Lotic Research Articles

Forest restoration improves habitat and water quality in tropical streams: A multiscale landscape assessment.

Forest restoration has been a common practice to safeguard water quality and stream health but it is unclear to which extent and pace forest restoration recovers stream ecosystem structure and functions. Also, stream health might be affected by the forest restoration type and the quality of the interventions. Here, we sought to evaluate the recovery of stream habitat and water quality through forest restoration in catchments dominated by pasturelands, and explored the relationship between landscape structure and stream ecosystem recovery. We sampled a total of 30 catchments during the dry season of 2023, covering six different classes (five catchment per class), based on the type and extent of forest cover: (i) all catchment area covered by native forest remnants, (ii) catchments mostly covered by old (26-37years) restored forests, (iii) catchments mostly covered by young (5-25years) restored forests, (iv) catchments in a pasture matrix with forest remnants around springs, (v) catchments in a pasture matrix with riparian buffers covered by pioneer vegetation (mostly herbs and shrubs), and (vi) catchments mostly covered by pastures. Data on stream water (e.g. temperature, nutrients and sediments) and habitat (e.g. substrate heterogeneity and volume of wood debris) quality were sampled and landscape metrics calculated by GIS at reach, riparian and catchment scales. In catchments covered by remnant and old restored forests, the water temperature and nutrient concentration were lower, and instream leaf banks were higher, whereas the number of large wood debris was higher in forest remnant catchments. Water temperature and ammoniacal-N correlated with Normalized Difference Vegetation Index (NDVI) at reach scale. Substrate heterogeneity and volume of woody debris correlated strongly with NDVI and proportion of forest over 38years old at the catchment scale. This outcome shows the potential application of forest structure (NDVI) and age for monitoring the stream ecosystem benefits of forest restoration. Overall, we found a gradient of recovery of both water and habitat quality progressing from more degraded (pasture-dominated catchments, pasture-dominated with forest remnants around springs, pasture-dominated with pioneer vegetation in riparian buffers) to more conserved catchments (young forest restoration, old forest restoration and old-growth conserved forests). In conclusion, the Atlantic Forest restoration contributed to improve water and habitat quality in streams, however these benefits were dependent on forest restoration age, extension and location.

Three-Dimensional Particle Tracking Velocimetry Investigation of Flow Dynamics Around Simplified Stones at Low Submergence: Implications for Instream Habitat

Shallow waterways such as rapids, tributaries and smaller streams can have important ecological functions in both free-flowing and regulated rivers. As more intermittent renewable energy is introduced to the energy system to reduce CO2 emissions, the operational conditions of hydropower plants are changing. This implies various flow scenarios that can lead to more locations with shallow depths and larger variations in water levels and velocities, resulting in increased impact on the riverine ecosystem. Accurate predictions of these impacts require an understanding of the flow dynamics near large roughness elements such as boulders or trees in shallow river regions. This study uniquely investigates the effect of relative submergence, i.e., water depth relative to boulder size, on the flow field, turbulence, and potential fish habitats around idealized stone shapes (hemispheres) in shallow open channel flow using time-resolved 3D particle tracking velocimetry. The results indicate that varying relative submergence significantly affects recirculation zones, velocity and vorticity distribution, as well as turbulent kinetic energy. Notably, larger regions of lower velocity downstream of the roughness elements were generated at lower submergences, which might be favorable for fish energy conservation. Valuable insights into ecohydraulic engineering and habitat restoration in shallow waterways can be gained by understanding the fundamental flow mechanisms at low submergence for the flow around large roughness elements.

