Effects of Cascade Reservoirs on Spatiotemporal Dynamics of the Sedimentary Bacterial Community: Co-occurrence Patterns, Assembly Mechanisms, and Potential Functions.

Dam construction as an important anthropogenic activity significantly influences ecological processes in altered freshwater bodies. However, the effects of multiple cascade dams on microbial communities have been largely overlooked. In this study, the spatiotemporal distribution, co-occurrence relationships, assembly mechanisms, and functional profiles of sedimentary bacterial communities were systematically investigated in 12 cascade reservoirs across two typical karst basins in southwest China over four seasons. A significant spatiotemporal heterogeneity was observed in bacterial abundance and diversity. Co-occurrence patterns in the Wujiang Basin exhibited greater edge counts, graph density, average degree, robustness, and reduced modularity, suggesting more intimate and stronger ecological interactions among species than in the Pearl River Basin. Furthermore, Armatimonadota and Desulfobacterota, identified as keystone species, occupied a more prominent niche than the dominant species. A notable distance-decay relationship between geographical distance and community dissimilarities was identified in the Pearl River Basin. Importantly, in the Wujiang Basin, water temperature emerged as the primary seasonal variable steering the deterministic process of bacterial communities, whereas 58.5% of the explained community variance in the neutral community model (NCM) indicated that stochastic processes governed community assembly in the Pearl River Basin. Additionally, principal component analysis (PCA) revealed more pronounced seasonal dynamics in nitrogen functional compositions than spatial variation in the Wujiang Basin. Redundancy analysis (RDA) results indicated that in the Wujiang Basin, environmental factors and in Pearl River Basin, geographical distance, reservoir age, and hydraulic retention time (HRT), respectively, influenced the abundance of nitrogen-related genes. Notably, these findings offer novel insights: building multiple cascade reservoirs could lead to a cascading decrease in biodiversity and resilience in the river-reservoir ecosystem.

Unraveling the antibiotic resistome in backwater zones of large cascade reservoirs: Co-occurrence patterns, horizontal transfer directions and health risks.

The widespread occurrence of antibiotic resistant genes (ARGs) throughout aquatic environments has raised global concerns for public health. However, the profiles and patterns of antibiotic resistome in backwater zone of cascade reservoirs, where water flow is slowed down, are still poorly understood. Here, we proposed a metagenomic analysis framework to comprehensively reveal the diversity, abundance, co-occurrence patterns and transfer direction of ARGs in cascade reservoirs system and evaluated their health risks through a procedure based on contigs. A total of 364 ARGs subtypes conferring resistance to different antibiotics classes were detected in our water samples, and the dominant ARGs (macB, bacA, vanRA, bcrA) were similar in different reservoirs. Meanwhile, the distribution of ARGs was influenced by the presence of biotic factors such as metal resistant genes (MRGs) and mobile genetic elements (MGEs), as well as abiotic factors such as dissolved oxygen (DO) and pH. Remarkably, ARGs (vanR, rosB, MexT) co-occurred with plasmids and virulence factor genes (VFGs), which can lead to the emergence and spread of highly virulent and antibiotic resistant bacteria in microbial communities. Overall, this study helps administrators to better understand the complex patterns of ARGs in backwater zones of large cascade reservoirs and provides a proper procedure for detecting the presence of high-risk of ARGs.

Control of Hydraulic Load on Bacterioplankton Diversity in Cascade Hydropower Reservoirs, Southwest China.

Hydroelectric reservoirs are highly regulated ecosystems, where the understanding on bacterioplankton has been very limited so far. In view of significant changes in river hydrological conditions by dam construction, hydraulic load (i.e., the ratio of mean water depth to water retention time) was assumed to control bacterioplankton diversity in cascading hydropower reservoirs. To evaluate this hypothesis, we investigated bacterioplankton composition and diversity using high-throughput sequencing and related environmental variables in eleven reservoirs on the Wujiang River, Southwest China. Our results showed a decrease of bacterioplankton diversity index with an increase of reservoir hydraulic load. This is because hydraulic load governs dissolved oxygen variation in the water column, which is a key factor shaping bacterioplankton composition in these hydroelectric reservoirs. In contrast, bacterioplankton abundance was mainly affected by nutrient-related environmental factors. Therefore, from a hydrological perspective, hydraulic load is a decisive factor for the bacterioplankton diversity in the hydroelectric reservoirs. This study can improve the understanding of reservoir bacterial ecology, and the empirical relationship between hydraulic load and bacterioplankton diversity index will help to quantitatively evaluate ecological effects of river damming.

