High-flow Season Research Articles

Multivariate statistical analyses for water and sediment quality index development: A study of susceptibility in an urban river

In this study, multivariate statistical analyses were performed to develop water and sediment quality indexes, allowing us (i) to select with reliability the most appropriate chemical variables for the evaluation of river quality susceptibility; (ii) to weight the influence of each variable based on monitored data; (iii) to consider possible synergism or antagonism derived from the combined effect of several pollutants; and (iv) to express the quality as a deviation from selected site-specific reference conditions. For the establishment of these threshold/maximum values, combining two biological indicators related to denitrifying bacteria in sediments turned out to be applicable to ensure compliance with the European water quality standard.The joint implementation of water and sediment quality indexes assisted us in the rapid detection of the deleterious effect of different anthropogenic contamination sources, as well as the influence of hydrological regime seasonality on river quality. In addition, metal-dependent water quality appeared to be coupled to sediment dynamics, since they were preferentially adsorbed onto sediments during low flow seasons, whereas there was potential for metal mobilization to water during sediment resuspension in high flow seasons. Therefore, an annual determination of sediment quality index was also recommended as suitable tool for prospective monitoring water quality, identifying those sites which could deserve special attention during certain periods, and planning future strategies for river quality improvement. However, two limitations were found: (1) sediment was not appropriate for water physicochemical quality early monitoring due to organic matter and nutrient continuous transformation; and (2) a multimetric index did not provide a concise and definitive quality information, thus a new tool for combining with quality index was proposed for specifically evaluate the water and sediment quality by identifying pollutant/s of concern at each location.

Stable H-O Isotopic Composition and Water Quality Assessment of Surface Water and Groundwater: A Case Study in the Dabie Mountains, Central China.

In order to understand the water cycle and assess the water quality for irrigation purposes in the Upper Pi River Basin (UPRB), which is the northern slope of the Dabie Mountains, 68 surface water and groundwater samples were collected and analyzed for H-O isotopes and hydrochemistry during the high-flow season in 2017 and 2018. The results show that ranges of hydrogen and oxygen isotopic composition (δ2H: −68.8‰ to −40.8‰, δ18O: −10.05‰ to −5.05‰) are controlled by the medium latitude and high altitude of the UPRB. Among different types of water, the δ2H and δ18O values can be ordered as follows: reservoir water < spring water ≈ river water < pond water. The water of the upstream medium and small reservoir is enriched with lighter isotopes that is likely related to more exchange with rainwater and less residence time; however, large reservoirs are similar to the upstream river and spring in terms of the H-O isotopic composition. Hydro-chemical facies are dominated by the Ca-HCO3 type in the UPRB, which reflects fresh recharged water from rainfall, and few samples are of the Ca-Cl type that is caused by intensive evaporation. The water quality for irrigation purposes was also evaluated. According to the Wilcox diagram, United States Salinity Laboratory (USSL) diagram, magnesium hazard, and Kelly’s ratio, all water samples have been considered suitable for irrigation water.

Effects of Change in Land Use Activities and Land Cover on Ragati River

Changes in land-use and land-cover have negatively affected the hydrology of Ragati River. It is one of the many rivers in Kenya that are considered as hot spots due to over abstraction especially during the dry season. This study aimed at evaluating the effects of land use and land cover changes which are important in ensuring sustainable use of the water resource. This study aimed at achieving the following objective: i)To analyze land use activities and land cover changes within Ragati River catchment area from 1990 to 2010; ii)To analyze the discharge of Ragati River during low flow and high flow seasons; iii) To assess the response of the sub-catchment to land-use and land cover change; iv) To evaluate the measures taken to mitigate negative effects of land use and land cover changes on the Ragati catchment. Satellite data was integrated in GIS to examine the extent of land-use and cover change in the sub-catchment. Both quantitative and qualitative data were used in this study. Various data types such as socio-economic data, land-use data, rainfall data and river flow data were collected in order to supplement information from landsat images. Analysis was done using classified landsat images of 1990, 2000 and 2009. SPSS and Ms Excel were used to analyse descriptive data. This study revealed that between 1988 and 2000 the dense forest decreased by 15%, land under perennial crops increased by 4.26%, cropland increased by 11%, settlement increased by 45.30% while closed woodland decreased by 4.6%. The results also show some change between 2000 and 2009. This can be attributed to increased campaign on reforestation of Mt. Kenya forest. Land under perennial crops decreased by 57%, crop land further by 4.04%, settlement increased by 49.41% while closed woodland decreased further by 3%. Over abstraction of water and river bank encroachment are some of the causes of changes in the volume of water flowing in the river. This study recommends that water users should be made aware of the importance of sustainable use of the water resource.

