Middle Yangtze Research Articles

How does the green efficiency of urban land use evolve in the urban agglomeration of China's middle Yangtze river?

AbstractThe urban agglomeration in the middle reaches of the Yangtze river (MYRUA) is the second largest national urban agglomeration in China, with an excellent ecological foundation. As the construction of urban agglomerations accelerated towards both extension and connotation, land prices of various cities rose, and the contradiction in the demand and utilization of land resources became acute. Therefore, exploring the spatiotemporal evolution and driving mechanisms of land green utilization efficiency within MYRUA had positive significance. This article focused on 28 prefecture‐level cities in the MYRUA and measured their urban land green use efficiency (ULGUE) from 2006 to 2018 using the super slack‐based measure model with undesirable output (Super‐SBM‐U). Then, the improved Fixed Malmquist–Luenberger index was developed to improve the accuracy of dynamic efficiency analysis. Further, the internal and external driving factors were explored using the index decomposition and the spatial error model with fixed effects. The results showed that the land use mode of the urban agglomeration in the middle reaches of the Yangtze River was relatively green, but efficiency gaps still existed. Although the green total factor productivity of the whole cluster has improved by 3.30% a year on average, the excessive pursuit of technological innovation has led to the stagnation of urban land management in various cities, and there are numerous inherent contradictions in urban development. But at the same time, feasible economic development directions, industrial upgrading, high‐quality employees, land marketization, and positive environmental behaviors are conducive to the green and sustainable development of ULGUE. This article depicted the internal land use characteristics of the MYRUA from the perspective of spatiotemporal evolution and efficiency, providing a theoretical reference for the green development and coordinated management of urban agglomeration land.

Characteristics of urban expansion in megacities and its impact on water-related ecosystem services: A comparative study of Chengdu and Wuhan, China

The megacity expansion that continues to occur in developing countries is an unavoidable trend at this stage, yet there exists a dearth of knowledge regarding the comprehensive impacts of urban expansion on water-related ecosystem services (WESs) within these areas. Taking Chengdu and Wuhan, two megacities in the upper and middle Yangtze River Economic Belt in China as the study area, this study first compare the magnitude, rate, spatial divergence, and the mode of urban expansion; then evaluate its impact on three typical types of WESs, i.e., water yield (WY), water purification (WP), and habitat quality (HQ) using spatial econometric models and bivariate Moran’s I. The research findings show that Chengdu’s suburbs tend to experience edge-based expansion along rivers, while Wuhan’s city center also spreads outward in edge-based from the river junctions of the Yangtze and Han rivers. Urban expansion negatively impacts all three types of WESs, and its effect in Chengdu is greater than in Wuhan. WY in the suburbs of Wuhan is severely impacted by urban expansion, while WP in the suburbs of Chengdu is as well. HQ in Chengdu’s city center suffers the largest negative impact. The study also reveals a spatial spillover effect of urban expansion, with extensive negative externalities on all WESs in the city centers of both cities and positive externalities on WY and WP in the southwestern suburbs of Chengdu. Based on these results, we suggest that megacities in developing countries should focus on protecting HQ in the city center and limiting cross-regional edge-based expansion in suburbs. Attention should be paid to the water-ecological protection of the suburb rivers. City managers should consider ecological zoning geared toward the conservation of WESs to address the adverse impacts of expansion.

Changes in riverbed morphology in the middle Yangtze River (Yichang–Chenglingji) after the construction of the Three Gorges Dam

ABSTRACTThe present study investigates the impact of the Three Gorges Dam (TGD) construction on riverbed morphology in the middle Yangtze River (YR). We analyse data collected on sand wave morphology, shallow surface topography, and section morphology after the completion of the dam. Specifically, we examine the spatial distribution characteristics of sand wave morphology and the riverbed evolution process in the Yicheng reach. Results reveal significant changes in the morphology of sand waves in the middle reach of the YR, with wavelengths ranging from 5.2124.15 to 5.0915.33 m in the upper and lower Jingjiang River, respectively. Additionally, sediment incipient velocity was found to be smaller than that of the sand wave, making it difficult for the sand wave to start as the bedload continued to coarsen. After impoundment, the riverbed is strongly scoured, the undercutting is serious, and the bed morphology changes dramatically.

