Mini-review on river eutrophication and bottom improvement techniques, with special emphasis on the Nakdong River

Multiple remotely sensed datasets and machine learning models to predict chlorophyll-a concentration in the Nakdong River, South Korea

The Nakdong River is a crucial water resource in South Korea, supplying water for various purposes such as potable water, irrigation, and recreation. However, the river is vulnerable to algal blooms due to the inflow of pollutants from multiple points and non-point sources. Monitoring chlorophyll-a (Chl-a) concentrations, a proxy for algal biomass is essential for assessing the trophic status of the river and managing its ecological health. This study aimed to improve the accuracy and reliability of Chl-a estimation in the Nakdong River using machine learning models (MLMs) and simultaneous use of multiple remotely sensed datasets. This study compared the performances of four MLMs: multi-layer perceptron (MLP), support vector machine (SVM), random forest (RF), and eXetreme Gradient Boosting (XGB) using three different input datasets: (1) two remotely sensed datasets (Sentinel-2 and Landsat-8), (2) standalone Sentinel-2, and (3) standalone Landsat-8. The results showed that the MLP model with multiple remotely sensed datasets outperformed other MLMs with 0.43 - 0.86 greater in R2 and 0.36 - 5.88 lower in RMSE. The MLP model demonstrated the highest performance across the range of Chl-a concentrations and predicted peaks above 20 mg/m3 relatively well compared to other models. This was likely due to the capacity of MLP to handle imbalanced datasets. The predictive map of the spatial distribution of Chl-a generated by MLP well captured the areas with high and low Chl-a concentrations. This study pointed out the impacts of imbalanced Chl-a concentration observations (dominated by low Chl-a concentrations) on the performance of MLMs. The data imbalance likely led to MLMs poorly trained for high Chl-a values, producing low prediction accuracy. In conclusion, this study demonstrated the value of multiple remotely sensed datasets in enhancing the accuracy and reliability of Chl-a estimation, mainly when using the MLP model. These findings would provide valuable insights into utilizing MLMs effectively for Chl-a monitoring.

Assessment of the impacts of constructing artificial structures on the water quality and hydrological environment of a meandering river

When an artificial structure is built in a river, the river changes significantly in water quality and hydraulic properties. In this study, the effects of the weirs constructed in the middle section of a river as a four major rivers restoration project in Korea on water quality and hydrological characteristics were analyzed. For multi-dimensional data analysis, a self-organizing map was applied, and statistical techniques including analysis of variation were used. As a result of analysis, the cross-sectional area of the river increased significantly after the construction of the weir compared to before the construction of the weir, and the flow velocity decreased at a statistically significant level. In the case of water quality, nitrogen, phosphorus, and suspended solids tended to improve after weir construction, and chlorophyll-a and bacteria tended to deteriorate. Some water quality parameters such as chlorophyll-a were also affected by seasonal influences. In order to improve the water quality deteriorated by the construction of the weir, it is necessary to consider how to improve the flow velocity of the river through partial opening or operation of the weir. In addition, in order to determine the effect of sedimentation of particulate matter due to the decrease in flow rate, it is necessary to conduct investigations on sediments around weirs in the future. PRACTITIONER POINTS: Compared to before the construction of the weir, there was no significant change in the flow rate of the river after the construction of the weir. In the case of chlorophyll-a and bacteria, the water quality was deteriorated after weir construction. To improve the deteriorated water quality, it is required to consider the fundamental management of each pollutant source and the flexible operation of both weirs. For some improved water quality parameters, further research is needed to determine whether these improvements are directly attributable to the construction of a weir.

Synthesis of high-crystallinity Zeolite A from rare earth tailings: Investigating adsorption performance on typical pollutants in rare earth mines

The ecological restoration of rare earth mines and the management of rare earth tailings have consistently posed global challenges, constraining the development of the rare earth industry. In this study, Zeolite A is efficiently prepared from the tailings of an ion-type rare earth mine in the southern Jiangxi Province of China. The resulting Zeolite A boasts exceptional qualities, including high crystallinity, a substantial specific surface area, and robust thermal stability. The optimum conditions for Zeolite synthesis are experimental determination and the adsorption properties of Zeolite A for typical pollutants (Cd2+, Cu2+, NH4+, PO43- and F-) in rare earth mines. The synthesised Zeolite A material is found to have strong adsorption properties. The adsorption mechanism is mainly cation exchange, and the priority of adsorption of pollutants is Cu2+> Cd2+ > NH4+ > PO43- > F-. Notably, the sodium Zeolite A material synthesized at room temperature can be effectively recycled multiple times. In summary, we propose a method to synthesise low cost and high adsorption zeolites using rare earth tailings. This will facilitate the reduction of rare earth tailings and the rehabilitation of rare earth mines. Our method has great potential as a rehabilitation technology for rare earth mines.

