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Abstract Freshwater fishery resources in Chinese rivers have been markedly impaired as the results of overfishing, damming, and watershed development. However, little is known about trophic ecology of fish assemblages in the rivers of China. In this study, fishes were collected from an upstream, midstream, and downstream site of the Moyangjiang River, Southern China, to investigate trophic niche structure using stable isotope analysis. We calculated several trophic niche metrics using δ13C and δ15N ratios of the fish assemblages at each study site. Results showed no significant differences in the δ13C ratios between the pelagic and benthic fish assemblages at each study site. This suggests a homogenous dietary base shared by all fish at each site, likely as the result of continuous water column mixing in this shallow and fast‐moving river. The upstream fish assemblage had the greatest species abundance and trophic diversity. The fish assemblage at the midstream site displayed the smallest trophic niche space (total area of the δ13C and δ15N bi‐plot space) and trophic diversity. The midstream site also showed the most depleted 13C, suggesting a more degraded habitat compared with the other two study sites. The fish assemblages at the downstream site displayed the greatest basal resource diversity, largest CR, and trophic niche space and trophic diversity. However, the fish assemblage at the downstream site also displayed the lowest species abundance, redundancy, and evenness of trophic spacing. These findings suggest that the trophic niche of the fish assemblage at the downstream site, which received resources from upstream flow, experienced greater watershed development and greater in situ production, demonstrating positive and negative impacts by anthropogenic activities. Future studies should also gather information on fish production, watershed development, and water quality, to aid in the interpretation of the stable isotope analysis of fish trophic niches.

Urbanization often exerts multiple effects on aquatic and terrestrial organisms, including changes in biodiversity, species composition and ecosystem functions. However, the impacts of urbanization on river phytoplankton in subtropical urbanizing watersheds remain largely unknown. Here, we explored the effects of urbanization on phytoplankton community structure (i.e., biomass, community composition and diversity) and function (i.e., resource use efficiency) in a subtropical river at watershed scale in southeast China over 6 years. A total of 318 phytoplankton species belonging into 120 genera and 7 phyla were identified from 108 samples. Bacillariophyta biomass showed an increasing trend with increasing urbanization level. The phytoplankton community shifted from Chlorophyta dominance in rural upstream waters to Bacillariophyta dominance in urbanized downstream waters. Furthermore, phytoplankton diversity and resource use efficiency (RUE = phytoplankton biomass/total phosphorus) were significantly decreased with increasing urbanization level from upstream to downstream. Phytoplankton RUE exhibited a significant positive correlation with species richness, but a negative correlation with phytoplankton evenness. The variation in environmental factors (turbidity, total nitrogen, NH 4 +-N, total phosphorus, PO 4 3−-P and percentage urbanized area) was significantly correlated with phytoplankton diversity and RUE. Overall, our results revealed the influence of urbanization on phytoplankton community structure and ecosystem function was due to its altering the environmental conditions. Therefore, human-driven urbanization may play crucial roles in shaping the structure and function of phytoplankton communities in subtropical rivers, and the mechanism of this process can provide important information for freshwater sustainable uses, watershed management and conservation. [Display omitted] [ABSTRACT FROM AUTHOR]

Aquatic insect growth is tightly linked to environmental temperature. Growth rate tends to increase with rising temperatures. Growth rate integrates different factors related to population fitness, being partly responsible for species distribution. We aim to estimate daily growth rates from mayfly nymphs reared under different thermal regimes in the field, during five different periods from 2016 to 2018. Twelve species of mayflies were reared in mesocosms in six streams (from three altitudinal levels with a mean elevation 725, 1069, and 1509 m.a.s.l.). Additionally, we transplanted nymphs between lowest and highest pairs of streams, thus rearing them under a different thermal regime. Temperature and other ambient variables were recorded at regular intervals. Daily growth rate (dgr) of most species resulted lower in the higher pair of streams (colder sites) than in the medium and lower streams (warmer sites). Transplant experiment also clearly showed this tendency: (1) nymphs transplanted to colder thermal regimes grew slower than those reared under their natural (warmer) regime and (2) nymphs transplanted to hotter thermal regimes grew faster than those reared under their original (colder) regime. Nymphs of three species (Americabaetis alphus, Leptohyphes eximius, and Cloeodes penai) did not show differences in growth among treatments. Our findings relating sensitiveness of Ephemeroptera nymphs to small temperature changes suggest that the distribution of some species will modify by increases in temperature derived from climatic change. [ABSTRACT FROM AUTHOR]

