Harmful algal blooms (HABs) threaten global freshwater ecosystems, yet reliable early warning indicators remain elusive, as traditional nutrient and temperature models often fail to capture the metabolic transition to explosive proliferation. This study investigates multi-component dissolved gas dynamics (O₂, CO₂, N₂, Ar, and TDG) as novel early warning indicators that directly reflect algal physiological activity rather than static environmental conditions, thereby bridging the gap between favorable conditions and bloom realization. We conducted continuous in-situ monitoring in the Pengxi River of the Three Gorges Reservoir, China, across five periods between 2023 and 2024. The results revealed three sequential gas-based indicators reflecting bloom progression. First, the Argon-Oxygen (Ar-O₂) correlation shifted from positive to negative, signaling the transition to algae-favorable conditions where photosynthetic oxygen production overrides temperature-driven solubility controls. Second, Photosynthetic Quotient (PQ) spikes exceeding 1.5 revealed cryptic subsurface algal accumulation before significant surface biomass appeared. Third, Total Dissolved Gas (TDG) supersaturation exceeding 105% marked a biophysical tipping point where micro-bubble formation accelerated bloom intensification through buoyancy-mediated aggregation. Unlike static environmental parameters, these gas dynamics directly reflect algal physiological activity and spatial distribution. Critically, the PQ‑spike stage defines the optimal intervention window, enabling reservoir operations to disrupt subsurface accumulation before surface manifestation and TDG‑driven escalation. Integrating these dissolved gas patterns into existing frameworks bridges the gap between environmental favorability and actual bloom occurrence. Our mechanism-based framework offers a transformative pathway for proactive HAB management in reservoir systems worldwide under intensifying climate pressures.

Minor water-level fluctuations are common in large, regulated reservoirs, yet their effects on air-water methane (CH4) emission in tributary bays remain unclear. We examined Xiangxi Bay (XXB) of the Three Gorges Reservoir (TGR) during three surveys spanning a drawdown-rise-drawdown sequence (0.19-0.25 m d-1) near the end of the TGR' annual drawdown. Vertical profiling of dissolved CH4 (CCH4), δ13C-CH4, environmental variables, velocity were combined with estimates of diffusive CH4 fluxes (FCH4). Minor fluctuations did not change the downstream-to-upstream pattern of FCH4 in XXB, but mean FCH4 during drawdown (WLDP) reached 55.38 μg m-2 h-1, 17.4 times the value during the water-level rise (WLRP; 3.18 μg m-2 h-1), the ratio reached 31.3 in the upstream. During WLRP, mixing depth (Dm) increased from 2.80 to 13.07 m, stratification stability (Rh) decreased from 8.07 to 4.21 m-1, and CCH4 fell to one-twelfth of the WLDP value. Velocity fields indicated that mainstem intrusion shoaled upstream during WLDP, potentially redistributing CH4-enriched bottom water toward the bay upstream surface. Layered water exchange during WLRP retained this water at depth and favored dilution and oxidation. Less negative surface CH4 stable carbon isotopes (δ13C-CH4) signature was consistent with oxidation, although water mass mixing may also have contributed. Stratification stability (Rh) was the most consistent predictor of FCH4; chlorophyll a (Chl.a) and total phosphorus (TP) provided additional predictive information during WLDP. Minor fluctuations can change diffusive CH4 flux by more than an order of magnitude without altering its longitudinal pattern. CH4 budgets for large riverine reservoirs should resolve short water-level rising and falling stages. Mitigation through water-level regulation still requires tests of fluctuation amplitude, rate, and duration.

