Chemical weathering and CO2 consumption in the upper Nyong Basin rivers (Central Africa): Insights on climatic and anthropogenic forcing in humid tropical environments.

A hydrological and hydrochemical database (produced by the M-TROPICS critical zone observatory) in the upper Nyong Basin from 1998 to 2017 was used to evaluate the river's response to climatic and anthropogenic forcing and examine chemical weathering processes. SiO2 and HCO3- constitute about 85 % of the Total dissolved solids (TDS) load, equivalent to 0.12 × 109 kg. y-1. Electrical conductivity (EC), Total dissolved solids (TDS), major cations, major anions (except F- and NO3-) and alkalinity (Alk) vary seasonally and follow a predictable model with discharge. Atlantic Meridional Mode oscillation controls the long-term water chemistry. Atmospheric input and silicate weathering are the main factors influencing the Nyong rivers chemistry. However, several indices supported the progressive water quality deterioration by human activities, namely: the excess of Cl- and SO42- after the substraction of atmospheric inputs, the basic pH observed for specific samples, long-term increase in the values of pH, EC, TDS, EC, Mg2+, Ca2+, F-, NO3-, HCO3-, Alk, SiO2 and Dissolved Organic Carbon. Runoff and physical erosion have an important control on chemical erosion in the upper Nyong Basin rivers. The chemical erosion rate (3.3 t.km-2.y-1) equals the silicate weathering rate. The CO2 consumption rate, in the Nyong rivers, is lower than the global average (98× 103 for silicate weathering and 246 × 103 mol.km-2.y-1 for chemical erosion) and estimated at 52.3 × 103 for silicate weathering and 54.1 × 103 mol.km-2.y-1 for chemical erosion. At Olama, the most downstream location of the monitoring setup, the Nyong River Basin consumed 1 × 109 mol.y-1 of CO2 by chemical erosion.

Spatio-temporal variation in water quality and phytoplankton community structure in Changwang, Meishe, and Wuyuan Rivers in Hainan Island, China.

For the first time, this study explored spatio-temporal variation in water quality and phytoplankton community structure in Changwang, Meishe, and Wuyuan Rivers in tropical Hainan Island, China. Phytoplankton samples and water were collected between March and December 2019 and analyzed using standard methods. Two-way ANOVA revealed significant spatial and seasonal variation in physico-chemical parameters (p < 0.05). Wuyuan had high TP (0.06 ± 0.04 mg L-1), TN (1.14 ± 0.71 mg L-1), NH4+-N (0.07 ± 0.09 mg L-1), Secchi depth (2.28 ± 3.79 m), salinity (3.60±5.50 ppt), and EC (332.50 ± 219.10 µS cm-1). At the same time, Meishe had high TP (0.07 ± 0.03 mg L-1), TN (1.04 ± 0.74 mg L-1), NH4+-N (0.07 ± 0.10 mg L-1), EC (327.61 ± 63.22 µS cm-1), and turbidity (40.25 ± 21.16 NTU). In terms of seasons, spring recorded high average TP, TN, NH4+-N, COD, and DO, while summer had a high temperature, Chl-a, salinity, and EC. Generally, the physico-chemical parameters met the China water quality standard limits (GB 3838-2002). Overall, 197 phytoplankton species belonging to Cyanophyta, Chlorophyta, Cryptophyta, Bacillariophyta, Pyrrophyta, Euglenophyta, Xanthophyta, and Chrysophyta were identified, with Cyanophyta being dominant. Phytoplankton density showed spatial changes varying from 18 × 106 cell L-1 to 84 × 106 cell L-1. The phytoplankton diversity ranged from 1.86 to 2.41, indicating a mesotrophic state. One-way ANOSIM showed no significant spatial dissimilarity in phytoplankton composition (R = 0.042, p = 0.771) but indicated a significant seasonal difference (R = 0.265, p = 0.001). Therefore, SIMPER analysis revealed that Lyngbya attenuata, Merismopedia tenuissima, Cyclotella sp., Merismopedia glauca, Merismopedia elegans, and Phormidium tenue contributed to the seasonal differences. Furthermore, CCA demonstrated that TP, TN, NH4+-N, COD, Chl-a, and Secchi depth greatly influenced the phytoplankton community. This study shows the spatio-temporal variation in water quality and phytoplankton communities, useful for managing riverine quality.