Sources, Water Quality, and Potential Risk Assessment of Heavy Metal Contamination in Typical Megacity River: Insights from Monte Carlo Simulation

Due to the intense human activities and rapid development of economy, dissolved heavy metals (DHMs) pose a significant threat to urban river ecosystems. Therefore, the distribution, sources, and potential risks of DHMs in the Chaobai River (typical urban river) were investigated via ICP-MS in detail. Results revealed considerable spatial heterogeneity of heavy metals with various concentrations from the upper to lower reach. Principal component analysis (PCA) revealed that V, Ni, As, Mo, and Pb mainly originated from a mixing process of industrial input and natural process, Cr and Cu were mainly derived from urban activities, and Zn was mainly influenced by agriculture activities. Furthermore, land use types within the buffer zone near sampling points were innovatively analyzed, revealing strong correlations between DHMs and regional land use patterns. Monte Carlo simulations were employed to assess the differentiated non-carcinogenic and carcinogenic risks associated with DHMs across four age groups. This study provided scientific references for the sustainable management of urban rivers and aquatic systems in such a megacity region.

The Optimization of River Network Water Pollution Control Based on Hydrological Connectivity Measures

Urbanization, driven by socio-economic development, has significantly impacted river ecosystems, particularly in plain city regions, leading to disruptions in river network structure and function. These changes have exacerbated hydrological fluctuations and ecological degradation. This study focuses on the central urban area of Changzhou using a MIKE11 model to assess the effects of four hydrological connectivity strategies—water diversion scheduling, river connectivity, river dredging, and sluice connectivity—across 13 different scenarios. The results show that water diversion, river dredging, and sluice connectivity scenarios provide the greatest improvements in water environmental capacity, with maximum increases of 54.76%, 41.97%, and 25.62%, respectively. The spatial distribution of improvements reveals significant regional variation, with some areas, particularly in Tianning and Zhonglou districts, experiencing declines in environmental capacity under sluice diversion and river-connectivity scenarios. In addition, the Lao Zaogang River is identified as crucial for improving the overall water quality in the network. Based on a multi-objective evaluation, combining environmental and economic factors, the study recommends optimizing water diversion scheduling at sluices (Weicun, Zaogang, and Xiaohe) with flow rates between 20–40 m3/s, enhancing connectivity at key river hubs, and focusing management efforts on the Lao Zaogang and Xinmeng rivers to strengthen hydrological and water quality linkages within the network.

Urban River Policies: Multilevel Analysis and Community Engagement in Curitiba.

This study investigates urban river policies, emphasizing the gaps in understanding the interactions between riverine communities and governance systems. Using empirical and theoretical methods, the research applies multivariate analysis and Structural Equation Modeling (SEM) to data from a representative sample of 1740 residents of Curitiba. The study maintains a 95% confidence level with a ±2.4% margin of error. Latent social, governmental, responsive, and environmental engagement variables are analyzed, highlighting the complexity of urban policies and the necessity for adaptable strategies. The findings suggest that greater social and governmental engagement correlates with more favorable perceptions of river quality. In contrast, responsive engagement exhibits a weaker relationship, while environmental engagement underscores ongoing challenges. The research introduces a theoretical framework supported by an analytical model, advocating for urban policies that account for contextual specificities and encourage collaborative engagement between governments and communities. The study concludes that implementing comprehensive strategies within this integrated framework is crucial for sustaining and enhancing urban river ecosystems, as demonstrated by the case of the Belém River in Curitiba.

Monitoring of Temporal and Spatial Variation of Water Quality in Agricultural Reservoir During Summer

ABSTRACTThis study investigated the spatiotemporal dynamics of water quality in a small‐scale agricultural reservoir, the Bangok Reservoir, during the summer. The study is motivated by the increasing importance of preserving river ecosystems, ensuring irrigation water quality and expanding agricultural water use. Seven water quality parameters were monitored at multiple points from July to October 2020, and Spearman correlation analysis was conducted to understand their spatial relationships. The results revealed that the Bangok Reservoir had reached a hypertrophic state, indicating excessive nutrient levels. While heavy rainfall and dilution effects posed challenges for accurate analysis, significant correlations were observed between chlorophyll‐a and total phosphorus, as well as between dissolved oxygen and pH. The trophic state index of Korea (TSIKO) indicated conditions favourable for algal growth due to phosphorus abundance. Higher concentrations of pollutants were found upstream, which was likely attributed to the inflow of livestock manure during rainfall and shallow water mixing. The study recommends the implementation of algae removal and reduction facilities in the upstream areas of the reservoir. These findings provide essential data for ongoing research on water quality in small‐scale agricultural reservoirs and emphasize the importance of continuous monitoring for better environmental management.

