Fish diversity is often challenging due to its labor-intensive and time-consuming, particularly when it comes to detecting rare and endemic fishes in fast-flowing waters. Environmental DNA metabarcoding has emerged as a powerful tool that enables a comprehensive understanding of freshwater biodiversity. In this study, we applied eDNA metabarcoding to assess fish diversity and assembly patterns in a highly dynamic and complex freshwater ecosystem characterized by diverse rare and endemic fish species. Specifically, three extensive surveys were performed to characterize the structure and spatial variation of fish communities within the Yangtze River Upstream Nature Reserve of Rare and Endemic Fishes. A total of 166 fish species, spanning8 orders, 25 families and 99 genera, were identified. Seasonal variations in alpha diversity were found to be significant. Changes in species composition across different sample sites were primarily associated with the species turnover rather than nestedness across the three seasons. This finding suggested that the conservation efforts should be applied to the whole Nature Reserve rather than a specific area. Endemic fishes were found to be concentrated in the core areas of the reserve, which serve as critical habitats for rare and endemic fish species. Therefore, stronger protection measures should be implemented in these core areas. This study also demonstrated the feasibility of using eDNA technology to evaluate fish diversity in fast-flowing rivers, particularly when conducting large-scale, rapid monitoring over extensive spatial areas.

Eutrophication remains a major challenge for lakes, where phosphorus (P) control has been central to management efforts. Despite substantial reductions in surface P inputs, the role of groundwater-derived P remains insufficiently understood. Notably, groundwater P is predominantly present as dissolved inorganic phosphorus (DIP), the most bioavailable form of P. In this study, we used radon (222Rn) mass balance to quantify lake-groundwater exchange in Lake Taihu. Results showed a clear seasonal variation in lateral groundwater discharge, with an estimated flux of 0.70 & times; 108 m3 d-1 in the wet season and 0.18 & times; 108 m3 d-1 in the dry season. The results of variance tests and path analysis showed that groundwater level and dissolved oxygen concentration contributed in opposite directions, promoting and inhibiting the synthesis of groundwater DIP, respectively. In addition, pH and the proportion of clay-rich sediments promoted the enrichment of DIP during the wet and dry seasons. The annual loading of DIP from groundwater sources was estimated to be 2,304.40 (95%CI: 1821.88 to 2585.12) t yr-1, accounting for more than 84% of the total external DIP input, approximately 7 times greater than that from surface water. Notably, about 1,954.38 t yr-1 was retained in the lake, indicating long-term internal accumulation. These results highlight groundwater as a dominant and persistent source of bioavailable P and underscore the need to incorporate subsurface fluxes into lake nutrient budgets for effective eutrophication management.

Water-level fluctuations caused by hydropower operations have the strongest effects in littoral zones, where they interfere with fish reproduction. Lithophilous species, which adhere eggs to gravel surfaces in these littoral zones, face heightened mortality as hydropeaking alternately inundates and exposes their spawning substrates. However, quantitative evidence connecting the intensity of hydropeaking with ecological impact of freshwater fish is still limited. This study addresses this knowledge gap by quantifying hatching success in fishes under daily hydropeaking conditions.Field experiments were conducted to determine the effect of dehydration duration on egg-hatching success, followed by the development of a water-level-fluctuation-based method to evaluate adhesive-egg spawning grounds and identify ecological thresholds for reservoir operation. Based on the effect of dewatering on fertilized egg survival, 6-h continuous exposure was identified as the ecological threshold for air-exposure-induced egg mortality. The spawning-ground deactivation rate was then proposed, defined as the ratio of the deactivated area (≥6 h exposure) to the total potential spawning area. Finally, it is recommended that discharge variations during the reproductive period be limited to ≤2600 m3 s-1 and water-level fluctuations be restricted to <1.5 m, to ensure that the majority of spawning grounds maintain intact ecological functions. The study elucidates the ecological mechanisms governing lithophilous fish egg hatching under daily hydropeaking and proposes ecological thresholds for reservoir operation. Furthermore, it provides a quantitative framework for assessing how hydropeaking affects lithophilous spawning habitats.