Previous studies suggested that strong positive correlations between microbial taxa destabilized the microbial communities, because high synchrony between taxa caused different taxa responded in tandem to the changes of environmental conditions. In this study, we used nutrient concentrations (nitrogen and phosphorus) as indicators for anthropogenic activities, and explored the effects of anthropogenic activities on the stability of bacterial communities in a subtropical river, Xiyuan River. We also used co-occurrence networks to indicate the synchrony between bacterial taxa. The nutrient concentrations were higher in the midstream and downstream areas than in the upstream area of Xiyuan River. As a result, the average proportion and correlation coefficient of positive correlations were higher in the midstream and downstream networks than in the upstream networks. This indicates that frequent anthropogenic activities increased the synchrony between taxa in the midstream and downstream communities, and therefore destabilized the bacterial communities. To further explore the mechanisms, we found that the changes of community stabilities associated with the changes of bacterial functions. Anthropogenic activity tolerant bacteria (e.g., nutrient removal, aromatic degradation and pathogen bacteria) and their linked bacterial taxa formed large and strong positive modules in the midstream and downstream networks, and thus destabilized the communities. Based on the perspective of microbial correlations, our results provide a new insight in the mechanisms of how anthropogenic activities alter riverine microbial communities. [ABSTRACT FROM AUTHOR]

Abstract Background The deep mechanisms (deterministic and/or stochastic processes) underlying community assembly are a central challenge in microbial ecology. However, the relative importance of these processes in shaping riverine microeukaryotic biogeography is still poorly understood. Here, we compared the spatiotemporal and biogeographical patterns of microeukaryotic community using high-throughput sequencing of 18S rRNA gene and multivariate statistical analyses from a subtropical river during wet and dry seasons. Results Our results provide the first description of biogeographical patterns of microeukaryotic communities in the Tingjiang River, the largest river in the west of Fujian province, southeastern China. The results showed that microeukaryotes from both wet and dry seasons exhibited contrasting community compositions, which might be owing to planktonic microeukaryotes having seasonal succession patterns. Further, all components of the microeukaryotic communities (including total, dominant, always rare, and conditionally rare taxa) exhibited a significant distance-decay pattern in both seasons, and these communities had a stronger distance-decay relationship during the dry season, especially for the conditionally rare taxa. Although several variables had a significant influence on the microeukaryotic communities, the environmental and spatial factors showed minor roles in shaping the communities. Importantly, these microeukaryotic communities were strongly driven by stochastic processes, with 89.9%, 88.5%, and 89.6% of the community variation explained by neutral community model during wet, dry, and both seasons, respectively. The neutral community model also explained a large fraction of the community variation across different taxonomic groups and levels. Additionally, the microeukaryotic taxa, which were above and below the neutral prediction, were ecologically and taxonomically distinct groups, which might be interactively structured by deterministic and stochastic processes. Conclusions This study demonstrated that stochastic processes are sufficient in shaping substantial variation in river microeukaryotic metacommunity across different hydrographic regimes, thereby providing a better understanding of spatiotemporal patterns, processes, and mechanisms of microeukaryotic community in waters.