Global patterns and drivers of coupling between anammox and denitrification processes across inland aquatic ecosystems

Denitrification and anammox collectively drive nitrogen loss from aquatic ecosystems, yet their global patterns and interactions remain unclear. To fill this gap in knowledge, we compiled a global dataset on anammox and denitrification, encompassing river, lake, wetland, and estuary ecosystems and comprising 2539 observations from 136 peer-reviewed papers. Here, we show that aquatic ecosystems with abundant denitrifying bacteria tend to have abundant anammox bacteria, but the abundance of anammox bacteria is lower than that of denitrifying bacteria. Importantly, we observed that hotspots for denitrification in aquatic ecosystems were also hotspots for anammox, and we explained the variation in anammox (21.55 (95% CI: 8.21–58.90) nmol-N g-1 day-1) and denitrification rates (171.76 (95% CI: 65.40–519.25) nmol-N g-1 day-1) across aquatic ecosystems. These results highlight that anammox should be included in models for accurate nitrogen budget assessment in aquatic ecosystems on a global scale, especially in the context of future climate warming.

Factors influencing the presence and relative abundance of Didymosphenia geminata: impact on the composition of diatom assemblages over a broad latitudinal gradient in Chilean rivers

This study explored the abiotic factors influencing Didymosphenia geminata blooms and the combined effect of D. geminata and abiotic factors on local diatom assemblages in 13 Chilean watersheds across a broad latitudinal range (37°–47° S). To understand the multi-scale drivers, we used generalized linear mixed-effects models to assess the abiotic factors most influential on D. geminata presence and relative abundance. We then explored the effects of D. geminata and abiotic factors on the α- and β-diversity of diatom assemblages using generalized linear models and permutational analyses of variance, respectively. Three watersheds lacked D. geminata and another three had low relative abundance. Seasonally, the relative abundance of D. geminata was significantly higher in spring than in autumn. We found positive correlations between elevation, river order, pH and D. geminata presence and relative abundance, while water temperature was negatively correlated with D. geminata relative abundance. Diversity indices were significantly influenced by watershed and to a lower extent by the presence of D. geminata. We found significant spatial (i.e. between watersheds along a south–north latitudinal gradient) and temporal (i.e. between seasons) variations in β-diversity of diatom assemblages. Moreover, increasing relative abundance of D. geminata in the assemblage was correlated with lower diatom diversity. In addition, the compositions of diatom assemblages with D. geminata were more alike and less variable than those in non-invaded sites. This study is a first attempt to understand the multi-scale drivers of diatom assemblages in Chilean lotic ecosystems.

Lightweight Multi-Scale Network for Segmentation of Riverbank Sand Mining Area in Satellite Images

Riverbank sand overexploitation is threatening the ecology and shipping safety of rivers. The rapid identification of riverbank sand mining areas from satellite images is extremely important for ecological protection and shipping management. Image segmentation methods based on AI technology are gradually becoming popular in academia and industry. However, traditional neural networks have complex structures and numerous parameters, making them unsuitable for meeting the needs of rapid extraction in large areas. To improve efficiency, we proposed a lightweight multi-scale network (LMS Net), which uses a lightweight multi-scale (LMS) block in both the encoder and decoder. The lightweight multi-scale block combines parallel computing and depthwise convolution to reduce the parameters of the network and enhance its multi-scale extraction ability. We created a benchmark dataset to validate the accuracy and efficiency improvements of our network. Comparative experiments and ablation studies proved that our LMS Net is more efficient than traditional methods like Unet and more accurate than typical lightweight methods like Ghostnet and other more recent methods. The performance of our proposed network meets the requirements of river management.

