Subtropical River Research Articles

Trophic niche diversity and redundancy across trophic positions in a subtropical river fish assemblage

Trophic niche diversity and redundancy across trophic positions in a subtropical river fish assemblage

The chemical characteristics and mixing behaviors of particulate organic matter from small subtropical rivers in coastal Gulf of Mexico

Rivers transport a substantial amount of dissolved and particulate organic matter (DOM and POM) to the oceans. In estuaries, the transition between DOM and POM due to dissolution or flocculation and their chemical compositions are key in determining their remineralization potential and thus influence the biogeochemical cycling of carbon and nitrogen. While much effort has been focused on trace metals, minerals, and nutrients in estuaries, the mixing behaviors of POM have been understudied despite their biogeochemical and ecological importance. In this work, water from several rivers from the northwestern Gulf of Mexico was mixed with seawater in different proportions, and the amount as well as the chemical characteristics of POM in the mixtures was measured. POM was characterized by organic carbon and nitrogen contents, carbon and nitrogen stable isotopes (δ13C and δ15N), and total hydrolyzable amino acids (THAA). The results showed that POM in most cases behaved conservatively after the mixing of river water with seawater. However, rapid POM formation was observed in one of the seven experiments, which was conducted in 2021 with Mission River water, a relatively pristine river. The amount of particulate organic carbon formed in this experiment (41 μM) was 12% of the dissolved organic carbon at a salinity of 6. The formed POM was more depleted in 13C and had a higher POC/PON ratio than expected for conservative mixing, suggesting that terrestrial DOM, likely sourced from lignins and/or lipids, may have contributed to the flocculates. The short duration of these laboratory experiments under dim or dark conditions excludes the potential effects from biotic or photochemical processes, and thus the observations should be attributed to the rapid physical transformation of DOM to POM or flocculation, particularly when the riverine OM is contributed more from terrestrial sources than in situ production. Overall, although POM mostly behaved conservatively, substantial POM can be produced from DOM under right conditions, and these results shed new light on the biogeochemical cycling of organic matter in subtropical estuaries.

Human-induced homogenization of microbial taxa and function in a subtropical river and its impacts on community stability

Combination of taxa and function can provide a more comprehensive picture on human-induced microbial homogenization. Here, we obtained 2.58 billion high-throughput sequencing reads and 479 high-quality metagenome-assembled genomes (MAGs) of planktonic microbial communities in a subtropical river for 5 years. We found the microbial taxa homogenization and functional homogenization were uncoupled. Although human activities in downstream sites significantly decreased the taxonomic diversity of non-abundant ASV communities (16S rRNA gene amplicon sequence variants), they did not significantly decrease the taxonomic diversity of abundant ASV and total observed MAG communities. However, the total observed MAG communities in downstream sites tended to homogenize into some specific taxa which encode human-activity-related functional genes, such as nutrient cycles, greenhouse gas emission, antibiotic and arsenic resistance. Those specific MAGs with high taxonomic diversity caused the weak heterogenization of total observed MAG communities in downstream sites. Moreover, functional homogenization promoted the synchrony among downstream MAGs, and these MAGs constructed some specific network modules might to synergistically execute or resist the human-activity-related functions. High synchrony also led to the tandem effects among MAGs and thus decreased community stability. Overall, our findings revealed the links of microbial taxa, functions and stability under human activity impacts, and provided a strong evidence to encourage us re-thinking biotic homogenization based on microbial taxa and their functional attributes.

Microeukaryotic plankton evolutionary constraints in a subtropical river explained by environment and bacteria along differing taxonomic resolutions.

Microeukaryotic plankton communities are keystone components for keeping aquatic primary productivity. Currently, variations in microeukaryotic plankton diversity have often been explained by local ecological factors but not by evolutionary constraints. We used amplicon sequencing of 100 water samples across five years to investigate the ecological preferences of the microeukaryotic plankton community in a subtropical riverine ecosystem. We found that microeukaryotic plankton diversity was less associated with bacterial abundance (16S rRNA gene copy number) than bacterial diversity. Further, environmental effects exhibited a larger influence on microeukaryotic plankton community composition than bacterial community composition, especially at fine taxonomic levels. The evolutionary constraints of microeukaryotic plankton community increased with decreasing taxonomic resolution (from 97% to 91% similarity levels), but not significant change from 85% to 70% similarity levels. However, compared with the bacterial community, the evolutionary constraints were shown to be more affected by environmental variables. This study illustrated possible controlling environmental and bacterial drivers of microeukaryotic diversity and community assembly in a subtropical river, thereby indirectly reflecting on the quality status of the water environment by providing new clues on the microeukaryotic community assembly.