A large amount of terrestrial nutrients are discharged into the ocean through rivers. However, the impact of seasonal hydrological variations on riverine nutrient concentrations and fluxes remains unclear, especially in the medium-sized subtropical rivers that are highly influenced by human activities. In this study, we investigated the monthly changes in nutrient concentrations (soluble reactive phosphorus, SRP; dissolved silicate, DSi; and dissolved inorganic nitrogen, DIN) in the North Stream (NS) and West Stream (WS) of the Jiulong River (JLR). The results show that the concentrations of SRP and DSi in the NS and the WS displayed a similar seasonal variability, which was different from the pattern of DIN. Hydrological conditions, chemical fertilizer loss and biogeochemical processes are responsible for the seasonal changes in the nutrients in the two streams, especially during extreme rain events. Nutrient concentrations in the NS exhibited a clockwise trajectory along with river discharge during rain events, while a reverse pattern in the WS was observed since it experienced a moderately long rain event. Different rainfall features between the two main tributaries resulted in the majority of nutrients being exported at the start of the rain event in the NS and the end of rain event in the WS. Indeed, the annual high flow (Q/Qm > 3) accounts for ~17.3% of the annual nutrient flux in the JLR even though this period spans only ~4.0% of a year, which suggests the importance of rain events on nutrient export in these subtropical rivers. Although the annual fluxes of DIN and SRP in the JLR were smaller than many rivers worldwide, higher areal yields of DIN and SRP were observed, indicating that the JLR is highly influenced by human activities. Our study systematically evaluated the response of nutrient concentrations to hydrological changes in two tributaries of the JLR, which is useful in better understanding the nutrient dynamics in medium-sized subtropical rivers.

Riparian soils and vegetation are important factors influencing the biodiversity and biogeochemical processes of river ecosystems. Riparian soils and vegetation form the foundation for multiple ecosystem services provided by river ecosystems. However, it remains poorly understood how riparian soils and vegetation interact with one another to maintain these services. In this study, we sampled four common types of riparian vegetation associated with the Beijiang River in South China. These included forestland, bamboo forest, mixed forest, and grassland ecosystems. Specifically, we analyzed the spatial distribution of riparian soils and their response to environmental factors (i.e., coverage and height of trees, shrubs and grass, distance to river, and altitude). Our results indicate that soil properties in riparian zones were affected significantly by vegetation type. In particular, clay content, soil organic carbon, and nitrate nitrogen content were significantly correlated with vegetation type. In contrast, changes in soil total nitrogen, total phosphorus, and available phosphorus content were not associated with vegetation type. Moreover, soil physical and chemical properties interacted with one an other, as well as with vegetation characteristics. This was indicated by the significant correlation observed between soil organic carbon, total nitrogen, total phosphorus, and soil texture, with structural characteristics of the four vegetation types. We also found that height and cover of trees and shrubs were significantly correlated with soil chemical properties. However, the effects of topographic variables such as altitude and distance to river were not significant. Results from this study can thus provide a basis for the ecological restoration and land management of degraded iparian zones.

[Display omitted] • Nitrification remains a key process throughout the year in both tributaries. • Ammonium uptake velocity in the NS is double WS's (8 m/yr) during low-flow periods. • Rainstorms trigger particle-associated and sedimentary denitrification. • Inhibitory effect of NH 4 + contributes to NO 3 − export to the adjacent Taiwan Strait. • Main N inputs are from sewage (NH 4 +), soil (NO 3 −) & shift from sewage to soil (PN). The sources and transformations of various nitrogen (N) species in rivers under the influence of anthropogenic inputs are the main concern of eutrophication research, especially in intermediate- and small-sized rivers, and are essential to understand for practical management. A key challenge lies in determining how N species can be altered by diverse biogeochemical processes along flow paths under different hydrological controls (e.g. river discharge). To better understand the response of in-stream N dynamics, we examined N transformation processes and identified the sources of various N species in the Jiulong River (JLR), a eutrophic river in China composed of two main tributaries: the North Stream (NS) and the West Stream (WS), using multiple isotopes (δ15N NO3 , δ18O NO3 , δ15N NO2 , δ15N NH4 , δ15N PN , δD H2 O and δ18O H2O). Our results reveal that hydrological and biogeochemical processes exert a joint control on river N dynamics. Key processes, such as nitrification and ammonium (NH 4 +) uptake, predominated in both tributaries during non-rainfall periods, while during rain events, nitrification remained significant. However, denitrification occurred differently in the NS (mainly on particles) and the WS (primarily in sediment), and was influenced by factors such as sediment characteristics and water flow patterns. We also observed differences in the uptake velocity of denitrification and NH 4 + between the two tributaries, highlighting the variable importance of these two processes. The main sources of N species varied based on hydrological conditions, with different contributions in the NS and WS. During non-rainfall periods, point sources significantly controlled N species compositions in both tributaries, with manure & sewage contributing 46 % and 42 % of the NH 4 +, 30 % and 50 % of the nitrate (NO 3 −), and sewage accounting for 62 % and 56 % of the particulate N (PN) in the NS and WS, respectively. During rainstorms, non-point sources became the predominant contributors of NO 3 − in both tributaries (soil organic N: NS = 48 %, WS = 38 %; chemical fertilizers: NS = 29 %, WS = 26 %) and PN (soil: NS = 52 %, WS = 57 %). The insights revealed using multiple isotopes serve as a foundational basis for advanced modeling and scenario development when managing subtropical rivers under complex human interferences and climate change. [ABSTRACT FROM AUTHOR]