Intensifying drought of Poyang Lake and potential recovery approaches in the dammed middle Yangtze River catchment

Study regionThe middle Yangtze River-Poyang Lake-catchment, China. Study focusThe natural hydrological regime (HR) of the study area was altered by extreme climate conditions and human activities, leading to frequent seasonal droughts. However, the ability of the existing reservoirs and proposed hydraulic projects to restore the HR is not yet clear. A hydrological model for the middle Yangtze River was used in combination with a lake water balance model to assess the recovery degree of the Poyang Lake HR under the regulation of the existing reservoirs and proposed hydraulic projects. New hydrological insights for the regionIn the past 20 years, the water level of Poyang Lake decreased by 5.9%, and the duration of the drought (lake level < 12 m) was prolonged by 39.5% compared to 1953–2002. The potential recovery capacity of the existing reservoirs (including the TGR and the lake catchment reservoirs) and the proposed hydraulic project (PLHP) at the discharging waterway at the north end of Poyang Lake was quantified for typical hydrological years. It was found that the HR in typical hydrological years can be entirely restored by joint regulation of the existing reservoirs, but that this method is not successful in extremely dry years. The PLHP can cope with extreme droughts, such as that in 2022, to restore the lake to a reasonable water level.

Variable depositional environments and organic matter enrichment of Early Cambrian shales in the Middle Yangtze region, South China

Organic-rich shales of the Lower Cambrian Shuijingtuo Formation (SJT) in the Middle Yangtze region (South China) represent an important target for shale gas exploration and development in China. The lower section of the SJT mainly consisting of organic-rich black shale deposits has been regarded to be a favorable target for shale gas exploration and received substantial attentions, while the upper section of the SJT primarily consisting of limestone, silty shale and black shale deposits has been rarely studied. Most recently, a major breakthrough of shale gas exploration and development has been made in the upper section shale deposits. In order to better understand the redox evolution of sedimentary environment throughout the whole Early Cambrian time and organic matter (OM) enrichment mechanism of the SJT shale deposits, geochemical investigations, including analyses of total organic carbon (TOC), as well as major and trace element compositions, were conducted on the upper section of the SJT shales from Well EYY1. Our results showed that the upper section of the SJT shale deposits was accumulated during the Cambrian Stage 3. The TOC values of the studied SJT shale samples ranged from 0.34 to 3.65 wt%. The lower part of the upper section of the SJT shale, which exhibited relatively high TOC contents (∼2.0 wt%), was deposited under anoxic bottom-water conditions during the early Cambrian Stage 3, while the upper part, showing relatively low TOC content (1.0 wt%), was deposited under suboxic bottom-water conditions with intermittent episodes of anoxic bottom-water conditions during the middle–late Cambrian Stage 3. The palaeo-productivity of the SJT shale was estimated to be generally low, and displayed little variation. The OM enrichment mechanism of the SJT shale deposits underwent dynamic changes due to relative sea-level changes in the Yangtze Ocean. The OM enrichment of the early and late Cambrian Stage 3 shale deposits was primarily controlled by reducing conditions, while that of the middle Cambrian Stage 3 shale deposits may have been influenced by episodic upwelling activities.

Evolution characteristics of mid‐channel bars with a typical high/low bank structure in the Middle Yangtze River