Impact of a megacity on the water quality of a tropical estuary assessed by a combination of chemical analysis and in-vitro bioassays.

Tropical estuaries are threatened by rapid urbanization, which leads to the spread of thousands of micropollutants and poses an environmental risk to such sensitive aqueous ecosystems. In the present study, a combination of chemical and bioanalytical water characterization was applied to investigate the impact of Ho Chi Minh megacity (HCMC, 9.2 million inhabitants in 2021) on the Saigon River and its estuary and provide a comprehensive water quality assessment. Water samples were collected along a 140-km stretch integrating the river-estuary continuum from upstream HCMC down to the estuary mouth in the East Sea. Additional water samples were collected at the mouth of the four main canals of the city center. Chemical analysis was performed targeting up to 217 micropollutants (pharmaceuticals, plasticizers, PFASs, flame retardants, hormones, pesticides). Bioanalysis was performed using six in-vitro bioassays for hormone receptor-mediated effects, xenobiotic metabolism pathways and oxidative stress response, respectively, all accompanied by cytotoxicity measurement. A total of 120 micropollutants were detected and displayed high variability along the river continuum with total concentration ranging from 0.25 to 78 µg L-1. Among them, 59 micropollutants were ubiquitous (detection frequency ≥ 80 %). An attenuation was observed in concentration and effect profiles towards the estuary. The urban canals were identified as major sources of micropollutants and bioactivity to the river, and one canal (Ben Nghé) exceeded the effect-based trigger values derived for estrogenicity and xenobiotic metabolism. Iceberg modelling apportioned the contribution of the quantified and the unknown chemicals to the measured effects. Diuron, metolachlor, chlorpyrifos, daidzein, genistein, climbazole, mebendazole and telmisartan were identified as main risk drivers of the oxidative stress response and xenobiotic metabolism pathway activation. Our study reinforced the need for improved wastewater management and deeper evaluations of the occurrence and fate of micropollutants in urbanized tropical estuarine environments.

The role of teleconnections and solar activity on the discharge of tropical river systems within the Niger basin.

The behavior of tropical river systems is driven by some internal and external factors. Understanding the role of these external forces, such as large-scale oscillations, on river discharge will provide insight into their dynamic complexities and modelling. In this study, the role of teleconnection patterns and solar activity on river discharges within the Niger basin was considered using both linear (correlation) and nonlinear (multifractal and joint recurrence analysis) statistical approaches. Correlation analysis suggests the existence of a linear relationship between tropical teleconnection patterns in the Atlantic and Pacific oceans with river discharge in the Niger basin. Nonlinear relationships were investigated using multifractal formalism and joint recurrence quantification analysis. A strong nonlinear relationship was observed between the teleconnection patterns and river discharge in Diola while other stations (Koulikoro, Ansongo, Niamey, Mopti, Kirango) showed no such relationship. The observation at Diola is attributed to its location (coastal) among other things. The multifractal strengths were found in the range of 0.58-2.86, suggesting fractal correlations between the parameters. There was no conclusive evidence of a linear and nonlinear relationship between solar activity and tropical river discharge within the Niger basin.

Biophysical and biochemical features' feedback associated with a flood episode in a tropical river basin model.

Global climate change scenarios such as frequent and extreme floods disturb the river basins by destructing the vegetation resulting in rehabilitation procedures being more costly. Thus, understanding the recovery and regeneration of vegetation followed by extreme flood events is critical for a successful rehabilitation process. Spatial and temporal variation of biochemical and biophysical features derived from remote sensing technology in vegetation can be incorporated to understand the recovery and regeneration of vegetation. The present study explores the flood impact on vegetation caused by major river basins in Sri Lanka (a model tropical river basin) by comparing pre-flood and post-flood cases. The study utilized enhanced vegetation index (EVI), normalized difference vegetation index (NDVI), the fraction of photosynthetically active radiation (FPAR), and gross primary productivity (GPP) of the Moderate Resolution Imaging Spectroradiometer (MODIS) platform. A remarkable decline in EVI, LAI, FPAR, GPP, and vegetation condition index was observed in the post-flood case. Notably, coupled GPP-EVI and GPP-LAI portrayed dependency of features and showed a significant impact triggered by the flood episode by narrowing the feature in post-flood events. EVI depicted the highest regeneration (0.333) while GPP presented the lowest regeneration (0.093) after the flood event. Further, it was revealed that 1.18 years have been on the regeneration. The regeneration of GPP and LAI remained low comparatively justifying the magnitude and impact of the flood event. The study revealed successful implications of vegetation indices on flood basin management of small to large tropical river basins.

