How does anthropogenic activity influence the spatial distribution of dissolved organic matter in rivers of a typical basin located in the Loess Plateau, China.

River ecosystems interact strongly with adjacent terrestrial environments and receive dissolved organic matter (DOM) from a variety of sources, all of which are vulnerable to human activities and natural processes. However, it is unclear how and to what extent human and natural factors drive DOM quantity and quality changes in river ecosystems. Here, three fluorescence components were identified via optical techniques, including two humic-like substances and one protein-like component. The protein-like DOM was mainly accumulated in anthropogenically impacted regions, while humic-like components exhibit the opposite trend. Furthermore, the driving mechanisms of both natural and anthropogenic factors on the variations in DOM composition were investigated using partial least squares structural equation modelling (PLS-SEM). Human activities, especially agriculture, positively influence the protein-like DOM directly by enhancing anthropogenic discharge with protein signals and also indirectly by affecting water quality. Water quality directly influences the DOM composition by stimulating in-situ production through a high nutrient load from anthropogenic discharge and inhibiting the microbial humification processes of DOM due to higher salinity levels. The microbial humification processes can also be restricted directly by a shorter water residence time during the DOM transport processes. Furthermore, protein-like DOM was more sensitive to direct anthropogenic discharge than indirect in-situ production (0.34 vs. 0.25), especially from non-point source input (39.1%), implying that agricultural industry optimization may be an efficient way to improve water quality and reduce protein-like DOM accumulation.

Effects of heavy metals and hyporheic exchange on microbial community structure and functions in hyporheic zone.

The responses of microbial communities in hyporheic zone to the eco-hydrological process have been a hotspot in river ecological health research. However, the impact of different metal pollution levels and hyporheic exchange on the microbial communities are still unclear. In this study, we further explored the effects of different degrees of heavy metals pollution and the strength of hyporheic exchange on the structures and functions of microbial community in hyporheic zone sediment ecosystem. Sediments were collected from the Weihe River to determine the concentrations of heavy metals, grain size distribution, and hydraulic conductivity, and the microbial information were obtained by eDNA technology. The comprehensive pollution status of the study area was at the slight and moderate level. The hydraulic conductivity (Kv) varied between 0.20 and 3.65 (m/d). The microbial community structures had complex temporal and spatial heterogeneity. The microbial molecular ecological network had modular characteristics and significant differences in different periods (p < 0.05). Metabolic functional genes in microbial communities had the highest relative abundance. In particular, there is a significant negative correlation between heavy metals and microorganisms (p < 0.05), with Cu and Zn contributing the most to microbial community changes (p < 0.05). Moreover, grain size had a significant impact on microorganisms, heavy metals and grain size significantly affect the predictive functions of microbial communities. Our in-depth research on microorganisms in the hyporheic zone provides references for monitoring and bioremediation of aquatic ecosystems.

Evaluation of water conservation function of Danjiang River Basin in Qinling Mountains, China based on InVEST model.

With the shortage of water resources becoming a global concern, the water conservation function has become one of the most important service functions and the key factor in the sustainable development of watershed ecosystem. The Danjiang River Basin as an important source of water for the middle route of China's South-to-North Water Diversion Project, its water conservation function has attracted extensive public attention under global climate change. In this study, InVEST water yield model based on Budyko hydrological method was employed to analyze the spatio-temporal dynamics of water conservation, and the response of water conservation to climate, land use and soil changes for the period from 2000 to 2019. The results show that the water conservation of Danjiang River Basin tends to decrease under the comprehensive influence of various factors. The spatial analysis of the importance of water conservation identified Shangnan County, the southern part of Danfeng County and the northern part of Shanyang County as important water conservation areas in the study area, which should be regarded as the key and priority protection areas in the regional water resource and ecological protection. The study provides insights for sustainable water management and ecological protection policies, and the InVEST model with localized parameters can also be applied to other areas lacking climate, hydrological and geological data.

