria Blooms in Lakes Using Improved Machine Learning Model Based on Multivariate Data

Cyanobacterial blooms in shallow lakes pose a significant threat to aquatic ecosystems and public health worldwide, highlighting the urgent need for advanced predictive methodologies. As impounded lakes along the Eastern Route of the South-to-North Water Diversion Project, Lakes Hongze and Luoma play a key role in water resource management, making the prediction of cyanobacterial blooms in these lakes particularly important. To address this, satellite remote sensing data were utilized to analyze the spatiotemporal dynamics of cyanobacterial blooms in these lakes. Subsequently, a precise machine learning model, integrating the Projection Pursuit Model and Random Forest (PP-RF) algorithms, was developed to predict the extent of cyanobacterial blooms, considering a range of influencing factors, including physical, chemical, climatic, and hydrologic variables. The findings indicated pronounced seasonal fluctuations in cyanobacterial blooms, with higher levels in summer than in other seasons. Key determinants for cyanobacterial blooms prediction included solar radiation, temperature and total nitrogen for Lake Hongze, while for Lake Luoma, significant predictors were identified as temperature, water temperature, and solar radiation. Compared with traditional data preprocessing methods, PP-RF model has advantages in addressing multicollinearity. This study provides a feasible method for predicting cyanobacterial blooms in impounded lakes within inter-basin water transfer projects. By inputting region-specific data, this model could be applied broadly, contributing to against the adverse effects of cyanobacterial blooms and provide scientific guidance for the protection and management of aquatic ecosystems.

Deciphering Planktonic Bacterial Community Assembly in the Storage Reservoir of the Long-Distance Water Diversion Project

Storage reservoirs are crucial components of long-distance water diversion projects, where water diversion may lead to changes in microbial diversity and community structure. Seasonal variations also drive alterations in microbial communities. However, the way that microbes assemble under the combined effects of water diversion and seasonal variations in the storage reservoir has not been extensively studied. Jihongtan Reservoir is the terminal storage reservoir of the Yellow River to Qingdao Water Diversion Project (YQWD), which had an average annual water diversion period exceeding 290 days in recent years. In this study, 16S rDNA amplicon sequencing was used to investigate the seasonal dynamics and assembly of planktonic bacterial communities during the water diversion period in Jihongtan Reservoir. The results indicate that planktonic bacteria were able to maintain stable diversity across all four seasons, while the community structure underwent significant seasonal succession. Water temperature (WT) was found to be the primary driving environmental factor influencing the seasonal dynamic of planktonic bacterial communities. Co-occurrence network patterns of planktonic bacterial communities varied across different seasons, particularly in spring and winter. The spring network displayed the most complexity, showcasing the highest connectivity and greater stability. In contrast, the winter network was simpler, exhibiting lower local connectivity but higher global connectivity and lower stability. The analysis of the neutral community model and null model revealed that the relative importance of deterministic and stochastic processes in governing planktonic bacterial community assembly varies seasonally. Stochastic processes (dispersal limitation) are more prominent in spring, summer, and autumn, while deterministic processes (heterogeneous selection) play a greater role in winter. This study is essential for gaining a comprehensive understanding of the effects of water diversion projects and offers valuable references for the assessment of other similar projects.

Differential impacts of water diversion and environmental factors on bacterial, archaeal, and fungal communities in the eastern route of the South-to-North water diversion project

Water diversion projects effectively mitigate the uneven distribution of water resources but can also influence aquatic biodiversity and ecosystem functions. Despite their importance, the impacts of such projects on multi-domain microbial community dynamics and the underlying mechanisms remain poorly understood. Utilizing high-throughput sequencing, we investigated bacterial, archaeal, and fungal community dynamics along the eastern route of the South-to-North water diversion project during both non-water diversion period (NWDP) and water diversion period (WDP). Our findings revealed competitive exclusion effects among bacterial and archaeal communities during the WDP, characterized by decreased species richness and increased biomass, while fungal biomass significantly declined. Distance-decay relationships suggested microbial homogenization during the WDP. Robustness analyses revealed reduced community stability during the WDP, with water diversion primarily influencing bacterial stability, while environmental factors had a greater impact on archaeal and fungal communities. Stochastic processes, primarily homogenizing dispersal and drift, intensified for bacterial and fungal communities during the WDP. Notably, only bacterial functional diversity decreased during the WDP, with increased relative abundance of chemoheterotrophic and organic compound catabolic bacteria and declined photoautotrophic bacteria. PLS-PM indicated that water diversion primarily shaped bacterial assembly processes and functional guilds, whereas environmental factors had a greater influence on archaeal communities. This study enhances our understanding of microbial dynamics during the WDP and underscores the importance of assessing both direct impacts and resulting environmental fluctuations.