Sources and geochemical behaviors of rare earth elements in suspended particulate matter in a wet-dry tropical river.

The processes of rock weathering and soil erosion, and hydrochemical characteristics are significantly affected by the climate in a basin. However, the sources of rare earth elements (REEs) in suspended particulate matter (SPM) under soil erosion, as well as the geochemical behaviors of REEs with changes in hydrochemical properties between seasons, have received little attention in the tropical monsoon zone. In this study, the temporal and spatial characteristics of the REEs in SPM were investigated in the Mun River (a wet-dry tropical river), Northeast Thailand. During the dry season, the compositions of the major elements and REEs in SPM were very similar to those in local soils. However, there was a clear difference between the compositions of these major elements and REEs in SPM and those in local soils during the rainy season. This suggests that the SPM and its REEs during the dry season were primarily derived from soil materials, while those during the rainy season were primarily derived from soil materials and products of rock weathering. The ∑REE contents in SPM decreased from 191.2 mg kg-1 to 170.6 mg kg-1 along the flow direction during the dry season, while they increased from 100.7 mg kg-1 to 135.3 mg kg-1 during the rainy season. The δEu (mean 1.26) and δGd (mean 1.58) values in SPM during the rainy season were higher than those (mean δEu 1.21 and mean δGd 1.12) during the dry season, and both of them were mainly controlled by the relative contributions of rock weathering products and soil materials to SPM. The results suggest that the temporal differences of REE geochemical characteristics in SPM were closely associated with SPM sources, while their spatial variations were mainly affected by the water-particle interaction in the tropical monsoon zone.

Bacterial Community Composition and Function of Tropical River Ecosystem along the Nandu River on Hainan Island, China.

Microorganisms play a pivotal role in nutrient cycling in aquatic ecosystems. Understanding bacterial diversity and its functional composition are crucial for aquatic ecology and protection. We investigated the bacterial community structure using 16S rRNA gene amplicons high-throughput sequencing in this study. Results showed that 105 amplicon sequence variants (ASVs) account for 43.8% of the total sequences shared by the Nandu River's lower, middle, and upper reach and the Songtao Reservoir. The dominant bacterial phylum in the Nandu River and its connected Songtao Reservoir were Proteobacteria and Actinobacteriota, respectively. The highest Chao1 and Shannon index values were found in the lower reach of the Nandu River. Beta diversity analysis showed the divergence in bacterial communities in the Nandu River and Songtao Reservoir, but not in different reaches. Among the water properties, based on the Mantel test, dissolved oxygen, total nitrogen, and nitrite significantly affected the bacterial communities. The functional profiles predicted by Tax4Fun2 showed that metabolism was the most abundant function group. The relative abundance of genetic information processing was significantly higher in the Songtao Reservoir than in the Nandu River, while the relative abundance of human diseases was significantly lower in the Songtao Reservoir than in the Nandu River. The appearance of the xenobiotics biodegradation and metabolism function group requires us to pay more attention to possible water pollution, especially at the upper reach of the Nandu River.

Traffic-related polycyclic aromatic hydrocarbons (PAHs) occurrence in a tropical environment.

Traffic-related PAH emissions over the urban area of Natal, Brazil, have shown a significant increase because of automobile usage and have become a major concern due to their potential effects on human health and the environment. Therefore, this research measured PAH contamination on major roads and river compartments in a tropical catchment (Pitimbu River) over an expanding urban area. Road PAH concentrations spanned from 692 to 2098 ng g-1 and suggest the predominance of heavy (diesel-powered) and light-duty (gasoline plus alcohol-powered) vehicle emission sources. High concentrations of naphthalene (515 ng g-1) and acenaphthylene (145 ng g-1) were found in river sediments, indicating oil-related spillage and low-temperature combustion sources. Diagnostic ratios indicated the prevalence of biomass, coal and petroleum combustion processes and refined oil products. The ecological risk assessment indicated an ecological contamination risk ranging between low and moderate because of naphthalene and acenaphthylene concentrations higher than ERL threshold values. Toxicity risks caused by PAHs were assessed by using the BaP-equivalent carcinogenic power (BaPE). Results indicated that both RDS and riverbed sediment samples are at low toxicity risk.