Abstract The dissolved organic matter (DOM) composition in river ecosystems can reflect human impacts on the river ecosystem, and plays an important role in the carbon cycling process. We collected water and phytoplankton samples at 107 sites in the Dongjiang River in two seasons to assess the impact of the sub-catchments land use composition. The results showed that (1) the forested sub-catchments had higher humic-like C1 (16.45%) and C2 (25.04%) and lower protein-like C3 (22.57%) and C4 (35.95%) than the urbanized and mixed forest-agriculture sub-catchments, while the urbanized sub-catchments showed an inverse trend (4.54%, 15.51%, 33.97% and 45.98%, respectively). (2) A significant variation in the proportion of C1 and C4 between the dry and rainy seasons was recorded in both the forested and the mixed forest-agriculture sub-catchments (p < 0.01), but only C4 showed an obvious seasonal variation in the urbanized sub-catchments (p < 0.01); while the DOM composition was mainly related to the proportion of urbanized and forested land year-round (p < 0.01). (3) No significant correlation was found in either season between the DOM composition and the agricultural land proportion (p > 0.05). Our findings indicated that the DOM composition was strongly dependent on the land use composition of the sub-catchments and varied seasonally, but the seasonal variation pattern could be disturbed by human activities in the extensively urbanized catchments. Notably, when considering bioavailability, the higher C4 proportion compared with those in temperate rivers implied a larger amount of CO 2 released from subtropical rivers into the atmosphere. Graphical abstract Image 1005 Highlights • The DOM composition was strongly dependent on the catchments land use structure. • Seasonal variation pattern of the DOM composition was observed. • This pattern could be disturbed by human activities in the urbanized catchments. • The high protein-like in subtropical rivers released more CO 2 into the atmosphere. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Over 85% samplings along the Daning River are supersaturated with CO 2. • p CO 2 levels in the tributaries are significantly higher than that in the main stem. • Human induced direct C influx and terrestrial environment considerably affect p CO 2. • TDN, TDP and T water significantly relate to p CO 2 in rainy season rather than in dry season. • CO 2 evasion accounts for 38.8% of riverine dissolved carbon delivery. Abstract Rivers and streams are important agents for global and regional carbon cycle, which represent a nonnegligible carbon source to the atmosphere. However, carbon dioxide (CO 2) degassing via water-air interface from inland waters currently has a large uncertainty in estimation. Here we examined the CO 2 partial pressure (p CO 2) and CO 2 degassing in the Daning River, a main tributary of the Yangtze in the Three Gorges Reservoir (TGR) region. The riverine p CO 2 showed obvious spatial and temporal variations on the main stem and its tributaries, ranging from 483.2 ± 293.9 μatm (May) to 2183.3 ± 1309.2 μatm (August) with an average of 1198.2 ± 1122.9 μatm. p CO 2 increased with the intensive agricultural practices and population on the lower reach of the tributary. The daytime average water-air CO 2 flux was 329.8 ± 470.2 mmol/m2/d, and yielded a total CO 2 efflux approximately 0.12 Tg CO 2 /y. The results also indicated that carbon loss via atmospheric exchange accounted for 38.8% of riverine dissolved carbon fluxes. There were close relations between p CO 2 and environmental parameters such as total dissolved nitrogen (TDN), total dissolved phosphorus (TDP) and water temperature in the rainy season, but not in the dry season. Our results demonstrated the essential need to highlight the importance of anthropogenic activities on global and regional CO 2 outgassing, and it is urgent to underscore the detailed biogeochemical processes in riverine p CO 2 , as well as the couplings between p CO 2 and environmental parameters. [ABSTRACT FROM AUTHOR]