Assessment of aquatic ecological health based on determination of biological community variability of fish and macroinvertebrates in the Weihe River Basin, China.

A healthy aquatic ecosystem plays an important role in the operation of nature and the survival of human beings. Understanding the mechanism of its interaction with the habitat process is conducive to formulating targeted ecological recovery plans. In this study, fish and macroinvertebrates were collected from 49 investigation sites in the Weihe River basin, China, during periods of the summer and the autumn of 2017. Cluster analysis and canonical correlation analysis (CCA) were used to analyze the similarity of community distribution of fish and macroinvertebrates and their response to environmental variables. The biological integrity index of fish (F-IBI) and benthic-macroinvertebrate (B-IBI) was introduced to evaluate the aquatic ecological health. The results showed that fish communities were more coherent than macroinvertebrate communities. The distinguished response to ecological factors was identified for fish and macroinvertebrates. The ecological factors of total nitrogen, conductivity and river width have significant effects on both fish and macroinvertebrate communities. In addition, the fish community was significantly influenced by chlorine, fluorine, pH and flow velocity, while the macroinvertebrate community was significantly influenced by bicarbonate and water depth. The differences in community structure and response to ecological factors between communities were amplified in their environmental quality scores. Although F-IBI and B-IBI tend to be consistent temporally, the correlation is not significant. B-IBI showed decreasing gradient of ecological health status in the downstream area, while F-IBI tended to be different across river systems, which further illustrated the differences in the response of fish and macroinvertebrates to environmental variables.

Comprehensive evaluation and scenario simulation for the water resources carrying capacity in Xi'an city, China.

The quantity and quality of water resources are of great importance in maintaining urban socio-economic development. Accordingly, substantial research has been conducted on the concept of the water resources carrying capacity (WRCC). In this study, analytic hierarchy process (AHP) and system dynamics (SD) models were combined to construct a multi-criteria evaluation system of the WRCC and a socio-economic/water resources SD model for Xi'an. The developmental trends of the society, economy, water supply/demand, and wastewater discharge were obtained from 2015 to 2020 using five scenarios designed for distinct purposes; these scenarios and trends were comprehensively evaluated using a combination of qualitative and quantitative analyses. The results indicated that the WRCC (0.32 in 2020) in Xi'an will shift from a normal to a poor state if the current social development pattern is maintained; therefore, we conclude that the socio-economic development of Xi'an is unsustainable. However, under a comprehensive scheme, the WRCC index (0.64 in 2020) will increase by 48% compared with the WRCC index under a business-as-usual scenario. Further, some practical suggestions, including the promotion of industrial reforms and the improvement of water-use efficiency and recycling policies, were provided for improving the regional WRCC.

Effects of chironomid larvae and Limnodrilus hoffmeisteri bioturbation on the distribution and flux of chromium at the sediment-water interface.

The impacts of chironomid larvae and the tubificid worm Limnodrilus hoffmeisteri on the distribution and flux of the heavy metal chromium (Cr) across the sediment-water interface were investigated with a 21-day laboratory microcosm experiment. The two studied species feature different bioturbation modes involving bioirrigation and upward bioconveyance. The Cr concentrations in the overlying water and pore water were measured and compared using treatments with bioturbation by a single species and by combinations of both species and a treatment with no organisms. The results indicated that both bioturbation modes significantly increased the Cr concentrations in the overlying water and pore water. The overlying water had lower Cr concentrations than the pore water. Little variation in the Cr concentrations was observed in the treatment without organisms. Both species enhanced the Cr flux from the pore water to the overlying water. The worm treatments had a great impact on the Cr concentration in the overlying water through intensive upward conveyance activity, while the chironomid larvae treatments exerted significant effects on the Cr variation in the pore water and Cr flux across the interface via bioirrigation activity. These findings reveal the importance of bioturbation in biogeochemical processes in freshwater ecosystems.

Spatiotemporal distribution of polycyclic aromatic hydrocarbons in sediments of a typical river located in the Loess Plateau, China: Influence of human activities and land-use changes.