AbstractAdjustments in mid‐channel bars (MCBs) often influence the stability of channel regime and the safety of river navigation in anabranching reaches. This study aims to investigate the evolution characteristics of the low and high parts of MCBs (L‐MCBs and H‐MCBs) in the Middle Yangtze River (MYR) and reveal the main factors influencing the deformation of these two components. Results show that the total exposed area of L‐MCBs decreased from 305 to 288 km2 in 2003–2019, whereas the H‐MCBs remained relatively stable. Taking the Nanyang bar with a typical high/low bank structure as an example, the deformation of the L‐MCB mainly occurred in the head zone, while the deformation of the H‐MCB was characterised by bank retreat in the tail zone and floodplain deposition on the top of the H‐MCB. In terms of influencing factors, the impacts of the altered flow–sediment conditions and the MCB's properties (soil composition and morphology characteristics) on the Nanyang bar deformation were investigated. On the one hand, the reduction in sediment concentration was the most direct factor causing the shrinkage of L‐MCBs, and the decrease in flood occurrence frequency made the deformation of H‐MCB less significant. On the other hand, the soil composition of the L‐MCB primarily consisted of fine sand with weak erosion resistance, while the large content of cohesive sediment made the H‐MCB difficult to be eroded by overbank flows; moreover, the riverbank of the H‐MCB in the bar‐tail zone was much steeper than that in the bar‐head zone, causing the deformation of the high part to mainly occur in the tail zone. Furthermore, the deformation of the high part was simulated using a bank erosion model at section scale, and the calculated safety factors of riverbank at a typical cross‐section show that the riverbank of the H‐MCB was the most unstable during the receding period.

Provenance changes of the Yangtze River Delta sediments since ∼3.6 Ma: Evidence from heavy mineral assemblages and detrital zircon U–Pb ages spectra

The Yangtze River drains from the eastern Tibetan Plateau and provides insights into the evolution of the Asian monsoon and uplift of the Tibetan Plateau. The age and evolution of the Yangtze River have been investigated, yet consensus remains lacking. This study aimed to investigate the sediment source changes of the Yangtze Delta since approximately 3.6 Ma. A new sediment core (YZQK2, 300.36 m) was subjected to high-resolution magnetostratigraphic, zircon U–Pb age spectral, and heavy mineral assemblage analyses. Magnetostratigraphy showed that 79.65, 171.22, and 293.4 m reflected the Matuyama–Brunhes, Gaussian–Matuyama, and Gilbert–Gaussian boundaries, respectively. The bottom age of the core was approximately 3.60 Ma. Between 3.60 and 3.44 Ma, heavy mineral assemblies were characterized as high reductive heavy minerals, pyrite, and siderite with the dominant peak of the detrital zircon U–Pb ages being 400–450 Ma. The sediments were primarily fed by tributaries draining into the middle Yangtze River, which were the Han River oriented in the Tongbai-Dabieshan orogenic belt and Xiang River and Gan River oriented in the Cathaysia Block. Between 3.44 and 2.79 Ma, the reductive heavy minerals, pyrite, and siderite disappeared, while garnet, apatite, sphene, and ilmenite showed a sharp increase. The dominant peak of detrital zircon U–Pb ages exhibited an extremely narrow range of 100–150 Ma. The sediments originated from the rock basin in the lower Yangtze River, the dominant river system, and were sourced locally or regionally from the adjacent Cretaceous granite plutons and volcanic rocks. The zircon age spectrum similar to the mainstream of the Yangtze River, containing amphibole, pyroxene, and magnetite from the upper to the middle and lower reaches of Yangtze River, has formed the Yangtze drainage pattern since 2.79 Ma. The sediment in the middle Yangtze reached the Yangtze Delta no later than 3.6 Ma, while the integration of the upper Yangtze into the Yangtze Delta occurred no earlier than 2.79 Ma. Our results provide novel insights into the established timeline of the Yangtze River.

Fluctuation in redox conditions and the evolution of early Cambrian life constrained by nitrogen isotopes in the middle Yangtze Block, South China