Variability in macroinvertebrate community structure and its response to ecological factors of the Weihe River Basin, China

Macroinvertebrates are sensitive to changes in the river environment and ecological status. Ecological variables over multi-spatial scales and macroinvertebrate community data were collected in June (normal flow season) and September (high flow season) of 2017 in the Weihe River Basin (WRB). A total of 14,377 individuals were identified, which were classified into 7 classes, 18 orders and 59 families. Macroinvertebrate community composition, density, biomass, the values of Pielou evenness index (E), Simpson diversity index (λ) were significantly different between normal flow season and high flow season. The dominant species (Tubificidae, Chironomidae and Baetidae) were the same in both seasons. The highest richness, abundance, density and biomass occurred at a stream bed depth of 0–10 cm. The results of canonical correspondence analyses (CCA) showed that ecological factors explained the major variation in macroinvertebrate community composition. Specifically, the increased nitrogen concentrations favored tolerant species, whereas high velocity and dissolved oxygen (DO) benefitted community taxa richness and biodiversity. The reduction of taxa richness, abundance, density and biomass in high flow season was related to the summer flood. Increased nutrient concentrations and macroinvertebrate habitat damage contributed to more tolerant, yet less diverse stream macroinvertebrate assemblages.

Hydrochemical Characteristics and Water Quality Assessment for the Upper Reaches of Syr Darya River in Aral Sea Basin, Central Asia

Based on water sampling of the upper reaches of the Syr River and its tributaries from the parts of Aral Sea Basin in Kyrgyzstan, the chemical compositions of river waters were systematically analyzed for revealing the hydrochemical characteristics and evaluating the water quality. Research indicates that there are some differences in ion concentration between the low-flow season (LFS) and high-flow season (HFS), but the hydrochemical classification reflected that all water samples fall in the calcium bicarbonate category, except that only three samples fall in the not dominant category during the LFS. The water quality classification shows that the water samples fall in the excellent to good categories for irrigation use. The analysis shows that the main ions of river waters come from the weathering of rocks, and the dissolution of carbonates is higher than that of silicates. Human activities have had an impact on the waterbody, especially inferred from the indicators of NH4-N and fecal coliform (FC). FC groups were detected in some rivers, in which the detection rate at the high-water level increased. The contents of potentially toxic elements are lower than international drinking water standards, but there are clustering differences between the LFS and HFS. There may be anthropogenic intrusions of Cu, Pb, and Zn during the LFS period and of Cu, Pb, Zn, and Cd during the HFS period. The results fill the gaps in the study of the hydrochemical composition and water quality assessment in the Aral Sea Basin and will also provide a basis for water resource management and for the study of water quality evolution in the future.

Trajectories of nitrate input and output in three nested catchments along a land use gradient

Abstract. Increased anthropogenic inputs of nitrogen (N) to the biosphere during the last few decades have resulted in increased groundwater and surface water concentrations of N (primarily as nitrate), posing a global problem. Although measures have been implemented to reduce N inputs, they have not always led to decreasing riverine nitrate concentrations and loads. This limited response to the measures can either be caused by the accumulation of organic N in the soils (biogeochemical legacy) – or by long travel times (TTs) of inorganic N to the streams (hydrological legacy). Here, we compare atmospheric and agricultural N inputs with long-term observations (1970–2016) of riverine nitrate concentrations and loads in a central German mesoscale catchment with three nested subcatchments of increasing agricultural land use. Based on a data-driven approach, we assess jointly the N budget and the effective TTs of N through the soil and groundwater compartments. In combination with long-term trajectories of the C–Q relationships, we evaluate the potential for and the characteristics of an N legacy. We show that in the 40-year-long observation period, the catchment (270 km2) with 60 % agricultural area received an N input of 53 437 t, while it exported 6592 t, indicating an overall retention of 88 %. Removal of N by denitrification could not sufficiently explain this imbalance. Log-normal travel time distributions (TTDs) that link the N input history to the riverine export differed seasonally, with modes spanning 7–22 years and the mean TTs being systematically shorter during the high-flow season as compared to low-flow conditions. Systematic shifts in the C–Q relationships were noticed over time that could be attributed to strong changes in N inputs resulting from agricultural intensification before 1989, the break-down of East German agriculture after 1989 and the seasonal differences in TTs. A chemostatic export regime of nitrate was only found after several years of stabilized N inputs. The changes in C–Q relationships suggest a dominance of the hydrological N legacy over the biogeochemical N fixation in the soils, as we expected to observe a stronger and even increasing dampening of the riverine N concentrations after sustained high N inputs. Our analyses reveal an imbalance between N input and output, long time-lags and a lack of significant denitrification in the catchment. All these suggest that catchment management needs to address both a longer-term reduction of N inputs and shorter-term mitigation of today's high N loads. The latter may be covered by interventions triggering denitrification, such as hedgerows around agricultural fields, riparian buffers zones or constructed wetlands. Further joint analyses of N budgets and TTs covering a higher variety of catchments will provide a deeper insight into N trajectories and their controlling parameters.

