Modeling the effect of climate change scenarios on water quality for tropical reservoirs

Impact of natural phenomena and anthropogenic activities on water quality is closely related with temperature increase and global warming. In this study, the effects of climate change scenarios on water quality forecasts were assessed through correlations, prediction algorithms, and water quality index (WQI) for tropical reservoirs. The expected trends for different water quality parameters were estimated for the 2030-2100 period in association with temperature trends to estimate water quality using historical data from a dam in Mexico. The WQI scenarios were obtained using algorithms supported by global models of representative concentration pathways (RCPs) adopted by the Intergovernmental Panel on Climate Change (IPCC). The RPCs were used to estimate water and air temperature values and extrapolate future WQI values for the water reservoir. The proposed algorithms were validated using historical information collected from 2012 to 2019 and four temperature variation intervals from 3.2 to 5.4 °C (worst forecast) to 0.9-2.3 °C (best forecast) were used for each trajectory using 0.1 °C increases to obtain the trend for each WQI parameter. Variations in the concentration (±30, ±70, and +100) of parameters related to anthropogenic activity (e.g., total suspended solids, fecal coliforms, and chemical oxygen demand) were simulated to obtain water quality scenarios for future health diagnosis of the reservoir. The results projected in the RCP models showed increasing WQI variation for lower temperature values (best forecast WQI = 74; worst forecast WQI = 71). This study offers a novel approach that integrates multiparametric statistical and WQI to help decision making on sustainable water resources management for tropical reservoirs impacted by climate change.

Monitoring of cyanobacteria and cyanotoxins in a Colombian tropical reservoir

Anthropogenic pollution and global climate change have resulted in favorable environmental conditions for increased frequency and duration of cyanobacterial blooms in aquatic systems. Cyanobacteria can produce toxic metabolites called cyanotoxins, which have become a worldwide concern as they threaten human and animal health. The presence of cyanobacteria and four cyanotoxins were evaluated in a Colombian reservoir. The reservoir was monitored for a year, with sampling campaigns every 3 months in seven stations. To identify and quantify cyanotoxins, the ultra-high-performance liquid chromatography coupled with triple quadrupole mass spectrometry (UHPLC-QqQ-MS/MS) technique was used, and the quantification of cyanobacteria was done by quantitative real-time PCR (qPCR) assay using a cyanobacterial-specific 16S rRNA gene fragment as a target. Cyanobacteria concentration was between 4.02 (± 0.11) × 10⁴ and 2.72 (± 0.28) × 10⁷ copies of Cyan 16S/μL, the minimum value corresponds to the station located in the central zone and the maximum to the station at the entrance of one of the tributary rivers. The presence of MC-RR, MC-LR, MC-YR, and NOD was detected in at least six of the seven sampling stations at different times of the year. In all cases, the concentration of the toxins detected was below 0.05 μg/L, so the guideline value established by the WHO for MC-LR was not exceeded.

Iron enrichment from hypoxic hypolimnion supports the blooming of Raphidiopsis raciborskii in a tropical reservoir

Occurring worldwide, blooms of Raphidiopsis raciborskii threaten the use of water resources especially in tropical and subtropical waterbodies. Its high flexibility in the uses of light and macronutrients (C, N, P) frustrates any bloom prediction and control based on macronutrients regulation. To identify the critical factors promoting periodic blooms of R. raciborskii, the trends of meteorological, hydrodynamic, physical, and chemical variables (including macro- and micronutrients: N, P, Fe) were analyzed in a Chinese tropical large reservoir (Dashahe reservoir) over five years. It was hypothesized that Fe availability, mediated by the mixing pattern of the reservoir, played a crucial role in the periodic blooms of the cyanobacterium. To have a more complete understanding, the effects of Fe on growth of a local R. raciborskii strain were tested in a monoculture experiment. The biomass and relative abundance of R. raciborskii in the reservoir showed a clear seasonal trend, with relative abundance > 50% in summer/autumn (July to October). Three habitat types along a dominance gradient were identified in the reservoir and 17 variables were used to compare them. Statistical analysis and habitat comparison showed that temperature and stratification, dissolved Fe and N concentrations in the epilimnion, and dissolved Fe and oxygen concentrations in the hypolimnion were the critical factors driving the dynamics of R. raciborskii in the study reservoir. The habitat dominated by R. raciborskii was characterized by a relatively low availability of macro resources (Z_eu/Z_m < 1, SRP < 0.01 mg/L, DIN < 0.3 mg/L) and by a high Fe availability supplemented from hypoxic hypolimnion. The dependence of growth on Fe concentration increase was confirmed in culture where the maximum was reached at 0.689 mg Fe /L. Our results suggest that a high Fe bioavailability, also originating from the hypoxic hypolimnion, influences the dynamics R. raciborskii and favors the blooms of the species. As a consequence, Fe concentrations in the water column as well as oxygen measurements along the water column should be routinely included in the monitoring programs aimed at predicting and controlling R. raciborskii blooms.