Phytoplankton functional groups as indicators of environmental changes in weir and non-weir sections of the lower Nakdong River, Republic of Korea

The Nakdong River underwent water impoundment after eight weirs were constructed as part of South Korea's Four Major River Restoration Project from 2009 to 2012. In this study, we aimed to confirm whether the assemblage of phytoplankton based on phytoplankton functional groups (PFGs), could indicate environmental changes in the weir section (WS) and non-weir section (NWS) of the lower Nakdong River after the construction of the weir. Thus, we examined the relationships between PFGs and gradients in environmental drivers, such as physicochemical, meteorological, and hydrological variables. Environmental gradients were observed between the WS and NWS in dissolved oxygen (DO), electric conductivity (EC), biochemical oxygen demand (BOD), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), dissolved total nitrogen (DTN), dissolved total phosphorus (DTP), ammonia nitrogen (NH3-N), nitrate nitrogen (NO3-N), and phosphorus (PO4-P), which were relatively higher in the WS. Seventeen PFGs were identified (A, B, C, D, E, F, G, H1, J, LM, LO, MP, P, T, W1, X1, and X2). Additionally, the LM and P groups, preferring an enriched lentic system more than other groups, were found to be the dominant PFGs that led the succession of assemblages. Traditional nutrients (N, P) and organic pollutants (BOD, COD) primarily affected the autochthonous growth of the most dominant PFGs in the WS as HRT (hydraulic retention time) increased. Furthermore, the hydrological variables associated with meteorological conditions have a synergistic effect on the composition of the major PFGs and chemical and physical variables in the WS. In other words, the WS may be a new source of inoculum that primarily determines the occurrence and maintenance of phytoplankton in the immediate downstream region (NWS). In particular, group LM (mainly potentially toxic Microcystis) developing in the upper weir impoundment is transported downstream, resulting in a high inoculation effect on further growth in the NWS during the summer monsoon season.

Strength, porosity and life cycle analysis of geopolymer and hybrid cement mortars for sustainable construction

Owing to the application of industrial wastes, geopolymers are generally regarded as a sustainable alternative to traditional construction materials. However, their lack of adoption on the industrial scale demands detailed investigations. This study conducts a comparative analysis of the compressive strength of different geopolymer and hybrid cement mortars with varying proportions of sodium hydroxide (from 5 to 25 wt%) and ordinary Portland cement (OPC) (from 15 to 35 wt%), respectively. The porosity of all designed mixtures was also analyzed using X-ray computed tomography (XCT) and water absorption tests. ReCiPe 2016 Midpoint (H) method was used for the Life cycle analysis of the geopolymer and hybrid cement mortars. Multi-criteria decision making (MCDM) approach was used to assess the sustainability potential of the designed mixtures based on compressive strength, porosity and overall environmental impact. Experimental results revealed that the increase in sodium hydroxide in geopolymer mortars up to 15 wt% offered its maximum compressive strength. Superior compressive strength was obtained at 35 wt% of OPC in hybrid cement mortars due to the formation of more C-S-H, C-A-S-H and N-A-S-H gels which fill up the voids and pores. Analysis of the macro and micro-porosity revealed that hybrid cement mortars yield denser structure than geopolymer mortars. Life cycle analysis based on 8 distinct impact categories showed that hybrid cement mortars outperform the geopolymers in all impact categories except 'mineral resource scarcity'. However, the overall environmental impact assessment using the 'coefficient of performance' depicts that hybrid cement mortars offer a significantly lower environmental burden than geopolymers. MCDM analysis shows that hybrid cement mortar with 5 wt% of sodium hydroxide and 35 wt% of OPC is the best choice for construction applications. This idea of sustainable hybrid cement mortar will be helpful for the construction industry to limit the environmental impact without compromising their structural performance.

Microcystis spp. and phosphorus in aquatic environments: A comprehensive review on their physiological and ecological interactions

Cyanobacterial blooms caused by eutrophication have become a major environmental problem in aquatic ecosystems worldwide over the last few decades. Phosphorus is a limiting nutrient that affects the growth of cyanobacteria and plays a role in dynamic changes in algal density and the formation of cyanobacterial blooms. Therefore, identifying the association between phosphorus sources and Microcystis, which is the most representative and harmful cyanobacteria, is essential for building an understanding of the ecological risks of cyanobacterial blooms. However, systematic reviews summarizing the relationships between Microcystis and phosphorus in aquatic environments are rare. Thus, this study provides a comprehensive overview of the physiological and ecological interactions between phosphorus sources and Microcystis in aquatic environments from the following perspectives: (i) the effects of phosphorus source and concentration on Microcystis growth, (ii) the impacts of phosphorus on the environmental behaviors of Microcystis, (iii) mechanisms of phosphorus-related metabolism in Microcystis, and (iv) role of Microcystis in the distribution of phosphorus sources within aquatic environments. In addition, relevant unsolved issues and essential future investigations (e.g., secondary ecological risks) have been highlighted and discussed. This review provides deeper insights into the relationship between phosphorus sources and Microcystis and can serve as a reference for the evaluation, monitoring, and effective control of cyanobacterial blooms.