Water quality assessment of the Ganges River during COVID-19 lockdown.

Ganges River water quality was assessed to record the changes due to the nation-wide pandemic lockdown. Satellite-based (Sentinel-2) water quality analysis before and during lockdown was performed for seven selected locations spread across the entire stretch of the Ganges (Rishikesh-Dimond Harbour). Results revealed that due to the lockdown, the water quality of the Ganges improved with reference to specific water quality parameters, but the improvements were region specific. Along the entire stretch of Ganges, only the Haridwar site showed improvement to an extent of being potable as per the threshold set by the Central Pollution Control Board, New Delhi, India. A 55% decline in turbidity at that site during the lockdown was attributed to the abrupt halt in pilgrimage activities. Absorption by chromophoric dissolved organic matter which is an indicator of organic pollution declined all along the Ganges stretch with a maximum decline at the downstream location of Diamond Harbour. Restricted discharge of industrial effluent, urban pollution, sewage from hotels, lodges, and spiritual dwellings along the Ganges are some of the reasons behind such declines. No significant change in the geographic trend of chlorophyll-a was observed. The findings of this study highlight the importance of regular monitoring of the changes in the Ganges water quality using Sentinel-2 data to further isolate the anthropogenic impact, as India continues the phase-wise opening amidst the pandemic.

Geochemical evaluation of potentially toxic elements determined in surface sediment collected in an area under the influence of gold mining.

In this present paper, the distribution of toxic metals and sediment quality were evaluated in five sampling points of the Itapicuru-Mirim River located in the city of Jacobina, Bahia, Brazil. The concentration of the elements arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), manganese (Mn), nickel (Ni), lead (Pb), and zinc (Zn) were determined from sediment samples collected from the superficial layer (0-10 cm) in November 2013. After the samples' total decomposition, the total concentration of metals was determined by inductively coupled plasma optical emission spectrometry (ICP OES), except for Hg, which was measured by inductively coupled plasma atomic emission spectrometry (ICP AES). A geochemical evaluation of sediment quality was performed using enrichment factor (EF), geoaccumulation index (Igeo), and pollution load index (PLIThe results of the total concentrations in the analyzed sediment samples were compared with the threshold effect levels (TELs) and probable (PELs) effect levels (PELs) in sediment quality guidelines (SQGs). For the fraction <75 µm, the decreasing order for the metal concentrations was (ppm): Fe (10.86%) > Mn (120.8) > Cr (122) > Zn (76.5) > Pb (49.6) > Cu (32.6) > Ni (28.6) > Hg (0.31) > Cd (0.13). Igeo suggested a moderate to seriously polluted category for Hg and moderately polluted for Cu. Generally, the results indicated probable risks to the biota caused by Cr, Ni, Pb, and Hg metals. However, only Hg, Cd, and Cu were of anthropogenic origin. Although the sediments are relatively preserved from pollution by these metals, there is a progressive deterioration of this compartment downstream of the Itapicuru-Mirim River in the city of Jacobina.

Combining geochemical and chemometric tools to assess the environmental impact of potentially toxic elements in surface sediment samples from an urban river.

This article investigates sediments collected from the banks of the Subaé River located in Todos os Santos Bay in the state of Bahia, Brazil, in 2018, twenty-five years after the closing of a former lead alloy processing plant. Ten sediment samples were collected at different points of the course of the river and its estuarine region. Chemometric tools were used to determine geochemical correlations between the organic matter content and concentration of sulfides and potentially toxic metals. The inorganic geochemical variables (enrichment factor [EF]) used in this evaluation were concentrations of the Pb, Cd, Cu, Zn, and Ni. Chemical element analyses were performed using ICP-OES. To assess the interaction between metals and sulfide or metals and organic matter, concentrations of Pb, Cd, Cu, Zn, Ni, sulfide, and the silt-clay fraction constituted the organic geochemical parameters selected to characterize the amount of organic matter present in Subaé River sediment samples, determining the carbon content (%TOC) to compose the matrix of the principal component analysis (PCA) and hierarchical cluster analysis. PCA showed that 88.3% of the samples were representative for assessing correlations between geochemical variables. A tendency toward binding was found among Cu, Cd, Ni, and sulfide, as well as the silt-clay fraction. The concentrations (mg kg-1) of lead, zinc, and copper were higher in both collection campaigns, ranging from 4.72 to 31.34, 12.76 to 54.24, and 5.34 to 31.37, respectively. Pb and Zn were presented in elemental form when assessed as a function of the pH and Eh of the environment. Except for Cd (EF: 0.51 to 5.49), the other elements exhibited little or no potential pollution in the aquatic environment of the Subaé River.