Microplastics (MPs) are ubiquitous in freshwater environments owing to their widespread use and persistence. Understanding the factors governing the vertical sedimentation of MPs is crucial for assessing their ecological risk and developing effective pollution control strategies. This review systematically examines the intrinsic particle characteristics, environmental drivers, and biological mediators that influence the settling behavior of MPs in freshwater systems. The density, shape, and size of MPs are critical determinants of their buoyancy, hydrodynamic properties, and aggregation potential. Environmental conditions, including hydrodynamics, water physicochemical properties, and surrounding media, regulate MP transport and deposition through complex interactions. Biological processes such as biofouling, ingestion and egestion by aquatic organisms, and bioturbation further modify the vertical distribution of MPs. Microbial colonization and extracellular polymeric substance (EPS) production alter MP density and surface properties, facilitating aggregation and sedimentation. Aquatic plants, zooplankton, and fish influence MP redistribution via interception, uptake, and excretion. Benthic macroinvertebrate bioturbation affects MP burial and resuspension at the sediment-water interface. Despite recent progress, knowledge gaps persist in understanding the synergistic effects of multiple factors under realistic environmental conditions. Future research should focus on developing integrated aggregation models that incorporate biofilm growth, environmental factors, and hydrodynamics, as well as creating high-precision vertical-flux models. Addressing these challenges will enhance the accuracy of MP fate predictions and inform targeted risk assessments and pollution control strategies for freshwater ecosystems.

The dry season is a distinct hydrological period that alters river flow conditions and reshapes fish habitat distribution. Understanding the spatial distribution and characteristics of fish habitats is vital for supporting effective management and conservation plans of the river ecology. This study utilized the hydroacoustic, the fishing methods, and two-dimensional hydrodynamic simulation to describe the distribution characteristics of fish habitats in the upper reaches of the Yangtze River during the dry season. The results showed the composition of the fish was dominated by Cyprinidae, with Pelteobagrus vachelli, Squalidus argentatus, and Megalobrama pellegrini as the dominant species. Most individuals were small-sized. There were 16 fish aggregation areas, with the majority of the fish aggregating at the river's bottom. The largest fish aggregation area measured 41,020 m2, the water depth ranged from 12.1 m to 55 m, and the depth-average velocity ranged from 0.29 m/s to 1.33 m/s. The fish aggregation areas were separated into three types of fish habitats based on geomorphological and hydraulic features: rock barriers, deep pools, and confluences. Fish primarily used low-velocity areas and deep pools in the main waterway as shelter. These findings can help balance the development of the waterway and ecology conservation and provide guidance for river ecosystems with similar hydrological characteristics.

Fish often swim in structured group formations (fish schooling). Hydrodynamics is one of the key external factors that might affect an organism’s swimming behavior, along with other aspects such as food resources and predators. Most previous studies on collective behavior of schooling fish have been conducted in relatively simplified tanks, with few focusing on collective behavior in complex flow environments with obstacles. In this study, complex hydrodynamic environments were constructed by arranging staggered cobbles in the flume to investigate the collective behavior and upstream tactics of juvenile fish under different hydrodynamic conditions, including low- (0.4 Ucrit) and high-flow (0.8 Ucrit) conditions. The results indicate the following: (1) Under high-flow conditions, schooling fish tend to swim side-by-side with stronger cohesion when confronted with the impact of high flow velocity; (2) in low-flow conditions, schooling fish display increased polarization, evidenced by reduced bearing angles between neighboring and focal fish, signifying improved group coordination; and (3) in high-flow conditions, schooling fish use the sheltered area behind obstacles to go upstream, while in none-obstructed zone, migrating upstream is challenging. This study provides a new approach for studying fish swimming behavior in natural-like environments and offers insights and theoretical references for fish habitat restoration works.