Estimation of nutrient sources and fate in groundwater near a large weir-regulated river using multiple isotopes and microbial signatures

The excessive input of nutrients into groundwater can accelerate eutrophication in associated surface water systems. This study combined hydrogeochemistry, multi isotope tracers, and microbiological data to estimate nutrient sources and the effects of groundwater-surface water interactions on the spatiotemporal variation of nutrients in groundwater connected to a large weir-regulated river in South Korea. δ11B and δ15N-NO3- values, in combination with a Bayesian mixing model, revealed that manure and sewage contributed 40 % and 25 % respectively to groundwater nitrate, and 42 % and 27 % to nitrate in surface water during the wet season. In the dry season, the source apportionment was similar for groundwater while the sewage contribution increased to 52 % of nitrate in river water. River water displayed a high correlation between NO3- concentration and cyanobacteria (Microcystis and Prochlorococcus) in the wet season. The mixing model using multiple isotopes indicated that manure-derived nutrients delivered with increased contributions of groundwater to the river during the wet season governed the occurrence of cyanobacterial blooms in the river. We postulate that the integrated approach using multi-isotopic and microbiological data is highly effective for evaluating nutrient sources and for delineating hydrological interactions between groundwater and surface water, as well as for investigating surface water quality including eutrophication in riverine and other surface water systems.

Spatio-temporal variations in water quality of a river-lake system during restoration treatments

To fill the knowledge gap about the functioning of the lake-river system subjected to restoration treatments, two tributaries, a shallow, restored lake and its outflow, were examined. The quality of water inflows, lake and outflow was compared before (BR), during sustainable (SR, deep water aeration, phosphorus inactivation and biomanipulation for 3 years) and limited lake restoration (LR, only aeration for 2 years). Physico-chemical parameters were analysed monthly at five stations. The nutrient concentrations at the inflows decreased over the years due to the improvement of water and sewage management in the catchment (in Mielcuch from 18.0 to 8.0 mgN L^-1 and 1.0 to 0.6 mgP L^-1). The decline at the outflow was the result of a better quality of water at the tributaries and SR in the lake. During LR, decrease of phosphorus concentration still occurred (0.11 mgP L^-1), but nitrogen concentration slightly increased (3.9 mgN L^-1). Although the outflowing waters still transported a high content of chlorophyll a and suspended solids during SR, their amount was lower (34.5 μg L^-1 and 17 mg L^-1, respectively) than that during BR and LR. During restoration, it is significant to monitor the water quality not only in the lake but also at the outflow. The slow deterioration of water quality at the outflow indicated that introducing changes in the applied restoration methods must be done carefully because the previously achieved effect may be lost. Hence, restoration of the upstream lake and good quality of its tributaries are of great importance for water bodies located downstream.

Hierarchical deep learning model to simulate phytoplankton at phylum/class and genus levels and zooplankton at the genus level

Harmful algal blooms (HABs) have become a global issue, affecting public health and water industries in numerous countries. Because funds for monitoring HABs are limited, model development may be an alternative approach for understanding and managing HABs. Continuous monitoring based on grab sampling is time-consuming, costly, and labor-intensive. However, improving simulation performance remains a major challenge in modeling, and current methods are limited to simulating phytoplankton (e.g., Microcystis sp., Anabaena sp., Aulacoseira sp., Cyclotella sp., Pediastrum sp., and Eudorina sp.) and zooplankton (e.g., Cyclotella sp., Pediastrum sp., and Eudorina sp.) at the genus level. The traditional modeling approach is limited for evaluating the interactions between phytoplankton and zooplankton. Recently, deep learning (DL) models have been proposed for solving modeling problems because of their large data handling capabilities and model structure flexibilities. In this study, we evaluated the applicability of DL for simulating phytoplankton at the phylum/class and genus levels and zooplankton at the genus level. Our work was an explicit representation of the taxonomic and ecological hierarchy of the DL model structure. The prerequisite for this model design was the data collection at two taxonomic and hierarchical levels. Our model consisted of hierarchical DL with classification transformer (TF) and regression TF models. These DL models were hierarchically connected; the output of the phylum/class level model was transferred to the genus level simulation model, and the output of the genus level model was fed into the zooplankton simulation model. The classification TF model determined the phytoplankton occurrence initiation date, whereas the regression TF model quantified the cell concentration of plankton. The hierarchical DL showed potential to simulate phytoplankton at the phylum/class and genus levels by producing average R^2, and root mean standard error values of 0.42 and 0.83 [log(cells mL^-1)], respectively. All simulated plankton results closely matched the measured concentrations. Particularly, the simulated cyanobacteria showed good agreement with the measured cell concentration, with an R² value of 0.72. In addition, our simulated result showed good agreement in peak concentration compared to observations. However, a limitation remained in following the temporal variation of Tintinnopsis sp. and Bosmia sp. Using an importance map from the TF model, water temperature, total phosphorus, and total nitrogen were identified as significant variables influencing phytoplankton and zooplankton blooms. Overall, our study demonstrated that DL can be used for modeling HABs at the phylum/class and genus levels.