The biofilm process for phosphorus (P) enrichment has low carbon requirements, high P recovery, and the potential for high P enrichment. Since the superiority of nitrite and nitrate for denitrifying phosphorus removal (DPR) and P enrichment has not been identified, a biofilm system was established to elucidate the effect of electron acceptors on DPR and P enrichment from the perspective of microbial properties, nutrient metabolism genes, and quorum sensing. The results indicated that nitrite increased intracellular P storage by 6.8 mg/g SS by promoting the proliferation of P, N, and C metabolism genes compared with nitrate, and increased Delta P/Delta N and P release to acetate absorption ratio (Prel/HAcupt) by 12.5 % and 19.0 %, respectively. Nitrite also affected the composition of EPS by upregulating the biosynthesis of amino acids and amino sugars, and nucleotide sugar metabolism, resulting in an increase in extracellular P storage by 15.2 mg/g SS. Moreover, nitrite accelerated microbial quorum sensing by stimulating the increase of quorum sensing functional genes, biofilm synthesis protein genes, and AHLs synthase genes, accelerating intracellular/extracellular P metabolism processes. The rise of TP storage led to changes in microbial metabolic patterns, accelerating the release of P during the anaerobic stage and increasing the P enrichment efficiency from 62.9 % to 74.5 %, resulting in the highest concentration of P enrichment solution (153.6 +/- 3.27 mg/L) was obtained under nitrite condition. This study provided a scientific basis for clarifying the mechanism of nitrite's effect in DPR systems and promoted the sustainable utilization of P resources.

Reservoirs function as critical nitrogen sinks within watersheds, yet their removal efficiencies remain poorly characterized. In this study, we quantified denitrification rates and excess dissolved nitrogen gas (N₂) in Xiangxi Bay, a eutrophic tributary of the Three Gorges Reservoir (TGR) to evaluate its capacity for nitrogen removal. The results indicated that water denitrification rates ranged from 5.16 to 65.31 nmol/(L·h), whereas those in sediments ranged from 2.56 to 7.75 nmol/(g·h). Both exhibited an increasing trend from downstream to upstream during the water impoundment period of the TGR. The in-situ variation of excess dissolved N2 (ΔN2) concentration (1.05-98.97 µmol/L) reflected the spatiotemporal distribution characteristics of denitrification rates and nitrogen removal capacity, indicating substantial nitrogen removal in Xiangxi Bay. Additionally, key factors affecting denitrification in water included dissolved organic carbon (DOC), chlorophyll a, and turbidity, whereas key factors in sediments were total organic carbon, total nitrogen, and DOC in pore water. Overall, the increase in organic matter in water and sediments triggered by algal growth/death and settling promoted denitrification, but excessive algal proliferation could lead to nitrogen limitation, thereby inhibiting denitrification. Therefore, preventing and managing algal blooms in tributaries could provide new insights for removing nitrogen in watersheds and safeguarding the health of aquatic ecosystems.

Accurate quantification of near-surface turbulence is essential for understanding the dynamics of turbulent mixing and mass transport in aquatic systems. However, field measurements of near-surface flow velocities often include contributions from surface gravity waves. For the quantification of turbulence and related transport processes, robust methods are needed to separate wave motion from the turbulent velocity fluctuations. In this study, we evaluated the performance of five different methods for wave-turbulence decomposition in estimating turbulent kinetic energy, Reynolds stress and turbulent kinetic energy dissipation rate. The methods include Ensemble Empirical Modal Decomposition (EEMD), Phase method (PH), Variational Mode Decomposition (VMD), Synchrosqueezed Wavelet Transform (SWT) and improved Synchrosqueezed Wavelet Transform (iSWT). We used these methods for a re-analysis of high-frequency velocity measurements from the water surface of the Kitinen River, Finland. The results show that the different methods remove the wave component to varying degrees, whereas the performance of the VMD method appeared insufficient. The estimated turbulent kinetic energy and Reynolds stresses were generally smaller than 30 % of those calculated from the unprocessed velocity measurements. In terms of energy spectra, the EEMD, PH, SWT and iSWT methods all provide a better removal of wave energy, but the EEMD and SWT methods resulted in substantial energy notches in the wave frequency band, resulting in a significant underestimation of the turbulent velocity fluctuations. In contrast, iSWT achieves the decomposition of wave and turbulence components by applying an optimal decomposition degree index popt, which maximizes the retention of turbulent velocity fluctuations. Application of the inertial dissipation method for estimating dissipation rates of turbulent kinetic energy from the spectra of separated turbulent velocities. The results showed that the iSWT method resulted in the longest inertial subrange, and allowed for most but also has very good robustness spectral fits for dissipation rates ranging from 1.33 x 10- 7 W/kg to 1.06 x 10-5 W/kg. Using dissipation rate estimates from an advanced methods explicitly considering wave-turbulence interactions as a reference, the iSWT method showed the closest agreement, whereas the dissipation rates estimated from velocities processed by the other four methods were generally lower. The newly proposed method is able to provide accurate estimates of dissipation rates by robustly separating the turbulence from wave-affected velocities compared to the four tested existing methods.

