Subtropical River Research Articles

Reconciliation of Spatiotemporal Influences on Two-Dimensional Distribution and Fate of Emerging Contaminants in a Subtropical River

This study involves elucidation of spatial and temporal influences on horizontal and vertical distribution and fate of contaminants of emerging concerns (CECs) in riverain systems. In total, 23 CEC...

The effects of clean energy production and urbanization on sources and transformation processes of nitrate in a subtropical river system: Insights from the dual isotopes of nitrate and Bayesian model

Source appointment and control of water pollution caused by nitrate in river are the key to water quality management as well as the basis for achieving aquatic environment sustainability. However, the effects of clean energy production (hydropower generation) and urbanization on the nitrate accumulation and transformation remain unclear. For that, this study quantified and identified the nitrate sources and key processes in a subtropical river, which has been impounded and urbanized in varying degree. The variations of dual isotopes and water chemistry indicated that NO3− in agriculture and forest land mainly from the chemical fertilizer and soil N, while manure and sewage were the main riverine NO3− sources. Nitrification was the dominate process while influenced greatly by inputs of manure and sewage, especially in urban area. Progressive denitrification has been found in the upper reaches of the Beixi River where the river impounded by dams for hydropower generation, with an estimated contribution of 36% (1%–68%). Results from Bayesian model showed that fertilizer and soil N contributed more than half of the NO3− in the wet season as a result of rainfall and surface runoff; while manure and sewage were the dominate NO3− sources in the dry season due to the urbanization. Land use including agriculture, dam construction, and urbanization have characterized the nitrate sources and pollution in the Jiulong River, and cascade dam impoundment could enlarge nitrate retention. We concluded that the combined use of dual isotopes of NO3−, hydrochemicals and the SIAR model offers an efficient approach to understand the nitrogen transformation and quantify the contributions of nitrogen source in river waters.

Perfluoroalkyl substances in water, sediment, and fish from a subtropical river of China: Environmental behaviors and potential risk

Perfluoroalkyl substances (PFAS) in water, sediment and fish were analyzed from a subtropical river, Jiulong River in the southeast of China, to character the sources, seasonal variations, bioconcentration and potential risk. PFAS in water, sediment, muscle and liver tissues of fish ranged from 2.5 to 410 ng L−1, 0.24–1.9 ng g−1 dw, 25–100 and 35–1100 ng g−1 ww, respectively. Generally, perfluorohexanoic acid (PFHxA) was the dominant compound in water, while, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) were the dominant compounds in sediment and fish tissues. High concentrations of PFAS in water were found near the machinery manufacturing and paper packaging plants in the north branch of Jiulong River. PFAS during the dry season were significantly (P < 0.01) higher than that during the normal season and wet season. The Kd of PFAS increased with the carbon chain length, and perfluoroalkyl sulfonic acids (PFSAs) exhibited higher Kd values than perfluoroalkyl carboxylic acids (PFCAs), indicating these long chain PFAS tended to be adsorbed by sediment. Long chain PFAS exhibited high bioconcentration factors (BCFs), while short and medium carbon chain PFAS had weak bioconcentration capacity. The hazard ratios (HR) suggested that frequent consumption of river fish may pose health risks to local population.

Climate change may impair electricity generation and economic viability of future Amazon hydropower

Numerous hydropower facilities are under construction or planned in tropical and subtropical rivers worldwide. While dams are typically designed considering historic river discharge regimes, climate change is likely to induce large-scale alterations in river hydrology. Here we analyze how future climate change will affect river hydrology, electricity generation, and economic viability of > 350 potential hydropower dams across the Amazon, Earth’s largest river basin and a global hotspot for future hydropower development. Midcentury projections for the RCP 4.5 and 8.5 climate change scenarios show basin-wide reductions of river discharge (means, 13 and 16%, respectively) and hydropower generation (19 and 27%). Declines are sharper for dams in Brazil, which harbors 60% of the proposed projects. Climate change will cause more frequent low-discharge interruption of hydropower generation and less frequent full-capacity operation. Consequently, the minimum electricity sale price for projects to break even more than doubles at many proposed dams, rendering much of future Amazon hydropower less competitive than increasingly lower cost renewable sources such as wind and solar. Climate-smart power systems will be fundamental to support environmentally and financially sustainable energy development in hydropower-dependent regions.