Effect of Gated Weir Opening on the Topography and Zooplankton Community of Geum River, South Korea

Hydrological changes affect not only the physicochemical factors and habitat structure of river ecosystems, but also the structure of biological communities sensitive to environmental changes, such as zooplankton. In this study, we investigate the effects of weir opening on environmental variables and topographic structures at Sejong Weir in South Korea and monitor the resulting changes in the structure and distribution of the zooplankton community. Weir opening led to increased dissolved oxygen and decreased conductivity, turbidity, chlorophyll a, total phosphorus, and total nitrogen and increased the diversity of topographic structures (reduced pool area and increase riffle and grassland/bare land areas) in the section downstream of Sejong Weir. Prior to weir opening (2015–2016), the cladoceran community was dominated by Chydrous spaeericus and Moina microcopa. After opening (2018–2019), the abundance of other cladoceran communities such as Bosmina groups (Bosmina longiseta, Bosmina fatalis, and Bosminopsis deitersi), Ceriodaphnia sp., and Daphnia obtusa increased. In contrast, the copepod species (Cyclops vicinus and Mesocyclops leukarti) were abundant before weir opening. We conclude that artificial weir opening helped maintain the unique environmental characteristics of the river ecosystem in terms of river continuity and led to a different zooplankton community composition in the new river environment.

Phytoplankton dynamics and implications for eutrophication management in an urban river with a series of rubber dams

Rubber dams are widely used in urban rivers for landscape construction and flood control. However, the increased water residence time by dams usually causes phytoplankton accumulation. Developing a greater understanding of the phytoplankton dynamics and the effecting factors is essential for the eutrophication control of dammed rivers. Here, we investigated the variations in biomass and structure of phytoplankton communities along an urban landscape river with 30 rubber dams, and the main controlling factors during a 2-yr field monitoring. The biomass of phytoplankton significantly increased from 12.7 μg/L-Chl a and 1.14 × 107 ind./L-cells at the natural river part above dams to 65.2 μg/L-Chl a and 1.16 × 108 ind./L-cells at the 30th dam on average. There were different dominant taxa of phytoplankton between river sections with and without dams in different seasons. As Bacillariophyta dominated at the natural river part above dams throughout the year, accounting for 64.6% on average, and dominated at the 13th and 30th dams during the cold seasons (69.6% on average). But during the warm seasons, Cyanophyta and Chlorophyta increased obviously in the dammed river sections and became dominant taxa at the 30th dam, accounting for 55.9% and 34.7% respectively. The α-diversity of phytoplankton decreased along the series of dams. While the β-diversity between river sections with and without dams increased because of species replacement. Redundancy analysis revealed that nutrients, flow velocity and temperature were the main factors influencing the spatial-temporal variation in phytoplankton community structure in this river. High-frequency monitoring data further indicated that phosphorus and discharge explained most of the variations in phytoplankton biomass within the 13th dam impoundment. It suggested that management strategies should focus on reducing the phosphorus input concentration under 0.164 mg/L and increase the discharge higher than 0.64 m3/s during warm seasons, to prevent phytoplankton bloom and further eutrophication problems in this dammed river.

The Spatial Distribution and Morphological Characteristics of Chum Salmon (Oncorhynchus keta) in South Korea

Chum salmon (Oncorhyncus keta) is a cold-water species reported to migrate within a wide range of habitats, including Korea, Japan, North America, and Russia, playing important roles in the river–sea nutrient cycle and food web. However, research on this species has not been widely performed in South Korea owing to its geographical location at the southern edge of migration. In this study, we analyzed the spatial distribution and morphological characteristics of chum salmon migrating to South Korea using the length–weight relationship. We also analyzed 3 years of catch, sex ratio, and individual information (total length (cm), weight (kg), n = 4400) from ten rivers (eight in the East coast and two on the South coast) with a total of 17 years of water quality and the distance they traveled (n = 50) using multivariate analysis. As a result, we discovered a trend of less migration in the southern part of South Korea for all individuals migrating to South Korea. Furthermore, the weight ratio of males/females was significantly different (p < 0.05). Based on the length–weight relationship analysis, the a and b values were between 0.0011 and 0.038 and 2.65 and 3.49, respectively. In the correlation analysis, catch is negatively correlated with distance traveled and temperature (p < 0.05), and positively correlated with pH, dissolved oxygen, distance, and female ratio (p < 0.05). This is possibly the result of differences in water quality during early life stages or the presence of an estuarine barrage at the mouth of the Nakdong River. This research may be used as fundamental distribution and morphological variations of chum salmon in South Korea.