Ebullition constitutes a major pathway for methane emissions from reservoirs, particularly in tributary bays with steep depth gradients. However, methane (CH4) bubble release characteristics remain poorly understood in Three Gorges Reservoir tributaries due to methodological limitations in quantitative monitoring. A novel underwater bubble generation device was employed to establish the target strength (TS) to bubble volume (V) relationship for echo sounder measurements. Sediment bubble release was monitored in situ in the Xiangxi River, a major tributary of the Three Gorges Reservoir (TGR). Site-specific acoustic TS-V relationships effectively enhanced the precision of acoustic bubble volume quantification. Fixed-point monitoring and comprehensive cruise surveys across depth gradients (0-70 m) revealed pronounced spatial heterogeneity in ebullition, with shallow areas (< 20 m) exhibiting fluxes 19-29 times greater than deep regions (> 40 m). A critical depth threshold of 40 m was identified, below which bubble formation is suppressed. Strong coupling between ebullition and dissolved CH4 distributions (R2 = 0.93) indicates bubble release drives dissolved methane concentrations more than sediment organic carbon content. These methodological advances provide essential tools for accurate reservoir methane emission assessments and highlight the importance of depth-dependent spatial variability in global carbon budget evaluations.

The construction of cascade hydropower dams in the Lancang River Basin of Southwest China significantly affects sediment nitrogen fractions. To assess these impacts, we contrasted the dammed Lancang River with the undammed Nujiang River. Using the recommended Ruttenberg sequential extraction process, we determined five nitrogen fractions: free nitrogen (F-N), exchangeable nitrogen (Ex-N), carbonate-associated nitrogen (CO 3 -N), ironmanganese oxides-bound nitrogen (IM-N), and organic nitrogen (Org-N). There were considerable differences between the properties of sediments across the two rivers and had their effects on nitrogen fraction distribution. The cascade reservoirs possessed higher levels of transferable nitrogen (TranN) and total inorganic nitrogen (TIN) than the natural river section. Nitrogen composition in the natural river was as per Org-N > CO 3 -N > IM-N > Ex-N > F-N, but at certain points in reservoirs, it altered to Org-N > IM-N > CO 3 -N > Ex-N > F-N. This means that cascade reservoirs favor the release of nitrogen and enhance bioavailable nitrogen. Dynamics of nitrogen fractions depend on environmental factors such as altered water level, reduced flow velocity, silt interception, resuspension of sediment, particle-size distribution, and mineral composition, and axis I explains over 70% of variance.