Data- and Model-Based Discharge Hindcasting over a Subtropical River Basin

This study aims to evaluate the performance of the Soil and Water Assessment Tool (SWAT), a simple Auto-Regressive with eXogenous input (ARX) model, and a gene expression programming (GEP)-based model in one-day-ahead discharge prediction for the upper Kentucky River Basin. Calibration of the models were carried out for the period of 2002–2005 using daily flow at a stream gauging station unaffected by the flow regulation. Validation of the calibrated models were executed for the period of 2008–2010 at the same gauging station along with another station 88 km downstream. GEP provided the best calibration (coefficient of determination (R) value 0.94 and Nash-Sutcliffe Efficiency (NSE) value of 0.88) and validation (R values of 0.93 and 0.93, NSE values of 0.87 and 0.87, respectively) results at the two gauging stations. While SWAT performed reasonably well in calibration (R value 0.85 and NSE value 0.72), its performance somewhat degraded in validation (R values of 0.85 and 0.82, NSE values of 0.65 and 0.65, for the two stations). ARX performed very well in calibration (R value 0.92, NSE value 0.82) and reasonably well in validation (R values of 0.88 and 0.92, NSE values of 0.76 and 0.85) at the two stations. Research results suggest that sophisticated hydrological models could be outperformed by simple data-driven models and GEP has the advantage to generate functional relationships that allows investigation of the complex nonlinear interrelationships among the input variables.

Elevated Temperatures Shorten the Spawning Period of Silver Carp (Hypophthalmichthys molitrix) in a Large Subtropical River in China

Global warming is influencing the life history traits of fishes globally. However, the impacts of elevated temperature on fish reproduction are diverse in different regions. Previous studies have revealed that the spawning timing of silver carp (Hypophthalmichthys molitrix) in the Pearl River, in China, has changed over the past decade. However, few studies have explored the potential reasons, which are critical for determining fishing-moratorium periods and developing sustainable fisheries. The current study used discharge suitability index (DSI), temperature suitability index (TSI), correlation and time-series analyses to determine (i) the optimal discharge and temperature for silver carp spawning; (ii) relationships among the thermal regime, hydrological parameters, and spawning timing based on an 11-year time-series dataset. Our results indicated that the most suitable discharge and temperature for silver carp spawning were 13,000–15,000 m3/s and 25–26°C, respectively. The start date of spawning fluctuated with a slight tendency to delay, while the spawning peak and end date obviously occurred earlier during the study period. Correlation analyses suggested that the increasing average temperature between January and March likely caused the initial spawning delay. Moreover, elevated temperatures in August and September probably promoted the anticipated end of silver carp spawning. However, increases in discharge did not significantly correlate with the start of spawning but were significantly and positively correlated with the spawning peak. These results indicated that elevated temperatures shorten the spawning period of silver carp in the Pearl River. Moreover, the initial spawning of silver carp seems to be triggered by temperature rather than changes in discharge; flow pulses can probably create more suitable spawning niches forH.molitrix. This study enhances our understanding of the effect of warming on fish reproduction in subtropical regions.

Influence of salinity on the horizontal distribution of soldier crab Mictyris guinotae in a subtropical river in Japan

ABSTRACT The horizontal distribution of Mictyris guinotae was studied in relation to gradients of salinity and sediment characteristics in sandy flats between 0 and 8.2 km from the mouth of a subtropical river. Lower salinity tolerance was also tested at seven salinity levels (0, 2, 4, 8, 12, 24 and 32) by a survival experiment for 14 days. Environmental clines were found in salinity but not in sediment characteristics. The upper limit of the occurrence of crab was 5.07 km upstream where seawater could reach during the spring flood tide. The densities of age-0 crabs tended to be higher at lower stations (0–0.65 km upstream), suggesting more recruits settle near the river mouth. Carapace length was significantly larger in the upper section (2.50–5.07 km upstream) in which salinity fluctuated or remained low (<10) for longer periods. It is unlikely that low salinity has a negative effect on post-larval growth. In contrast, body size was smaller in the lower section, possibly due to the effect of density. Some individuals died at the end of the experiment at salinity ≤12. The upper limit of appearance was explained by the salinity of riverine water and low salinity tolerance of the crab.