Influence of urbanization on size and biomass of a small-sized characid fish (Diapoma alburnum) in a subtropical river basin of southern Brazil.

The coastal lagoons of the Tramandaí River basin are dynamic ecosystems characterized by high biodiversity. They provide important ecosystem services, such as water supply for human consumption, industry, agriculture, animal husbandry, leisure activities, tourism and fishing. Constant increases in the human population and the use and occupation of the land around the lagoons has brought growing demands for their resources, resulting in increased pressure that compromises these ecosystems. Understanding how biological populations respond to these anthropogenic pressures is essential. This study examined whether the degree of urbanization around 23 lagoons in the Tramandaí River basin influences the size and biomass of the characid fish species Diapoma alburnum. Specimens were collected between 2009 and 2012 using standardized drag nets. All specimens were measured for standard length to determine average length (size) per lagoon, while all fish collected per lagoon were weighed together to determine average biomass per lagoon by dividing by the number of specimens. Urbanization around the lagoons was measured using satellite images representing artificial nocturnal light as a proxy for urbanization. Nocturnal light intensity was measured within 1 km, 3 km, and 5 km buffers around each lagoon. The resulting urbanization index ranged 6 - 44% for the 5 km buffer, from 3 - 55% for the 3 km buffer and 1 - 65% for the 1 km buffer. Regression analyses showed a significant positive relationship with increasing urbanization around lagoons for D. alburnum average biomass in the 3 km and 5 km buffers and for D. alburnum average length in the 3 km buffer. Although urbanization around the lagoons is not fully established, the results indicate its impact on the size and biomass of D. alburnum.

Altitudinal distribution and body condition of migratory fish juveniles Megaleporinus obtusidens (Valenciennes) and Salminus brasiliensis (Cuvier) in the main channel of the Uruguay River, Southern Brazil.

In this study, we analyzed the altitudinal distribution and body condition of the juvenile migratory fish Megaleporinus obtusidens and Salminus brasiliensis in the Uruguay River, a South American subtropical river. We used the presence/absence data and condition factor (K) of juveniles as indicators to try to characterize some recruitment patterns of migratory fish in the main channel of the Uruguay River. Gillnet sampling was conducted during two reproductive years, 2015-2016 and 2016-2017 (November and March of each reproductive year), in three Uruguay River stretches at different levels of altitude. The nets remained in the water for 10-12 hours per night or day for 10 days during each field trip (NOV-15, MAR-16, NOV-16, and MAR-17). The abundance of M. obtusidens juveniles were similar between the two reproductive years, whereas S. brasiliensis capture was higher during the 2015-2016 reproductive year. The probability of finding M. obtusidens juveniles in the Uruguay River was negatively correlated (p < 0.05) with the altitudinal gradient of the sampling points, as shown by the GLM model. For S. brasiliensis, the logit model indicated a non-significant probabilistic relationship (p > 0.05) between the presence of juveniles of this fish species and the altitude gradient. Regarding the body condition of juveniles, the water quality of the sampling points did not affect the juvenile condition factor for either species; however, a positive relationship (p < 0.05) was found with the water level for both species, even with an adjusted R2 of 0.13 for M. obtusidens and 0.48 to S. brasiliensis. Altitude gradient and water level were factors that may affect the juvenile distribution and body condition of these species, respectively, in the Uruguay River. Therefore, this information can be used in hydropower generation policies to conserve migratory fish populations in the Uruguay River Basin.