The phytoplankton community in the river is closely related to the location of the river and the impact of human activities. To summarize the patterns of phytoplankton community changes in rivers and to analyze the reasons for these patterns and differences, we sampled the three tributaries of the Dongjiang River at different latitudes in the dry and rainy season for three years. The results showed that the three rivers were mesotrophic, lightly eutrophic and moderately eutrophic respectively. From the south to the north, the water temperature and nutrition showed an increasing trend. In two different seasons, the differences in the water temperature and dissolved oxygen were clear. In the dry season, results of the multidimensional scaling (MDS) analysis indicated that the phytoplankton community structures in the Li River and Qiuxiang River were similar. Regardless of the number of species, the cell abundance or the dominance index, Bacillariophyta were found to be dominant. Chlorophyta was dominant in the Danshui River. In the rainy season, Bacillariophyta, Bacillariophyta-Chlorophyta and Chlorophyta-Cyanophyta became the dominant types in the Li River, Qiuxiang River and Danshui River, respectively. These different patterns in phytoplankton community variation were affected by both the water quality and temperature. [ABSTRACT FROM AUTHOR]

Whether bacterioplankton are assembled in the same way as microeukaryotes is a key question that has been answered only partially in microbial ecology. In particular, relating distribution patterns to the underlying ecological processes for plankton communities in highly dynamic ecosystems, such as river−reservoirs subjected to anthropogenic impacts, remains largely unstudied. Here, we analyzed taxonomic distribution patterns, and unraveled community assembly processes underlying the core and random bacterioplankton and microeukaryotes from a subtropical river−reservoir system. These plankton domains were modelled using the spatial abundance distributions (SpADs) of the operational taxonomic units (OTUs) as a proxy for abundant and rare taxa delineation. Both bacterioplankton and microeukaryote plankton communities exhibited significant distance−decay relationships, and samples were grouped depending on reservoir or river habitats. The neutral community model showed that 35−45% of the plankton community variation could be explained by neutral processes. The phylogenetic null model revealed that dispersal limitation accounted for the largest percentage of pairwise comparisons (42−68%), followed by environmental selection (18−25%). We concluded that similar prevalence of ecological processes acting on particular subsets of the bacterioplankton and microeukaryotes might have resulted from similar responses to environmental change, potentially induced by human activities in the watershed.

The phytoplankton community in the river is closely related to the location of the river and the impact of human activities. To summarize the patterns of phytoplankton community changes in rivers and to analyze the reasons for these patterns and differences, we sampled the three tributaries of the Dongjiang River at different latitudes in the dry and rainy season for three years. The results showed that the three rivers were mesotrophic, lightly eutrophic and moderately eutrophic respectively. From the south to the north, the water temperature and nutrition showed an increasing trend. In two different seasons, the differences in the water temperature and dissolved oxygen were clear. In the dry season, results of the multidimensional scaling (MDS) analysis indicated that the phytoplankton community structures in the Li River and Qiuxiang River were similar. Regardless of the number of species, the cell abundance or the dominance index, Bacillariophyta were found to be dominant. Chlorophyta was dominant in the Danshui River. In the rainy season, Bacillariophyta, Bacillariophyta-Chlorophyta and Chlorophyta-Cyanophyta became the dominant types in the Li River, Qiuxiang River and Danshui River, respectively. These different patterns in phytoplankton community variation were affected by both the water quality and temperature.