Daily variation feeding of Astyanax lacustris (Lütken 1875) in a subtropical river

This study aimed to know the daily variation of Astyanax lacustris (Lütken 1875) feeding in a lotic environment. Fish were caught with a net for three days every three hours in the summer to capture ten individuals per hour in a stretch of the Ijuí River, Middle Uruguay River, Brazil. The captured specimens had their stomachs removed and the content analyzed and separated into seven food categories with the aid of stereomicroscope. For the analysis of food items were used the frequency of occurrence methods, volumetric method and applied the Alimentary Index (IAi). Two hundred stomachs were analyzed, of which 95% had food content. The most abundant items were algae and autochthonous insects. Astyanax lacustris feeds throughout the day especially early in the morning (9 hours) and reduces its feeding at night (24 hours and 3 hours). It was also verified variation of AI of different food items throughout the day. It is concluded that A. lacustris feeds throughout the day, especially in the daytime and that throughout the 24 hours it varies its diet due to photoperiod and food availability.

Seasonal Surface and Bottom Temperature-salinity Anomaly of a Subtropical River in Response to Sea Surface Elevation

Seasonal Surface and Bottom Temperature-salinity Anomaly of a Subtropical River in Response to Sea Surface Elevation

Vegetation disturbances and flood energy during an extreme flood on a sub‐tropical river

AbstractThe advent of 2D hydraulic modelling has improved our understanding of flood hydraulics, thresholds, and dynamic effects on floodplain geomorphology and riparian vegetation at the morphological‐unit scale. Hydraulic concepts of bed shear stress, stream power maxima, and energy (cumulative stream power) have been used to characterize floods and define their geomorphic effectiveness. These hydraulic concepts were developed in the context of reach‐averaged, 1D hydraulic analyses, but their application to 2D model results is problematic due to differences in the treatment of energy losses in 1D and 2D analyses. Here we present methods for estimating total and boundary resistance from 2D modelling of an extreme flood on a subtropical river. Hydraulic model results are correlated with observations of the flood impacts on floodplain geomorphology and the riparian vegetation to identify thresholds and compute variants of flood energy. Comparison of LiDAR data in 2011 and 2014 shows that the 2011 flood produced 2–4 m of erosion on floodplain bars that were previously forested or grass‐covered. Deposition on flood levees, dunes, and chute bars was up to 3.4 m thick. Various hydraulic metrics were trialled as candidates for thresholds of vegetation disturbance. The accuracy of thresholds using metrics extracted at the flood peak (i.e. boundary resistance and stream power maxima) was similar to that using energy as a threshold. Disturbance to forest and grass on vegetated bars was associated with stream powers of &gt;834 W/m2and unit flows of &gt;26 m2/s, respectively. Correlation of the hydraulic metrics with erosion and deposition depths showed no substantial improvement in using flood energy compared to metrics extracted at the flood peak for describing erosion and deposition. The extent of vegetation disturbances and morphological adjustments was limited for this extreme flood, and further 2D studies are needed to compare disturbance thresholds across different environments.

Deciphering the signatures of weathering and erosion processes and the effects of river management on Li isotopes in the subtropical Pearl River basin