Land use composition, configuration and nutrient: Key drivers of benthic metabolism in streams

Benthic metabolism is an essential process which plays a critical role in the carbon and nutrient cycles and in the transfer of material and energy in stream ecosystems. Benthic algae are one of principal primary producers in stream food webs that converts dissolved inorganic carbon to organic matter, and algae are more palatable basal resources for aquatic consumers due to its higher unsaturated fatty acid content than terrestrial carbon. Despite the importance of algae, few studies have investigated how the benthic metabolism is influenced by distal factors, such as upland land use and landscape configuration, and proximal factors, such as stream temperature, nutrient availability, and light availability at various spatial scales. Here, we measured the benthic metabolism and proximal factors at the outlet of 14 selected sub-catchments in a subtropical river. Land use and landscape metrics at the catchment, riparian, and reach scales were processed as distal factors. Using stepwise regression models, we found that the proximal factors, such as water temperature (WT) and total phosphorus (TP), significantly increased primary production, and that nitrate-nitrogen (NO3--N), pH, and electrical conductivity (EC) decreased gross primary production (GPP). Also, TP was positively correlated with respiration (R) while NO3--N negatively impacted R. Structural equation modeling demonstrated that the patch cohesion index negatively affected GPP by restraining WT and TP at the catchment scale while the increased shape index elevated the WT and TP to promote GPP. Our results suggested that stream metabolism is vulnerable to anthropogenic disturbances, such as upland land use and landscape patterns. Thus, land use planning and administration, particularly at catchment scale, should consider how to minimize the influence of human activities on ecosystem function in streams.

Environmental DNA and remote sensing datasets reveal the spatial distribution of aquatic insects in a disturbed subtropical river system

Biodiversity datasets with high spatial resolution are critical prerequisites for river protection and management decision-making. However, traditional morphological biomonitoring is inefficient and only provides several site estimates, and there is an urgent need for new approaches to predict biodiversity on fine spatial scales throughout the entire river systems. Here, we combined the environmental DNA (eDNA) and remote sensing (RS) technologies to develop a novel approach for predicting the spatial distribution of aquatic insects with high spatial resolution in a disturbed subtropical Dongjiang River system of southeast China. First, we screened thirteen RS-based vegetation indices that significantly correlated with the eDNA-inferred richness of aquatic insects. In particular, the green normalized difference vegetation index (GNDVI) and normalized difference red-edge2 (NDRE2) were closely related to eDNA-inferred richness. Second, using the gradient boosting decision tree, our data showed that the spatial pattern of eDNA-inferred richness could achieve a high spatial resolution to 500 m reach and accurate prediction of more than 80%, and the prediction efficiency of the headwater streams (Strahler stream order = 1) was slightly higher than the downstream (Strahler stream order >1). Third, using the random forest algorithm, the spatial distribution of aquatic insects could reach a prediction rate of over 70% for the presence or absence of specific genera. Overall, this study provides a new approach to achieving high spatial resolution prediction of the distribution of aquatic insects, which supports decision-making on river diversity protection under climate changes and human impacts.

Damming exacerbates the discontinuities of phytoplankton in a subtropical river in China

Phytoplankton is sensitive to changes in river ecosystems. Increasing dams disrupt the continuity of river ecosystems. However, the spatial impacts of dams on phytoplankton have not been well documented. In this study, using multiple statistical analyses, the relationships between environmental drivers and phytoplankton community structures in natural background reaches, reservoirs, and corresponding post-dam reaches were explored in the Jiulong River with multiple cascaded dams, which encountered eutrophication and algal blooms in the past 15 years. Results illustrated that damming exacerbated longitudinal discontinuities of phytoplankton communities. The relative abundance of phytoplankton varied in three types of river sections. The average phytoplankton abundance in the reservoirs (1.62 × 105 cell·L−1) was higher than those in the natural background reaches (5.15 × 104 cell·L−1) and the corresponding downstream reaches (4.55 × 104 cell·L−1). The total β diversity ranged from 0.38 to 0.89 with an average of 0.64 and dominated by species replacement and least by species richness. The water environmental factors and hydraulic parameters rather than nutrients were more attributable to phytoplankton community variability in three river sections. These findings facilitate the management of rivers with multiple cascade dams by releasing environmental flows, jointly operating cascade hydropower stations, and developing nutrient reduction schemes to mitigate the negative impacts of damming in the river.