Spatio-temporal variability of trace metals and major ions in a small tropical river, southwest coast of India.

Sita, a small tropical river originating in the Western Ghats, southwestern India, was chosen for this study to understand the spatial and temporal variability of dissolved trace elements and major ions. Sampling was done in monsoon, post-monsoon and pre-monsoon seasons across the river catchment. Dissolved elements like Zn, Ni, Cu and Co showed high concentrations during monsoon which could be because of the intense chemical weathering of silicate rocks in the catchment, surface runoff and wet atmospheric deposition. In the remaining seasons, the river is fed by the groundwater and anthropogenic effluents which is contributing to higher concentrations of dissolved Pb, Fe and Cd. The results are analysed using factor analysis (SPSS 12), which helped to delineate the sources of metal assemblages. Factor 1 is dominated by major ions (Na, K, Cl, Mg, Ca), which could be naturally sourced from atmospheric deposition and weathering of rocks. Factor 2 is dominated by trace metals (Cd, Pb, Co) and sulphates (SO4) which may have anthropogenic sources such as the discharge of domestic and industrial effluents and agricultural effluents.

Consumer trophic positions respond variably to seasonally fluctuating environments.

The effects of environmental seasonality on food web structure have been notoriously understudied in empirical ecology. Here, we focus on seasonal changes in one key attribute of a food web, consumer trophic position. We ask whether fishes inhabiting tropical river-floodplain ecosystems behave as seasonal omnivores, by shifting their trophic positions in relation to the annual flood pulse, or whether they feed at the same trophic position all year, as much empirical work implicitly assumes. Using dietary data from the Tonle Sap Lake, Cambodia, and a literature review, we find evidence that some fishes, especially small piscivores, increased consumption of invertebrates and/or plant material during the wet season, as predicted. However, nitrogen stable isotope (δ15 N) data for 26 Tonle Sap fishes, spanning a broader range of functional groups, uncovered high variation in seasonal trophic position responses among species (0 to ±0.52 trophic positions). Based on these findings, species respond to the flood pulse differently. Diverse behavioral responses to seasonality, underpinned by spatiotemporal variation at multiple scales, could be central for rerouting matter and energy flow in these dynamic ecosystems. Seasonally flexible foraging behaviors warrant further study given their potential influence on food web dynamics in a range of fluctuating environments.

Tropical surface water quality studies: Implications for the aquatic fate of N-methyl carbamate pesticides.

Water quality assessment was conducted on the Ruiru River, a tributary of an important tropical river system in Kenya, to determine baseline river conditions for studies on the aquatic fate of N-methyl carbamate (NMC) pesticides. Measurements were taken at the end of the long rainy season in early June 2013. Concentrations of copper (0.21-1.51 ppm), nitrates (2.28-4.89 ppm) and phosphates (0.01-0.50 ppm) were detected at higher values than in uncontaminated waters, and attributed to surface runoff from agricultural activity in the surrounding area. Concentrations of dissolved oxygen (8-10 ppm), ammonia (0.02-0.22 ppm) and phenols (0.19-0.83 ppm) were found to lie within normal ranges. The Ruiru River was found to be slightly basic (pH 7.08-7.70) with a temperature of 17.8-21.2°C. The half-life values for hydrolysis of three NMC pesticides (carbofuran, carbaryl and propoxur) used in the area were measured under laboratory conditions, revealing that rates of decay were influenced by the electronic nature of the NMCs. The hydrolysis half-lives at pH 9 and 18°C decreased in the order carbofuran (57.8 h) > propoxur (38.5 h) > carbaryl (19.3 h). In general, a decrease in the electron density of the NMC aromatic ring increases the acidity of the N-bound proton removed in the rate-limiting step of the hydrolysis mechanism. Our results are consistent with this prediction, and the most electron-poor NMC (carbaryl) hydrolyzed fastest, while the most electron-rich NMC (carbofuran) hydrolyzed slowest. Results from this study should provide baseline data for future studies on NMC pesticide chemical fate in the Ruiru River and similar tropical water systems.