Over the past decade, restoration of the Kissimmee River in central Florida has received considerable attention from local, state, national, and international media. In terms of areal extent, project cost, and ecological evaluation it is one of the largest and most comprehensive river restoration projects in the world. The goal of reestablishing ecological integrity involves restoring the physical attributes and the hydrologic processes that were lost after channelization of the river in the 1960s. The project is expected to restore over 80 km2 of floodplain wetlands and reestablish over 70 km of river channel. Restoration construction began in 1999; to date, three construction phases have been completed, with the final phase of construction slated for completion in 2019. Restoration evaluation is widely viewed as a critical component of any restoration project. Equally important is the dissemination of information gained from restoration evaluation programs. This introductory article presents a brief overview of project history and outlines the approach and logic of the Kissimmee River Restoration Evaluation Program. The following papers present the results of ecological studies conducted before and after completion of the first phase of restoration construction. This first phase reestablished flow through 23 km of reconnected river channels and seasonally inundated a large portion (approximately 2,900 ha) of the floodplain within the Phase I project area. Although these studies present interim responses prior to full hydrologic restoration, results suggest that the ecosystem is responding largely as predicted by performance measures developed prior to restoration construction. [ABSTRACT FROM AUTHOR]

Aim Dendritic ecological networks ( DENs), such as river systems, combine features that challenge the traditional conceptual views and empirical approaches applied to metacommunities. As a result of their dendritic branching geometry and stream flow directionality, they are strongly hierarchical and asymmetrical. We analysed the metacommunity structure of benthic diatoms in a large-scale river system with the aim of evaluating the importance of potential causal influences. Furthermore, we hypothesized that metacommunities of diatoms that are strongly attached to their substrata show a different spatial structure than metacommunities of other, more weakly attached diatoms. Location The study was carried out in the Dong River, a 32,275 km2 subtropical river network located in southern China. Methods We surveyed benthic diatom communities during three seasons (dry, intermediate and wet). Using partial redundancy analysis, we partitioned community variation among environmental models and different spatial eigenfunction models to evaluate the influence of alternative dispersal pathways (overland versus water course dispersal), stream directionality, man-made dams and diatom functional traits on diatom metacommunity structure. Results Models based on hydrological connections and water directionality represent spatial patterns better than overland distances, suggesting that the dynamics of benthic diatom metacommunities are mainly confined to the river network and influenced by the prevailing water flow. We found significant effects of man-made dams on the spatial structure of important limnological variables and diatom metacommunity structure. The metacommunity of strongly attached diatoms also showed a weaker signature of flow directionality than that of other growth forms, especially in seasons with high water levels. Main conclusions We conclude that the consideration of among-site connectivity, flow directionality and species traits is key to a better understanding of the spatial ecology of passively dispersing microbial organisms in river systems. [ABSTRACT FROM AUTHOR]

A large amount of terrestrial nutrients are discharged into the ocean through rivers. However, the impact of seasonal hydrological variations on riverine nutrient concentrations and fluxes remains unclear, especially in the medium-sized subtropical rivers that are highly influenced by human activities. In this study, we investigated the monthly changes in nutrient concentrations (soluble reactive phosphorus, SRP; dissolved silicate, DSi; and dissolved inorganic nitrogen, DIN) in the North Stream (NS) and West Stream (WS) of the Jiulong River (JLR). The results show that the concentrations of SRP and DSi in the NS and the WS displayed a similar seasonal variability, which was different from the pattern of DIN. Hydrological conditions, chemical fertilizer loss and biogeochemical processes are responsible for the seasonal changes in the nutrients in the two streams, especially during extreme rain events. Nutrient concentrations in the NS exhibited a clockwise trajectory along with river discharge during rain events, while a reverse pattern in the WS was observed since it experienced a moderately long rain event. Different rainfall features between the two main tributaries resulted in the majority of nutrients being exported at the start of the rain event in the NS and the end of rain event in the WS. Indeed, the annual high flow (Q/Qm > 3) accounts for ~17.3% of the annual nutrient flux in the JLR even though this period spans only ~4.0% of a year, which suggests the importance of rain events on nutrient export in these subtropical rivers. Although the annual fluxes of DIN and SRP in the JLR were smaller than many rivers worldwide, higher areal yields of DIN and SRP were observed, indicating that the JLR is highly influenced by human activities. Our study systematically evaluated the response of nutrient concentrations to hydrological changes in two tributaries of the JLR, which is useful in better understanding the nutrient dynamics in medium-sized subtropical rivers. [ABSTRACT FROM AUTHOR]