Seasonal geochemical trends and pollution assessment of bottom sediments in the São Francisco hydrographic basin, Brazil: the Três Marias Reservoir

The Três Marias Reservoir is the ninth largest reservoir in Brazil, becoming crucial for national strategic development. However, many anthropic activities may affect the sediment quality, promoting the need for a proper environmental assessment. This research appraised the seasonal influences on the Três Marias Reservoir's sediment geochemistry, elucidating possible anthropogenic impacts. The concentrations of Mg, Al, Ca, Cr, Fe, Co, Cu, Cd, Ti, Mn, Ni, Zn, Ba, and Pb were measured in 78 samples of bottom sediments regarding the two seasons of the area, a dry winter and rainy summer. The median ± 2 median absolute deviation (MAD) settled the geochemical background and environmental thresholds for the two seasons. The sediment quality guidelines CONAMA 344/12 highlight the possible adverse ecological effects of pollutants. The hierarchical clustering analysis, the geoaccumulation index, and the pollution load index delineated the polluted zones. The pollution load index ranges from 0.25 to 2.28 in the dry season and 0.56 to 2.11 in the rainy season, defining three affected zones in the reservoir. Forestry and agriculture are the probable pollution sources, reaching warning levels that should be considered in further environmental strategies.

Iron and manganese mobilisation due to dam height increase for a tropical reservoir in South East Asia

The aim of this research was the analysis of the effect of a dam height raise on the water quality of a tropical reservoir used for drinking water purposes in South East Asia. Analyses of iron, manganese, pH and ammonia were performed over a 5-year period from daily water sampling at the reservoir. In addition, high-frequency monitoring data of nitrate, ammonium, pH and blue-green algae were obtained using a monitoring probe. The results showed that due to the raising of the reservoir water level, previously oxic sediments became submerged, triggering an increase in iron and manganese in particular due to the establishment of reducing conditions. Manganese concentrations with values up to 4 mg L^-1 are now exceeding guideline values. The analysis strongly indicated that both iron and manganese have a seasonal component with higher iron and manganese concentrations during the wet season. Over a three-year period afterwards, concentrations did not go back to pre-raise levels. The change in water quality was accompanied by a change in pH from previous values of around 5 to pH values of around 6.5. Geochemical simulations confirmed the theory that the increasing concentrations of iron and manganese are due to the dissolution of MnO₂ and ferric oxyhydroxides oxidising organic matter in the process. This study showed that changes in reservoir water levels with the establishment of reducing conditions can have long-term effects on the water quality of a reservoir.

Epipelon, phytoplankton and zooplankton responses to the experimental oligotrophication in a eutrophic shallow reservoir

Epipelon can contribute to the maintenance of shallow lake oligotrophication. Herein, we simulated oligotrophication by diluting eutrophic water and evaluated epipelon biomass and structure and potential relationships with phytoplankton and zooplankton communities. Dilutions of 25-75% negatively impacted phytoplankton biomass and zooplankton diversity and increased Rotifera density. Additionally, the 25% dilution increased Copepoda density, but had no effect on Cladocera. On both experimental days, epipelon chlorophyll-a and algal density responded to oligotrophication, but the algal biomass response was less pronounced after 14 days. Ceratium furcoides was dominant in the phytoplankton, while diatom species were dominant in the epipelon. We observed that experimental oligotrophication can influence both the biomass and taxonomic structure of the algal and zooplankton communities. Overall, we concluded that experimental oligotrophication negatively impacted the phytoplankton biomass and favored the development of the phototrophic epipelon; however, a large reduction in eutrophication (>50%) is required for a significant algal response in the benthic environment of a shallow tropical reservoir.