Contribution for lake phytoplankton community succession under multi-source artificial hydrological regulation

In aquatic ecosystems, phytoplankton, as key primary producers, are extremely sensitive to environmental changes. Artificial hydrological regulation, as a water resource management strategy, affects the water quality and ecological environment of the receiving water area. Particularly under multi-source replenishment conditions, the spatiotemporal nonlinear response of water quality poses a challenge to quantify the contribution of multi-source replenishment to the phytoplankton community succession in large lakes. For example, Baiyangdian Lake (BL) has implemented multi-source artificial hydrological regulation measures, including the diversion of the Yellow River into the BL (Xiaobai River), the South-to-North Water Diversion Project (Baigouyin River) and the replenishment of upstream reservoirs (Xiaoyi River, Bao River and Fu River). Physicochemical parameters and phytoplankton community data were surveyed from 2017 to 2020 in July (summer) and October (autumn). The ecological impacts of artificial hydrological regulation were investigated by quantifying the correlations between physicochemical parameters and phytoplankton communities and by quantifying the contribution of artificial hydrological regulation to key factors. The results show that under the artificial hydrological regulation, the phytoplankton community was still dominated by Chlorophyta and Cyanophyta, but the emergence of Xanthophyta and Chrysophyta indicated that the cleanliness of the water column was improved, and the biodiversity indicators showed a fluctuating upward trend, which suggested that the succession trend was favorable. Ammonia nitrogen (NH3-N), total phosphorus (TP), and dissolved oxygen (DO) are the key factors affecting phytoplankton community succession in BL. The replenishment sources contribution index (RSCI) was used to characterize the spatiotemporal response of the key factors, and it was found that Fu River has the highest contribution of more than 30%, followed by Xiaoyi River with a contribution of more than 25%. The spatial distribution of RSCI shows that the area with high RSCI values (>0.9) of Fu River accounts for more than 50% of the total area, the area with high RSCI values (>0.9) of Xiaoyi River accounts for more than 35%. Under the comprehensive influence of the RSCI of each replenishment source, the phytoplankton biodiversity index of BL increased from north to south. This study provides basic support for BL in realizing the optimized artificial hydrological regulation strategy and enhancing the ecological restoration benefits in the receiving water area.

Seasonal Dynamics of Eukaryotic Microbial Communities in the Water-Receiving Reservoir of the Long-Distance Water Diversion Project, China

Inter-basin water transfer projects, such as the Yellow River to Qingdao Water Diversion Project (YQWD), are essential for addressing water scarcity, but impact local aquatic ecosystems. This study investigates the seasonal characteristics of eukaryotic microbial communities in the Jihongtan Reservoir, the main water-receiving body of YQWD, over a one-year period using 18S rDNA amplicon sequencing. The results showed that the eukaryotic microbial diversity did not exhibit significant seasonal variation (p > 0.05), but there was a notable variance in the community structure (p < 0.05). Arthropoda and Paracyclopina, representing the most dominant phylum and the most dominant genus, respectively, both exhibited the lowest abundance during the winter. The Chlorophyta, as the second-dominant phylum, demonstrates its higher abundance in the spring and winter. The Mantel test and PLS-PM (Partial Least Squares Path Modeling) revealed that water temperature (WT), dissolved oxygen (DO), and pH influenced the seasonal dynamic of eukaryotic microbial communities significantly, of which WT was the primary driving factor. In addition to environmental factors, water diversion is likely to be an important influencing factor. The results of the co-occurrence network and robustness suggested that the spring network is the most complex and exhibits the highest stability. Moreover, keystone taxa within networks have been identified, revealing that these key groups encompass both abundant and rare species, with specificity to different seasons. These insights are vital for understanding the seasonal variation of microbial communities in the Jihongtan Reservoir during ongoing water diversions.