Ephemeroptera, Plecoptera and Trichoptera (Insecta) Abundance, Diversity and Role in Leaf Litter Breakdown in Tropical Headwater River.

Leaf litter decomposition in a tropical stream was examined in two types of leaf packs; single species leaf packs of Pometia pinnata and two species leaf packs of equal combination of Pometia pinnata and Dolichandrone spathacea leaves. Both leaf packs were immersed in a river and weekly examined for remains of decomposed leaves and presence of EPT. In the control leaf packs, leaves in the two species leaf packs treatments decomposed within 35 days, faster than in single species leaf packs which decomposed after 42 days. In the presence of EPT, the leaf breakdown took 28 days in two species and 35 days for single species leaf packs. Higher abundance of EPT was observed in single species leaf packs but its diversity was higher in two species leaf packs. Litter breakdown in the stream was faster in the presence of EPT and softer leaves of D. spathacea with higher nitrogen content underwent faster decomposition and sustained higher numbers of EPT.

The influence of heavy metals on toxicogenetic damage in a Brazilian tropical river.

Anthropogenic activities in tropical rivers favor the eutrophication process, which causes increased concentration of heavy metals. The presence and bioaccumulation of metals are directly related to the presence of genotoxic damage in aquatic organisms. Thus, we evaluated the presence of heavy metals (Fe, Zn, Cr, Cu and Al) and performed toxicogenetic tests in surface (S) and bottom (B) of water samples of the Poti river (Piaui/Brazil). Cytotoxicity and genotoxicity tests were performed in Allium cepa, and micronucleus (MN) and comet assay were performed in Oreochromis niloticus. The chemical analysis showed concentrations above the limit for Cu, Cr, Fe and Al according to Brazilian laws, characterizing anthropogenic disturbance in this aquatic environment. Toxicogenetic analysis presented significant cytotoxic, mutagenic and genotoxic effects in different exposure times and water layers (S and B), especially alterations in mitotic spindle defects, MN formations, nuclear bud and DNA strand breaks. Correlations between Fe and cytotoxicity, and Al and mutagenicity were statistically significant and point out to the participation of heavy metals in genotoxic damage. Therefore, Poti river water samples presented toxicogenetic effects on all bioindicators analyzed, which are most likely related to heavy metals pollution.

Passive sampling for spatial and temporal monitoring of organic pollutants in surface water of a rural-urban river in Kenya.

Passive sampling is an emerging monitoring strategy for surface waters and can be applied in a range of environments including remote locations. Silicone rubber (SR) as a robust single-phase passive sampler was applied to characterize the spatial and temporal variability of organochlorine pesticides (OCPs), polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs) and three phthalates, namely, dibutyl phthalate (DBP), benzyl butyl phthalate (BBP) and bis(2-ethylhexyl)phthalate (DEHP) in a tropical river traversing rural and urban catchments. OCPs and PCBs were not detected. Up to 31.8ng/L of freely dissolved concentrations of PAHs were quantified and were dominated by the lower molecular weight members. Mean concentrations of DBP, BBP and DEHP were 72.6ng/L, 3.9ng/L and 7.1ng/L respectively. However, in sampling for phthalates using SR, quality control and assurance remains the key challenge and must always be ensured. Spatial variability in concentrations was evident and could be related to land use. Temporal variability was not significant.

Spatial variation and risk assessment of trace metals in water and sediment of the Mekong Delta.

The Mekong Delta, is home to 17 million inhabitants and faces numerous challenges relating to climate change, environmental degradation and water issues. In this study, we assess trace metals concentrations (Al, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Mo, Cd, Hg, Pb) in the water, suspended particulate matter and surface sediments of the Tien River, the Northern branch of the Mekong Delta, during both dry and rainy seasons. Metal concentrations in the dissolved and suspended particle phases remain in the low concentration range of the main Asian Tropical River. During transportation in the riverine part, we evidenced that V, Cr, Co, As and Pb are dominant in the particulate phase while Mo, Ni and Cu dominate in the dissolved fraction. In the salinity gradient, dissolved U, V, Mo exhibit conservative behaviour while Ni, Cu, As, Co and Cd showed additive behaviour suggesting desorption processes. In the surface sediment, metal concentrations are controlled by the particle-size, POC contents and Fe, Al and Mn - oxy(hydr)oxides. Calculated Enrichment Factor and Geoaccumulation Index evidenced As enrichment while the calculated mean effect range median quotients evidenced a low to medium ecotoxicological potential effects range in the surface sediments.