AbstractThe Ediacaran–Cambrian (E-C) transition (∼542–517 Ma) witnessed the rapid evolution of Cambrian animals, which was accompanied by carbon cycling anomalies and a significant increase in the concentration of oxygen in Earth’s atmosphere. The mechanisms stimulating the evolution of complex eukaryotes, however, remain problematic, especially concerning the link between biological evolution and contemporaneous changes in the oceanic environment. In this study, integrated δ13Ccarb–δ13Corg–δ15N compositions were analysed from the YD-4 core samples to understand redox fluctuations and nitrogen cycling of the middle Yangtze Block across the E-C transition. Two negative δ13Ccarb excursions (N1 and N2) and a positive δ13Ccarb excursion (P1) are identified from the studied samples and are supposedly of primary origin. Constrained by of the U-Pb age, biolithology and pattern of isotopic variation, N1, P1 and N2 are comparable to the Basal Cambrian Carbon Isotope Excursion (BACE), Zhujiaqing Carbon Isotope Excursion (ZHUCE) and Shiyantou Carbon Isotope Excursion (SHICE). We interpreted the decreased δ15N values in this study as resulting from intensified atmospheric nitrogen fixation driven by enhanced denitrification associated with expanded marine anoxia, as well as partial ammonium assimilation, while increased δ15N values suggest weakened denitrification associated with an amplified oxic water mass. The temporal coincidence of N1 and N2, with two episodes of negative δ15N excursions, and of P1, with a positive δ15N excursion, suggests that variable oceanic redox conditions and nitrogen bioavailability may have influenced the evolution of the Cambrian eukaryote-dominated community.

Socioeconomic and seasonal effects on spatiotemporal trends in estrogen occurrence and ecological risk within a river across low-urbanized and high-husbandry landscapes

Estrogen pollution is a persistent issue in rivers. This study investigated the occurrence, spatiotemporal variation mechanisms, sources, and ecological risks of estrone (E1), 17β-estradiol (E2), estriol (E3), 17α-ethinylestradiol (EE2), diethylstilbestrol (DES), and bisphenol-A (BPA) in the waters of the Zijiang River, a tributary of the middle Yangtze River. The results revealed elevated detection frequencies and estrogen concentrations in the dry season compared to the wet season, mainly due to the precipitation dilution effect. Total estrogen concentration ranged from 21.2 to 97.5 ng/L in the dry season, which was significantly correlated to spatial distributions of animal husbandry and population. Among the estrogens studied in the river, E2, BPA, and EE2 were predominant. The collective sources of E1, E2, E3, and EE2 were traced back to human and husbandry excrement, whereas BPA emitted from daily life products, contributing to 55.5% and 42.7% of the total estrogen concentration, respectively. Particularly, the average and median E1, E2, and EE2 concentrations in the river exceeded the environmental quality standards of the European Union. The total estrogenic activity dominated by EE2 exceeded the 1 ng E2/L threshold, with levels exceeding 10 ng E2/L during the dry season. The risk quotients exhibited a high ecological risk of E1 and EE2 to fish and a moderate to high ecological risk of E1 to crustaceans, EE2 to mollusks, and E2 to fish. Therefore, E1, E2, and EE2 pollution of the river may lead to both high estrogenic potency and moderate or high ecological risk; thus, they should be considered priority pollutants in the river. These results yield valuable insights into the spatiotemporal change mechanisms, sources, and ecological risks of estrogens in river water of low-urbanization and rural watersheds.

Long-distance recovery of nonuniform suspended load in the Middle Yangtze River owing to upstream damming

The amount of suspended sediment entering the Middle Yangtze River (MYR) was greatly reduced owing to the operation of the Three Gorges Dam (TGD) and other upstream dams, which has caused the process of sediment transport in the MYR at a severe non-equilibrium state. A detailed investigation was conducted into the transport of nonuniform suspended load in the MYR using about 30-year observed data: (i) firstly, the spatiotemporal variations in fractional sediment fluxes were investigated during the pre- and post-TGD periods. The total sediment flux decreased after the TGD operation, and the magnitude for the coarse fraction (d > 0.125 mm) was smaller than those for the fine (d < 0.031 mm) and medium (d = 0.031–0.125 mm) fractions; moreover, the sediment flux for each fraction continuously recovered along the 955-km-long reach, with the recovery distance for the coarse fraction being much shorter than the other two fractions; it was mainly attributed to the fact that the riverbed could supply abundant coarse sediment to the flow, and this fraction experienced a considerably higher recovery degree than the other two fractions; (ii) secondly, the sediment budgets for the fine, medium and coarse fractions were calculated to assess the contributions of different sediment sources to the recovery of graded sediments in the MYR. About 50 %, 29 % and 14 % of the increased fractional sediment amounts for fine, medium and coarse fractions were originated from the tributary confluence, while the remaining sediment was supplied from the riverbed during the process of channel erosion. Therefore, the recovery of suspended load was largely attributed to the supply of sediment from the riverbed, especially for the coarse fraction; (iii) finally, the impacts of the altered flow-sediment regime and the bed-material composition on sediment recovery efficiency were analysed. The sediment recovery efficiency greatly increased after the TGD operation, which was caused by the reduced sediment concentration; and the sediment recovery efficiency for the coarse fraction was much larger than those for the fine and medium sediment fractions. It is attributed to the fact that the channel can provide abundant coarse sediment to the flows in the MYR, while the sediment recovery efficiencies of fine and medium fractions were restricted by the low degree of sediment supply from the riverbed.