Molecular responses to inorganic and organic phosphorus sources in the growth and toxin formation of Microcystis aeruginosa

Toxic cyanobacteria bloom is a ubiquitous phenomenon worldwide in eutrophic lakes or reservoirs. Microcystis, is a cosmopolitan genus in cyanobacteria and exists in many different forms. Microcystis aeruginosa (M. aeruginosa) can produce microcystins (MCs) with strong liver toxicity during its growth and decomposition. Phosphorus (P) is a typical growth limiting factor of M. aeruginosa. Though different forms and concentrations of P are common in natural water, the molecular responses in the growth and MCs formation of M. aeruginosa remain unclear. In this study, laboratory experiments were conducted to determine the uptake of P, cell activity, MCs release, and related gene expression under different concentrations of dissolved inorganic phosphorus (DIP) and dissolved organic phosphorus (DOP). We found that the growth of M. aeruginosa was promoted by increasing DIP concentration but coerced under high concentration (0.6 and 1.0 mg P/L) of DOP after P starvation. The growth stress was not related to the alkaline phosphatase activity (APA). Although alkaline phosphatase (AP) could convert DOP into algae absorbable DIP, the growth status of M. aeruginosa mainly depended on the response mechanism of phosphate transporter expression to the extracellular P concentration. High-concentration DIP promoted MCs production in M. aeruginosa, while high-concentration DOP triggered the release of intracellular MCs rather than affecting MCs production. Our study revealed the molecular responses of algal growth and toxin formation under different P sources, and provided a theoretical basis and novel idea for risk management of eutrophic lakes and reservoirs.

Soluble reactive phosphorous determination in wastewater treatment plants by automatic microanalyzers

The analysis of soluble reactive phosphate (SRP) in water is key to control water quality. In order to continuous monitor orthophosphate content in water during treatment processes and in the effluents of wastewater treatment plants, conventional procedures, usually performed in a laboratory, must be adapted. This means pursuing efforts on miniaturizing systems to operate in situ and automating analytical methods to work on-line. The design, construction and evaluation of an automatic and low cost cyclic olefin copolymer (COC)-based spectrophotometric microanalyzer, capable of operating in unattended conditions, is presented to monitor soluble reactive phosphorous, as orthophosphate ion, in wastewater samples coming from sewage treatment plants. The microsystem, constructed by CNC micromilling and using a multilayer approach, integrates microfluidics to carry out the phosphomolybdenum blue (PMB) reaction and an optical flow-cell for the spectrophotometric orthophosphate determination in a single polymeric substrate smaller than a credit card. It is connected to a compact optical detection system composed by a LED emitting at 660 nm and a PIN-photodiode, both integrated in a PCB. Flow management is automatically performed by programmed microvalves and micropumps, which control autocalibration processes and allow unattended operation. Analytical features after the optimization of the microfluidic platform and the chemical and the hydrodynamic variables, were a linear range from 0.09 to 32 mg L^-1 P and a detection limit of 0.03 mg L^-1 P with a sampling rate of 24 samples h^-1, demonstrating the microanalyzer suitability for SRP monitoring in water. Moreover, real samples were analyzed obtaining promising results.

Combined use of calcium nitrate addition and anion exchange resin capping to control sedimentary phosphorus release and its nitrate‑nitrogen releasing risk

Calcium nitrate (Ca(NO₃)₂) addition can be used to control the release of phosphorus from sediments, however it can also cause an increase in the concentration of nitrate‑nitrogen (NO₃^--N) in the water column. The risk of NO₃^--N release from the Ca(NO₃)₂-injected sediments may be reduced by the placement of the anion exchange resin (AER) capping layer. In this study, the effectiveness of the combined use of Ca(NO₃)₂ addition and AER capping to prevent the liberation of phosphorus from sediments was investigated, and the reduction of the risk of NO₃^--N released from the Ca(NO₃)₂-injected sediment by the AER capping was also evaluated. The combined application of Ca(NO₃)₂ addition and AER capping could tremendously reduce the amount of soluble reactive phosphorus (SR-P) in the overlying water, with SR-P reduction rates of 75.9-98.7%. Furthermore, it could cut down the contents of high-resolution diffusive gradients in thin films (DGT)-labile phosphorus in the sediments, resulting in the formation of phosphorus static layer in the upper sediments. The combined treatment using Ca(NO₃)₂ and AER had a relatively small effect on the contents of mobile phosphorus in the sediments, but it could greatly increase the amount of residual phosphorus in the top 30mm sediments (increased by 27.7-42.9%). The amount of NO₃^--N in the overlying water under the action of the combined treatment method using Ca(NO₃)₂ and AER was much lower than that under the action of the single Ca(NO₃)₂ treatment during the early stage of sediment remediation. In conclusion, the combined use of Ca(NO₃)₂ addition and AER capping is a more promising strategy for the control of sedimentary phosphorus release than the single use of Ca(NO₃)₂ addition from the point of view of both the control efficiency of P release from sediments and the releasing risk of the added nitrate.