Active biomonitoring of stream ecosystems: untargeted metabolomic and proteomic responses and free radical scavenging activities in mussels.

Many contaminants from scattered sources constantly endanger streams that flow through heavily inhabited areas, commercial districts, and industrial hubs. The responses of transplanted mussels in streams in active biomonitoring programs will reflect the dynamics of environmental stream conditions. This study evaluated the untargeted metabolomic and proteomic responses and free radical scavenging activities of transplanted mussels Sinanodonta woodiana in the Winongo Stream at three stations (S1, S2, S3) representing different pollution levels: low (S1), high (S2), and moderate (S3). The investigation examined untargeted metabolomic and proteomic responses in the gills and 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) activities in the gills, mantle, and digestive glands. Metabolomic analysis revealed a clear separation between mussel responses from the three stations after 28 days of exposure, with specific metabolites responding to different pollution levels. Proteomic analysis identified β-Actin protein in all stations. The β-Actin protein sequence of unexposed mussels had coverage of 17%, and increased to 23% at S1 on day 28 and 34% at S2 and S3 on day 28. All tissues showed increased DPPH and ABTS activities from day 3 to day 28, mainly in stations S2 and S3. These findings underscore the impact of pollution levels on the metabolomic and proteomic responses of S. woodiana and the importance of these discoveries as early indicators (biomarkers) of long-term aquatic environmental problems. In the face of current environmental challenges, this research raises concerns about the health of water bodies. It underscores the importance of developing robust, standardized, and dependable analytical techniques for monitoring the health of aquatic environments.

The impacts of alien species on river bioassessment.

The extent of alien taxa impacts on river ecosystem health is unclear, but their frequency continues to rise. We investigated 1) the prevalence of including alien taxa in common bioindicators used in river bioassessment, 2) the effect of alien taxa on the richness and abundance of natives, and 3) whether including alien taxa in bioassessment tools increased their sensitivity to river degradation. In the 17 countries analyzed fish represented the greatest number of alien species (1726), followed by macrophytes (925), macroinvertebrates (556), and diatoms (7). Yet, alien species are only distinguished from natives in some fish and macrophyte indices. In addition, the analyses of 8 databases with fish, macroinvertebrate, or macrophyte data showed that abundance of alien taxa was associated with different stressors and pressures resulting in river degradation, and had a significant effect on native community composition. When alien species were accounted for, there was a strong negative correlation between the values of a fish index with alien richness and abundance while when alien taxa was not or only partially considered the results varied. Thus, we recommend: 1) Include specific metrics for alien species in biological quality indices. 2) Increase the investigation of alien taxa of small organisms (e.g. diatoms, small benthic invertebrates). 3) Eliminate sites with confirmed biological invasions for use as reference sites. 4) Remove alien from calculations of total richness and diversity. 5) Identify to the species level in biomonitoring programs. 6) Avoid legislation and management that protect alien species. 7) Encourage behaviors that prevent alien invasions of aquatic biota.

Season and Flow Drive Productivity of a Regulated River

Flow regimes of river ecosystems worldwide have undergone substantial changes because of water resource development, altering the way in which organic matter is generated and cycled throughout entire river catchments. Flow–ecology studies have focused on structural variables measured at small spatial scales. This creates a challenging mismatch when applying adaptive flow management for ecosystem functioning at a catchment or regional scale. Here, we sought to inform flow management by evaluating the drivers of ecosystem metabolism in a regulated river and assessing our ability to predict metabolism at unmonitored locations. We estimated rates of ecosystem metabolism from high-frequency monitoring of dissolved oxygen concentration at eight sites on the Lachlan River of Australia’s Murray–Darling Basin. We then applied a spatio-temporal stream network model to predict metabolism at unmonitored locations using only remotely sensed and gauging station predictor variables. Gross primary productivity (GPP) was higher at sites with lower mean annual discharge, and strong seasonal patterns in rates of productivity tended to be disrupted by rising flows. Similarly, ecosystem respiration (ER) was higher at sites with lower mean annual discharge and lower annual flow variation, but increased slightly in response to higher daily flows. Predictions at validation sites were generally accurate, albeit with substantial site-to-site variation. Our results suggest that flow changes may have altered metabolic rates from conditions prior to water abstraction and dam construction. These findings will assist in managing flows for ecosystem function outcomes and support extrapolation from monitored sites to the broad scales required for evaluating catchment-scale outcomes of river management.