Runoff variation characteristics, association with large-scale circulation and dominant causes in the Heihe River Basin, Northwest China

The water resources in arid and semi-arid regions are critical to providing reliable sources of water for food production and ecosystem functioning. In this study, continuous wavelet transform and wavelet coherence were used to analyse the runoff periodicity and relationship with climate indices, respectively. Additionally, the double mass curve (DMCD), the slope changing ratio of cumulative quantity (SCRCQ) and the Choudhury–Yang equation (Budyko-CY) methods for different potential evapotranspiration data (E0(E0-20 cm, E0-PM, E0-H)) were used to separate the impacts of climate changes and anthropogenic activities on runoff variations. The results demonstrated that the flow regimes in high and low flow seasons were not obvious shifts, and that after implementation of the Ecological Water Diversion Project (EWDP), ecosystems were gradually restored in the downstream portion of the Heihe River Basin (DHRB). Periodicities of 1–7 years and 1–5.8 years were detected in Yingluoxia and Zhengyixia, respectively. Additionally, on a 1–148.2 month timescale, the monthly runoff with AO, NAO, PDO, and AMO had significant resonance periodicity and a 1–48 month Spearman's lag correlation. On the annual and high-flow, climate changes dominant determinant to an increase of runoff for the DMCD and Budyko-CY in period 2, SCRCQ and Budyko-CY in period 3 for different E0 in the upstream (UHRB). In the midstream (MHRB) region, anthropogenic activities played a dominant role in deducing the runoff by the SCRCQ and Budyko-CY methods for different E0 values in period 3. During the low-flow season, the impact of human activities in the UHRB and climate changes in the MHRB was significant for period 2 and 3. Therefore, the impacts of climate change and human activities on runoff changes caused by the introduction of different E0 on different timescales should be fully considered in the future.

Geochemical characteristics of dissolved heavy metals in Zhujiang River, Southwest China: spatial-temporal distribution, source, export flux estimation, and a water quality assessment.

To investigate the sources and spatial-temporal distribution of dissolved heavy metals in river water, and to evaluate the water quality, a total of 162 water samples were collected from 81 key sampling points in high and low flow seasons separately in the Zhujiang River, Southwest China. Ten dissolved heavy metals (V, Cr, Mn, Co, Ni, Cu, Mo, Cd, Ba, and Pb) in the Zhujiang River water exhibit little variation at temporal scale, but vary with a significant spatial heterogeneity. Furthermore, different metals present different variation trends along the main channel of the Zhujiang River. Our results suggest that Ba (14.72 μg L−1 in low flow season and 12.50 μg L−1 in high flow season) and Cr (6.85 μg L−1 in low flow season and 7.52 μg L−1 in high flow season) are consistently the most abundant metals in the two sampling periods. According to the water quality index (WQI values ranged from 1.3 to 43.9) and health risk assessment, metals investigated in Zhujiang River are below the hazard level (all hazard index (HI) < 1). Application of statistical approaches, including correlation matrix and principal component analysis (PCA), identify three principal components that account for 61.74% of the total variance, the results conclude that the anthropogenic heavy metals (V, Cr, Ni, and Cu) are greatly impacted by the dilution effect, and the heavy metals in Zhujiang River are mainly presented a natural sources signature from the perspective of entire basin. Moreover, our results reveal that the estimated export budget of several heavy metals including V (735.6 t year−1), Cr (1,561.1 t year−1), Ni (498.2 t year−1), and Mo (118.9 t year−1) to the ocean are higher than the world average.