Lithium (Li) isotopes signatures in sedimentary archives are promising proxies to reconstruct paleo-weathering rates and paleo-seawater composition. Nonetheless, the interpretation of the seawater Li isotope composition evolution over the Cenozoic is still debated. In this aim, the study of large rivers can provide constraints for continental weathering flux to the ocean. However, in the recent past, human activities like flow regulation via reservoir construction, have significantly modified the hydrodynamic of riverine systems, impacting the transfer of matter from the continent to the ocean. Hence, these effects need to be considered and investigated when unravelling the signature of natural processes from river borne materials.We report Li concentrations and Li isotope compositions (δ7Li) for the dissolved and sediment loads of subtropical river waters from the Xijiang basin, the largest tributary of the Pearl River (also known as the Zhujiang), Southern China. These rivers display low dissolved Li concentrations. Although they are potentially impacted by carbonate dissolution in the upstream karst terrains, the Li budget of these rivers is dominated by silicate weathering. The dissolved Li isotope compositions of rivers vary over a range from +16‰ to +27‰. These values are high compared to the estimated composition of the bedrock (~0‰), and are explained by the preferential incorporation of the light 6Li in solid weathering products formed during the incongruent dissolution of silicate rocks. We show that the variations of the dissolved δ7Li values across the basin reflect changes in silicate weathering intensity which controls the formation of secondary phases. The high δ7Li values in the headwaters reflect greater degree of secondary phase formations associated with the higher erosion rates in the mountainous part of the basin, whereas the lower δ7Li values in the lowlands characterized by low soil erosion rates and more abundant rainfall reflect a more intensive weathering. This interpretation is supported by the positive relationship between soil erosion rates and the Li dissolved fluxes. The isotope compositions of the suspended particulate material (SPM) transported by rivers in the Xijiang Basin (from −3.8‰ to −0.7‰) are systematically lower than the composition of the bedrock. We observe that the δ7Li of most SPM samples collected during the wet season are lower than those of samples collected during the dry season. We interpret this seasonality as a consequence of seasonal changes in the sediment properties due to water regulation. In spite of the influence induced by river managements on the river sediments, our data also highlight that the imprint of weathering and erosion processes is still preserved in the sedimentary load of the Xijiang as suggested by the relationship between the SPM δ7Li values and the weathering intensity defined as the ratio between the chemical weathering rates and the total denudation rates. Altogether, our study confirms the robustness of Li isotopes as a proxy of weathering and erosion processes, even in this heavily-managed river system.

Anthropogenic activities change the relationship between microbial community taxonomic composition and functional attributes.

Functional redundancy is considered common in microbial systems, but recent studies have challenged this idea. The mechanism for this contradictory result is not clear. However, in this study, we hypothesize that strong environmental filtering which links to the anthropogenic activities is able to weaken microbial functional redundancy. We used metagenome and 16S rRNA gene high-throughput sequencing to investigate planktonic microbial communities in a subtropical river. We found that the weak anthropogenic activities might result in a low selection pressure in the river upstream area. Therefore, the microbial community functional attributes were stable although the community composition changed with the water temperature and NO3 -N in upstream area (this indicates functional redundancy). However, the strong anthropogenic activities in river downstream area selected pollutant-degraded functions (e.g. nitrogen metabolism, toluene, xylenes and ethylbenzene degradation) and potentially pollutant-degraded (tolerant) microbes, and therefore caused the microbial community composition synchronously changed with the variation of community functional attributes. Our results reveal that strong environmental filtering which associates with the anthropogenic activities not only has effects on microbial community composition and community functional attributes but also on their relationships. These results provide a new insight to refine the functional redundancy idea.

N2O emission and its influencing factors in subtropical streams, China

BackgroundRivers and streams are one of the primary sources of nitrous oxide (N2O) which is an important greenhouse gas with great global warming potential. Yet, over the past century, human activities have dramatically increased reactive nitrogen loadings into and consequently led to increased N2O emission from the river ecosystems. Here, we carried out a study in two subtropical rivers, i.e., Jinshui River and Qi River with slight and intense human disturbance in their respective catchments in China. The study intended to explore spatial variability and seasonality in N2O emissions, and the relative importance of physicochemical variables, nitrification and denitrification potentials, and functional genes abundance influencing N2O emissions.ResultsN2O concentration, N2O saturation, and N2O flux of Jinshui River peaked in high flow season. N2O concentration, N2O saturations, and N2O flux in Qi River and downstream of Jinshui River were significantly higher than that in other areas in normal and low flow seasons. N2O concentration was positively correlated with water temperature, water NO3−, and DOC, negatively correlated with water NH4+ and DOC/NO3− (the ratio of dissolved organic carbon to NO3− in water), and positively correlated with potential nitrification rate in high flow season, but not correlated with functional genes abundance. Both rivers had lower N2O saturation and flux than many freshwater systems, and their EFr-5 (N2O emission factor for river) was lower than the recommended values of IPCC.ConclusionsWhile the two rivers were moderate sources of N2O and N2O emissions in river systems were normally elevated in the summer, areas with intense human disturbance had higher N2O concentration, N2O saturations, and N2O flux than those with slight human disturbance. Physicochemical variables were good indicators of N2O emissions in the river ecosystems.