Adaptive shifts in plant traits associated with nitrogen removal driven by phytoremediation strategies in subtropical river restoration

Phytoremediation, which is commonly carried out through hydroponics and substrate-based strategies, is essential for the effectiveness of nature-based engineered solutions aimed at addressing excess nitrogen in aquatic ecosystems. However, the performance and mechanisms of plants involving nitrogen removal between different strategies need to be deeply understood. Here, this study employed in-situ cultivation coupled with static nitrogen tracing experiments to elucidate the influence of both strategies on plant traits associated with nitrogen removal. The results indicated that removal efficiencies in plants with substrate-based strategies for ammonium nitrogen and nitrate nitrogen were 30.51–71.11 % and 16.82–99.95 %, respectively, which were significantly higher than those with hydroponics strategies (25.98–58.18 % and 7.29–79.19 %, respectively). Similarly, the plant nitrogen uptake rates in the substrate-based strategy also generally showed higher levels compared to hydroponics strategies (P < 0.05). Meanwhile, the microorganisms-mediated nitrous oxide emission rates in the substrate-based strategy during summer (unamended: 0.00–0.58 μg/g/d; potential: 3.35–7.65 μg/g/d) were obviously lower than those in the hydroponics strategy (unamended: 2.23–11.70 μg/g/d; potential: 9.72–43.09 μg/g/d) (P < 0.05). Notably, analysis of similarity tests indicated that the influences of strategy on the above parameters generally surpass the effects attributable to interspecies plant differences, particularly during summer (R > 0, P < 0.05). Based on statistical and metagenomic analyses, this study revealed that these differences were driven by the stabilizing influence of substrate-based strategy on plant roots and enhancing synergistic interplay among biochemical factors within plant-root systems. Even so, phytoremediation strategies did not significantly alter the characteristics of plants with regards to their tendency towards ammonium nitrogen uptake (up to 87.68 %) and dissimilatory nitrate reduction to ammonium as primary biological pathway for nitrogen transformation which accounted for 53.66–96.47 % nitrate removal. In summary, this study suggested that the substrate-based strategy should be a more effective strategy for enhancing the nitrogen removal ability of plants in subtropical river restoration practices.

Bioaccumulation and sources of metal(loid)s in fish species from a subtropical river in Bangladesh: a public health concern.

Toxic metals and freshwater fish's metalloid contamination are significant environmental concerns for overall public health. However, the bioaccumulation and sources of metal(loids) in freshwater fishes from Bangladesh still remain unknown. Thus, the As, Pb, Cd, and Cr concentrations in various freshwater fish species from the Rupsha River basin were measured, including Tenualosa ilisha, Gudusia chapra, Otolithoides pama, Setipinna phasa, Mystus vittatus, Glossogobius giuris, and Pseudeutropius atherinoides. An atomic absorption spectrophotometer was used to determine metal concentrations. The mean concentrations of metal(loids) in the fish muscle (mg/kg) were found to be As (1.53) > Pb (1.25) > Cr (0.51) > Cd (0.39) in summer and As (1.72) > Pb (1.51) > Cr (0.65) > Cd (0.49) in winter. The analyzed fish species had considerably different metal(loid) concentrations with seasonal variation, and the distribution of the metals (loids) was consistent with the normal distribution. The demersal species, M. vittatus, displayed the highest bio-accumulative value over the summer. However, in both seasons, none of the species were bio-accumulative. According to multivariate statistical findings, the research area's potential sources of metal(loid) were anthropogenic activities linked to geogenic processes. Estimated daily intake, target hazard quotient (THQ), and carcinogenic risk (CR) were used to assess the influence of the risk on human health. The consumers' THQs values were < 1, indicating that there were no non-carcinogenic concerns for local consumers. Both categories of customers had CRs that fell below the permissible range of 1E - 6 to 1E - 4, meaning they were not at any increased risk of developing cancer. The children's group was more vulnerable to both carcinogenic and non-carcinogenic hazards. Therefore, the entry of metal(loids) must be regulated, and appropriate laws must be used by policymakers.