The Loess Plateau, as the key energy base of China, has sensitive responses to the global changes, and receives polycyclic aromatic hydrocarbons (PAHs) from anthropogenic activities. However, understanding how anthropogenic and climate factors affect synergistically the PAHs distribution in this vulnerable ecological environment is deficient. Here the spatiotemporal distribution of PAHs in sediments from a typical river of the Loess Plateau were investigated. The PAHs were mainly from coal combustion in the range of 194-514 ng g-1, and their concentrations were generally higher in normal season than wet season as the dilution effect of high river discharge and strong precipitation. The interactive effects of land-use and precipitation showed PAHs enriched in forest-grass land were transferred into rivers through surface and subsurface runoff during light rainfall, resulting in the increase of the PAHs concentrations in river sediments. In contrast, large precipitation in wet season would obscure any spatial variations. In addition, human activities, especially energy production, directly enhanced PAHs accumulation in river sediments due to the emission from the production processing of oil and coal, and indirectly influenced the PAHs by impacting the per capita GDP. These findings had important implications for the management and prediction of PAH accumulation.

[Diversity of Zooplankton and Niche Characteristics of Keystone Species in the Weihe River Based on eDNA].

Environmental DNA(eDNA), a new tool for monitoring the biodiversity of aquatic ecosystems, is able to analyze characteristics of biodiversity from a microscopic perspective. Based on eDNA data collected from the Weihe River, diversity indexes, non-metric multidimensional scaling, cluster analysis, and correlation network analysis were employed to explore the diversity and community structure of zooplankton focusing on the niche differentiation of keystone species and environmental adaptability. The eDNA approach identified three types of zooplankton including Rotifera, Cladocera, and Copepoda, among which the dominant species was Brachionus calyciflorus. The zooplankton community shows significant differences in species composition, abundance, diversity and spatial distribution characteristics(P<0.01). The average values of the Chao1 index, ACE index, Shannon index, and Simpson index were 22.25, 22.38, 2.32, and 0.68, respectively. The downstream biodiversity is significantly higher than in the upstream area. Non-metric multidimensional scale analysis and hierarchical cluster analysis further showed that the zooplankton community structure shows distinct regional differences. The keystone species in the community have a high degree of connection with other species, with a high node degree, centrality, and modularity. The niche breadth(Bi) of the operational taxonomic units(OTUs) of the keystone species varied from 0.38 to 0.80. The medium niche species accounted for 63% of all keystone species. The average niche overlap index(Qik) was 0.72, and the degree of overlap was generally high. RDA analysis further identified that water environmental variables were closely related to changes in the zooplankton community structure and niche differentiation. For example, total nitrogen and water temperature were the main limiting factors, which play important roles in shaping the zooplankton community structure.

Surface Water-Groundwater Interaction in the Guanzhong Section of the Weihe River Basin, China.

As a crucial agricultural and economic development zone since the Qin Dynasty (221 to 206 BC), the Guanzhong section of the Weihe River basin is facing serious water resource shortages due to population growth and regional development. Its water resource amount per capita is only 361 m3 , about 1/6 of the average in China and less than 1/20 of the average in the world. Surface water and groundwater (SW-GW) interaction, having a significant influence on the spatiotemporal distribution of water resources, was qualitatively and quantitatively investigated during a wet year based on stable isotopes and hydrochemistry. The results show that the recharge pattern in the north part varies with season, that is, 40% of the surface water recharge comes from groundwater in the dry season, but 93% of the groundwater recharge comes from surface water in the rainy season. In the south part, groundwater is always recharged by surface water, with contributions of 47% and 61% in the rainy and dry seasons, respectively. For the main stream, the recharge pattern is complicated and varies with season and site. This study will provide useful information about SW-GW interaction at basin scale. Integrated management of groundwater and surface water could improve the efficiency of regional water resources utilization and promote accurate and sustainable water management in the semi-arid basin.