Diatoms have been regularly used as bioindicators to assess water quality of surface waters. However, diatom-based indices developed for a specific geographic region may not be appropriate elsewhere. We sampled benthic diatom assemblages in the upper Han River, a subtropical river in China, to evaluate applicability of 14 diatom-based indices used worldwide for water quality assessment. A total of 194 taxa from 31 genera were identified in the dry season and 139 taxa from 23 genera in the wet season. During the dry season, significant relationships were found for all but one of the diatom-based indices (Index Diatom Artois-Picardie) with one or more physical and chemical variables including nutrients and ion concentrations in river waters. The Biological Diatom Index (IBD) and diatom-based eutrophication/pollution index (EPI-D) were strongly related to trophic status and ionic content, while Watanabe's Index was related to organic pollution and conductivity. Yet, the diatom indices showed weak relationships with physical and chemical variables during the wet season. It suggests that diatom-based indices developed in Europe can be applied with confidence as bioindicators of water quality in subtropical rivers of China, at least during base-flow conditions. [ABSTRACT FROM AUTHOR]

Purpose: River sediments play a crucial role in the storage and transformation of organic matter (OM). Nutrient dynamics are controlled by the interaction of several key parameters, i.e. river discharge, channel geometry and vertical exchanges of water (upwelling vs. downwelling zones). The main aim of this study was to evaluate the effect of channel forms and discharge variation on nutrient spiralling in the hyporheic zone (HZ) of streams. Materials and methods: Four experimental flow manipulations (EFM) were carried out at two reaches with different channel forms (straight vs. sinuous) in an oligotrophic subtropical river in Australia. Flow manipulation consisted of reducing the river width with a temporary dam, diverting and concentrating the main water flux on two different geomorphological units (riffle vs. gravel bar), in order to simulate flooding conditions. Hyporheic waters were analysed for their physicochemical characteristics and nutrient (nitrates + nitrites = NO and soluble reactive phosphorus [SRP]) and OM contents at two depths (10 and 50 cm) within the bed sediments, both upstream and downstream of the geomorphological units. Results and discussion: The physicochemical parameters clearly demonstrated the existence of hyporheic flow paths, characterized by the alternation of downwelling and upwelling areas, with more consistent gradients in gravel bars than in riffles. The HZ acted as source for NO and SRP, but this role varied between geomorphological units and reaches. The effect of EFM differed between sampling points, irrespective of the type of geomorphological unit. In gravel bars, a flush out during high discharge was observed for NO, SRP and particulate organic matter (POM) at the sinuous channel, whereas storage and removal were recorded at the straight channel for SRP and NO, respectively. At the riffle of the sinuous channel, very fine POM accumulated, while removal was noticed for POM. In contrast, at the riffle of the straight channel, SRP accumulated in the HZ and NO was removed out of the HZ. Conclusions: Nutrient dynamics in the HZ and the response to flow increases were not governed by the geomorphological unit type. Other parameters that determine water residence time in the sediments, such as local heterogeneity in sediment characteristics (grain size, porosity and hydraulic conductivity), channel sinuosity, reach slope and the size and form of the gravel bar, may be more significant explanatory variables for understanding OM and nutrient dynamics in the HZ. This study emphasizes the need for caution in making generalisations about the role of river sediment in nutrient storage and the impact of floods on nutrient dynamics. [ABSTRACT FROM AUTHOR]

Prokaryotic and eukaryotic microbes are key organisms in aquatic ecosystems and play pivotal roles in the biogeochemical cycles, but little is known about genetic diversity of these communities in subtropical rivers. In this study, microbial planktonic communities were determined by using denaturing gradient gel electrophoresis (DGGE) analysis from the Jiulong River, southeast China, and their relationships with local environmental factors were studied. The Betaproteobacteria (26%) and Dinophyceae (26%) were the most dominant taxa in prokaryotic and eukaryotic clones derived from DGGE bands, respectively. Further, both cluster and ordination analyses of prokaryotic and eukaryotic DGGE fingerprinting resulted in three identical groups from the 15 sites, which were closely related with the environmental factors. Partial redundancy analysis (partial RDA) revealed that agricultural pollution (phosphorus and nitrogen) and saltwater intrusion (conductivity and salinity) were the main factors impacting microbial community composition, by explaining more than two-thirds of the total variation in both prokaryotic (67.0%) and eukaryotic (70.5%) communities. Moreover, the robust and quantifiable relationship between DGGE results and environmental variables indicated that the community-level molecular fingerprinting techniques could support the physicochemical assessment of riverine water quality and ecosystem health. [ABSTRACT FROM AUTHOR]