Accelerated River Meander Migration on the Tibetan Plateau Caused by Permafrost Thaw

AbstractThe migration of rivers in permafrost landscapes has critical implications for riverine infrastructure, ecosystem stability, and carbon cycling, yet its magnitude and underlying mechanisms remain poorly understood. Here, we leverage four decadal satellite imagery, hydrological observations, and permafrost modeling to investigate meander migration dynamics on the Tibetan Plateau. Our data show that the migration rates of permafrost rivers have increased by 34.6% from 1987 to 2022, in response to the combined effects of increased discharge, riverbank destabilization driven by ground ice melt and extended thawing days (increased by 35 days). In contrast, rivers flowing across seasonally frozen ground exhibited a decline in migration rate by 11.1%, driven by vegetation greening and riverbank stabilization. In a future warming climate for the Tibetan Plateau, the migration rates of permafrost rivers are anticipated to further accelerate, potentially threatening riverine infrastructure safety and aquatic ecosystems, and intensifying the permafrost carbon cycle.

Dynamics of a Reaction–Diffusion–Advection System From River Ecology With Inflow

ABSTRACTThis paper is to study the population dynamics of a single species model and a two‐species competition model from river ecology with inflow. One interesting feature in these models is that species can flow into the river through the upstream end due to advective movement while both diffusive and advective movements will cause population loss at the downstream end. We first determine necessary and sufficient conditions for persistence of a single species, in terms of the critical habitat size and the critical advection rate. For the competition model, we investigate the joint effects of advection rates, interspecific competition intensities, and boundary conditions on global dynamics of the system. It shows that the strategy of smaller advection is beneficial for species to survive. Our results partially extend the previous works.

Biodiversity spatial distribution of benthic macroinvertebrate assemblages is influenced by anthropogenic disturbances at multiple spatial extents.

Understanding the patterns and mechanisms of biodiversity and its organization in space is essential for developing effective conservation strategies. Zeta diversity is an index of how taxa are shared by several sites, providing information on how ecological filters, including anthropogenic disturbances, influence biodiversity distribution. This study documents how anthropogenic disturbances at multiple spatial extents affect the spatial variation of benthic macroinvertebrate assemblages in lotic ecosystems. To test the relation between zeta diversity and anthropogenic disturbances, we used three disturbance metrics. (a) For in-stream disturbances, we used the percentage of fine sediment in the substrate (PCT_FN). (b) For local/riparian disturbances, we used the Local Disturbance Index (LDI). (c) For catchment disturbances we used the Catchment Disturbance Index (CDI). Our results showed that differing anthropogenic disturbances were differently important for spatial biodiversity variation in benthic macroinvertebrate assemblages. Relatively rarer taxa were usually more susceptible to in-stream and local/riparian-scale disturbances or local environmental filters. On the other hand, relatively common taxa were usually more related to catchment-scale disturbances or landscape resistance to dispersal. These results indicate that conservation efforts to maintain headwater ecosystem biodiversity must incorporate multiple spatial extents because relatively rare and relatively common taxa appear to be affected to different degrees by different anthropogenic disturbances at different spatial extents.

Multiple stressor effects act primarily on microbial leaf decomposers in stream mesocosms.