The biotic condition of dams run-of-the-river in sequence: adaptation of a multimetric index based on the Neotropical fish fauna.

The synergistic effects of run-of-the-river (ROR) on fish fauna, though still poorly understood, are amplified when dams form reservoir cascades. In an attempt to assist in this understanding, we used an adaptation of the Reservoir Fish Assemblage Index (RFAI) to evaluate the biotic conditions of the Rio das Antas Energy Complex, in the Neotropical Patos Lagoon ecoregion. We evaluated the attributes of the fish fauna from the point of view of the complex, for each reservoir and between different sections. Fish samplings were performed quarterly at nine sites for 2 years (2015-2017). We proposed 26 metrics, related to aspects of composition, reproduction, feeding, habitat, and tolerance, selected based on criteria of variability, responsiveness, and redundancy. The final RFAI score was distributed in four categories of biological status, based on the best-observed condition. Eight metrics composed the final index, among which, only the proportion of nektonic individuals + nektobenthic (PNNbI) correlated with all sections. The final RFAI was classified as poor in all sections of the reservoir cascade, showing no significant differences between the sampling sites. This scenario indicates that dams in sequence intensify and homogenize the amplitude of fragmentation impacts on fish fauna. The final RFAI proved to be representative of the transition sections, emphasizing the importance of these reaches in impounded environments. Although the disturbance scales presented here are applicable mainly to ROR systems with reduced discharge section, the index was developed so as to allow its replicability in any dammed water course.

Spatiotemporal patterns and co-occurrence patterns of dissimilatory nitrate reduction to ammonium community in sediments of the Lancang River cascade reservoirs.

Dissimilatory nitrate reduction to ammonium (DNRA) is an important nitrate reduction pathway in freshwater sediments. Many studies have focused on the DNRA process in various natural habitats. However, the joint operation of cascade reservoirs will affect the physical and chemical properties of sediments, which may change the DNRA process and bacterial community pattern in the surface sediments of cascade reservoirs. Our study was the first to investigate the spatiotemporal distribution patterns of potential DNRA rate, nrfA gene abundances, and DNRA bacterial community diversity in surface sediments of the Lancang River cascade reservoirs. The results of slurry incubation experiments combined with the 15N isotope tracer experiment ascertained that the potential rates of DNRA were 0.01-0.15 nmol-N cm-3 h-1, and qPCR results indicated that the abundance range of nrfA was 1.08 × 105-2.51 × 106 copies g-1 dry weight. High throughput sequencing of the nrfA gene revealed that the relative abundance of Anaeromyxobacter (4.52% on average), Polyangium (4.09%), Archangium (1.86%), Geobacter (1.34%), and Lacunisphaera (1.32%) were high. Pearson and RDA correlation analysis exhibited that nrfA gene abundance was positively correlated with altitude, pH, OC, and sand concentration. Anaeromyxobacter was positively correlated with reservoir age and DNRA potential rate. The deterministic environmental selection process plays a crucial role in the formation of the DNRA bacterial community. Network analysis displayed that the dominant DNRA genus was the key population of the DNRA microbial community in the sediments of Lancang River cascade reservoirs. This study reveals that the variation of DNRA bacterial activity and community structure is largely driven by the construction of cascade reservoirs, and provides a new idea for further understanding the characteristics of the DNRA community in the cascade reservoir ecosystem.

[Variation in Phosphorus Concentration and Flux at Zhutuo Section in the Yangtze River and Source Apportionment].