New lake in a changing world: the construction and filling of a small hydropower reservoir in the tropics (Rio de Janeiro, Brazil)

Climate change has affected rainfall patterns in tropical regions, where simultaneous demands for water and energy, habitat loss, declining biodiversity, and spread of invasive species have reflected a rapidly changing world underway. In Brazil, hydropower generation accounts for 64% of the electricity matrix, which presently includes 1007 small hydropower plants (SHPs) having many others under construction or planned. This paper aimed to evaluate changes in water quality, plankton communities, and benthic macroinvertebrates during dam construction, filling, and the first year of operation of a SHP. Suspended solids, turbidity, and silica were variables that highlighted the impact of this construction on the river. Fast changes in water quality (increases in calcium, chlorides, and nitrate) and on aquatic communities (i.e. euglenophyceans and testate amoebae increased in numbers) were detected during the filling phase. Following SHP construction, the concentrations of metals and total phosphorus tended to decrease. Two striking findings observed in the aquatic communities from the riverine conditions to the new lake were the increase in picocyanobacteria abundance, expanding population stocks throughout the river basin, and the constant presence of the invasive mollusc Corbicula fluminea in the macroinvertebrate assemblage, revealing once again its resistance to environmental variability. The lake soon became a natural trap for ions from the drainage basin, as revealed by the increase in electrical conductivity, ammonium, potassium, and magnesium concentrations and the abundance of cyanobacteria, highlighting the need for watershed management to improve ecological conditions in the lake.

Detected cyanotoxins by UHPLC MS/MS technique in tropical reservoirs of northeastern Colombia

This study focused on the detection and quantification of eight cyanotoxins in water samples in three reservoirs located in the eastern department of Antioquia, Colombia. The reservoirs are a source of water supply and hydroelectricity, and also generate economic activities in fishing and recreation. Between May 2015 and October 2016, 8 samplings were carried out at times of high temperatures, which ranged from 20 to 29 °C. This period was selected because of a significant or strong El Niño phenomenon, according to the World Meteorological Organization. For the study, 270 integrated samples were taken from the photic zone (PZ) and the surface of the reservoirs, at each of the three sampling points. The samples were analyzed by the analytical technique of ultra-high-performance liquid chromatography coupled to triple quadrupole mass spectrometry (UHPLC MS/MS). The quantification performed for six microcystins (MCs), a nodularin (NOD) and a cylindrospermopsin (CYN), showed positive results well above 1 μg L^-1. In the water of the Abreo Malpaso and Peñol reservoirs, microcystin-LR (MC-LR), microcystin-YR (MC-YR) and [D-Asp³,(E)-Dhb⁷]- microcystin-RR toxins were detected at levels of considerable concentration, especially between May and September 2015, when there was no rainfall in this region. In the Playas reservoir, positive results for [D-Asp³,(E)-Dhb⁷]-MC-RR were detected from May to November 2015, with the highest concentrations being reached in dry season. The temperatures reached and the changes in climatic conditions witnessed during the monitoring period of this study were important factors in the production of cyanotoxins. This was evidenced in this work by the high concentrations of detected cyanotoxins and their absence in periods of rain, as happened from the second quarter of 2016 until the end of the study in October. This is the first study of the detection and quantification of cyanotoxins in tropical reservoirs of northeastern Colombia using the UHPLC MS/MS analytical technique, which allowed the toxins to be unequivocally detected and confirmed. A method was developed and validated, proving to be sensitive, reproducible and accurate. For each of the toxins (microcystin-LR (MC-LR), microcystin-RR (MC-RR), microcystin-YR (MC-YR), [D-Asp³,(E)-Dhb⁷]- microcystin-RR, microcystin-LW (MC-LW), microcystin-LF (MC-LF), nodularin (NOD) and cylindrospermopsina (CYN)) the correlation coefficients (R²) were in a range between 0.9907 and 0.9999. Verification of the accuracy of the method was performed through a calibration curve in solvent. The recovery percentages of the accuracy and precision tests of the method for low level, medium level and high level were in a range between 64% and 115% for all the cyanotoxins. The validation of the cyanotoxin method shows that it is possible to detect them individually in natural water with a quantification limit (LOQ) of approximately 0.05 μg L^-1.