A large sample analysis of European rivers on seasonal river flow correlation and its physical drivers

Abstract. The geophysical and hydrological processes governing river flow formation exhibit persistence at several timescales, which may manifest itself with the presence of positive seasonal correlation of streamflow at several different time lags. We investigate here how persistence propagates along subsequent seasons and affects low and high flows. We define the high-flow season (HFS) and the low-flow season (LFS) as the 3-month and the 1-month periods which usually exhibit the higher and lower river flows, respectively. A dataset of 224 rivers from six European countries spanning more than 50 years of daily flow data is exploited. We compute the lagged seasonal correlation between selected river flow signatures, in HFS and LFS, and the average river flow in the antecedent months. Signatures are peak and average river flow for HFS and LFS, respectively. We investigate the links between seasonal streamflow correlation and various physiographic catchment characteristics and hydro-climatic properties. We find persistence to be more intense for LFS signatures than HFS. To exploit the seasonal correlation in the frequency estimation of high and low flows, we fit a bi-variate meta-Gaussian probability distribution to the selected flow signatures and average flow in the antecedent months in order to condition the distribution of high and low flows in the HFS and LFS, respectively, upon river flow observations in the previous months. The benefit of the suggested methodology is demonstrated by updating the frequency distribution of high and low flows one season in advance in a real-world case. Our findings suggest that there is a traceable physical basis for river memory which, in turn, can be statistically assimilated into high- and low-flow frequency estimation to reduce uncertainty and improve predictions for technical purposes.

Spatial and Seasonal Variation of O and H Isotopes in the Jiulong River, Southeast China

The stable isotope technique of oxygen and hydrogen (δ18O and δ2H) and deuterium excess (d-excess) was used to investigate distribution characteristics in June 2017 and January 2018 in the Jiulong River, southeast China. The results revealed that (1) seasonal isotopic composition was mainly controlled by precipitation. It enriched lighter water isotopes in winter more than in summer because of the aggravating effect of low temperature and great rainfall. (2) Spatial distribution of the North, West, and South River showed increasing enrichment of heavy isotopes in that order. In the high-flow season, the continuous high-flow made δ18O and δ2H homogeneous, despite increasing weak evaporation along water-flow paths in the West and South River. In the low-flow season, there was a decreasing trend in the middle and lower reaches of the North and West main stream and an increasing trend in the South River. (3) O and H isotopic geochemistry exhibited natural and anthropogenic influence in hydrological process, such as heavy rainfall and cascade reservoirs. The results showed that O and H isotopes are indeed useful tracers of the water cycle.

Pharmaceutical residues in streams near concentrated animal feeding operations of Korea – Occurrences and associated ecological risks

Concentrated animal feeding operations (CAFOs) have been suggested to be the most significant source of pharmaceutical release into the environment. However, limited information is available on the occurrence of veterinary pharmaceutical residues and the associated ecological risks to the aquatic environment near CAFO areas. In this study, ten commonly used veterinary antibiotics, including sulfonamides, tetracyclines, and cephalosporins, along with three analgesics, were measured in water samples collected from the streams that run near two CAFOs in Korea in 2013 (n = 16) and 2014 (n = 10). In addition, the associated ecological risks were estimated by calculating risk quotient. The pharmaceuticals were detected in a higher amount in the samples collected downstream from the CAFO than in those collected upstream. Acetaminophen, sulfamethazine, sulfathiazole, and oxytetracycline were detected at maximum concentrations of 38.8 μg/L, 21.3 μg/L, 17.4 μg/L, and 16.9 μg/L, respectively. Relatively higher concentrations of pharmaceuticals were observed in locations adjacent to the CAFO and the downstream area, suggesting the influence of the CAFO. Except for acetaminophen, lower concentrations of the target pharmaceuticals were detected in the samples collected during the high-flow season. The concentrations of most of the target pharmaceuticals exceeded the risk quotient of one, suggesting potential ecological effects in the areas affected by CAFOs. Our observations show that the water environment near a CAFO could be heavily affected by veterinary pharmaceuticals and analgesic drugs that are also frequently used among humans. Hence, the ecological consequences of pharmaceutical residues in the water bodies near CAFOs warrant further investigation.