Baseline seasonal investigation of nutrients and trace metals in surface waters and sediments along the Saigon River basin impacted by the megacity of Ho Chi Minh (Vietnam).

The Saigon River, Southern Vietnam, crosses one of the most dynamic developing Megacity in Southeast Asia: Ho Chi Minh City (HCMC). The increased economic, industrial, and domestic developments may affect the environmental quality of water and halieutic resources. In this study, we evaluated the seasonal (dry and wet seasons) biogeochemical state of the Saigon River during two snapshot campaigns conducted along the river basin upstream from HCMC; the Saigon River was characterized by slightly acidic (pH 5.7-7.7) and oxygen-depleted water (dissolved oxygen (DO), 0.36-5.18 mg l-1). Nutrients (N-NH4 = 0.01-2.41, N-NO3 = 0.14-2.72, and P-PO4 = ~0-0.42 mg l-1), DOC (2.2-8.0 mg l-1), POC, and trace metal(oid) (As, Cd, Cr, Cu, Zn, and Hg) concentrations were low showing a good quality of the upstream river. In the urban center area, DO dropped to 0.03 mg l-1 accompanied with a rise of nutrient concentrations (e.g., N-NH4, up to 17.7 mg l-1) likely originating from wastewater discharges. Trace metal concentrations also rose sharply (e.g., Cr and Hg rose up to 10-fold higher) in both water and sediments but remained under the World Health Organization (WHO) and Vietnamese concentration guidelines. In the downstream estuarine area, the intrusion of marine waters diluted water flowing from HCMC, leading water quality to return close to the state observed upstream from HCMC. In general, levels of nutrient and metal contaminations along the Saigon River during both seasons appear moderate regarding to Vietnamese and WHO guidelines although the urban area is highlighted as the major contributor for metal(oid) emissions. Finally, we showed that apart from wastewater and industrial discharges that affect the river quality, metal(oid) partitioning between solid and solution is controlled by the change in water geochemistry along the continuum during both seasons, such as DO (e.g., for As and Cr) and pH (e.g., for Pb) which drives their sorption/dissolution dynamics.

Spatial and temporal variation of phytoplankton in a tropical eutrophic river / Variação espacial e temporal do fitoplâncton em um rio eutrófico tropical

Abstract This study aims to evaluate the environmental factors determining of the changes in phytoplankton structure in spatial (upper, middle and lower course) and seasonal (dry and rainy period) scales in a eutrophic river (Almada River, northeastern Brazil). In the study period, total accumulated rainfall was below of the historic average, resulting in flow reduction, mainly in rainy period. High orthophosphate concentration was found at the sampling sites. Phytoplankton chlorophyll a increased from upstream to downstream. Geitlerinema splendidum (S1) and Chlamydomonas sp. (X2) were the most abundant species in the upper course and several species of diatoms (D), Euglenophyceae (W1, W2) and Chlorophyceae (X1) in the middle and lower course. The functional groups were found to be characteristic of lotic ecosystem, shallow, with low light availability, rich in organic matter and eutrophic environments. We conclude that phytoplankton community structure was sensitive to change of the river flow and nutrient availability in spatial and seasonal scale in a tropical river.

Resumo O objetivo deste estudo foi avaliar os fatores ambientais determinantes das mudanças na estrutura do fitoplâncton em escala espacial (alto, médio e baixo curso) e temporal (seca e chuva) em um rio eutrófico tropical (rio Almada, nordeste do Brasil). No período de estudo, a precipitação acumulada mensal foi abaixo da média histórica, resultando na redução da vazão, principalmente no período chuvoso. A concentração de ortofosfato foi elevada nos pontos de amostragem. A clorofila a do fitoplâncton aumentou de montante para jusante. Geitlerinema splendidum (S1) e Chlamydomomas sp. (X2) foram as espécies mais abundantes no alto curso e várias espécies de diatomáceas (D), Euglenophyceae (W1, W2) e Chlorophyceae (X1) no médio e/ou baixo curso. Os grupos funcionais foram característicos de ambientes lóticos, rasos, com baixa disponibilidade de luz, rico em matéria orgânica e eutrófico. Concluímos que a estrutura da comunidade fitoplanctônica foi sensível às mudanças na vazão e na disponibilidade de nutrientes em escala espacial e temporal em um rio tropical.