Metabolism in a deep hypertrophic aquatic ecosystem with high water-level fluctuations: a decade of records confirms sustained net heterotrophy

Long-term and seasonal changes in production and respiration were surveyed in the Valle de Bravo reservoir, Mexico, in a period during which high water-level fluctuations occurred (2006-2015). We assessed the community metabolism through oxygen dynamics in this monomictic water-body affected by strong diurnal winds. The multiple-year data series allowed relationships with some environmental drivers to be identified, revealing that water level-fluctuations strongly influenced gross primary production and respiratory rates. Production and respiration changed mainly vertically, clearly in relation to light availability. Gross primary production ranged from 0.15 to 1.26 gO₂ m^-2 h^-1, respiration rate from -0.13 to -0.83 gO₂  m^-2 h^-1 and net primary production from -0.36 to 0.66 gO₂  m^-2 h ^-1 within the production layer, which had a mean depth of 5.9 m during the stratification periods and of 6.8 m during the circulations. The greater depth of the mixing layer allowed the consumption of oxygen below the production layer even during the stratifications, when it averaged 10.1 m. Respiration below the production layer ranged from -0.23 to -1.38 gO₂ m^-2 h^-1. Vertically integrated metabolic rates (per unit area) showed their greatest variations at the intra-annual scale (stratification-circulation). Gross primary production and Secchi depth decreased as the mean water level decreased between stratification periods. VB is a highly productive ecosystem; its gross primary production averaged 3.60 gC m^-2 d^-1 during the 10 years sampled, a rate similar to that of hypertrophic systems. About 45% of this production, an annual average net carbon production of 599 g C m^-2 year^-1, was exported to the hypolimnion, but on the average 58% of this net production was recycled through respiration below the production layer. Overall, only 19% of the carbon fixed in VB is buried in the sediments. Total ecosystem respiration rates averaged -6.89 gC  m^-2 d^-1 during 2006-2015, doubling the gross production rates. The reservoir as a whole exhibited a net heterotrophic balance continuously during the decade sampled, which means it has likely been a net carbon source, potentially releasing an average of 3.29 gC m^-2 d^-1 to the atmosphere. These results are in accordance with recent findings that tropical eutrophic aquatic ecosystems can be stronger carbon sources than would be extrapolated from temperate systems, and can help guide future reassessments on the contribution of tropical lakes and reservoirs to carbon cycles at the global scale. Respiration was positively correlated with temperature both for the stratification periods and among the circulations, suggesting that the contribution of C to the atmosphere may increase as the reservoirs and lakes warm up owing to climate change and as their water level is reduced through intensification of their use as water sources.

Effects of increased zooplankton biomass on phytoplankton and cyanotoxins: A tropical mesocosm study