From disruption to adaptation: Response of phytoplankton communities in representative impounded lakes to China's South-to-North Water Diversion Project

Impounded lakes are often interconnected in large-scale water diversion projects to form a coordinated system for water allocation and regulation. The alternating runoff and transferred water can significantly impact local ecosystems, which are initially reflected in the sensitive phytoplankton. Nonetheless, limited information is available on the temporal dynamics and assembly patterns of phytoplankton community in impounded lakes responding to continuous and periodic water diversion. Herein, a long-term monitoring from 2013 to 2020 were conducted to systematically investigate the response of phytoplankton community, including its characteristics, stability, and the ecological processes governing community assembly, in representative impounded lakes to the South-to-North Water Diversion Project (SNWDP) in China. In the initial stage of the SNWDP, the phytoplankton diversity indices experienced a decrease during both non-water diversion periods (8.5 %∼21.2 %) and water diversion periods (5.6 %∼12.2 %), implying a disruption in the aquatic ecosystem. But the regular delivery of high-quality water from the Yangtze River gradually increased phytoplankton diversity and mediated ecological assembly processes shifting from stochastic to deterministic. Meanwhile, reduced nutrients restricted the growth of phytoplankton, pushing species to interact more closely to maintain the functionality and stability of the co-occurrence network. The partial least squares path model revealed that ecological process (path coefficient = 0.525, p < 0.01) and interspecies interactions in networks (path coefficient = -0.806, p < 0.01) jointly influenced the keystone and dominant species, ultimately resulting in an improvement in stability (path coefficient = 0.878, p < 0.01). Overall, the phytoplankton communities experienced an evolutionary process from short-term disruption to long-term adaptation, demonstrating resilience and adaptability in response to the challenges posed by the SNWDP. This study revealed the response and adaptation mechanism of phytoplankton communities in impounded lakes to water diversion projects, which is helpful for maintaining the lake ecological health and formulating rational water management strategies.

Assembly mechanism and stability of zooplankton communities affected by China's south-to-north water diversion project

Water diversion can effectively alleviate water resource shortages and improve water environmental conditions, while also causing unknown ecological consequences, in particular, the assembly mechanism of zooplankton communities in the affected areas will become more complex after long-term water transfer. Taking Nansi Lake, the second largest impounded lake along the eastern route of China's South to North Water Diversion Project (SNWDP), as an example, the composition and diversity of zooplankton communities in the lake area and estuaries during the water diversion period (WDP) and non-water diversion period (NWDP) were studied. The potential assembly process of zooplankton communities was further explored, and the stability of communities in different regions during different periods was compared. The related results indicated that the changes in water quality conditions induced by water diversion had a relatively weak impact on the zooplankton communities. In the assembly mechanism of zooplankton communities, stochastic process played a more important role during both WDP or NWDP, and the proportion of deterministic process was relatively higher during NWDP, which may be related to the greater role of total nitrogen (TN) in the assembly of the zooplankton communities. The network analysis and cohesion calculation results showed that the stability of the zooplankton communities in the lake area sites was higher than that in the estuary sites, and the stability during NWDP was higher than that during WDP. In sum, the stability of zooplankton communities displayed a degree of change affected by water diversion activities, but the community assembly was not significantly influenced by the water quality fluctuations after about relatively long-term water diversion. This study provides an in-depth understanding of the ecological effects of water diversion on the biological communities in the affected lake, which is beneficial to the management and regulation of long-term water diversion projects.

The dynamic changes in phytoplankton and environmental factors within Dongping Lake (China) before and after the South-to-North Water Diversion Project