Chemical weathering of small catchments on the Southeastern Tibetan Plateau I: Water sources, solute sources and weathering rates

Hydro-geochemical study of small catchment provides important information to identify water and solute sources, understand chemical weathering processes and their controlling factors. In this work, 44 small catchments on the southeastern Tibetan Plateau were investigated. Stream, precipitation, glacier and spring waters in both high and low flow seasons and bed rocks samples were analyzed with a main purpose to understand the processes controlling the stream water chemistry and quantify the weathering rates. The stream waters are mainly recharged by precipitation and glacier meltwater. Glacier meltwater and precipitation account for 25.8% and 73.9% of the total discharge in high flow season, and 44.4% and 54.1% in low flow season on average. Hydrograph separation and chemical mass balance are jointly used to estimate the contributions of major reservoirs (precipitation, glacier, spring, carbonates and silicates) to the total dissolved loads of the streams. Rock weathering accounts for ~90% of the total dissolved cations for most streams. Silicate and carbonate weathering account for 15.9% and 75.2% of total dissolved cations in high flow season, and 9.5% and 77.2% in low flow season on average. Lack of basic hydrological data in the ungauged remote area is a problem for quantified weathering study. The Noah LSM model is applied to obtain the annual runoff of these un-gauged catchments in this study. Based on these approaches, the chemical weathering rates and total denudation rates (TDR) are calculated for each of the small catchments. The silicate cation weathering rates (SCWR) range between 0.6 and 5.2 t/km2/yr, with the area-weighted mean value about 1.8 t/km2/yr. The TDR range between 8.9 and 1907.9 t/km2/yr. The comparisons between the small catchments and with other river basins in different tectonic and climatic environments indicate that lithology, climatic factors (temperature and runoff) and physical erosion rate are the key parameters controlling chemical weathering rate. The average SCWR of the small catchments is about 6 times higher in high flow season than in low flow season, which could be attributed to the higher temperature and runoff in high flow season. Meanwhile, the positive relationship between SCWR and TDR supports the view that physical erosion has an important effect on chemical weathering in the Tibetan Plateau.

First report on OH-PAHs in South African Clarias gariepinus bile from an urban impacted system

The concentrations of selected hydroxylated polycyclic aromatic hydrocarbons (OH-PAHs) were determined in the bile of the African sharptooth catfish Clarias gariepinus from impoundments in the urban impacted Klip River system in Soweto, South Africa. Fish were sampled from three impoundments (Lenasia, Fleurhof, and Orlando dams) during the early high-flow season (September/October) of 2013. Biliary OH-PAHs were analysed using a high-pressure liquid chromatograph coupled to a tandem mass spectrometer (HPLC-MS/MS). Seven of the thirteen targeted metabolites were present in the fish of Soweto. The ΣOH-PAHs ranged between 0.1 and 1 876 ng ml−1, with greatest ΣOH-PAH mean at Orlando (947 ng ml−1) followed by Fleurhof (371 ng ml−1). The most dominant metabolite in the sampling area was 2–,3–OH fluorene, ranging between not detected and 1 429 ng ml−1, with the greatest mean at Orlando (709 ng ml−1). PAH metabolites quantified in C. gariepinus most likely originated from the sediments. The hepatosomatic index of the C. gariepinus increased proportionally with the biliary OH-PAH concentrations. To the authors’ knowledge this data on biliary OH-PAH for fish is the first for South Africa.

Morphodynamics of the Qiantang Estuary, China: Controls of river flood events and tidal bores

The importance of seasonal variations in river discharge on the morphological development of estuaries has been recognized in recent years, yet in situ observations about such variations are rare. Here we report a long-term dataset of bathymetry in the middle reach of the Qiantang Estuary, China, characterized by the presence of a large inner bar. Moreover, a hydrographic survey was carried out in the Yanguan reach where one of the largest tidal bores in the world occurs, covering a spring-neap tidal cycle in 2015. Meanwhile, detailed seasonal bathymetric data together with daily river discharges of 2015 were collected. The bed morphology shows strong seasonal and inter-annual variations. During the high flow season, the river flow erodes the bed and transports a large amount of sediments seaward. A good power-law relationship exists between the high river discharge and the channel volume at the upper estuary. Flood tides dominate under usual river flow condition. In particular, the tidal bore during spring and intermediate tides is characterized by large current velocity and high suspended sediment concentration, and transports a large amount of sediment landward. Over a year, a dynamic morphological equilibrium can be maintained. Moreover, the estuary has also been significantly influenced by the large-scale embankment in recent decades, constraining the lateral thalweg migration, bank erosion and point bar deposition, which usually occur in natural sinuous estuaries.