Discharge dynamics of agricultural diffuse pollution under different rainfall patterns in the middle Yangtze river

Rainfall plays a crucial role in influencing the loss of agricultural diffuse pollution. The middle Yangtze River region is well-know for its humid climate and numerous agricultural activities. Thus, this study quantitatively analyzed the concentration and distribution of nitrogen (N) and phosphorus (P) load and loss in a major tributary of the middle Yangtze River under different rainfall patterns by using sampling analysis and SWAT model simulation. The total nitrogen (TN) and nitrate-nitrogen (NO3−) concentrations were 1.604–3.574 and 0.830–2.556 mg/L, respectively. The total phosphorous (TP) and Soluble Reactive Phosphorus (SRP) were 2–148 and 2–104 μg/L, respectively. The modeling results demonstrated that higher rainfall intensity led to greater load and loss flux of diffuse pollutant at the outlet. Organic nitrogen (ORGN) is the main nitrogen form transported from the subbasin to the reach, while organic phosphorus (ORGP) and inorganic phosphorus (INORGP) were transported at similar amounts. Under the condition of conventional rainfall, the outlet reaches mainly transported NO3−, and ORGN gradually increased when rainstorm events occurred. The ratio of INORGP to ORGP was relatively stable. During extreme rainfall event, rainfall is the dominant element of agricultural diffuse pollution. A strong positive correlation exists between rainfall intensity and pollution loss during rainstorms. Storm rain events were the main source of TN and TP losses. Few storm rain days generated pollutants that accounted for a large proportion of the total loss, and their impact on TP loss was significantly greater than that of TN. The influence of storm rain on TN is mainly the increase in runoff, while TP is sensitive to the runoff and sediment transport promoted by rainfall. By setting different precipitation scenarios, it was confirmed that under the same rainfall amount, short-term storm rain has the most significant impact on the TN load, whereas TP load may be influenced more by the combined effects of rainfall duration and intensity. Therefore, to reduce the impact of agricultural diffuse pollution, it is important to take targeted measures for the rainstorm days.

Spatiotemporal Variations of Aerosol Optical Depth and the Spatial Heterogeneity Relationship of Potential Factors Based on the Multi-Scale Geographically Weighted Regression Model in Chinese National-Level Urban Agglomerations

Investigating the spatiotemporal variation characteristics of aerosol optical depth (AOD) and its driving factors is essential for assessing atmospheric environmental quality and alleviating air pollution. Based on a 22-year high-resolution AOD dataset, the spatiotemporal variations of AOD in mainland China and ten national urban agglomerations were explored based on the Mann–Kendall trend test and Theil–Sen median method. Random forest (RF) and multiscale geographically weighted regression (MGWR) were combined to identify the main driving factors of AOD in urban agglomerations and to reveal the spatial heterogeneity of influencing factors. The results showed that areas with high annual average AOD concentrations were mainly concentrated in the Chengdu–Chongqing, Central Plains, Shandong Peninsula, and Middle Yangtze River urban agglomerations. Southern Beijing–Tianjin–Hebei and its surrounding areas revealed the highest AOD pollution during summer, whereas the worst pollution during the remaining three seasons occurred in the Chengdu–Chongqing urban agglomeration. Temporally, except for the Ha-Chang and Mid-Southern Liaoning urban agglomerations, where the average annual AOD increased, the other urban agglomerations showed a decreasing trend. Among them, the Central Plains, Middle Yangtze River, Guanzhong Plain, and Yangtze River Delta urban agglomerations all exhibited a decline greater than 20%. According to the spatial trends, most urban agglomerations encompassed much larger areas of decreasing AOD values than areas of increasing AOD values, indicating that the air quality in most areas has recently improved. RF analysis revealed that PM2.5 was the dominant factor in most urban clusters, followed by meteorological factors. MGWR results show that the influencing factors have different spatial scale effects on AOD in urban agglomerations. The socioeconomic factors and PM2.5 showed strong spatial non-stationarity with regard to the spatial distribution of AOD. This study can provide a comprehensive understanding of AOD differences among urban agglomerations, and it has important theoretical and practical implications for improving the ecological environment and promoting sustainable development.