Effects of Urban Hydrology on Plastic Transport in a Subtropical River

Effects of Urban Hydrology on Plastic Transport in a Subtropical River

Spatial and Temporal Distribution of Bacterioplankton Molecular Ecological Networks in the Yuan River under Different Human Activity Intensity.

Bacterioplankton communities play a crucial role in freshwater ecosystem functioning, but it is unknown how co-occurrence networks within these communities respond to human activity disturbances. This represents an important knowledge gap because changes in microbial networks could have implications for their functionality and vulnerability to future disturbances. Here, we compare the spatiotemporal and biogeographical patterns of bacterioplankton molecular ecological networks using high-throughput sequencing of Illumina HiSeq and multivariate statistical analyses from a subtropical river during wet and dry seasons. Results demonstrated that the lower reaches (high human activity intensity) network had less of an average degree (10.568/18.363), especially during the dry season, when compared with the upper reaches (low human activity intensity) network (10.685/37.552) during the wet and dry seasons, respectively. The latter formed more complexity networks with more modularity (0.622/0.556) than the lower reaches (high human activity intensity) network (0.505/0.41) during the wet and dry seasons, respectively. Bacterioplankton molecular ecological network under high human activity intensity became significantly less robust, which is mainly caused by altering of the environmental conditions and keystone species. Human activity altered the composition of modules but preserved their ecological roles in the network and environmental factors (dissolved organic carbon, temperature, arsenic, oxidation–reduction potential and Chao1 index) were the best parameters for explaining the variations in bacterioplankton molecular ecological network structure and modules. Proteobacteria, Actinobacteria and Bacteroidetes were the keystone phylum in shaping the structure and niche differentiations in the network. In addition, the lower reaches (high human activity intensity) reduce the bacterioplankton diversity and ecological niche differentiation, which deterministic processes become more important with increased farmland and constructed land area (especially farmland) with only 35% and 40% of the community variation explained by the neutral community model during the wet season and dry season, respectively. Keystone species in high human activity intensity stress habitats yield intense functional potentials and Bacterioplankton communities harbor keystone taxa in different human activity intensity stress habitats, which may exert their influence on microbiome network composition regardless of abundance. Therefore, human activity plays a crucial role in shaping the structure and function of bacterioplankton molecular ecological networks in subtropical rivers and understanding the mechanisms of this process can provide important information about human–water interaction processes, sustainable uses of freshwater as well as watershed management and conservation.

Spatial distribution, source apportionment, and associated risks of trace metals (As, Pb, Cr, Cd, and Hg) from a subtropical river, Gomti, Bangladesh

Sediment samples from the Gomti River basin were investigated to determine and evaluate trace metal concentrations, their biological effect, and potential ecological and human health risks for adults and children. The mean concentrations of trace metals were organized in the descending order of mercury (Hg) (0.08 mg/kg) > chromium (Cr) (0.06 mg/kg) > lead (Pb) (0.05 mg/kg) > arsenic (As) (0.02 mg/kg) > cadmium (Cd) (0.01 mg/kg). The current study illustrated that metals were attributed to the area from natural sources and different anthropogenic sources especially from industries. However, the concentration levels were lower than the sediment quality guidelines (SQGs) based on the effect-range classifications of threshold effect concentration (TEC) and probable effect concentration (PEC). Therefore, the concentration of the elements showed no adverse biological effects on aquatic organisms. The evaluated potential ecological risk index also revealed the low toxicity to the aquatic environment. Moreover, the applied sediment quality indices, geo-accumulation index (Igeo), contamination factor (CF), and contamination severity index (CSI) suggested that the contamination levels of the elements were in the acceptable range, and the contamination had not notably impacted on the sediment quality. The risk index (HI) concerning age groups was significantly less than the threshold limit of 1 indicating that the contamination had no non-carcinogenic risk effect. The total carcinogenic risk (TCR) was less than a risk value of 1 × 10−4. Hence, the current study suggests that immediate remediation is not required due to an absence of alarming conditions in the study area. Proper monitoring of the attribution of the metal elements should be done for the betterment of human and environmental health.

Dissimilatory nitrate reduction and functional genes in two subtropical rivers, China.