Phosphorus (P) is a pollutant of great concern in the Yangtze River Basin. The Xiangjiaba Reservoir and Xiluodu Reservoir on the lower reach of the Jinsha River began to operate in 2012 and 2013, respectively, which greatly changed the concentrations of suspended sediment and characteristics of P form and transport in the reservoirs and the downstream reach from Yibin to Jiangjin of the Yangtze River. The Zhutuo section is representative in the water quality of the Yibin-Jiangjin reach, which can not only reflect the comprehensive effects of the formation of the two reservoirs and changes in the aquatic environment in the Min-Tuo Rivers but also reflect the quality of water flowing into the Three Gorges Reservoir. The runoff, concentrations and fluxes of suspended sediments (SS), and P concentrations and fluxes at Zhutuo section were studied during 2002-2019, and the source of P was apportioned based on the principle of river base flow. The results showed that in the past 18 years, the concentrations and fluxes of total phosphorus (TP) and particulate phosphorus (PP) at Zhutuo section in the wet season were higher than those in the level and dry seasons; the rule of positive correlation between PP and SS concentrations remained unchanged. From 2002 to 2019, the concentrations and fluxes of TP, PP, and dissolved P (DP) generally increased first and then decreased, and the operation of the Xiangjiaba Reservoir was a time node for the trend turning. Compared with that in the period from 2002-2012, the SS concentration and flux decreased by 94% and 77%, TP and PP concentrations decreased by 46% and 70%, and TP and PP fluxes decreased by 58% and 74%, respectively, during 2014-2019. The decline mainly occurred in the wet season, followed by that in the level season. After the formation of the two reservoirs, the relationship between water and sediment and the form of P greatly changed, and the proportion of DP in TP increased significantly, whereas the proportion of PP was the opposite. The TP pool in overlying water in the dry and level seasons shifted from mainly particulate to mainly dissolved. The change in water and sediment conditions was the main driving force for the significant change in P concentration, flux, and form. Before the operation of the Xiangjiaba Reservoir, the Jinsha River was the maximum contributor to the whole and diffuse source part of the TP load at Zhutuo section among the contributing catchment sub-basins; however, the Minjiang River became the largest contributor after the operation. The average TP load at Zhutuo section from 2017-2019 was 3.575×104 t·a-1 (after deducting the natural background value), of which the contribution of diffuse sources and point sources accounted for 68% and 32%, respectively. The Minjiang River represented 49%, 43%, and 62% of the total TP load, diffuse source TP load, and point source TP load at Zhutuo section, respectively. Considering the load contribution and pollution intensity, the key area for P pollution control in the area upstream of the Three Gorges Reservoir was the Min-Tuo River Basin.

Impact of cascade reservoirs on nutrients transported downstream and regulation method based on hydraulic retention time.

Cascade reservoirs construction has modified the nutrients dynamics and biogeochemical cycles, consequently affecting the composition and productivity of river ecosystems. The Jinsha River, as the predominant contributor to runoff, suspended sediment (SS), and nutrients production within the Yangtze River, is a typical cascade reservoir region with unclear transport patterns and retention mechanisms of nutrients (nitrogen and phosphorus). Furthermore, how to regulate nutrients delivery in the cascade reservoirs region is also an urgent issue for basin water environment study. Therefore, we monitored monthly variations in nitrogen and phosphorus concentrations from November 2021 to October 2022 in the cascade reservoirs of the Jinsha River. The results indicated that the concentrations and fluxes of total phosphorus (TP) and particulate phosphorus (PP) decreased along the cascade of reservoirs, primarily due to PP deposited with SS, while opposing trends for total nitrogen (TN) and dissolved total nitrogen (DTN), which might be the consequences of human inputs and the increase of dissolved inorganic nitrogen discharged from the bottom of the reservoirs. Moreover, the positive average annual retention ratios for TP and PP were 10% and 16%, respectively, in contrast to the negative averages of -8 % for TN and -11% for particulate nitrogen (PN). The variability in runoff-sediment and hydraulic retention time (HRT) of cascade reservoirs played crucial roles in the retention of TP and PP. A regulatory threshold of HRT = 5.3 days in the flood season was obtained for controlling the balance of TP based on the stronger relationship between HRT and TP retention ratio. Consequently, the HRT of these reservoirs could be managed to control nutrients delivery, which was of particular significance for basin government institutions. This study enhances our comprehension of how cascade reservoirs influence the distribution and transport patterns of nutrients, offering a fresh perspective on nutrients delivery regulation.