At the global level, stream ecosystems are influenced by multiple anthropogenic stressors such as eutrophication, habitat deterioration, and water scarcity. Multiple stressor effects on stream biodiversity are well documented, but multiple stressor effects on stream ecosystem processes have received only limited attention. We conducted one mesocosm (stream channel) and one microcosm (feeding trial) experiment to study how combinations of reduced flow, increased nutrient concentrations, and increased fine sediment coverage would influence fungal and macroinvertebrate decomposer assemblages and their active contribution to leaf decomposition. In the stream channels, increased fine sediment coverage significantly reduced fungal biomass, occurrence frequencies of most aquatic hyphomycete species, and microbial leaf decomposition rates compared to untreated controls. Macroinvertebrate-induced leaf decomposition rates were mainly correlated to total fungal biomass and community composition. Neither increased nutrient concentrations, nor reduced flow conditions significantly influenced leaf decomposer communities or decomposition rates. The feeding trials revealed significantly reduced leaf consumption in the freshwater amphipod Gammarus pulex when feeding on leaf material from treatments with increased fine sediment coverage in the mesocosm experiment. When offered a food choice between sterile, unconditioned leaf material and leaf material from treatments with increased fine sediment coverage, G. pulex foraged mainly on sterile material. This study showed that increased fine sediment coverage can alter the flux of energy and material in the detrital food chain through bottom-up regulation of leaf conditioning by fungal decomposers. Our results suggest that increasing attention should be given to mitigating fine sediment transport and deposition in stream systems to preserve ecosystem functioning within the detrital food chain.

Distinct response of nitrogen metabolism to exogenous cadmium (Cd) in river sediments with and without Cd contamination history.

The role of metal resistance on nitrogen metabolism function and community resilience against Cd is important for elucidating the evolutionary dynamics of key ecological functions in river ecosystems. In this study, the response of nitrogen transforming function to Cd exposure in river sediments from the Yangtze River Basin with varying levels of heavy metal contamination history (Cd-contaminated and Cd-free sediments) was compared to understand how Cd influenced nitrogen metabolism under varying metal resistance conditions. The results showed that chronic and persistent Cd pollution of sediments caused an elevation of transport efflux metal resistance genes (MRGs) and a reduction in the uptake MRGs, leading to a stronger tolerance to Cd for Cd-contaminated sediment than Cd-free ones. Specifically, denitrification, anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA) respectively responded to Cd through different mechanisms. Exogenous Cd (5-100 mg kg-1) influenced denitrification rates (-70 %-100 % deviation to control group) by regulating key genera (Thiobacillus, Magnetospirillum, Sideroxydans etc.) and gene clusters for denitrification. Both adaptive nature of anammox bacteria and co-regulation of key genera (Candidatus_Scalindua, Candidatus_Jettenia, Planctomyces etc.) and gene hzsA were drivers of differential responses in sediments from various contamination history. Environmental factors rather than contamination history, key genera or genes were probably critical ones determining Cd-resistance in DNRA, being more tolerant to Cd in sediments with higher TOC and NH4+. Stimulation of N2O reduction process (genera Gemmatimonas and Gemmatirosa and genes nosZ) in Cd-contaminated sediments by exogenous Cd lowered N2O emission risk, whereas the reverse was true for Cd-free sediments. These results enrich our understanding about the linkages among MRGs and nitrogen reduction functions in river.

Longitudinal metagenomic analysis on antibiotic resistome, mobilome, and microbiome of river ecosystems in a sub-tropical metropolitan city.