Responses of water quality and phytoplankton assemblages to remediation projects in two hypereutrophic tributaries of Chaohu Lake

Water shortages and the presence of point and diffuse source pollution have caused a serious deterioration in water quality in two tributaries (the Tangxi River and Shiwuli River) of Chaohu Lake, China. To reduce nutrient pollution and suppress harmful algal blooms (HABs), hard engineering and ecological remediation projects were implemented. A post-project investigation from 2013 to 2016 was carried out to evaluate the outcome of the remediation projects by monitoring the seasonal and spatial variations in water quality and the phytoplankton community. In the Tangxi River, the average total phosphorus (TP) concentrations in the four seasons were below 0.5 mg L^-1, with the lowest concentration (0.29 ± 0.12 mg L^-1) found in autumn. Remediation measures including sediment dredging, riparian buffer zone creation, downstream wetland park construction, and water augmentation using reclaimed water and filtered lake water might combine to promote P source mitigation. Moreover, the percentage of bloom-forming cyanobacteria (i.e., Microcystis, Aphanizomenon, Anabaena, Oscillatoria, Phormidium and Planktothrix) in the phytoplankton assemblage and the biomass of the dominant species indicated successful HAB control. In the Shiwuli River, water quality improvements and phytoplankton responses have been observed since 2015 after the upgrading of a local wastewater treatment plant (WWTP) with effluent that was used for flow augmentation. Nevertheless, there is still room for improvement via increasing the river self-purification ability (e.g., the creation of downstream wetlands and riparian buffer zones) and promoting water augmentation according to the experience gained in the remediation projects of the Tangxi River.

Seasonally varying effects of environmental factors on phytoplankton abundance in the regulated rivers

This study investigates a seasonally varying response of phytoplankton biomass to environmental factors in rivers. Artificial neural network (ANN) models incorporated with a clustering technique, the clustered ANN models, were employed to analyze the relationship between chlorophyll a (Chl-a) and the explanatory variables in the regulated Nakdong River, South Korea. The results show that weir discharge (Q) and total phosphorus (TP) were the most influential factors on temporal dynamics of Chl-a. The relative importance of both variables increased up to higher than 30% for low water temperature seasons with dominance of diatoms. While, during summer when cyanobacteria predominated, the significance of Q increased up to 45%, while that of TP declined to about 10%. These tendencies highlight that the effects of the river environmental factors on phytoplankton abundance was temporally inhomogeneous. In harmful algal bloom mitigation scenarios, the clustered ANN models reveals that the optimal weir discharge was 400 m³/s which was 67% of the value derived from the non-clustered ANN models. At the immediate downstream of confluence of the Kumho River, the optimal weir discharge should increase up to about 1.5 times because of the increase in the tributary pollutant loads attributed to electrical conductivity (EC).

Why Algae Release Volatile Organic Compounds-The Emission and Roles

A wide spectrum of volatile organic compounds (VOCs) are released from algae in aquatic ecosystems. Environmental factors such as light, temperature, nutrition conditions and abiotic stresses affect their emission. These VOCs can enhance the resistance to abiotic stresses, transfer information between algae, play allelopathic roles, and protect against predators. For homogeneous algae, the VOCs released from algal cells under stress conditions transfer stress information to other cells, and induce the acceptors to make a preparation for the upcoming stresses. For heterogeneous algae and aquatic macrophytes, the VOCs show allelopathic effects on the heterogeneous neighbors, which benefit to the emitter growth and competing for nutrients. In cyanobacterial VOCs, some compounds such as limonene, eucalyptol, β-cyclocitral, α-ionone, β-ionone and geranylacetone have been detected as the allelopathic agents. In addition, VOCs can protect the emitters from predation by predators. It can be speculated that the emission of VOCs is critical for algae coping with the complicated and changeable aquatic ecosystems.