Effluent discharge from wastewater treatment plants alters nitrogen and sulfur cycling in the hyporheic zone (HZ), potentially shifting microbial communities to alternative stable states. However, these transitions remain poorly understood in such specific subsurface environments. Here, we characterized and predicted multiple stable states of communities in the HZ of representative effluent-dominated rivers by integrating molecular techniques, alternative stable states theory, and machine learning models. The results revealed the existence of bistable states in terms of microbial taxa, functional genes, and metabolic pathways. The potential analysis demonstrated that with increases in nitrogen and sulfur loading, the taxonomic composition shifted from a state with higher diversity and lower stability to one with more prominent interspecific competition. The regime shift in metabolic functions was likely the initial transformation, as it was subsequently followed by alterations in the taxonomic composition. Optimized random forest and XGBoost models combined with network embedding achieved over 90% accuracy in predicting taxonomic composition and metabolic functions, outperforming stand-alone machine learning models. The generated results demonstrated that the accurate description and prediction of microbial responses to anthropogenic disturbances, e.g., effluent discharge, required the joint evaluation of variability in community structure and metabolic function.

Serving as a novel type of biocompatible carrier, the applications of Bio-wax in fluvial ecological remediation inevitably require consideration of hydrodynamic conditions. However, few studies have explored the biofilm formation and community assembly on Bio-wax carriers under varying shear stresses. Here, the biofilm formation patterns and community assembly mechanisms on two carriers, i.e., the high-density polyethylene (HDPE) and the Bio-wax in response to three shear stress conditions were revealed. The maximum growth rates of biofilms on Bio-wax were increased by 29.9 %, 72.8 %, and 206.8 % compared to those attached to HDPE in three reactors, demonstrating the superiority of Bio-wax in developing more stable biofilms under fierce shear stress. The microbial community generated on Bio-wax exhibited higher richness and diversity, as well as an enrichment of more keystone species. An increase in shear stress remarkably enhanced the deterministic assembly processes, simultaneously increasing the modularity of the phylogenetic molecular ecological networks. Intriguingly, a medium hydrodynamic condition with tau value of 0.3 Pa was proven most suitable for cultivating biofilms on Bio-wax, supported by the highest specific growth rate and a more complex network structure. This study confirms the advantage of Bio-wax carriers in developing biofilms, both in quantity and quality under varying shear stresses, providing theoretical support for their applications in diverse scenarios.

Microbial corrosion of hydraulic concrete structures (HCSs) has received increasing research concerns. However, knowledge on the morphology of attached biofilms, as well as the community structures and functions cultivated under variable nutrient levels is lacking. Here, biofilm colonization patterns and community structures responding to variable levels of ammonia and sulfate were explored. From field sampling, NH4+-N was proven key factor governing community structure in attached biofilms, verifying the reliability of selecting target nutrient species in batch experiments. Biofilms exhibited significant compositional differences in field sampling and incubation experiments. As the nutrient increased in batch experiments, the growth of biofilms gradually slowed down and uneven distribution was detected. The proportions of proteins and β-D-glucose polysaccharides in biofilms experienced a decrease in response to elevated levels of nutrients. With the increased of nutrients, the mass losses of concretes exhibited an increase, reaching a highest value of 2.37 % in the presence of 20 mg/L of ammonia. Microbial communities underwent a significant transition in structure and metabolic functions to ammonia gradient. The highest activity of nitrification was observed in biofilms colonized in the presence of 20 mg/L of ammonia. While the communities and their functions remained relatively more stable responding to sulfate gradient. Our research provides novel insights into the structures of biofilms attached on HCSs and the metabolic functions in the presence of high level of nutrients, which is of significance for the operation and maintenance of hydraulic engineering structures.