Restoration of complex river ecosystems requires an understanding of the availability of habitat for populations and multi-scale choices. Habitat unit (HU) mapping classifies river habitats, aiding in establishing the connection between the physical and biological conditions of rivers. Mid-scale HU classification can effectively predict fish habitat utilization patterns and has been developed and applied in numerous small to medium-sized river classifications. However, the delimitation of habitat units (HUs) for large rivers remains in its preliminary stages. This study used a two-dimensional (2D) hydrodynamic numerical model to calculate hydrodynamic data and forms ecological expert opinions based on the swimming abilities of typical fish species and geomorphic characteristics. A HU mapping procedure was constructed to describe the mesohabitat heterogeneity of HUs in the fluctuating backwater areas of the upper Yangtze River. Results indicated: (1) Rich diversity of habitats in the investigated river segment. HUs exhibit evident patterns in their planar, longitudinal, and lateral distribution. (2) Discharge magnitude influences the stability of HUs, leading to transitions between HU types. During the storage and falling periods, no dominant HU was observed, whereas the fast channel unit dominates during flood period, resulting in a reduction of the preferred habitat of fish. (3) Field monitoring of fish communities validates the potential of HU mapping in describing habitat utilization, which HUs such as pools and riffles were favoured by fish, while fast channel and slackwater unit were actively avoided, confirming the efficacy of the procedure. This research holds significant implications for habitat restoration in river management.

This study investigates the impacts of hydrological regime alterations on fish community structure and ecological types in the Yibin section of the Yangtze River. Utilizing eDNA metabarcoding data from 2021, combined with historical records, the study offers a comprehensive analysis of fish community structure, structural evolution, and shifts in dominant species. The Indicators of Hydrologic Alteration-Range of Variability Approach method (IHA-RVA) was applied to calculate the degree of hydrologic alteration after the dam impoundment. Then, the Spearman correlation method is used to identify a number of key indicators driving shifts in species composition. Results indicate that fish species richness increased during the early impoundment phase of the Three Gorges Dam, corresponding with rising water levels. However, following the operation of the Xiangjiaba Dam, species richness began to decline. The traditionally dominant Cyprinidae family has diminished in prevalence, while taxonomic diversity and community complexity have increased. Rheophilic species remain dominant, though the proportion of limnophilic species has risen, now constituting nearly 50% of the total species composition. Altered hydrological conditions, particularly increased long-term dry season flow, have emerged as key drivers of changes in fish community structure. These findings provide crucial insights for the conservation of fish resources in the upper Yangtze River and the ecological management of the Jinsha River cascade dams.

Changes in fish habitat induced by dam construction in the downstream river have been well documented in an increasing body of literature. Fish habitats in the backwater zone of dams have undergone dramatic changes, which have still remained poorly understood. This study aims to provide a more complete understanding of fish habitat distribution and clarify fish habitat utilization patterns in the upstream dam in order to adopt effective remediation activities. This study was conducted in the Three Gorges Dam (TGD) as it is the world's largest installed capacity hydropower station, with a backwater zone length of about 660 km. Four major Chinese carp (FMCC) were selected as target fish species, and fish habitat distribution upstream of TGD was assessed by integrating the swimming ability and environmental preference of different fish species into their critical life stages. Assessment results at different life stages of the target fish species showed that June was a critical period for spawning activities FMCC in the fluctuating backwater zone of TGD. The riverbank was an important feeding habitat for FMCC, and the high-velocity flow in the centre of the channel exceeded the limits of the swimming ability of target fish. The fish habitat distribution in the fluctuating backwater zone of TGD was heavily affected by runoff from April to September and by dam operation from October to March of the next year. This work provide valuable information about river conservation and management in the upstream of TGD.

近年来,长江上游大量集中采砂造成河床地貌大幅改变,进而改变了该河段原有的水流条件.采砂可能会改变鱼类生境,但采砂后的河道鱼类生境现状尚不明确.分别在2019年1月以及2020年6月、12月对洛碛河段进行了3次水声学调查,探明了洛碛河段的鱼群生境现状.基于采砂前后实测地形,分析了洛碛河段采砂前后的地形、流速、水深等参数变化.通过构建鱼类栖息地模型,评价了采砂坑内的鱼类生境状况.结果表明:3次调查均在采砂坑深潭监测到鱼群,说明采砂坑深潭已逐渐形成可供鱼类栖息的新生境;鱼群密度最大值出现在2020年6月,为170.0 ind./1000 m3,最小值出现在2019年1月,为81.8 ind./1000 m3;2020年6月监测到的鱼类平均体长最大,达到了17.2 cm,最小平均体长出现在2019年1月,为14.8 cm;洛碛河段采砂后形成的深潭的水深大于20 m,流速低于1.2 m/s;栖息地评价模型显示,采砂坑在洪水期为鱼类提供了缓流适宜生境.