Environmental DNA metabarcoding reveals the effect of environmental selection on phytoplankton community structure along a subtropical river

The Dongjiang River, a major tributary of the Pearl River system that supplies water to more than 40 million people in Guangdong Province and neighboring regions of China, harbors rich biodiversity, including many endemic and endangered species. However, human activities such as urbanization, agriculture, and industrialization have posed serious threats to its water quality and biodiversity. To assess the status and drivers of phytoplankton diversity, which is a key indicator of aquatic ecosystem health, this study used Environmental DNA (eDNA) metabarcoding combined with machine learning methods to explore spatial variations in the composition and structure of phytoplankton communities along the Dongjiang River, including its estuary. The results showed that phytoplankton diversity exhibited spatial distribution patterns, with higher community structure similarity and lower network complexity in the upstream than in the downstream regions. Environmental selection was the main mechanism shaping phytoplankton community composition, with natural factors driving the dominance of Pyrrophyta, Ochrophyta, and Cryptophyta in the upstream regions and estuaries. In contrast, the downstream regions was influenced by high concentrations of pollutants, resulting in increased abundance of Cryptophyta. The random forest model identified temperature, dissolved oxygen, chlorophyll a, NO2−, and NH4+ as the main factors influencing the primary phytoplankton communities and could be used to predict changes during wet periods. This study provides valuable insights into the factors influencing phytoplankton diversity and community composition in the Dongjiang River, and demonstrates the application value of eDNA metabarcoding technique in large-scale, long-distance river biodiversity monitoring.

Environmental DNA biomonitoring reveals the human impacts on native and non-native fish communities in subtropical river systems

Subtropical rivers are one of the hotspots of global biodiversity, facing increased risks of fish diversity changes and species extinction. However, until now, human impacts on native and non-native fish communities in subtropical rivers still lack sufficient effort. Here, we used the environmental DNA (eDNA) approach to investigate fish communities in the Dongjiang River of southeast China, a typical subtropical river, and explored the effects of regional land use and local water pollution on fish taxonomic and functional diversity. Our data showed that 90 species or genera of native fish and 15 species or genera of non-native fish were detected by the eDNA approach, and there was over 85% overlap between eDNA datasets and historical records. The taxonomic and functional diversity of all, native and non-native fish communities showed consistent spatial patterns, that is, the upstream of the tributary was significantly higher than that of the mainstream and downstream. Land use and water pollution such as COD and TP were the determinants in shaping the spatial structure of fish communities, and water pollution explained 31.56%, 29.88%, and 27.80% of the structural variation in all, native and non-native fish communities, respectively. The Shannon diversity and functional richness of native fish showed a significant downward trend driven by COD (pShannon = 0.0374; pfunctional = 0.0215) and land use (pShannon = 0.0159; pfunctional = 0.0441), but they did not have significant impacts on non-native fish communities. Overall, this study emphasizes the inconsistent response of native and non-native fish communities to human impacts in subtropical rivers, and managers need to develop strategies tailored to specific fish species to effectively protect water security and rivers.

Variation of material fluxes in a large heterogenic subtropical river: A combination of climate change and human damming influences

Study regionThe West River, the main tributary of the Pearl River, China Study focusA hydrological model of the West River was successfully established and used to quantify the role of basin-wide spatiotemporal change of the impacts of climate change and human damming activities in the variation of material fluxes (water discharge and sediment load) at the river outlet at both interannual and seasonal time scales of the period from 1975 to 2017. New hydrological insights for the regionThe interannual variations of water discharge and sediment load were controlled by a single factor locally: runoff variation was positively correlated with the ENSO index (r = 0.56, p < 0.001) and the dams caused a 75.1% reduction of the annual sediment load in the post-dam period (2009–2017). Seasonally, climate change can alter the monthly water discharge by − 12–23% in the latter half of an abnormal year, while the dams caused a 4.6% reduction and a 12.4% increase in the summer and winter sediment load, respectively. There was a strong interaction between the two influencing factors seasonally, particularly during the wetter summer or drier winter, wherein the dominant driving mechanism in the monthly variation of material fluxes can switch. For the river basin, the impacts of climate change and dams are spatially variable. The basin-wide spatiotemporal change of the impacts of climate change and human damming activities can complicate the hydrological signal at the river outlet.

Urbanization reduces fish taxonomic and functional diversity while increases phylogenetic diversity in subtropical rivers

Urbanization is considered as a major cause of widespread biodiversity loss in freshwater ecosystems. However, multidimensional fish diversity patterns driven by urbanization in subtropical rivers are still unclear. We studied fish taxonomic, functional, and phylogenetic diversity in rivers under a gradient of watershed urbanization (Maozhouhe, Guanlanhe and Shenzhenhe rivers are in urbanized watersheds while Pingshanhe and Dapengwan rivers are in forested watersheds) in Shenzhen, a megacity with rapid urbanization in south China. Fish assemblages showed significant differences between the forested and urbanized watersheds in both the wet and dry seasons. Rivers in the forest watersheds showed significantly higher taxonomic and functional but lower phylogenetic diversity patterns. Rivers in the urbanized watersheds were largely occupied by non-native species. Overall, there was a distinct difference in river fish assemblages between forested and urbanized watersheds, and urbanization largely affected multidimensional fish diversity. These results will help us to take more effective conservation and restoration measures, and provide an essential reference for ecological management and sustainability of urban rivers.