Substantial nutrient inputs from reservoir impoundment typically increase sedimentation rate and primary production. This can greatly enhance methane (CH4) production, making reservoirs potentially significant sources of atmospheric CH4. Consequently, elucidating CH4 emissions from reservoirs is crucial for assessing their role in the global methane budget. Reservoir operations can also influence hydrodynamic and biogeochemical processes, potentially leading to pronounced spatiotemporal heterogeneity, especially in reservoirs with complex tributaries, such as the Three Gorges Reservoir (TGR). Although several studies have investigated the spatial and temporal variations in CH4 emissions in the TGR and its tributaries, considerable uncertainties remain regarding the impact of reservoir operations on CH4 dynamics. These uncertainties primarily arise from the limited spatial and temporal resolutions of previous measurements and the complex underlying mechanisms of CH4 dynamics in reservoirs. In this study, we employed a fast-response automated gas equilibrator to measure the spatial distribution and seasonal variations of dissolved CH4 concentrations in XXB, a representative area significantly impacted by TGR operations and known for severe algal blooms. Additionally, we measured CH4 production rates in sediments and diffusive CH4 flux in the surface water. Our multiple campaigns suggest substantial spatial and temporal variability in CH4 concentrations across XXB. Specifically, dissolved CH4 concentrations were generally higher upstream than downstream and exhibited a vertical stratification, with greater concentrations in bottom water compared to surface water. The peak dissolved CH4 concentration was observed in May during the drained period. Our results suggest that the interplay between aquatic organic matter, which promotes CH4 production, and the dilution process caused by intrusion flows from the mainstream primarily drives this spatiotemporal variability. Importantly, our study indicates the feasibility of using strategic reservoir operations to regulate these factors and mitigate CH4 emissions. This eco-environmental approach could also be a pivotal management strategy to reduce greenhouse gas emissions from other reservoirs.

Clarifying reactive oxygen species (ROS) variation in the presence of co-existing anions is significant for understanding the catalytic effect of magnetite (Fe3O4)-induced advanced oxidation processes (AOPs) in natural environment, yet this remains controversial. Herein, we compare the specific impacts of NO3-, SO42-, and Cl- on ROS (•OH, SO4•-, O2•-, and 1O2) exposure concentration in H2O2 and peroxydisulfate (PDS) systems catalyzed by Fe3O4, as well as how these variations affect the catalytic efficiency by developing kinetic model. In both two systems, NO3- demonstrates no discernible effect on ROS, whereas SO42- inhibits the exposure of all ROS and thus micropollutants degradation. Through theoretical calculation, it is proposed that SO42- primarily exerts its influence through affecting the electronic structure over catalyst surface. Regarding Cl-, it affects ROS exposure mainly by reacting with ROS. It shows inhibitory effect on 1O2 in both systems, but its suppressive impact on •OH is markedly more pronounced in H2O2 system compared to PDS system, which may be related to its rapid reactivity with SO4•-. Besides, the chlorine radicals (mainly ClO•) generated through the reaction of Cl- may exert a selective influence on micropollutants degradation. This study can help to re-understand the influence behavior of co-existing anions during AOPs.

As a key nutrient for primary productivity in freshwater ecosystems, the effect of upper cascade reservoirs on the downstream phosphorus (P) loading is subject to an ongoing scientific debate in the Mekong River. To investigate the effects of cascade reservoirs on the forms of P and its bioavailability in surface sediment, two rounds of field research were carried out both in a free-flowing river (Nu River) and in the Upper Mekong River (Lancang River) during winter and summer. Results showed that while the P was trapped with sediments and environmental parameters changed significantly, high relative abundance of bioavailable phosphorus (BioP, generally include Fe-bound P, Al-bound P, and Organic P) in sediment was detected in reservoirs and at near the border between China and Myanmar. The increased Bio-P in reservoirs may be caused by sediments sorting, anaerobic surface sediments, appropriate temperature, longer residence time and algae-nutrient positive feedback loops. Accumulated Bio-P in sediments may elevate the phosphorus release to overlying water and exacerbate eutrophication in reservoirs. Considered the intense anthropogenic phosphorus loading, nutrients enrichment risk in the lower Mekong River should receive more attention in further research.