The upstream of the Yangtze River has abundant fish resources and is home to the four major Chinese carps (FMCCs), which are economically important fish in the Yangtze River that reside in the fluctuating backwater area (FBA) of the Three Gorges Reservoir (TGR). Their spawning habitat has been influenced by the Three Gorges Dam (TGD) operations. This study investigated the effects of dam operation on spawning habitat suitability and the potential spawning sites of FMCCs, using a physical habitat suitability model combined with a two-dimensional (2D) hydrodynamic numerical model. The habitat model was verified by field measurements of the fish egg density in the FBA, illustrating the capability of predicting the spawning availability of FMCC by comparing the habitat suitability index (HSI) distribution and the weighted usable area (WUA) with the simulation scenarios before and after the TGD construction. The habitat model was then applied to evaluate the effects of TGD operation on spawning habitat suitability and potential spawning grounds in the FBA. These results implied that the TGD operation had negative effects on spawning habitat suitability and produced temporal changes in the available WUA in the FBA. The predictions revealed that impoundment of the TGD decreased the suitability of the hydrodynamic factors. The main spawning period was delayed from April to May, and the variation in the water level induced by the reservoir operation scheme was likely the main factor. The HSI distribution in the FBA showed that the potential spawning grounds decreased to one site located in the Mudong-Luoqi River section. Furthermore, there was a rapid reduction in WUAs, accounting for approximately 18% of that seen in 2001 (before the TGD operation). The results of this study can aid in the management of the spawning grounds of FMCCs in the TGR and provide efficient methods for improving spawning habitat suitability, such as adjusting the pool level of the TGD during the falling stage.

The waterway from Chongqing to Yibin in the upper Yangtze River provides strong support for the socioeconomic development in Western China, while it also has a negative influence on the river ecosystem, such as river fragmentation, wetland disconnectivity, fish habitat, and biodiversity. Accurately characterizing the exploitation, economic efficiency, and ecological pressure of the waterway from Chongqing to Yibin is of great significance for the further development of the upper Yangtze River. In this study, based on statistical data from the National Bureau of Statistics of China, the waterway development, economic benefits, and ecological pressure of the waterway from Chongqing to Yibin were evaluated. The sustainability index (SI) was introduced to comprehensively assess the sustainability of waterways using the hierarchical model, which integrates the exploitation ratio (ER), economic efficiency index (EEI), and ecological pressure index (EPI). A relatively low ER (41.04%), a low consistency index (CI, 0.31), a high EEI (9037.18 USA$/gha), and a low EPI (0.33) yielded an SI of 0.70, indicating that the waterway from Chongqing to Yibin in the upper Yangtze River is in a sustainable development stage. Compared with the typical global Golden Inland Waterways, the waterway from Chongqing to Yibin still has great development potential under the prioritizing ecological protection.

Blue water and green water are of great importance for food production and maintenance of ecosystem. Blue/green water flow on landscape scale and its seasonal variation remains unclear. This study attempt to analyze characteristics of green and blue water flow on landscape scale.In this study, FLEX-Topo model was adopted. And upstream of Heihe River Basin (UHB) was selected as the study area, which was divided through topographic information into four landscapes, such as riparian area, grass hillslope, forest hillslope and bare soil/rock. Based on the analysis of the simulation during the period of 1979-2015, characteristics of green and blue water flow of the four landscapes was presented. The results showed that (1) FLEX-Topo proved to be an efficient approach for catchment hydrological process simulation as well as for green and blue water study; (2) Annual green water flow(GWF) of riparian area was 325.19 mm/a while its blue water flow(BWF) was 151.92 mm/a. GWF and BWF of grass hillslope was 270.50 mm/a and 199.91 mm/a, respectively; GWF of forest hillslope was 424.75 mm/a which is much greater than its BWF (42.11mm/a);as for bare soil/rock, GWF (227.96 mm/a) was a litter smaller than BWF (295.32 mm/a). From the results, its can be concluded that:(1)on annual scale, majority of precipitation and water reserve in riparian area, grass hillslope, forest hillslope became green water flow, especially in forest hillslope. More than half of precipitation in bare soil/rock turn to blue water flow; (3) seasonal variation of both green water flow and green water flow of the four hydrological landscapes synchronized with precipitation.