Dam construction reshapes sedimentary pollutant distribution along the Yangtze river by regulating sediment composition

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.

Decoupling the effects of climate, topography, land use, revegetation, and dam construction on streamflow, sediment, total nitrogen and phosphorus in the Yangtze River Basin

Evaluating changes in streamflow, sediment, and nutrient fluxes, as well as quantifying their influencing factors, is crucial for regional water resource protection. While the relationships between major influencing factors and these indicators have been widely studied, the quantitative contributions of the separate and interactive effects of these influencing factors have not been fully explored. This study quantitatively evaluated the changing characteristics of streamflow, sediment discharge, total nitrogen (TN) and total phosphorus (TP), as well as the separate and interactive effects of various major influencing factors such as-rainfall, temperature, evapotranspiration (ET), revegetation, dam construction, and land use change-by applying the GeoDetector method to account for their spatial heterogeneity and contributions. Our findings reveal that the influence of these factors has gradually intensified over time, with dam construction and land use change emerging as the most significant contributors to changes in sediment discharge and TN, respectively. Notably, the interactive effects between dam capacity and vegetation cover on streamflow and sediment discharge was twice as strong as their separate impacts, highlighting the effectiveness of integrating dam construction with reforestation to control erosion and sediment transport. Similarly, the interaction of dam capacity and land use change had a 1.5 times greater impact on TN and TP than their separate effects, indicating that reducing fertilizer application at the source and in the meantime implementing direct interception measures are more effective ways to control water pollution. These findings provide a solid foundation for policymakers to develop integrated water management strategies targeting multiple factors simultaneously, that address both water quantity and quality concerns in the Yangtze River Basin and similar regions.

Spatial distribution of potential nitrogen reduction rates and associated microbial communities revealed by metagenomic analysis in Yangtze River sediments

Understanding the intricacies of nitrogen reduction processes and the composition of associated microbial communities is crucial for illuminating the reactions of ecosystems and their functions to persistent nitrogen inputs. To enhance research on the nitrogen reduction process, we determined the potential rates, quantified the relevant genes, and analyzed the macro factors in the sediments of the Yangtze River. The results showed that dissimilatory reduction of nitrate to ammonium (DNRA) dominated the N-reduction processes in the Yangtze River sediment, with average rates of 0.89 ± 0.71 nmol N g-1 h-1. Meanwhile, denitrification and anammox rates were 0.73 ± 0.74 and 0.07 ± 0.07 nmol N g-1 h-1, respectively. The Three Gorges Dam (TGD) caused higher potential rates (nmol N g-1 h-1) of denitrification (1.38), anammox (0.12), DNRA (1.48), and N2O depletion (1.49 nmol g-1 h-1) in the Three Gorges Reservoir (TGR) compared to other river reaches. The average copy numbers (copies·g-1) of nrfA (2.96 × 106), narG (8.17 × 105), nirS (6.10 × 106), nosZ (2.77 × 106), and hzsB (3.68 × 105) in TGR sediments were higher than those in the other reaches. The TGD's interception of fine sediments and nutrients enhanced microbial gene abundance, thereby favoring N-reduction processes and resulting in N2O depletion in reservoir sediments. Moreover, the TGD caused a decreased contribution gap between DNRA and denitrification in the TGR (2%) compared with the upper (35%) and lower (18%) reaches, while causing predominant anammox (50%) in the middle reach. Metagenomic results suggested that sediment particle size, along with organic carbon and inorganic nitrogen concentrations, influenced N reduction rates by affecting narG, norB and C, nrfA and H, and hzsB and C. This study reveals the spatial pattern of the N-reduction rate in the Yangtze River sediments and quantitatively defines the intensity of dam effects on sediment N-reduction rate.