Modelling of phosphorus and nonuniform sediment transport in the Middle Yangtze River with the effects of channel erosion , tributary confluence and anthropogenic emission.

Phosphorus (P) transport plays a crucial role in the aquatic ecology of natural rivers. However, our understanding still remains unclear that how P transport is affected in a river-lake connected system downstream of a dam. This system usually undergoes both severe channel degradation and complex exchange of flow-sediment-phosphorus between the mainstem and tributaries. In the current study, a method was proposed firstly to determine the individual contribution of different sources to P recover based on the calculation of phosphorus budget; then an integrated model was developed, covering the modules of flow, nonuniform sediment and phosphorus transport. The application of the proposed method in the 955-km-long Middle Yangtze River (MYR) shows that the type of P transportation was predominantly changed from particulate phosphorus to dissolved phosphorus after the operation of the Three Gorges Project (TGP), but a significant longitudinal recovery of total phosphorus (TP) flux was observed. The TP flux exporting from the MYR was mainly from the Upper Yangtze River (44%), and 12%, 18% and 26% of that were originated from channel erosion, tributary confluence and anthropogenic emission. Moreover, the effects were investigated of nonuniform sediment transport and bed-material coarsening on P transport in the MYR, based on the proposed integrated model. Obtained results show that the TP transport process in the MYR was more reasonable simulated using the nonuniform sediment mode, and it is also confirmed that the process of bed-material coarsening after the TGP operation would lead to the decrease of particulate phosphorus flux in the MYR.

Nanopore structures and controlling factors of the Early Cambrian shale reservoir in the Middle Yangtze Platform, South China

At present, the characteristics and controlling factors of nanopore structure in shale reservoirs are still not well understood owing to the concurrence of both organic matter (OM) and inorganic mineral (IM) pores. This issue is more prominent for the Early Cambrian shale in South China owing to the greatly variable nanopore structures of the samples from different areas. In this study, a set of Early Cambrian shale samples was taken from an exploration well in the Middle Yangtze Platform, a newly-determined shale gas play with a great potential outside the Sichuan Basin, their OM-removed (OMR) samples were obtained by a high-temperature oxidation method, and the pore structures of the bulk shales and their OMR-samples were analyzed by low-pressure gas adsorption experiments. The OM nanopore structure parameters of shale were calculated, and the controlling factors of OM and IM nanopores in the shale were investigated. The results show that the shale nanopores are mainly mesopores, their OM nanopores, especially OM micropores, are obviously more developmental compared with the IM nanopores. The TOC content is the dominant influencing factor of the shale nanopores. The brittle minerals support the formation of interparticle pores, and provide protection to OM pores. The clay minerals provide a certain amount of nanopores, but the strong compaction result in rich micropores. The carbonate minerals decrease the shale nanopores owing to the strong cementation. The favorable area of shale gas in the Early Cambrian strata in the studied area was predicted by the TOC content and preservation conditions.