Zooplankton are important biocontrol agents for algal blooms in temperate lakes, while their potential in tropical and subtropical environments is not well understood. The aim of the present study was to evaluate the influence of increased zooplankton biomass on phytoplankton community and cyanotoxins (microcystins and saxitoxin) content of a tropical reservoir (Ipojuca reservoir, Brazil) using in situ mesocosms. Mesocosms consisted of 50L transparent polyethylene bags suspended in the reservoir for twelve days. Phytoplankton populations were exposed to treatments having 1 (control), 2, 3 and 4 times the biomass of zooplankton found in the reservoir at the beginning of the experiment. Filamentous cyanobacteria such as Planktothrix agardhii and Cylindrospermopsis raciborskii were not negatively influenced by increasing zooplankton biomass. In contrast, the treatments with 3 and 4 times zooplankton biomass negatively affected the cyanobacteria Aphanocapsa sp., Chroococcus sp., Dolichospermum sp., Merismopedia tenuissima, Microcystis aeruginosa and Pseudanabaena sp.; the diatom Cyclotella meneghiniana; and the cryptophyte Cryptomonas sp. Total microcystin concentration both increased and decreased at different times depending on zooplankton treatment, while saxitoxin level was not significantly different between the treatments and control. The results of the present study suggest that zooplankton biomass can be manipulated to control the excessive proliferation of non-filamentous bloom forming cyanobacteria (e.g. M. aeruginosa) and their associated cyanotoxins.

Effective utilization of distiller's grain soluble-an agro-industrial waste in the feed of cage-reared minor carp Labeo bata in a tropical reservoir, India

A 60-day feeding trial was conducted to evaluate the efficacy of distiller's grain soluble (brewery waste) as a prospective ingredient to substitute expensive and high demand feed component, soybean meal for farming Labeo bata in cages installed in tropical reservoir. Two isonitrogenous and isocaloric diets comprising brewery waste (49.2 % CP) as test diet and soybean meal (44.4 % CP) as reference diet were formulated and extruded to obtain 2-mm floating pellets. The efficacy of the diets was tested in terms of survival (%), live weight gain (%), SGR (%/day), FCR, PER and ANPU and recorded 65 ± 0.2, 96 ± 8.1, 1.9 ± 0.1, 2.5 ± 0.02, 1.4 ± 0.1, 20.3 ± 2.0 and 66 ± 0.6, 112 ± 9.8, 2.2 ± 0.1, 2.2 ± 0.2, 1.6 ± 0.1 and 20 ± 2.1, respectively, for soybean and brewer's waste-based formulated feed. The analyses of results revealed that survival, growth parameters and biochemical composition of whole body tissue did not differ significantly (p > 0.05) despite complete replacement of soybean meal by brewery waste. However, the cost estimate of diet revealed marked reduction of feed cost of Rs. 9.2/kg (33.8 %) in the test diet as compared to the reference diet. The study suggests that brewery waste could effectively replace soybean meal without effecting survival and growth of the fish. The finding thus may pave a productive way for reducing environmental pressure of disposal of an agro-industrial waste.

Distribution of three non-essential trace metals (Cadmium, Mercury and Lead) in the organs of fish from Aiba Reservoir, Iwo, Nigeria

The distribution of non-essential trace elements in some vital organs of 11 fish species from Aiba Reservoir, Iwo, Nigeria was assessed between November 2010 and June 2011. The fish species belong to seven families; family Mormyridae, family Cyprinidae, family Hepsetidae and family Channidae each with one species; family Bagridae and family Clariidae each with two species; and family Cichlidae with three species. All families, except Clariidae and Channidae, are common in the daily catch from the reservoir. Atomic absorption spectrophotometry was used to determine the levels of cadmium, mercury and lead in fish organs. The concentration of toxic trace metals in fish ranged from 0.001 to 0.100 ppm (Cd), 0.000–0.067 ppm (Hg) and 0.001–0.125 ppm (Pb) dry weight. This study shows similarity (p > 0.05) in the distribution of Cd, Hg and Pb among fish species; and a non-uniform distribution of toxic trace metals within fish organs with Kidney > Liver > Gill ≥ Intestine ≥ Muscle. Canonical variate analysis shows clear discrimination of Clarias macromystax and Channa obscura for gill trace metal levels of Cd, Hg and Pb while Labeo senegalensis and Oreochromis niloticus were discriminated for liver trace metal values of Cd and Pb only when compared to other fish species studied. The discrimination of some fish species based on trace metals in the gills and liver suggests different regulatory strategies for trace metal accumulation. Variation due to comparison among different fish species from the same water body suggests that accumulation may be species dependent. Differential accumulation of toxic trace metals in fish organs makes them good bioindicators of freshwater contamination.