Nitrous oxide (N2O) Emissions at the Air-Water-Sediment Interfaces of Cascade Reservoirs in the Yunnan-Guizhou Plateau: Spatial Patterns and Environmental Controls.

The construction of cascade reservoirs can interfere with the natural hydrologic cycles of basins, causing negative environmental effects such as altering the emission patterns of the Nitrous oxide (N2O), a potent greenhouse gas. To elucidate the impact of cascade reservoirs construction on river N2O emissions, we utilized the thin boundary model and the incubation experiments to estimate the N2O fluxes at the air-water interface and at the water-sediment interface of cascade reservoirs on the Yunnan-Guizhou Plateau, respectively. Additionally, we explored the influence of various factors, with particular emphasis on damming, on N2O emissions and production. Moreover, we identified the main pathways of N2O production and proposed management strategies to mitigate N2O emissions from cascade reservoirs. The findings revealed that N2O fluxes at the air-water interface and the water-sediment interface were 4.73 ± 1.32 µmol · m-2 · d-1 and 15.56 ± 1.98 µmol · m-2 · d-1, respectively. Influenced by temperature, dissolved oxygen (DO), resource substances (active nitrogen substrates and dissolved organic carbon (DOC)) and reservoir properties (scale, hydraulic retention time (HRT), reservoir age, etc.), the N2O concentration and flux exhibited notable spatial heterogeneity, gradually increasing downstream. Temperature has a significant direct impact on N2O flux, as well as indirect effects through DO and resource chemicals. Furthermore, the correlation between dissolved oxygen utilization rate (AOU) and net N2O flux (△N2O) indicated that N2O emissions at the water-air interface were primarily attributable to nitrification, whereas those at the water-sediment interface were predominantly driven by denitrification. These findings not only enhance our comprehension of N2O emissions at various interfaces of cascade reservoirs but also offer theoretical backing for the formulation of management strategies aimed at efficiently mitigating N2O emissions from continuously dammed rivers.

The cumulative effects of cascade reservoirs control nitrogen and phosphorus flux: Base on biogeochemical processes.

The reservoir serves as a water source, a flood control structure, a navigational aid, and also impacts the downstream ecosystem as well as the reservoir zone. However, debate exists about effectiveness of cascade reservoirs in controlling the transportation of nutrients, particularly in the Yangtze River basin, which has been significantly affected by reservoir development. This research develops a new model X-NPSEM (X with Nitrogen and Phosphorus Steady-state Reservoir Model) based on biogeochemical processes of nitrogen and phosphorus reaction for investigating the dynamic storage capacity of cascade reservoirs at both reservoir- and watershed scales. Then the cumulative effects of cascade reservoirs and the related mechanism were investigated in Fujiang watershed, China. Based on the results, cascade reservoirs retained 16.3 % of nitrogen fluxes and 37.6 % of phosphorus fluxes annually. Downstream reservoirs have higher retention rates of phosphorus (0.48/d) compared to upstream reservoirs (0.10/d), mainly due to inflow sediment. Nitrogen retention rates show seasonal variations: wet season (0.21/d) and dry season (0.17/d). These fluctuations in nitrogen retention are primarily influenced by changes in temperature rather than other factors such as operation period, nitrogen and phosphorus concentration, or the nitrogen/phosphorus ratio. In upstream, the concentration of sediment entering the reservoir plays a decisive role in the transformation of P retention from sink to source. The X-NPSRM coupler model could be used for global reservoir operation and watershed management.

Variation in sediment sources and the response of suspended sediment grain size in the upper Changjiang River Basin following the large dam constructions.