Dissimilatory nitrate reduction processes, including denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA), are important pathways of nitrate transformation in the aquatic environments. In this study, we investigated potential rates of denitrification, anammox, and DNRA in the sediments of two subtropical rivers, Jinshui River and Qi River, with different intensities of human activities in their respective catchment, China. Our objectives were to assess the seasonality of dissimilatory nitrate reduction rates, quantify their respective contributions to nitrate reduction, and reveal the relationship between dissimilatory nitrate reduction rates, functional gene abundances, and physicochemicals in the river ecosystems. Our results showed higher rates of denitrification and anammox in the intensively disturbed areas in autumn and spring, and higher potential DNRA in the slightly disturbed areas in summer. Generally, denitrification, anammox, and DNRA were higher in summer, autumn, and spring, respectively. Relative contributions of nitrate reduction from denitrification, anammox, and DNRA were quite different in different seasons. Dissimilatory nitrate reduction rates and gene abundances correlated significantly with water temperature, dissolved organic carbon (DOC), sediment total organic carbon (SOC), NO3-, NH4+, DOC/NO3-, iron ions, and sulfide. Understanding dissimilatory nitrate reduction is essential for restoring nitrate reduction capacity and improving and sustaining ecohealth of the river ecosystems.

Estimating submarine groundwater discharge at a subtropical river estuary along the Beibu Gulf, China

Estimating submarine groundwater discharge at a subtropical river estuary along the Beibu Gulf, China

Event loading drives distribution of the organochlorine pesticide metabolite DDE in a sub-tropical river system, Brisbane River, Australia

Past catchment practices can contribute to environmental impacts for decades following their cessation. We examine the distribution of the prevalent organochlorine pesticide, dichlorodiphenyltrichloroethane (DDT) and its metabolites (DDE, DDD) in the sediments of a sub-tropical river system (Brisbane River, Australia). This study aimed to identify sources of DDT, DDE, DDD into the lower reaches of the Brisbane River. Annual sediment sampling of the lower Brisbane River over a period of 15 years (2001–2015) revealed a significant increase in sediment DDT, DDE and DDD content following major floods. A regional survey detected elevated sediment DDT, DDE and DDD content at 32 of 79 sites sampled; however, these were generally below guideline trigger values. DDE was the sole fraction at all but one site with creek systems dominated by intensive cropping practices identified as legacy sources and major flood events as a driver of elevated sediment DDE content in the lower reaches.

Spatial-temporal variability of indirect nitrous oxide emissions and emission factors from a subtropical river draining a rice paddy watershed in China

Indirect nitrous oxide (N2O) emissions from rivers draining agricultural watersheds are of increasing concerns due to riverine abundant sources of nitrogen loaded through leaching and runoff. However, the seasonal variation of N2O emissions from agricultural drainage rivers is poorly explored, especially the uncertainty in quantifying indirect N2O emission factors (EFs) from these aquatic environments. Here, a two-year study (2014-2016) was conducted to quantify indirect N2O emissions from a river draining a rice paddy watershed in subtropical China. Indirect N2O fluxes were simultaneously determined using the floating chamber method (chamber-based) and the gas exchange modeling approach (model-based) based on the measurement of dissolved N2O concentration. Results showed that seasonal dissolved N2O concentration and N2O fluxes had a similar variation pattern, with the highest and the lowest levels in summer and winter, respectively. The annual mean of model-based N2O fluxes (20.24 ± 3.34 μmol m−2 d−1) was generally in agreement with chamber-based N2O fluxes (18.70 ± 3.56 μmol m−2 d−1). The indirect emission factor of N2O was highly dependent on the surface water NO3−-N concentration. Annual mean indirect EF of N2O from the drainage river was estimated to be 0.00051, which was significantly lower than the default EF5r value (0.0025) proposed by the Intergovernmental Panel on Climate Change (IPCC). These results suggest that the use of IPCC default value might have overestimated indirect N2O emissions from agricultural impacted riverine systems. Our study also highlights that more extensive in-situ measurements are required for monitoring indirect N2O emissions from agricultural impacted waters with different drainage characteristics, which would benefit for refining the IPCC EF5r default value to further constrain global N2O budget.