Rivers play an important role as reservoirs and sinks for antibiotic resistance genes (ARGs). However, it remains underexplored for the resistome and associated mobilome in river ecosystems, and hosts of riverine ARGs particularly the pathogenic ones are rarely studied. This study for the first time conducted a longitudinal metagenomic analysis to unveil the resistome, mobilome, and microbiome in river water, by collecting samples from 16 rivers in Hong Kong over a three-year period and using both short-read and long-read sequencing. Results revealed that aminoglycoside, bacitracin, β-lactam, macrolide lincosamide-streptogramin, and sulfonamide were the predominant ARG types in the river water samples. Riverine ARGs exhibited high spatial variations in abundance and diversity. Environmental factors such as fecal coliform count, Escherichia coli count, 5-day biochemical oxygen demand (BOD5), dissolved oxygen (DO), and total organic carbon (TOC) had a significant correlation to the absolute concentrations of ARGs. Nanopore sequencing was used to reveal the physical genetic linkage of mobile genetic elements (MGEs) with ARGs in river water samples. The results showed that qacEdelta, transposase, integrase, and Tn916 had a high prevalence in ARG-carrying long reads. Host tracking using ARG-carrying reads identified 23 pathogenic bacteria species that harbored ARGs. Some ARGs were shared by different bacterial groups. This study presented a nuanced insight of resistome in river water by a longitudinal metagenomic analysis and deepened our understanding of common and divergent riverine antimicrobial resistant risk across the regional patterns.

Does what we find depend on how we sample? Measured streambed microplastic concentrations can be affected by the choice of sampling method.

Microplastics (MPs) are prevalent in rivers worldwide and can adversely impact riverine ecosystems. To sample for MPs in streambeds, a variety of different sampling techniques is applied, including (i) scooping, (ii) coring, (iii) freeze coring, (iv) resuspension method, and (v) piezometer sampling. These common sampling techniques capture different parts of the streambed and different sampling volumes. However, the resulting MP concentrations are usually reported without discussing how the sampling method may affect MP recovery, thus limiting rigorous inter-study comparisons. To better understand the impact of these techniques on MP recovery in different streambed environments, we conducted mesocosm experiments. Two mesocosms were filled with sediment of different composition and spiked with a known concentration of polyamide (PA) fragments prior to employing the various sampling techniques. MP recovery rates from scooping, coring and freeze coring varied by a factor of two compared to the input concentration, while resuspension and piezometer techniques overestimated PA recovery by three to nine times. Furthermore, variations in recovered PA particle size distributions emphasize the importance of selecting methods based on research objectives and streambed characteristics. Our results underline the need for a critical evaluation of reported streambed microplastic concentrations with a focus on the chosen sampling technique.

Effects of reduced flow gradient on benthic biofilm communities' ecological network and community assembly.

The intensification of human activities has led to flow reduction and cut-off in most global rivers, seriously affecting riverine organisms and the biogeochemical processes. As key indicators of river ecosystems' structure and function, benthic biofilms play a critical role in driving primary production and material cycling in rivers. This research aimed to investigate the characteristics of microbial communities' complexity and stability during river flow reduction. Benthic biofilms were grown in artificial channels and subjected to eight gradients of flow reduction (represented by flow velocity from 0.4 to 110cm/s). Biofilms' biodiversity, ecological networks and community assembly of bacteria, fungi and algae were investigated by high-throughput sequencing. Results showed significant differences in community composition and structure under different flow conditions. The eight flow gradients' microbial communities were divided into three groups: low, medium and high flows. The flow reduction led to significant decreases in bacterial and fungal communities' Chao1 index. Low flow conditions enriched the bacterial phyla Oxyphotobacteria, Alphaproteobacteria and Mollicutes, but significantly decreased the fungal phylum Chytridiomycota. Lowering flow reduced the fungal network's number of nodes and increased the algal network's number of edges. Cross-domain interactions network analysis showed a gradual increase in node and edge numbers with decreasing flow, while decreasing average path length. The neutral model predicted stochastic processes primarily drove biofilm community assembly, and that model's explanations decreased as the flow gradient decreased. The null model analysis revealed diffusion limitation as the most common stochastic ecological process for bacterial and algal communities, with reduced flow reducing heterogeneous selection and increasing diffusion-limited processes. This study provides an in-depth analysis of flow reduction's effects on biofilm communities' ecological networks and community assembly.