Assessment of environmental flows using hydrological methods for Krishna River, India

Krishna River is significantly affected due to Srisailam dam from past 30 years. The impact of this hydraulic structure drastically reduced the minimum flow regime on the downstream, which made the river nearing to decaying stage. In the present paper, Environmental Flow called minimum flow values released for the dam are estimated with the help of three hydrological methods viz., Range of variability Approach (RVA), Desktop Reserve Model (DRM), and Global Environmental Flow Calculator (GEFC). DRM method suggested considering the intermediate values obtained from among the three methods to preserve the ecosystem on the downstream of the river, which amounts to an average annual allocation of 9378 Million Cubic Meter (MCM) which is equal to 23.11% of mean annual flow (MAF).In this regard GEFC and RVA methods accounted for 22 % and 31.04% of MAF respectively. The results indicate that current reservoir operation policy is causing a severe hydrological alteration in the high flow season especially in the month of July. The study concluded that in the case of non-availability of environmental information, hydrological indicators can be used to provide the basic assessment of environmental flow requirements. It is inferred from the results obtained from the study, that the new reservoir operations can fulfil human water needs without disturbing Environmental Flow Requirements.

The Impacts of Land Use Patterns on Water Quality in a Trans-Boundary River Basin in Northeast China Based on Eco-Functional Regionalization.

The relationships between land use patterns and water quality in trans-boundary watersheds remain elusive due to the heterogeneous natural environment. We assess the impact of land use patterns on water quality at different eco-functional regions in the Songhua River basin during two hydrological seasons in 2016. The partial least square regression indicated that agricultural activities associated with most water quality pollutants in the region with a relative higher runoff depth and lower altitude. Intensive grazing had negative impacts on water quality in plain areas with low runoff depth. Forest was related negatively with degraded water quality in mountainous high flow region. Patch density and edge density had major impacts on water quality contaminants especially in mountainous high flow region; Contagion was related with non-point source pollutants in mountainous normal flow region; landscape shape index was an effective indicator for anions in some eco-regions in high flow season; Shannon’s diversity index contributed to degraded water quality in each eco-region, indicating the variation of landscape heterogeneity influenced water quality regardless of natural environment. The results provide a regional based approach of identifying the impact of land use patterns on water quality in order to improve water pollution control and land use management.

Geochemistry of the dissolved loads during high-flow season of rivers in the southeastern coastal region of China: anthropogenic impact on chemical weathering and carbon sequestration

Abstract. The southeastern coastal region is one of the most developed and populated areas in China. Meanwhile, it has been severely impacted by acid rain over many years. The chemical compositions and carbon isotope compositions of dissolved inorganic carbon (δ13CDIC) in river water in the high-flow season were investigated to estimate the chemical weathering and associated atmospheric CO2 consumption rates as well as the acid-deposition disturbance. Mass balance calculations indicated that the dissolved loads of major rivers in the Southeast Coastal River Basin (SECRB) were contributed to by atmospheric (14.3 %, 6.6 %–23.4 %), anthropogenic (15.7 %, 0 %–41.1 %), silicate weathering (39.5 %, 17.8 %–74.0 %) and carbonate weathering inputs (30.6 %, 3.9 %–62.0 %). The silicate and carbonate chemical weathering rates for these river watersheds were 14.2–35.8 and 1.8–52.1 t km−2 a−1, respectively. The associated mean CO2 consumption rate by silicate weathering for the whole SECRB was 191×103 mol km−2 a−1. The chemical and δ13CDIC evidence indicated that sulfuric and nitric acid (mainly from acid deposition) were significantly involved in the chemical weathering of rocks. There was an overestimation of CO2 consumption at 0.19×1012 g C a−1 if sulfuric and nitric acid were ignored, which accounted for about 33.6 % of the total CO2 consumption by silicate weathering in the SECRB. This study quantitatively highlights the role of acid deposition in chemical weathering, suggesting that the anthropogenic impact should be seriously considered in estimations of chemical weathering and associated CO2 consumption.

