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Isotopic niches reflect the basic structure and functioning of river food webs; however, their response to riverine environments remains unclear. We used stable isotope analysis and community-wide metrics to quantify how invertebrate niches vary with environmental changes along a large subtropical river in China. Eight niche metrics, which had higher values in the wet than in the dry season, increased from headwaters to the middle river and decreased sharply near the estuarine industrial zones. The δ13C value of > - 23.8‰, which indicated consumption of epilithic diatoms, separated the invertebrates between the upper and mid-lower reaches. The δ15N values > 9.4‰ identified site-specific nitrogen sources from manure (e.g., animal effluent) and domestic sewage in agricultural area. The output of mixing models showed a downstream shift in carbon utilization by invertebrates from autochthonous periphyton and submerged hydrophytes to allochthonous C3 plants. Principle component (PC) and cluster analysis decomposed and grouped 40 environmental variables into 4 PCs that explained 84.5% of the total variance. Hierarchical partitioning revealed that the second and first PCs, which were driven mainly by biological indicators and habitat characteristics, had the highest explanatory power for niche ranges and areas (e.g., Bayesian ellipse), respectively. Our results suggest that reducing anthropogenic pressures (e.g., habitat loss and water pollution) on river ecosystems through measures, such as protecting diatom-dominated biofilms in riffles and controlling nitrogen loading in rural regions, may produce the greatest impact for river management. Graphical abstract.

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To understand the heavy metal pollution in agricultural soils along the East River basin and assess the pollution related health effect to local residents, interviews and archived data were obtained to identify the study sites affected by polluted tailing. Soil samples were collected and tested for heavy metal content and the Comprehensive Pollution Index (CPI). The degree of pollution of agricultural soils in the area was assessed using GIS-based Spatial distribution map of heavy metals and the trend of soil heavy metal risk. Two villages (Matian and Zhudui) near the East River were included in this study for health effect assessment. A total of 193 residents aged 15 or above from each village were tested for the present status of chronic diseases. Convenient sampling method was used to collect blood samples from 78 residents for heavy metal concentration. The contents of Pb, Cd, As, Zn, and Cu in the agricultural soils were all over the standards with a moderate to severe CPI. Among these metals, Cd was the highest followed by Pb, and Cu was the lowest. The contents of Pb, Cd, As, and Zn tend to be higher in soils closer to the river. The prevalence of chronic diseases was over 30%, which is significantly higher than the report from the national central region (23.15%). The average blood lead level (BLL) among children under 14 years is 7.42 µg/dL. Although the adults in Matian village had a significantly higher BLL (χ2 = 8.70, p = 0.03) as compared to Zhudui village, there was no significant difference for the prevalence of chronic diseases between the two villages (χ2 = 3.23, p = 0.09). The mean BLL of children and the proportion of children with BLL ≥ 10 µg/dL in this study are equivalent to the national average. The higher BLL concentration and prevalence of chronic diseases in adults might be due to their long-term exposure to heavy metal contamination environment and higher background level of heavy metals. Findings from this study will form the baseline information for local government to the development of effective approaches to control the heavy metal contamination and reduce the pollution related adverse health effect on local residents.

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Due to the heterogeneous distribution of resources along large rivers, understanding prey utilization by basin-scale fish assemblages remains a challenge, and thus, recognizing regional fish trophic guilds and indicator species is important. We analyzed the stomach contents of 96 fish species along the subtropical East River in China and identified 8 prey items (29 subcategories). Site-specific differences in fish diet composition (DC) revealed longitudinal shifts in utilized prey taxa, from upstream lotic to downstream semi-lentic items, and these were characterized by a decrease in the proportions of epilithic diatoms and aquatic insect larvae (Ephemeroptera and Chironomidae) accompanied by an increase in bivalves (Corbicula and Limnoperna), shrimps and fishes, and organic sediments. The relative prey consumption weighted by fish abundance and biomass indicated that decreasing insect consumption and increasing detritus consumption were two fundamental vectors governing fish-centered feeding pathways. Seventeen prey-oriented fish guilds that were clustered based on DC matrix determined the spatial variation in the fish trophic structure. The cumulative presence of (a) upstream guilds reliant on insects and epiphytes, (b) midstream guilds reliant on hydrophytes, molluscs, and nekton, and (c) downstream guilds reliant on detritus, annelids, and plankton resulted in a longitudinal increase in guild richness, but this continuity was interrupted near the industrialized estuary. The most abundant 28 fish species across the guilds were selected as trophic indicator species; their spatial distribution significantly (p < 0.05) explained >80% of the environmental and prey variables identified. These species signified the availability of predator-prey links in distinct habitats and the key environmental factors supporting these links. With a high contribution (>51%) of exotic species, an increase in detritivores downstream distinguishes the subtropical East River from temperate rivers. Particularly, in the disturbed lower reaches, the dominance of detritivores prevailed over the predicted increase in other feeding groups (e.g., omnivores and carnivores).

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Drought is a complex natural hazard that may have destructive damages on societal properties and even lives. Generally, socioeconomic drought occurs when water resources systems cannot meet water demand, mainly due to a weather-related shortfall in water supply. This study aims to propose a new method, a heuristic method, and a new index, the socioeconomic drought index (SEDI), for identifying and evaluating socioeconomic drought events on different severity levels (i.e., slight, moderate, severe, and extreme) in the context of climate change. First, the minimum in-stream water requirement (MWR) is determined through synthetically evaluating the requirements of water quality, ecology, navigation, and water supply. Second, according to the monthly water deficit calculated as the monthly streamflow data minus the MWR, the drought month can be identified. Third, according to the cumulative water deficit calculated from the monthly water deficit, drought duration (i.e., the number of continuous drought months) and water shortage (i.e., the largest cumulative water deficit during the drought period) can be detected. Fourth, the SEDI value of each socioeconomic drought event can be calculated through integrating the impacts of water shortage and drought duration. To evaluate the applicability of the new method and new index, this study examines the drought events in the East River basin in South China, and the impact of a multi-year reservoir (i.e., the Xinfengjiang Reservoir) in this basin on drought analysis is also investigated. The historical and future streamflow of this basin is simulated using a hydrologic model, Variable Infiltration Capacity (VIC) model. For historical and future drought analysis, the proposed new method and index are feasible to identify socioeconomic drought events. The results show that a number of socioeconomic drought events (including some extreme ones) may occur in future, and the appropriate reservoir operation can significantly ease such situation.

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The occurrence and distribution of 11 antibiotics in multiple environmental media including river water, suspended particle, sediment, and soil of the East River catchment, South China, were systematically characterized from multiple spatial (area and point) and temporal (seasonal variation and diurnal variation) scales. The results indicated that six to eight antibiotics were detected in these media. The predominant antibiotics and their concentrations varied with environment media because of the varied contamination sources, physicochemical properties of antibiotics, and media. For their spatial distribution, the river environment including water phase, suspended particles, and sediments showed a similar feature with the rule of C delta > C lower reach > C middle reach, which may be related to the industrial level and population density. However, the antibiotics in the soils showed a close relationship with the land use types. A diurnal variation of antibiotics at river sections was mainly affected by the tidal change and diurnal domestic sewage discharge feature. Source analysis indicated that domestic sewage was the main source for antibiotic contamination in the river, while irrigation and fertilization using river water and animal wastes were the main reasons for antibiotic contamination in the soils. However, antibiotics may be redistributed in different media in a catchment.

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