Short-term exposure to ozone and mortality from AIDS-related diseases: A case-crossover study in the middle Yangtze River region, China

BackgroundPrevious investigations have predominantly concentrated on the influence of ozone (O3) on general population mortality. However, a noticeable gap exists regarding the attention directed towards susceptible demographics, specifically individuals afflicted by human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS). MethodsA dataset comprising 1467 AIDS-related fatalities from 2013 to 2020 was amassed from the Hubei Provincial Center for Disease Control and Prevention. Daily maximum 8-h average O3 levels and meteorological parameters were extracted from the ChinaHighAirPollutants dataset and the National Meteorological Science Data Center, respectively. A time-stratified case-crossover methodology was employed to scrutinize the connection between short-term exposure to O3 and AIDS-related deaths. ResultsA rise of one interquartile (IQR) in O3 concentration, lagged by 4 days, was associated with a 15% [95% confidence intervals (CIs): 2, 31] increase in AIDS-related deaths. Notably, males demonstrated heightened susceptibility to the adverse consequences of O3, marked by an odds ratio of 1.20 (95% CIs: 1.05, 1.37) at lag 4 day. Additionally, patients aged over 65 years exhibited escalated vulnerability to brief O3 exposure. Marriage status and educational attainment emerged as influential factors modifying the interplay between O3 and AIDS-related mortality. ConclusionsOur study presents novel evidence spotlighting the deleterious repercussions of O3 on mortality in the HIV/AIDS population.

Species-specific adaptations, individual condition, and life-history-induced dispersal strategies of fish larvae along a transverse gradient in a large river

Early life-history properties reveal a considerable variation among riverine fish species and may play a major role in the downstream drift of larval fish. This role, however, remains largely unexplored. Assemblages of drifting fish larvae and their condition during the peak seasons were analyzed along a cross-section from littoral to midstream zones in a free-flowing section in the middle Yangtze River. Assemblage structure and larval body injuries revealed pronounced transverse patterns. These patterns were mainly explained by hydraulic gradients. Highest species occurrence was recorded in the littoral zone. Larval abundance of most species and larval body injury rates were higher in littoral and sublittoral zones. Relative inshore and nearshore larval abundance revealed a significant negative relationship with species-specific initial body size, and varied among species. Larval abundance, body injury rate, and feeding intensity were all higher at night. Our findings revealed considerable habitat preferences of drifting larval fish and suggest important adaptations for reducing starvation- and predation risks. These factors should be considered in species conservation and river management.

Nonstationary frequency analysis and uncertainty quantification for extreme low lake levels in a large river-lake-catchment system

Extreme hydrological events have become increasingly frequent on a global scale. The middle Yangtze River also faces a substantial challenge in dealing with extreme flooding and drought. However, the long-term characteristics of the extreme hydrological regime have not yet been adequately recognized. Moreover, there is uncertainty in the extreme value estimation, and this uncertainty needs to be distinguished and quantified. In this study, we investigated the nonstationary frequency characteristics of extreme low lake levels (ELLLs), taking the Poyang Lake as an example. Daily lake levels from 1960 to 2022 were utilized to estimate the return level using the generalized Pareto distribution (GPD). The uncertainty from three sources, i.e., the parameter estimator, threshold selection, and covariate, was quantified via variance decomposition. The results indicate that (1) the parameter estimator is the predominant source of uncertainty, with a contribution rate of approximately 87 %. The total uncertainty of the covariate, threshold, and interaction term is only 13 %. (2) Two indexes, namely the annual minimum water level (WLmin) and the days with peak over the 90 % threshold per year (DPOT90), decreased (0.01–0.03 m/year) and increased (0.17–1.39 days/year), respectively, indicating a progressively severe drought trend for Poyang Lake. (3) The return level with return period of 5 to 100 years significantly decreased after the early 21st century. A large spatial heterogeneity was identified for the variation in the return level, and the change rate of the return level with a 100-year return period ranged from 5 % to 40 % for the whole lake. (4) The ELLLs had a stronger correlation with the catchment discharge than with the Yangtze River discharge and the large-scale atmospheric circulation indices. This study provides a methodology with reduced uncertainty for nonstationary frequency analysis (NFA) of ELLLs exemplified in large river–lake systems.