Elemental composition and speciation trends in Upper Mazowe River, a typical sub-tropical river ecosystem impacted by gold mining and agriculture in Zimbabwe

This study was carried on Upper Mazowe River in Zimbabwe which connects with Luenha River in Mozambique, which is a tributary of the Zambezi River. Zambezi River is the fourth longest river in Africa flowing into the Indian Ocean, which makes the study of regional and global interest. Heavy metal aqueous speciation has received relatively little attention compared to sediment speciation in the Upper Mazowe River, and yet heavy metal behaviour and fate in surface waters are greatly affected by the heavy metal chemical form in solution. The aim of the study was to determine the elemental composition and speciation of heavy metal(oid)s. This study focused on water and sediment speciation of Al, As, Cu, Hg, Mn, Fe, and Pb using Visual MINTEQ Version 3.1 computer modelling and sequential extraction techniques. Twenty-eight different water column elemental species, comprising 3, 2, 3, 1, 7, 7 and 5 species each, and 35 sediment phase elemental species, comprising 6, 6, 6, 2, 6, 5, and 4 species each for Al, As, Cu, Hg, Mn, Fe, and Pb respectively, were identified. Of note is the fact that Hg, mainly derived from artisanal gold mining activities, exists entirely as the fulvic acid complex, FA2Hg (aq), and As exists as H3AsO3 and H2AsO3− in the water phase. Exchange equilibrium reactions across the water-sediment interface were observed for Mn2+, Fe2+, Cu2+, and Pb2+. Reactions leading to the formation of the species were identified. Species interactions within the water column and sediment phase and species interrelationships with physicochemical parameters are discussed. H3AsO3 formation is predicted to be temperature sensitive and the aluminium hydroxide-fulvic acid complex /FA2(AlOH) (aq) is oxic favourable.

Aquatic Macrophytes Metal and Nutrient Concentration Variations, with Implication for Phytoremediation Potential in a Subtropical River System

Human activities have resulted in an increase in metal pollution within aquatic ecosystems, with most of these metals ending up being taken by macrophytes. Thus, these macrophytes provide an opportunity to study metal pollution dynamics and the role that macrophytes play in potentially translocating and accumulating these metals. Here, we studied three macrophyte species, namely Phragmites australis, Schoenoplectus corymbosus, and Typha capensis, and assessed their potential to be utilized in the phytoremediation of metals in an Austral subtropical river across three seasons. We measured P, K, Ca, Mg, B, Fe, Zn, Cu, and Mn concentrations in macrophyte roots, stems, and leaves, and we further quantified the metal bioconcentration factor (BCF). The N, Ca, and Mg concentrations were generally high in P. australis leaves across all seasons. In general, high Na, Mg, and Ca concentrations were observed in T. capensis across seasons. The bioconcentration factor (BCF) values were generally low (&lt;1) in most macrophyte parts for most metals during the cool-dry season, with the exception of Na, which had high BCF values &gt; 1 (i.e., accumulators) across the different macrophyte parts. We found that P. australis and S. corymbosus have the potential to accumulate metals such as B, Na, Mg, Ca, and N and also have high phytoremediation potential for the studied metals. We found that the studied macrophytes were good at phytoremediation within the river system; however, for any treatment of polluted systems, it is better to use a combination of different macrophytes, as some were better at translocating certain metals than others.