The amount, patterns, and particle size composition of suspended sediment in the upper Changjiang Basin has been altered significantly due to the cascade reservoirs construction. A dam breach may disrupt sedimentation in the reservoir, channel erosion downstream of the dam, and the cycle of nutrients and contaminants adherent to the dam. This study is based on the long time-series field data of water discharge, sediment flux, and suspended sediment grain size of the upper Changjiang River Basin from 1973 to 2019. Four significant stepwise reduction periods in sediment load were identified by employing the M-K test, namely, 1973-1992, 1993-2002, 2003-2012, and 2013-2019. Based on the results, sediment load reduction in the upper Changjiang River (CJR) is remarkably correlated with an increase in the capacity of large reservoirs. Jialingjiang River became the largest sediment source area for upper CJR in 2013-2019. Moreover, the variation range of suspended sediment median particle size gradually narrowed with decreasing sediment load during the four periods in the upper CJR. The silt content of suspended sediment increased while clay and sand contents decreased which would maybe a factor of decreasing of P flux in the upper CJR. After the impounding of the Xiangjiaba and Xiluodu Reservoirs in the Jinshajiang River, the fining trend of suspended sediment along the main stem of upper CJR was cut off in 2013-2019, which may be ascribed to sediment management of the cascade reservoirs regulation and the sediment resuspension from the channel erosion in the river downstream. These observations can also serve as a reference for future studies about the effects of cascade reservoirs on aquatic ecology and environmental changes.

Defining the sources and the fate of nitrate by using dual isotopes and a Bayesian isotope mixing model: Water-nitrate management in cascade dams of Lancang river.

A multi-isotopes approach involving the use of stable nitrate isotopes (δ15N-NO3- and δ18O-NO3-) combined with stable water isotopes (δD-H2O and δ18O-H2O) and SIAR model as tracers can help identify the nitrogen source and understand the transformation process in a river-cascade reservoirs system. In this study, we identify the potential impact of the N source in the Lancang River basin, clarified the seasonal variations in the isotope values and estimated the probability distribution and proportional contribution of multi-terminal NO3--N sources using Bayesian isotope mixing model. In addition, we investigate the factors that led to the seasonal variations of the stable isotopes and evaluated the relationship between the uncertainty of the contribution ratio of the N sources and isotopic variations in the river-cascade reservoirs system. The NO3--N is the main component of DIN (dissolved inorganic nitrogen), accounting for 68.1 % of DIN. The ratios of δ15N-NO3- and δ18O-NO3- ranged from +4.2 ‰ to +10.3 ‰, and from +5.9 ‰ to +9.3 ‰ in the Lancang River. The δD-H2O and δ18O-H2O of the surface waters ranged from -109.47 ‰ to -76.44 ‰, and from -15.13 ‰ to -11.61 ‰, respectively. The SIAR model analysis results show that nitrification of livestock and poultry manure is the main source of NO3--N in the upstream natural reach, accounting for 40.2 %. There is little difference between the wet season and the dry season. Nitrification of soil organic nitrogen is the main source of NO3--N in the cascade development reach, accounting for 42.3 %. The contribution rate of atmospheric precipitation to nitrate concentration in both sampling periods is low (<5 %). This study provides a useful insight for reservoir water environmental managers to verify cascade development river pollution contributors and to better apply remedial solutions.

Sedimentation supports life-cycle CH4 production and accumulation in a river valley reservoir: A hierarchical Bayesian modeling approach.