The stable nitrogen isotope ratios (δ15N) of particulate organic matter (POM) and periphyton were measured at sites along the Dashahe River, a short subtropical watercourse in SE China. δ15N values of both POM and periphyton tended to increase from upstream to downstream, ranging from 1.1 to 7.4 for periphyton and 1.6 to 7.2 for POM. The δ15N values in both POM and periphyton were depleted at upstream sites with lower concentrations of total nitrogen (TN) and total phosphorus (TP) but enriched at downstream sites with relatively high TN and TP. In addition, δ15N values for POM and periphyton showed a significant negative correlation with distance to the most polluted section of the river, implicating human impact in the pattern of increasing δ15N. This study confirms δ15N in POM and periphyton as an excellent indicator of anthropogenic nitrogen pollution in rivers.

Whether bacterioplankton are assembled in the same way as microeukaryotes is a key question that has been answered only partially in microbial ecology. In particular, relating distribution patterns to the underlying ecological processes for plankton communities in highly dynamic ecosystems, such as river–reservoirs subjected to anthropogenic impacts, remains largely unstudied. Here, we analyzed taxonomic distribution patterns, and unraveled community assembly processes underlying the core and random bacterioplankton and microeukaryotes from a subtropical river–reservoir system. These plankton domains were modelled using the spatial abundance distributions (SpADs) of the operational taxonomic units (OTUs) as a proxy for abundant and rare taxa delineation. Both bacterioplankton and microeukaryote plankton communities exhibited significant distance–decay relationships, and samples were grouped depending on reservoir or river habitats. The neutral community model showed that 35–45% of the plankton community variation could be explained by neutral processes. The phylogenetic null model revealed that dispersal limitation accounted for the largest percentage of pairwise comparisons (42–68%), followed by environmental selection (18–25%). We concluded that similar prevalence of ecological processes acting on particular subsets of the bacterioplankton and microeukaryotes might have resulted from similar responses to environmental change, potentially induced by human activities in the watershed. [ABSTRACT FROM AUTHOR]

Although seasonality is a key driver of environmental fluctuation in aquatic ecosystems, there exists limited knowledge on the factors controlling the distribution of antibiotic resistance genes (ARGs) across seasons at a watershed scale. Here we used high-throughput quantitative PCR to quantify 285 ARGs conferring resistance to most major classes of antibiotics, reveal their spatial and seasonal distribution patterns, and depict the underlying mechanisms in a subtropical riverine ecosystem under low and high human pressures, in Xiamen city, southeast China. Our results showed that spatial differences in ARG richness and abundance overwhelmed their seasonal variations, with only ARGs that confer resistance to sulfonamide and vancomycin being significantly different across seasons. Only a few abundant ARGs (19 ARGs) could contribute to >70% of the total ARGs abundance and were found in all seasons. The significantly higher number of ARGs in the summer rainy period than other seasons coincided with high number of significant edges in ARG co-occurrence networks. Summer rainfall had strong dilution effect on ARGs in upstream waters and enrichment effect in downstream waters. The variance partitioning analysis indicated that the environment explained larger variance of ARG profiles than mobile genetic elements (MGEs), spatial predictors and the rainfall. Nevertheless, strong and significant correlations between transposase gene absolute abundance and aminoglycoside, chloramphenicol, MLS, multidrug and tetracycline classes of resistance genes inferred the role of MGEs on ARG distribution. Overall, our results imply that the modelling and management of ARGs in highly dynamic ecosystems could be better implemented by considering priority genes that dominate at spatial and seasonal gradients., (Copyright © 2019 Elsevier B.V. All rights reserved.)