Effects of land-use pattern and physiochemical conditions on phytoplankton communities in a German lowland catchment

To understand the lowland riverine phytoplankton community (of species and functional groups), and to investigate the effects of abiotic factors on phytoplankton variance during the high-flow season, 59 sites of the Treene catchment, northern Germany, were visited in December 2014, where 327 species belonging to six taxonomic groups and 21 phytoplankton functional groups were observed. Bacillariophyta, especially benthic pennales (corresponding to the functional group TB), were dominant in most of the study area. Cyanobacteria (specifically indicated by the functional groups M and S1), which are typically found in lake environments, were also found in the main stream of the River Treene. Furthermore, Euglenophyta (represented by the functional groups W1 and W2) showed a higher percentage in the headwaters. A redundancy analysis indicates that the land-use pattern and physiochemical condition co-contributed to the phytoplankton community variance. The total explained variance (68.4 %) of phytoplankton data was partitioned into three parts: purely physiochemical factors contributed to 24.1 % of the explanation, followed by land-use characteristics (15.6 %), and their shared effects (28.7 %). The functional groups W1 and W2 exhibited a strong positive correlation to the percentage of agricultural land use. The percentages of urban land use and phosphate concentration in the stream were interrelated with the functional groups Lo and M, which indicates mesotrophic to eutrophic water conditions. The unexplained variance (31.6 %) may result from hydrological regimes, which will be taken into account in subsequent studies.

Estuarine Circulation Patterns in a Complex Geometry Estuary: Dinh An Estuary, Mekong River

Estuarine circulation (EC) is a natural phenomenon that contributes to the evolution of estuaries worldwide. This paper uses the Dinh An estuary, Mekong River as a case study to investigate patterns of EC in a complex cross-section estuary. An Acoustic Doppler Current Profiler (ADCP) measured a series of river discharge and current profiles at a transect near the mouth of the estuary between 15 and 30 September 2009. Tidal elevation was also recorded concurrently with the river discharge by using a leveling rod. The observed data reveal that the river-discharge phase lagged 6 h behind the tidal phase in the high-flow season (May to October). Four main types of EC were observed during the reversal time in the ebb tide with river discharges in a range of −5000 m3/s to 13,600 m3/s. There were two main flood tide patterns, which corresponded to the discharges from −6000 m3/s to 5000 m3/s. The classical EC type (seawater flows landward near the bed and river water flows seaward near the surface) appeared only in the deepest channel in the ebb tide as saline intrusion induced a density imbalance. The results suggest that tidal phase and bed geometry play important roles in forming EC patterns in estuaries. This paper provides essential information related to flow structure in complex bathymetry estuaries for further studies on saline intrusion, environmental processes, sediment dynamic and estuarine morphology.

Tracing the origin and geochemical processes of dissolved sulphate in a karst-dominated wetland catchment using stable isotope indicators

Sulphur plays a critical role in the biogeochemistry of wetlands. SO42− is the sulphur form that is most commonly assimilated by plants, and it is the medium of sulphur transformation in wetland ecosystems. The Caohai wetland is an important ecosystem on the Yunnan–Guizhou Plateau, SW China, and karst water is the most important water source in this wetland. In this study, the compositions of multiple isotopes (δ34SSO4 and δ18OSO4) were measured in groundwater, river water and wetland water in the Caohai catchment to elucidate the solute sources and behaviours of sulphate. The results suggest that SO42− in groundwater is mainly derived from sulphide oxidation. Groundwater contributes a significant amount of sulphate to river water, and it is the main sulphate source of wetland water during the high-flow season. In contrast, rain water is the main sulphate source of wetland water during the low-flow season due to the reduction in groundwater recharge. These data indicate that groundwater is not only an important water source in the karst-dominated wetland catchment but also provides pathways by which sulphate is transported to the wetland. In addition, due to the effects of dissimilatory sulphate reduction in the wetland, the sulphate isotope composition in wetland water was higher than those in the groundwater and inflowing rivers, which indicates that biogeochemical processes in wetland ecosystems influence the sulphate content and composition in karst water.