Reservoirs have been recognized as a source of methane (CH4). With the gradual increase in the number of the world's reservoirs, predicting the long-term variation of reservoir CH4 emissions is important to understand the global change in carbon cycling due to reservoir creation and operation. Here, we first categorized the origins and transport of organic carbon (OC) by reservoir creation and operation into the following four aspects: a) the decomposition of flooded organic matter, b) the sedimentation of OC from upstream sediment inputs, c) the transition of the aquatic ecosystem from lotic to lentic type, stimulating the production of autochthonous OC; and d) reservoir as the collector of anthropogenic OC inputs from surrounding communities. It was assumed that OC from the four aspects jointly determined the production and accumulation of reservoir CH4 concentration, supporting life-cycle reservoir CH4 emissions. A hierarchical Bayesian model of reservoir CH4 concentration was established and calibrated by observed monthly datasets in 2018 in the Xiangjiaba Reservoir (XJB), a river valley dammed reservoir in the upper Yangtze River, China. The model explained the relative contributions of the four aspects to reservoir CH4 production and accumulation. Approximately 78% of the CH4 concentration was contributed by the decomposition of flooded organic matter during the first 10 years after impoundment. However, the contribution of flooding faded away after 10 years of impoundment. With the increase in reservoir age, sedimentation of OC dominantly determined the reservoir CH4 production and accumulation. Scenario analysis of the XJB's life cycle demostrated that the CH4 concentration in the XJB would reach its peak approximately 70 - 80 years after impoundment. In the cascade system, the upstream reservoir will help to reduce sediment OC input, and to mitigate downstream reservoir CH4 production and accumulation. Our effort provided a new modeling approach for long-term management strategies to reduce reservoir CH4 emissions under global change.

Effect of cascade damming on microplastics transport in rivers: A large-scale investigation in Wujiang River, Southwest China.

Rivers are the important channels for transporting microplastics into the ocean from land. Prosperous dam construction changed the connectivity of rivers, thereby reducing the flux of microplastics to the ocean. However, this process currently lacks verification for the large-scale watersheds. In this study, we investigated the Wujiang River in China to evaluate the interception of cascade dams on microplastics. The results showed that: 1) The midstream exhibits a high abundance of microplastics (606.6-1046.2 items·kg-1) while the upstream and downstream reach exhibits relatively low pollution levels. The small-sized microplastics of 0-0.5 mm are easily migrated into downstream while the large-sized microplastics of 0.5-5 mm tend to deposit. 2) Ten kinds of plastic materials were found, in which polyethylene and polypropylene, originated from the developed tourism and fishery, account for 74.2% in all samples. 3) The earliest microplastics were found in the sediments of 1962. The abundance of microplastics in the sediments in seven reservoirs increased over time, impling the contribution of increasing human activities. 4) Positive correlations between the abundance of microplastics in sediments and local gross domestic product (GDP) (n = 33, R2 = 0.89, p < 0.05) and negative correlations between microplastics abundance and reservoir basin area (n = 33, R2 = 0.42, p < 0.05) revealed that GDP and watershed area are the key factors that control the distribution of microplastics. Our results help to understand the migration of microplastics between terrestrial and marine ecosystems.

Terrigenous organic carbon drives methane dynamics in cascade reservoirs in the upper Yangtze China.

Methane (CH4) emissions from freshwaters to the atmosphere have a profound impact on global atmospheric greenhouse gas (GHG) concentrations. Anthropogenic footprints such as dam construction and reservoir operation significantly changed the fate and transport of CH4 in freshwaters. The source of particulate organic carbon (POC) in reservoirs is a critical factor controlling CH4 production and emissions. However, little is known of how reservoir operation mediates the transport of POC and regulates CH4 accumulation in cascade hydroelectric reservoirs. Here, spatial and temporal variations in POC and CH4 were explored in the Xiluodu (XLD) and Xiangjiaba (XJB) reservoirs which are deep valley cascade reservoirs located in the main channel of the upper Yangtze River. Based on the δ13C-POC and N/C mole ratio of particulate organic matter, the results of multi-endmember stable isotope mixing models by a Bayesian model showed that terrigenous POC and autochthonous POC accounted for approximately 55% ± 18% and 43% ± 19% (SD, n = 179) of POC, respectively. Together with other hydrological and environmental parameters, we found that the input of terrigenous POC was dominantly influenced by water level variations and flow regulation due to reservoir operation. The cumulative effect of POC caused by cascade dams was not apparent. Terrigenous POC were more likely to drive CH4 accumulation in our study. Evident low level of CH4 in both reservoirs were likely affected by low sedimentation of POC and microbial CH4 oxidation. We hope our study could provide a conceptual framework for further modeling of CH4 dynamics in cascade reservoirs.

Dam cascade unveils sediment methylmercury dynamics in reservoirs.