Identifying sources and risks of polycyclic aromatic hydrocarbons in the Yarlung Tsangpo river: A seasonal study

The presence of polycyclic aromatic hydrocarbons (PAHs) in the Yarlung Tsangpo River (YTR) poses substantial threats to both regional ecological security and human health. While it is imperative to accurately assess and pinpoint the risks and sources of PAHs in the YTR, a comprehensive understanding of their seasonal variations in composition and distribution is currently lacking. In a thorough investigation aimed at evaluating seasonal fluctuations, sources, potential hazards, and the impact of natural and anthropogenic factors on PAHs concentrations in the YTR, enlightening findings emerged. During the dry season, concentrations of the sum of 16 PAHs (Ʃ16PAHs) ranged from 18.74 to 60.01 ng/L, with low molecular weight PAHs (2-3 rings) prevailing, primarily originating from biomass combustion. Conversely, in the wet season, concentrations varied from 10.35 to 27.04 ng/L, with high molecular weight PAHs (5-6 rings) dominant, largely sourced from transportation emissions. The increased rainfall during the wet season led to a dilution of PAHs concentrations, resulting in lower levels compared to the dry season. Factors such as population density, energy consumption, and energy production exhibited positive correlations with PAHs levels, influencing their spatio-temporal distribution. The spatial distribution of PAHs concentrations was found to be associated with ecological and health risks, with risk quotient analysis indicating lower ecological risks, but heightened risks during in the dry season. Moreover, through monte carlo simulation and risk assessment, it was revealed that there were elevated carcinogenic risks for adults during the dry season. Prioritizing reductions in concentrations of specific PAHs such as DahA and BaP proved effective in mitigating the risk of carcinogenic risk. This study furnishes valuable reference data for the management and regulation of PAHs in the YTR, offering insights crucial for safeguarding the river's ecosystem and the well-being of the communities reliant on it.

COVID-19 lockdown measures affect polycyclic aromatic hydrocarbons distribution and sources in sediments of Chaohu Lake, China

The COVID-19 pandemic has profoundly impacted human activities and the environment globally. The lockdown measures have led to significant changes in industrial activities, transportation, and human behavior. This study investigates how the lockdown measures influenced the distribution of polycyclic aromatic hydrocarbons (PAHs) in the sediments of Chaohu Lake, a semi-enclosed lake. Surface sediment samples were collected in summer of 2020 (lockdown have just been lifted) and 2022 and analyzed for 16 priority PAHs. The range of ΣPAHs concentrations remained similar between 2020 (158.19-1693.64 ng·g-1) and 2022 (148.86-1396.54 ng·g-1). Among the sampling sites, the west lake exhibited similar PAHs concentrations characteristics over the two years, with higher levels observed in areas near Hefei City. However, the east lake exhibited increased ΣPAHs concentrations in 2022 compared to 2020, especially the area near ship factory. PAHs source analysis using principal component analysis-multiple linear regression (PCA-MLR) revealed an increased proportion of petroleum combustion sources in 2022 compared to 2020. The isotope analysis results showed that organic matter (OM) sources in the western lake remained relatively stable over the two years, with sewage discharge dominating. In contrast, the eastern lake experienced a shift in OM sources from sewage to C3 plants, potentially contributing to the increased PAH levels observed in the eastern lake sediments. Ecological risk assessment revealed low to moderate risk in both 2020 and 2022. Health risk evaluation indicated little difference in incremental lifetime cancer risk (ILCR) values between the two years, with only benzo[a]pyrene (BaP) posing a high risk among the carcinogenic PAHs. Children generally faced higher health risks compared to adults. This study reveals pandemic-induced changes in PAH pollution and sources in lake sediments, offering new insights into the impact of human activities on persistent organic pollutants, with implications for future pollution control strategies.