Ecological health analysis of wetlands in the middle reaches of Yangtze River under changing environment

ABSTRACT Changes in wetland ecosystems have a critical impact on the local ecology and species diversity. Different development scenarios and policies are key factors influencing their changes. Therefore, we studied changes of wetlands in the middle Yangtze River basin (MYRB) in 2001–2020, and a patch-generated land use simulation (PLUS) model and random forest (RF) method were applied to predict and analyze the changes under different scenarios in the MYRB in the future (i.e.2035–2095). The results indicated that: (1) The regions with high wetland proportions were concentrated in the central and eastern MYRB in 2001–2020, with a 1.5% decrease in overall wetland area; (2) The RF could simulate the future ecological quality with training and testing accuracies of 0.98 and 0.92, respectively; (3) Remote Sensing Ecological Index (RSEI) less than 0.5 in the central and eastern regions and 13.3% reduction in the northwest in the SSP245 scenario. In general, the study provides a basis for future regional studies of ecosystem quality and provides data to support wetland conservation and management.

The cause and inspiration of the nonlinear impact of the Three Gorges Dam operation on stage-discharge relation in downstream channels

Evaluating the effects of dam operations on downstream hydrological regimes is crucial to cope with the threat of flood. Some channels in dammed rivers have experienced erosion and counterintuitive higher flood water levels. However, the impacts of dam-induced channel changes and dam-altered streamflow on downstream hydrological regimes have been less thoroughly evaluated. Herein, we integrated the Distributed Time Variant Gain Model and a proposed grey model to thoroughly decouple the effects of TGD-altered Yangtze River streamflow and TGD-induced river channel changes on the stage-discharge relation in Jingjiang reach of the middle Yangtze River basin, from 2003 to 2014. The stage-discharge relation in Jingjiang reach is presented by the relation between water level at Chenglingji station (Zcl) and discharge at Luoshan station (Qls). The results indicate that the TGD operation increases Zcl by 0.01 ∼ 0.06 m and 0.02 ∼ 0.48 m on average when Qls is below 12,000 and exceeds 24000 m3/s, respectively. However, the TGD operation decreases Zcl by 0.01 m on average when Qls is around 18000 m3/s. Hence, the TGD operation has a nonlinear impact on downstream stage-discharge curves, which results from the TGD interception of the release, water–sediment regulation of the TGD, and the erosion of the fine-grained channel. Given that the causes of the nonlinear impact are widespread, we speculated that the resulting changes in downstream stage-discharge curves influenced by the water–sediment regulation of dams in fine-grained rivers will be nonlinear and exhibit the hook curve. Meanwhile, the purposed grey model helps to evaluate the impacts of dam-induced river channel changes on downstream hydrological regimes in data-scarce rivers. Our work helps to thoroughly evaluate the impacts of dam operations on downstream hydrological regimes and the resulting flood threat.

Decoupling of nitrogen, phosphorus and biogenic silica in floodplain sediments in response to land use change and hydrological alterations

Human activities are altering global biogeochemical cycles of nitrogen (N), phosphorus (P) and biogenic silica (BSi). In order to reveal nutrient dynamics in response to human disturbances, this study explores burial rates and ratios of N, P and BSi in 210Pb-dated sediment cores from eight lakes in the middle Yangtze River floodplain. Mean burial rates of N, P and BSi increased substantially from the early 20th century, except for a hiatus in the 1970s. Before the 1960s, the stable ratios of N:P, N:BSi and P:BSi indicated the nutrient supply from clastic sources, while arable land expansion enhanced nutrient burial in lakes. Although damming increased the retention of N, P and BSi in upstream reservoirs after the 1960s, anthropogenic N and P release to the lakes outpaced the retention by dams, as suggested by continuous increases in N and P burial rates. The concurrent increase in BSi burial rate suggested that nutrient enrichment promoted diatom production at the earlier stage, while rising N:BSi and P:BSi ratios and declining BSi burial rate from the 1990s indicated the Si depletion with further eutrophication. Spatial heterogeneities among the lakes were linked to site-specific factors such as hydrological connectivity and macrophyte growth. Synchronous increases in N, P and BSi burial rates before the 1960s mainly responded to deforestation and arable land expansion; thereafter, the decoupling of N, P and BSi likely resulted from intensive farming (e.g., fertilizer usage) and damming. Given new dam construction and land use change in river basins, the BSi depletion relative to N and P is likely occurring in riverine floodplains worldwide.