Methylmercury (MeHg) can be bioaccumulated through food chains and adversely affect human health. Reservoirs are reported to stimulate MeHg production, however, the characteristics of MeHg dynamics in cascade reservoirs and the associated relations to sedimentation as well as reservoir properties remained unclear. Here we investigated sediment MeHg dynamics in eight cascade reservoirs in the upper Mekong River. We found significant differences in sediment MeHg concentration between the reservoirs, showing an increase with fluctuations along the reservoirs cascade. However, a novel relationship was found between sediment%MeHg (MeHg/Hg) and the ratio of reservoir hydraulic residence time (HRT) to reservoir age. This relationship is formed by the joint effects of the original deposit of Hg and organic carbon (OC) before impoundment and the subsequent sedimentation of Hg and OC after impoundment. The original deposit is continuously transformed as the reservoir ages, whereas the latter is driven by the annual hydrological cycle and HRT, of which the HRT is dominant. This finding cannot be easily revealed in a single reservoir or by comparing multiple reservoirs in different rivers. The discovery is of great significance to understand Hg geochemical cycling in reservoirs, which is quickly increasing in rivers worldwide.

Spatiotemporal variation of phosphorus in the Three Gorges Reservoir: impact of upstream cascade reservoirs.

The impoundment of the Three Gorges Reservoir (TGR) and upstream cascade reservoirs (UCRs) has altered the hydrologic regime, with inevitable effects on phosphorus transport processes in the TGR. In order to investigate the effects of impoundment, long-term monitoring data of flow rate, suspended sediment (SS), and phosphorus fractions of six stations in the TGR basin were collected and divided into three periods, period 1 (P1) (1985-2002), period 2 (P2) (2003-2012), and period 3 (P3) (2013-2017), based on the periodic impoundment time. The results indicated that the impoundment of the TGR and UCRs considerably decreased the SS concentration. Efficient sediment interception by the UCRs led to a dramatic decline in the concentrations of total phosphorus (TP) and particulate phosphorus (PP) in the mainstream, while the total dissolved phosphorus (TDP) showed a general increasing trend from 2004 to 2017. Different phosphorus fractions in the mainstream exhibited seasonal variations; among them, the concentrations of TP and PP were highest in the wet season, while the highest TDP concentration occurred in the dry season. Further analysis indicated that the seasonal distribution of TP was significantly homogenized in P3. Additionally, the SS concentration was positively correlated with the concentrations of TP and PP in the mainstream, while the correlations in P3 were significantly lower than that in P2. The findings can provide a scientific reference for future investigations dedicated to the long-term effects of the UCRs on the eco-environment in the TGR as well as the downstream.

Seasonal variation of nitrogen biogeochemical processes constrained by nitrate dual isotopes in cascade reservoirs, Southwestern China.

The increase of affected river reaches by reservoirs has drastically disturbed the original hydrological conditions, and subsequently influenced the nutrient biogeochemistry in the aquatic system, particularly in the cascade reservoir system. To understand the seasonal variation of nitrogen (N) behaviors in cascade reservoirs, hydrochemistry and nitrate dual isotopes (δ15N-NO3- and δ18O-NO3-) were conducted in a karst watershed (Wujiang River) in southwest China. The results showed that NO3--N accounted for almost 90% of the total dissolved nitrogen (TDN) concentration with high average concentration 3.8 ± 0.4 mg/L among four cascade reservoirs. Higher N concentration (4.0 ± 0.8 mg/L) and larger longitudinal variation were observed in summer than in other seasons. The relationship between the variation of NO3--N and dual isotopes in the profiles demonstrated that nitrification was dominated transformation, while assimilation contributed significantly in the epilimnion during spring and summer. The high dissolved oxygen concentration in the present cascade reservoirs system prevented the occurrence of N depletion processes in most of the reservoirs. Denitrification occurred in the oldest reservoir during winter with a rate ranging from 18 to 28%. The long-term record of surface water TDN concentration in reservoirs demonstrated an increase from 2.0 to 3.6 mg/L during the past two decades (~ 0.1 mg/L per year). The seasonal nitrate isotopic signature and continuously increased fertilizer application demonstrated that chemical fertilizer contribution significantly influenced NO3--N concentration in the karst cascade reservoirs. The research highlighted that the notable N increase in karst cascade reservoirs could influence the aquatic health in the region and further investigations were required.