Hysteresis analysis of nitrate dynamics in the Neuse River, NC

Anthropogenic activities have caused N saturation in many terrestrial ecosystems. The transfer of nutrients and sediments to freshwater environments has resulted in water quality impairments including eutrophication, increased turbidity, ecosystem acidification, and loss of biodiversity. Storm events account for the transport of a large proportion of nutrients and sediments found in watersheds on an annual basis. To implement effective water-quality management strategies, the importance of surface and subsurface flow paths during storm events and low flow conditions need to be quantified. The increased availability of optical in-situ sensors makes high-frequency monitoring of catchment fluxes practical. In this study, we present a high-resolution nitrate monitoring record over a 10-year period in the Neuse River Basin near Clayton, North Carolina. The relationship between discharge and nitrate concentration for 365 storm events are categorized into hysteresis classes that indicate different transport mechanisms into the river. Storm events over the entire period of this study are divided between clockwise, counter-clockwise, and complex hysteresis patterns, indicating multiple nitrate flow paths during different seasons and years. Logistic regression of a suite of environmental variables demonstrates that antecedent soil moisture is a significant factor in determining the storm hysteresis class, with the odds of counter-clockwise hysteresis increasing by 10.3% for every 1 percentage point increase in the soil moisture. There is also an overlying seasonal effect, which indicates that dry soil conditions and frequent small storms during summer leads to greater nitrate transport on the rising limb, in contrast to slower, groundwater-driven inputs during the rest of the year.

Stabilization of High-Organic-Content Water Treatment Sludge by Pyrolysis

Water treatment sludge from algal blooms were analyzed and compared with general water treatment sludge as the pyrolysis temperature was varied from 300 °C to 900° C. Elemental analysis showed that the water treatment sludge in the eutrophication region has ~12% carbon content, higher than that (8.75%) of general water treatment sludge. X-ray diffraction (XRD) analysis of both types of sludge showed that amorphous silica changed to quartz and weak crystalline structures like kaolinite or montmorillonite were decomposed and changed into stronger crystalline forms like albite. Fourier transform infrared spectroscopy (FT-IR) peaks of humic/fulvic acid that indicated the affinity to combine with heavy metals disappeared above 700 °C. Toxicity characteristic leaching procedure (TCLP), conducted to determine the heavy metal leaching amount of pyrolyzed water treatment sludge, showed the lowest value of 5.7 mg/kg at 500 °C when the humic acid was not decomposed. At 500 °C, the heavy metal leaching ratio to the heavy metal content of high organic content water treatment sludge and low organic content water treatment sludge were 1.87% and 3.19%, respectively, and the water treatment sludge of higher organic content was more stable. In other words, pyrolysis of water treatment sludge with high organic content at 500 °C increases the inorganic matter crystallinity and heavy metal leaching stability.

Overcoming Interfacial Scaling Using Engineered Nanocelluloses: A QCM-D Study

Nucleation of sparingly soluble species, such as the inorganic salts of calcium, magnesium, and phosphorous, followed by their growth at solid-liquid interfaces has turned into a major concern in water-based industries. Increased resistance against heat, mass, and momentum transfer is the main drawback of the so-called scaling phenomenon. Although phosphorous-, nitrogen-, and sulfur-based antiscaling macromolecules offer adequate antiscaling performance, their potential negative environmental impacts render them less desirable. Despite recent efforts in developing green antiscalants, there has been no promising green solution based on biomass due to its chemical inertness. Here, we use quartz crystal microbalance with dissipation monitoring (QCM-D) to evaluate the real-time performance of an emerging family of nanoengineered anionic hairy cellulose crystals, bearing dicarboxylated amorphous cellulose chains, with a charge density of more than 5.5 mequiv per g, in preventing the nucleation and growth of calcium carbonate, the most common industrial scale. Remarkably, a CaCO₃ mass deposition rate ∼0 (complete scale inhibition) is obtained when less than 10 ppm of the hairy nanocellulose is added to an already scaled surface under a harsh supersaturated condition at 50 °C. Motivated by their threshold antiscaling effect, we show that coating planar silica surfaces with hairy nanocelluloses may result in scale-resistant interfaces. This research envisions how engineered hairy nanocelluloses may have practical implications for developing scale-resistant interfaces based on the most abundant biopolymer in the world.

Extreme weather event may induce Microcystis blooms in the Qiantang River, Southeast China

A severe cyanobacterial bloom in the mainstem of a large Chinese river was first reported from China. The Qiantang River is the longest river in the Zhejiang province, southeast China. It provides drinking water supply to ~ 16 million people, including Hangzhou city. Fifteen sites along the Qiantang River (including upper, middle (Fuchunjiang Reservoir), and lower reaches and tributaries) were sampled between August 13 and September 9, 2016 to conduct a preliminary examination of the outbreak of Microcystis blooms. Laboratory investigation revealed that Microcystis spp. are dominant in the Fuchunjiang Reservoir (an overflow reservoir on the mainstem of the Qiantang River) with an extremely high cell density of 2.3 × 10⁸ cells/L, leading to a severe bloom in the mainstem of the Qiantang River. Investigations of the meteorological, hydrological, and nutrient characteristics associated with the bloom indicated that extremely dry (6.8 mm rainfall from August 13 to September 9, 2016) and hot (32 consecutive days of temperatures > 30 °C from July 20 to August 31, 2016) weather might be the key factors triggering the bloom. Additionally, the extremely low flow of the tributary, Lanjiang River (142 ± 56 m³/s from August 13 to September 9), and its high nutrient background, favored the bloom. While nutrient reductions are important, the most immediate and effective management approach might be to implement appropriate minimal flow conditions to mitigate the blooms.