Serving as a vital linkage between surface water and groundwater, the hyporheic zone (HZ) plays a fundamental role in improving water quality and maintaining ecological security. In arid or semi-arid areas, effluent discharge from wastewater treatment facilities could occupy a predominant proportion of the total base flow of receiving rivers. Nonetheless the relationship between microbial activity, abundance and environmental factors in the HZ of effluent-receiving rivers appear to be rarely addressed. In this study, a spatiotemporal field study was performed in two representative effluent-dominated receiving rivers in Xi'an, China. Land use data, physical and chemical water quality parameters of surface and subsurface water were used as predictive variables, while the microbial respiratory electron transport system activity (ETSA), the Chao1 and Shannon index of total microbial community, as well as the Chao1 and Shannon index of denitrifying bacteria community were used as response variables, while ETSA was used as response variables indicating ecological processes and Shannon and Chao1 were utilized as parameters indicating microbial diversity. Two machine learning models were utilized to provide evidence-based information on how environmental factors interact and drive microbial activity and abundance in the HZ at variable depths. The models with Chao1 and Shannon as response variables exhibited excellent predictive performances (R-2: 0.754-0.81 and 0.783-0.839). Dissolved organic nitrogen (DON) was the most important factor affecting the microbial functions, and an obvious threshold value of similar to 2 mg/L was observed. Credible predictions of models with Chao1 and Shannon index of denitrifying bacteria community as response variables were detected (R-2: 0.484-0.624 and 0.567-0.638), with soluble reactive phosphorus (SRP) being the key influencing factor. Fe (II) was favorable in predicting denitrifying bacteria community. The ESTA model highlighted the importance of total nitrogen in the ecological health monitoring in HZ. These findings provide novel insights in predicting microbial activity and abundance in highly-impacted areas such as the HZ of effluent-dominated receiving rivers.

Disentangling the underlying processes of plant adaptations to multiple abiotic stressors is crucial regarding promissory species for the restoration of riparian ecosystems prone to suffering extreme flood and drought events in the context of global climate change and human activities. Distylium chinense is a dominant evergreen shrub, distributed in the riparian areas of the Yangtze River in China. Here, one field study and five controlled experiments (Control, CK; single drought, D; single flooding, FF; from drought to recovery to full flooding, D-R-FF; from full flooding to recovery to drought, FF-R-D) were conducted. More hypertrophied lenticels, adventitious roots, and the increased stem-base hypertrophy of D. chinense were observed under the D-R-FF condition compared with FF and FF-R-D. Interestingly, the increase of the net photosynthetic rate (Pn) coincidentally occurred with the increase of heme degradation by heme oxygenase (r = 0.608, p = 0.003). Pn of D. chinense in D-R-FF was about twice as much as that in FF-R-D. The enhanced photosynthetic performance was functionally coupled with the adequate water supply to promote the tolerance of D. chinense to alternate drought–flooding condition compared with no any flooding condition. The accumulation of soluble sugar was highest under D, followed by FF-R-D, FF and D-R-FF, which showed that soluble sugar accumulation over the drought period could trigger the recovery growth of flooded plants in later flooding. These data provided the first insights into the tolerance mechanisms by a suite of morphological alterations and physiological adaptations, especially in the enhanced photosynthetic performance of D. chinense under alternating drought and flooding stresses. So, D. chinense could be considered as a prominent shrub species in the restoration practices of wetlands, riparian areas, and other flood-prone forests.