Hydrodynamics Regulate Longitudinal Plankton Community Structure in an Alpine Cascade Reservoir System.

Previous studies report significant changes on biotic communities caused by cascade reservoir construction. However, factors regulating the spatial-temporal plankton patterns in alpine cascade reservoir systems have not been fully explored. The current study explored effects of environmental factors on the longitudinal plankton patterns, through a 5-year-long study on the environmental factors and communities of phytoplankton and zooplankton in an alpine cascade reservoir system located upstream of Yellow River region. The findings showed that phytoplankton and zooplankton species numbers in the studied cascade reservoir system were mainly regulated by the hydrological regime, whereas nutrient conditions did not significantly affect the number of species. Abundance and biovolume of phytoplankton in cascade reservoirs were modulated by the hydrological regime and nutrient conditions. The drainage rate, N:P ratio, and sediment content in cascade reservoirs were negatively correlated with abundance and biovolume of phytoplankton. Abundance and biovolume of zooplankton were not significantly correlated with the hydrological regime but showed a strong positive correlation with nutrient conditions in cascade reservoirs. Shannon-Wiener index (H') and the Pielou index (J) of phytoplankton were mainly regulated by the hydrological regime factors, such as drainage rate and sediment content in cascade reservoirs. However, temperature and nutrient conditions were the main factors that regulated the Shannon-Wiener index (H') and the Pielou index (J) of zooplankton. Species number, abundance, and biovolume of phytoplankton showed a significant positive correlation with those of zooplankton. Hydrodynamics and nutrient conditions contributed differently in regulating community structure of phytoplankton or zooplankton. These findings provide an understanding of factors that modulate longitudinal plankton community patterns in cascade reservoir systems.

Nitrous oxide emissions from cascade hydropower reservoirs in the upper Mekong River.

Nitrous oxide is a powerful greenhouse gas, and its emissions from single reservoirs have been extensively studied; however, it still remains unclear about nitrous oxide emission patterns in cascade reservoirs. In this study, nitrous oxide emissions from cascade hydropower reservoirs were investigated using the thin boundary layer model in the heavily dammed upper Mekong River. Meanwhile, sediment denitrification for nitrous oxide production was analysed using the stable isotope method and the quantitative polymerase chain reaction method. Our results demonstrated that nitrous oxide emissions (0.47-1.08 µg m-2h-1) in the upper Mekong River were much lower than the global mean level (19.60 µg m-2h-1), but were increased by dam constructions; nitrous oxide emissions exhibited an increase trend along the flow direction in the cascade reservoirs. Sediment accumulation by dams supplied sufficient nitrogen substrates and organic carbon, creating hotspots of denitrification at the transition zone in reservoirs. As the elevation decreased, the increase in temperature enhanced microbial denitrification at the active zone, and thereby increased nitrous oxide production with the prolonged residence time. This study advanced our knowledge on nitrous oxide emissions from cascade hydropower systems.

Hydropower reservoirs on the upper Mekong River modify nutrient bioavailability downstream.

Hydropower development is the key strategy in many developing countries for energy supply, climate-change mitigation and economic development. However, it is commonly assumed that river dams retain nutrients and therefore reduce downstream primary productivity and fishery catches, compromising food security and causing trans-boundary disputes. Contrary to expectation, here we found that a cascade of reservoirs along the upper Mekong River increased downstream bioavailability of nitrogen and phosphorus. The dams caused phytoplankton density to increase with hydraulic residence time and stratification of the stagnant reservoirs caused hypoxia at depth. This allowed the release of bioavailable phosphorus from the sediment and an increase in dissolved inorganic nitrogen as well as a shift in nitrogen species from nitrate to ammonium, which were transported downstream by the discharge of water from the base of the dam. Our findings provide a new perspective on the environmental impacts of river dams on nutrient cycling and ecosystem functioning, with potential implications for sustainable development of hydropower worldwide.