Distribution and sources of polycyclic aromatic hydrocarbons in cascade reservoir sediments: influence of anthropogenic activities and reservoir hydrology

The construction of dams has caused disruptions to river connectivity, leading to alterations in the deposition of hydrophobic organic contaminants in reservoir sediments. Further investigation is warranted to explore the impact of cascade reservoirs with differing hydrological characteristics on polycyclic aromatic hydrocarbons (PAHs) distribution in sediment. This study examines the presence of 30 PAHs in the sediments collected from six cascade reservoirs situated in the Wujiang River basin during January and July 2017. The results showed that Σ30 PAHs ranged from 455-3000 ng/g dw (mean 1030 ng/g dw). Anthropogenic activities and reservoir hydrology determined the distribution trend of PAHs in sediments, with an overall increase from upstream to midstream and then a decrease downstream. The PAH levels were highly linked to the secondary industry (P < 0.05). This was further supported by the relationship between the PAH emissions from coal combustion and traffic sources analyzed by the positive matrix factorization model and economic parameters in the wet season (P < 0.01). At the same time, reservoir age (RA) showed a positive correlation with PAH concentrations (P < 0.05), while hydraulic retention time (HRT) exhibited a negative correlation with PAH levels (P = 0.03). The relationship between total organic carbon (TOC) and PAHs in stream sediments worldwide was nonlinear (P < 0.01), with PAH concentrations initially rising and then falling as TOC levels increased. Concerns regarding carcinogenic risk were raised due to contributions from coal and vehicular sources, with the risk increasing with RA.

Humic substance mitigated the microplastic-induced inhibition of hydroxyl radical production in riparian sediment

Hydroxyl radicals (·OH) generated during the flooding-drought transformation process play a vital role in affecting nutrient cycles at riparian zone. However, information on the processes and mechanisms for ·OH formation under the influence of microplastics (MPs) remains unclear. In this study, the effects of MPs on ·OH production from riparian sediments with different biomass [e.g., vegetation lush (VL) and vegetation barren (VB)] were studied. The results showed that presence of MPs inhibited the production of ·OH by 27 % and 7.5 % for VB and VL sediments, respectively. The inhibition was mainly resulted from the MP-induced reduction of the biotic and abiotic mediated Fe redox processes. Spectral analysis revealed that VL sediments contained more high-molecular-weight humic-like substances. Presence of MPs increased the abundances and activities of Proteobacteria, Acidobacteria and Actinobacteria, which were conducive to the changes in humification and polar properties of organic matters. The reduced humic- and fulvic-like substances were accumulated in the flooding period and substantially oxidized during flooding/drought transformation due to the enhanced MP-mediated electron transfer abilities, thus mitigated the MP-induced inhibition effects. Therefore, in order to better understanding the biogeochemical cycling of contaminants as influenced by ·OH and MPs in river ecosystems, humic substances should be considered systematically.

Historical behaviors of microplastic in estuarine and riverine reservoir sediment

This study investigates the sedimentation behaviors of microplastics (MPs) within a typical meso-scale river estuary, the Yalu River Estuary (YRE) and its riverine reservoir. It analyzes sediment cores in two habitats of Yalu River, revealing changing MPs abundance over time. Results highlight significant differences in riverine and estuarine MPs deposition. Reservoir sample contains more MPs in fragments. Color variations are notable in estuarine samples but minimal in reservoir sample. After 1980, estuarine cores show an increase in coarser MPs, likely due to growth of aquaculture activities. Although sediment accumulates at 1/10 of the rate in reservoir compared to estuary, MPs in reservoir sediments exceeds estuarine level by over threefold. A possible mechanistic framework is then proposed to discuss the varying MPs behaviors in the two habitats, indicating reservoirs accumulate MPs at a higher rate due to the barrier effect of an upper-stream reservoir, stable hydrodynamics, and weak salinity-induced buoyancy.