Nitrogen (N) pollution is a major threat to river ecosystems worldwide. Elucidating the community structure of N-cycling microorganisms in rivers is essential to understanding how ecosystem processes and functions will respond to increasing N inputs. However, previous studies generally focus on limited functional genes through amplicon sequencing or quantitative PCR techniques and cannot cover all N-cycling microorganisms. Here, metagenomic sequencing and genome binning were used to determine N-cycling genes in water, channel sediments, and riparian soils of the Yangtze River, which has been heavily polluted by N. Additionally, the denitrification and anaerobic ammonium oxidation (anammox) rates that reflect N removal potential were measured using 15N isotope pairing technique. Results showed that functional genes involved in organic N metabolism (i.e., organic degradation and synthesis) and nitrate reduction pathways (i.e., dissimilatory and assimilatory nitrate reduction to ammonium and denitrification) were more abundant and diverse than other N-cycling genes. A total of 121 metagenome-assembled genomes (MAGs) were identified to be involved in N-cycling processes, and the key MAGs were mainly taxonomically classified as Alphaproteobacteria and Gammaproteobacteria. The abundance and diversity of most N-cycling genes were higher in soils and sediments than in water, as well as higher in downstream and midstream than in upstream sites. These spatial variations were explained not only by local environment and vegetation but also by geographical and climatic factors. N removal process (i.e., denitrification and anammox) rates were significantly related to the abundance or diversity of several N-cycling genes, and climate and edaphic factors could regulate denitrification and anammox rates directly and indirectly through their effects on functional genes. Overall, these results provide a new avenue for further understanding the biogeographic patterns and environmental drivers of N-cycling microorganisms in rivers from the metagenomic perspective.

Abstract Human-induced disturbances such as dam construction and regulation often alter the duration，frequency and seasonality of flooding and thus substantially influence plant characteristics in the hydro-fluctuation zones. However, the effect mechanism of anti-seasonal hydrological alterations on vegetation distribution patterns and niche characteristics in the water level fluctuation zones (WLFZs).is still unclear. In this study, 368 quadrats were selected to investigate the effects of the anti-seasonal hydrological regimes on the foristic composition, species diversity and niche characteristic in the hydro-fluctuation zone of the Three Gorges Reservoir (TGR), a unique riparian ecosystem, China. The results showed that the number of species per square meter (S), the Shannon-Wiener diversity index (H) and Simpson dominance index (D) of the plant guilds in the TGR increased significantly with elevation, which was inconsistent with humped diversity–disturbance relationship of the intermediate disturbance hypothesis, while the opposite trend was observed for the Pielou evenness index (E). The H, D, S and E from upstream to downstream firstly showed a significant increasing trend (p<0.05), reached the highest in the middle reaches, and then decreased in the lower reaches. The vegetation was classified into 12 guild types but the vegetation composition showed a significant variation with a transition from xerophytes to mesophytes and hygrophytes with the increasing flooding time. Cynodon dactylon was the most dominant species based on its highest important value and niche breadth. And high niche breadth had a high niche overlap between species. Therefore, anti-seasonal hydrological alterations precipitated substantial reduction of plant diversity, species competition and exclusion among species by expanding the niche in the guilds. The vegetation in the unique riparian ecosystems was still in the primary stage of plant community succession with low species diversity, high niche overlap, intense competition and obvious single-species dominant communities. Compared to total nitrogen, total phosphorus and soil organic matter factors, the hydrological alteration filtering was more important in explaining the plant guild patterns and niche characteristics. Therefore, there may be some differences in the governance strategies adopted in different areas of the novel riparian ecosystems for vegetation restoration efforts of the riparian forests.

Human-induced disturbances such as dam construction and regulation have led to widespread alterations in hydrological processes and thus substantially influence plant characteristics in the hydro-fluctuation zones (HFZs). To reveal utilization of limited resources and mechanisms of inter-specific competition and species co-existence of plant communities based on niche breadth and overlap under the different HFZs of the Three Gorges Reservoir (TGR) in China, we conducted a field investigation with 368 quadrats on the effects of hydrological alterations on plant diversity and niche characteristics. The results showed anti-seasonal flooding precipitated the gradual disappearance of the original diverse niches, resulting in the reduction of plant species richness and functional diversity and more obvious competition among plant species with similar resource requirements. Annuals, perennials and shrubs accounted for 71.23%, 27.39% and 1.37%, respectively, suggesting that annuals and flood-tolerant riparian herbs were favored under such novel flooding conditions. A consistent increase in species number, Shannon-Wiener diversity index and Simpson dominance index with altitude was inconsistent with hump-shaped diversity-disturbance relationship of the intermediate disturbance hypothesis, while the opposite trend was observed for the Pielou evenness index. This species distribution pattern might be caused by several synergetic attributes (e.g., the submergence depth, plant tolerant capacity to flooding, life form, dispersal mode and inter-specific competition). Vegetation types shifted from xerophytes to mesophytes and eventually to hygrophytes with the increasing flooding time in the HFZs. Hydrological alterations proved to be the paramount driver of vegetation distribution in the different HFZs. The niche analysis provided the first insights on the mechanisms of resource utilization and inter-specific competition, of which annuals could germinate quickly after soil drainage to achieve the greatest competitive advantages and occupy a larger niche space than other plants. Vegetation was still in the early stage of primary succession in the novel riparian forests. Therefore, vegetation restoration strategies should be biased towards herbaceous plants, due to annuals with better environmental adaptability, supplemented by shrubs and small trees. To establish a complete reference system for vegetation restoration, natural vegetation monitory plots in the different succession stages should be established in the different HFZs of the TGR, and their environmental conditions, community structures and inter-specific relationships further analyzed.