Dongping Lake is one of the most important regulation and storage lakes along the eastern route of the South-to-North Water Diversion Project in China, the water quality condition of which directly influences the safety of water diverting, because it serves as a Yangtze River water redistribution control point. However, the changes in algae, and in environmental factors affecting their community structures, before and after the water diversion project are rarely reported. In this study, the temporal variations of phytoplankton abundance were examined based on monthly samples collected at three stations from May 2010 to April 2022. The total abundance of algae greatly decreased after the water diversion project was implemented, with a relatively stable biodiversity and evenness before and after the water translocation. Multiple statistical methods were used together with the water quality indices (WQIs) and the nutrient status index (TSIM) to evaluate overall water condition and analyse relationships among environmental factors. The WQIs demonstrated a general "Good" water quality with a seasonal differentiation, and that water conditions during water transfer periods were better than during non-water transfer periods, which may be ascribed to the improved hydraulic conditions and purified water environment during water transfer periods. Redundancy analysis showed that water temperature, ammonia nitrogen, water transparency, and total phosphorus were the most important environmental factors, with relatively decreased contribution rates towards phytoplankton communities after the water translocation. Importantly, some dominant phytoplankton genera of Chlorophyta, Bacillariophyceae, and Cyanophyceae were similarly affected by water transparency, and nitrogen and phosphorus nutrients in summer after the water translocation. These research findings helped us gain a comprehensive understanding of the changing patterns of water quality and microalgae and their relationships before and after the water diversion project, providing a guidance for future lake management in regulating hydraulic conditions and improving water quality of Dongping Lake.

Integrated evaluation of the impact of water diversion on water quality index and phytoplankton assemblages of eutrophic lake: A case study of Yilong Lake

Water diversion has been widely utilized to enhance lake water quality and mitigate cyanobacterial blooms. However, previous studies have mainly focused on investigating the effects of water diversion on water quality or aquatic ecological health. Consequently, there is limited research investigating the combined impact of water diversion on the water quality and the ecological health of eutrophic lakes, and whether the WQI and phytoplankton assemblages demonstrate similar patterns following water diversion. In this study, the effects of water diversion on the ecosystem health of eutrophic lakes were comprehensively evaluated based on the WQI indices and phytoplankton assemblages during the NWDP-21 and WDP-22. The results showed that the annual mean of WQI increased from 52.02 to 54.36 after water diversion, which improved the water quality of the lake, especially NH3-N and TN decreased by 58.6% and 15.2%, respectively. The phytoplankton assemblages changed significantly before and after water diversion, and we observed that the total biomass of phytoplankton decreased by 12.3% and phytoplankton diversity indices (Shannon-Wiener diversity, Pielou evenness, and Simpson index) increased by 8.6%-8.9% after water diversion, with an improvement in the connectivity and stability of the phytoplankton. Notably, enhanced adaptations of rare sub-communities for resource use in water diversion environments, and water diversion inhibited the dispersal ability of dominant functional groups, and the effects of hydrological disturbances on the structure of phytoplankton assemblage favored the ecological health of eutrophic lakes. VPA analysis further reveals that water diversion alters the drivers of phytoplankton functional group biomass and phytoplankton diversity. The results of the PLS-PM analysis clarify that water diversion indirectly impacts the total phytoplankton biomass and phytoplankton diversity primarily by modifying light availability. Significant correlations are observed between the dominant functional groups biomass and diversity indices of WQI. The trends in changes observed in water quality indices and phytoplankton following water diversion align with the evaluation of water ecological health. This study provides valuable guidance for the ecological management of the diversion project in Yilong Lake and serves as a reference for similar projects in other lakes.

Spatio-temporal dynamics of phytoplankton in a diversion reservoir and the major influencing factors: taxonomic versus functional groups classification

Identifying factors affecting phytoplankton dynamics is crucial to the management of aquatic ecosystems. A lot of scholars have conducted intensive studies on phytoplankton in lake or reservoirs, but not many studies have been conducted on diversion reservoirs. To explore the seasonal and spatial variation of phytoplankton communities and their relationship with environmental factors in the context of water diversion, a case study was carried out at XiKeng (XK) reservoir in South China. In this study, month-by-month water samples and phytoplankton were collected from this reservoir from December, 2021, to July, 2022. The results showed that the phytoplankton community was characterized by significant spatial and temporal variations. There were significant differences in phytoplankton abundance and structure in the reservoirs in terms of time. The abundance of phytoplankton cells and the proportion of Cyanobacteria in the reservoir showed a trend of increasing from autumn to spring and then decreasing from spring to summer, while the functional group evolved from S1 in autumn to SN in spring and summer. The abundance of phytoplankton was influenced by the dynamic water division and the characteristics of the reservoir itself, resulting in a spatial distribution characteristic of AIII > AII > AI. Water temperature (WT) and nutrients were the key factors driving the changes in phytoplankton abundance and community structure in the reservoir. These findings will deepen our understanding of the spatial and temporal dynamics of phytoplankton community structure in diversion reservoirs and provide a basis for freshwater water ecological management strategies.