Local environmental and spatial factors are associated with multiple facets of riverine fish β‐diversity across spatial scales and seasons

Abstract Disentangling the factors governing community structure across various spatial and temporal scales is a core task in community ecology and biogeography, because it could provide valuable insights into biodiversity conservation and management. The relative role of environmental (a proxy for niche‐based mechanism) and spatial (a proxy for dispersal) effects in shaping ecological communities depends on diversity facets, network positions, seasonality and biological characteristics of the focal organisms. However, such patterns are not well understood for fish communities in large river systems, especially if all of these patterns are examined simultaneously. We investigated fish taxonomic, functional and phylogenetic total β‐diversity (and their turnover and nestedness phenomena) at different spatial scales (basin vs. section scales) and seasons (wet vs. dry seasons) in a subtropical river system (the Chishui River basin, China). We also quantified the relative importance of environmental variables and spatial factors in shaping fish β‐diversity by using generalised dissimilarity modelling. At the basin scale, fish taxonomic and phylogenetic β‐diversity across seasons mainly reflected turnover, yet turnover and nestedness contributed likewise to total functional β‐diversity. Taxonomic and phylogenetic β‐diversity were strongly correlated (r from 0.39 to 0.87), whereas functional and both taxonomic and phylogenetic facets were relatively weakly correlated (r from 0.27 to 0.65). Generalised dissimilarity modelling suggested that phylogenetic β‐diversity phenomena were consistently more influenced by environmental variables (wet 16.1%, dry 22.4%) than by spatial factors (wet 6.7%, dry 4.8%). However, for both taxonomic and functional facets, environmental variables (turnover 13.8%, nestedness 6.4%) and spatial factors (turnover 4.3%, nestedness 9.4%) accounted for a greater proportion of variations in turnover and nestedness, respectively. At the section scale, only phylogenetic total β‐diversity and nestedness, as well as taxonomic nestedness significantly differed between the headwater and downstream sections. Furthermore, headwater communities were primarily explained by spatial (15.7%) and environmental (12.7%) variables in the wet season, while environmental factors (25.9%; spatial 7.7%) dominated in the dry season. By contrast, downstream communities were more influenced by environmental factors (17.9%; spatial 8.3%) in the wet season, whereas environmental (11.0%) and spatial variables (11.5%) contributed likewise to β‐diversity in the dry season. Our results highlight that, compared with taxonomic β‐diversity, accounting for turnover and nestedness of functional and phylogenetic β‐diversity helps to reveal distinct biodiversity patterns, and potential environmental and spatial controls. Given that community–environment relationships and the relative contribution of environmental and spatial factors varied between seasons and between the basin and section scales, our study underlines the importance of long‐term dynamics and spatial scales in metacommunity studies. Our findings also suggest that maintaining environmental heterogeneity and adequate hydrological connectivity would be essential for the conservation of fish diversity. Overall, our study advances current knowledge of fish biodiversity patterns and their maintenance mechanisms by adding insights from multi‐faceted diversity and seasonality, which has rarely been done in large subtropical rivers.

WITHDRAWN: The drivers of benthic macroinvertebrates communities along a subtropical river system: Sediment chemistry or water quality?

WITHDRAWN: The drivers of benthic macroinvertebrates communities along a subtropical river system: Sediment chemistry or water quality?

Impacts of human activities and climate change on water and sediment evolution in four large subtropical river basins in China

Human activities and climate are two key factors affecting hydrological and soil erosion processes. Understanding their impacts on water discharge and sediment transport is very important for sustainable development of watersheds, particularly in subtropical regions with high precipitation and high sediment loads. We here compared the long-term dynamics of water discharge and sediment transport in four large contiguous watersheds in subtropical China and explored the importance of the main influencing factors including climate, nature endowment such as geology, and human activities. Significant differences have been observed between watersheds in terms of sediment and water discharge (p < 0.001). Pairwise comparison among the four basins demonstrated significant differences in sediment modulus among 5 of the 6 pairs (p < 0.05). The sediment modulus in the Lishui Basin was significantly higher than those in other basins, mainly due to the substantial presence of steep slopes in the mountainous region, which is prone to soil erosion. Temporally, sediment transport in the four basins was well synchronized showing significant decreasing trends with abrupt change points (p < 0.001). In contrast, no trends and abrupt change points were detected in water discharge, mainly influenced by precipitation. Human activities played a predominant role to the reduction of sediment transport (80% to 102%) compared with those of climate (-2% to 19%) across the four basins. Additionally, our study also showed land cover had various basin-independent impacts on the change of sediment transport. Through cross-basin comparisons of the long-term changes in water discharge and sediment transport, our study revealed the similarities and differences in their driving mechanisms, providing valuable information for land use planning and water resource management in large subtropical basins.