Bioretention systems offer advantages in controlling non-point source pollution from runoff rainwater. However, the systems frequently encounter challenges, including insufficient stability of nitrogen and phosphorus removal. Limited research has been performed on bioretention systems which integrate actual data from non-point source pollution cases for the quantitative and qualitative refinement of initial and non-initial rainwater. Moreover, the potential linkages between amended media and microbial communities in bioretention systems with the addition of novel functional filler have not been explored. In this study, a system for treating both initial and non-initial rainwater was established through measurements including iron-modified biochar (FeBC) packing and the optimization of the layer structures. In system treating initial rainwater, the systems loaded with FeBC maintained stable NH4+-N and NO3--N removal rates of over 95% and 80%, respectively under 12 rainfall simulation events. After a 10-day antecedent drying duration (ADD), the removal rates for NH4+-N and PO43--P remained above 78% and 85%. In systems designed to process non-initial rainwater, increasing the height of the transition layer effectively enhanced the NH4+-N removal stability. Meanwhile, increasing the height of the drainage layer could promote PO43--P removal rates to over 75%. The addition of FeBC facilitated the growth of certain denitrifiers improved overall NO3--N removal during successive rainfall events. The microbial communities may adapt to variations in the external environment by enhancing the synthesis of ribosome and the metabolism of pyrimidine and purine, further improving the stability of NH4+-N removal. This study provides a theoretical basis for the precise enhancement of nitrogen and phosphorus removal and the design of bioretention systems for differentiated treatment of rainwater, guiding their design and applications in different regions.

Nitrogen (N) cycling in rivers is particularly active and dynamic due to excess nutrient inputs worldwide. However, the multidimensional spatial patterns of the activity and community structure of N-cycling microorganisms in rivers remain unclear, limiting our understanding of river ecological functions, especially N removal capacity. Here, we measured the nitrification and denitrification rates and identified nitrifying and denitrifying microorganisms using high-throughput sequencing of archaeal amoA, bacterial amoA, nirK, and nirS genes in channel sediments, riparian rhizosphere soils, and riparian bulk soils of 30 N-polluted rivers across China. Results showed that in the lateral dimension, nitrification rates in sediments did not differ significantly from those in rhizosphere and bulk soils, but denitrification rates were higher in sediments than in bulk soils. However, the archaeal amoA gene abundance in sediments was considerably lower than that in rhizosphere and bulk soils, and bacterial amoA gene abundance in sediments was greater than that in rhizosphere soils. In the vertical dimension, both nitrification and denitrification rates in riparian bulk soils decreased with soil depth, and topsoils harbored more nitrifying and denitrifying microbes than subsoils. Denitrification but not nitrification rates increased with latitude and altitude but decreased with increasing mean annual temperature and precipitation. Overall, these results provide new insights into the multidimensional spatial patterns of river N cycling at a large scale, which is crucial to evaluating the N removal function of global rivers.