A retrospective analysis of heavy metals and multi elements in the Yangtze River Basin: Distribution characteristics, migration tendencies and ecological risk assessment

The Yangtze River is the third longest river in the world with more than 6300 km, covering 0.4 billion people. However, the aquatic ecosystem of the Yangtze River has been seriously damaged in the past decades due to a rapid development of economic and industrialization along the coast. In this study, we first established a dataset of fifty elements, including nine common heavy metals (HMs) and forty-one other elements, in the Yangtze River Basin through the collection of historical data from 2000 to 2020, and then analyzed their spatiotemporal distribution characteristics. The results indicated that the Three Gorges Reservoir (TGR), a region formed by the construction of the Three Gorges Dam (TGD), may act as a sink for these elements from upstream regions. The concentrations of seven elements in surface water and 13 elements in sediment obviously increased from the upstream region of the TGR to the TGR. In addition, ten elements in the surface water and 5 elements in the sediments clearly decreased, possibly because of the interception effects of the TGD. On a timescale, Cr obviously tended to migrate from the water phase to the sediment; Pb tended to migrate from the sediment to the water phase. In the ecological risk assessment, all common HMs in surface water were supposed to have negligible risks as protecting 90 % of aquatic organisms; Cd (210.2), Hg (58.0) and As (43.1) in sediment posed high and moderate ecological risks using the methodology of the potential ecological risk index. Furthermore, Hunan Province is at considerable risk according to the sum of the potential risk index (314.8) due to Cd pollution (66.8 %). These fundamental data and results will support follow-up control strategies for elements and policies related to aquatic ecosystem protection in the Yangtze River Basin.

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

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

Microbial community structure and denitrification responses to cascade low-head dams and their contribution to eutrophication in urban rivers

Low-head dams are one of the most common hydraulic facilities, yet they often fragment rivers, leading to profound changes in aquatic biodiversity and river eutrophication levels. Systematic assessments of river ecosystem structure and functions, and their contribution to eutrophication, are however lacking, especially for urban rivers where low-head dams prevail. In this study, we address this gap with a field survey on microbial community structure and ecosystem function, in combination with hydrological, environmental and ecological factors. Our findings revealed that microbial communities showed significant differences among the cascade impoundments, which may be due to the environment heterogeneity resulting from the cascade low-head dams. The alternating lentic-lotic flow environment created by the low-head dams caused nutrient accumulation in the cascade impoundments, enhancing environmental sorting and interspecific competition relationships, and thus possibly contributing to the reduction in sediment denitrification function. Decreased denitrification led to excessive accumulation of nutrients, which may have aggravated river eutrophication. In addition, structural equation model analysis showed that flow velocity may be the key controlling factor for river eutrophication. Therefore, in the construction of river flood control and water storage systems, the location, type and water storage capacity of low-head dams should be fully considered to optimize the hydrodynamic conditions of rivers. To summarize, our findings revealed the cumulative effects of cascade low-head dams in an urban river, and provided new insights into the trade-off between construction and decommissioning of low-head dams in urban river systems.

Dam construction alters planktonic microbial predator‒prey communities in the urban reaches of the Yangtze River

While dam construction supports social and economic development, changes in hydraulic conditions can also affect natural aquatic ecosystems, especially microbial ecosystems. The compositional and functional traits of multi-trophic microbiota can be altered by dam construction, which may result in changes in aquatic predator-prey interactions. To understand this process, we performed a large-scale sampling campaign in the urban reaches of the dam-impacted Yangtze River (1 995 km) and obtained 211 metagenomic datasets and water quality data. We first compared the compositional traits of planktonic microbial communities upstream, downstream, and in a dam reservoir. Results showed that Bacteroidetes (R-strategy) bacteria were more likely to survive upstream, whilst the reservoir and downstream regions were more conducive to the survival of K-strategy bacteria such as Actinobacteria. Eukaryotic predators tended to be enriched upstream, whilst phototrophs tended to be enriched in the reservoir and downstream regions. Based on bipartite networks, we inferred that the potential microbial predator-prey interactions gradually and significantly decreased from upstream to the downstream and dam regions, affecting 56% of keystone microbial species. Remarkably, functional analysis showed that the abundance of the photosynthetic gene psbO was higher in the reservoir and downstream regions, whilst the abundance of the KEGG carbohydrate metabolic pathway was higher upstream. These results indicate that dam construction in the Yangtze River induced planktonic microbial ecosystem transformation from detritus-based food webs to autotroph-based food webs.