Effects of phosphorus sources on volatile organic compound emissions from Microcystis flos-aquae and their toxic effects on Chlamydomonas reinhardtii

There is diverse phosphorus (P) in eutrophicated waters, but it is considered as a crucial nutrient for cyanobacteria growth due to its easy precipitation as insoluble salts. To uncover the effects of complex P nutrients on the emission of volatile organic compounds (VOCs) from cyanobacteria and their toxic effects on other algae, the VOCs from Microcystis flos-aquae supplied with different types and amount of P nutrients were analyzed, and the effects of VOCs and their two main compounds on Chlamydomonas reinhardtii growth were investigated. When M. flos-aquae cells were supplied with K₂HPO₄, sodium pyrophosphate and sodium hexametaphosphate as the sole P source, 27, 23 and 29 compounds were found, respectively, including furans, sulfocompounds, terpenoids, benzenes, aldehydes, hydrocarbons and esters. With K₂HPO₄ as the sole P source, the VOC emission increased with reducing P amount, and the maximum emission was found under Non-P condition. In the treatments of M. flos-aquae VOCs under Non-P condition and two main terpenoids (eucalyptol and limonene) in the VOCs, remarkable decreases were found in C. reinhardtii cell growth, photosynthetic pigment content and photosynthetic abilities. Therefore, we deduce that multiple P nutrients in eutrophicated waters induce different VOC emissions from cyanobacteria, and P amount reduction caused by natural precipitation and algal massive growth results in more VOC emissions. These VOCs play toxic roles in cyanobacteria becoming dominant species, and eucalyptol and limonene are two toxic agents.

An oxygen slow-releasing material and its application in water remediation as oxygen supplier

In this study, an oxygen slow-releasing material (OSRM) consisting of calcium peroxide (CaO₂), stearic acid (SA) and quartz sand was used to improve oxygen supply during bioremediation. The oxygen-releasing rates of CaO₂ powder and OSRM with different SA contents were investigated. The efficacy of OSRM as an oxygen supplier was assessed by water remediation experiments using activated sludge. Results showed that CaO₂ powder was effectively embedded by SA under anhydrous conditions. The oxygen-releasing rate decreased with increasing SA contents. Moreover, the OSRM exhibited higher oxygen-releasing capacity, and more effective pH control ability than CaO₂ powder. The water remediation experiments showed better removal of COD and [Formula: see text] with OSRM as the oxygen supplier. These results provided detailed information when CaO₂ was applied as the oxygen supplier in water remediation, which can serve as references for field application of bioremediation.

Real-time eutrophication status evaluation of coastal waters using support vector machine with grid search algorithm

The development of techniques for real-time monitoring of the eutrophication status of coastal waters is of great importance for realizing potential cost savings in coastal monitoring programs and providing timely advice for marine health management. In this study, a GS optimized SVM was proposed to model relationships between 6 easily measured parameters (DO, Chl-a, C1, C2, C3 and C4) and the TRIX index for rapidly assessing marine eutrophication states of coastal waters. The good predictive performance of the developed method was indicated by the R² between the measured and predicted values (0.92 for the training dataset and 0.91 for the validation dataset) at a 95% confidence level. The classification accuracy of the eutrophication status was 86.5% for the training dataset and 85.6% for the validation dataset. The results indicated that it is feasible to develop an SVM technique for timely evaluation of the eutrophication status by easily measured parameters.

Phosphorus leakage from fisheries sector - A case study in Thailand

Although phosphorus (P) is an essential element needed for all lives, excess P can be harmful to the environment. The objective of this study aims to determine P flows in the fisheries sector of Thailand consisting of both sea and freshwater activities of captures and cultures. Currently, the annual fisheries catch averages 3.44 ± 0.50 Mt. Most comes from marine capture 1.95 ± 0.46 Mt, followed by coastal aquaculture 0.78 ± 0.09 Mt, freshwater aquaculture 0.49 ± 0.05 Mt, and inland capture 0.22 ± 0.01 Mt. Of this total, about 11% is contained in fresh products directly sold in local markets for consumption, while 89% is sent to processing factories prior to being sold in local markets and exported. The quantities of P entering the fisheries sector come from captures, import of fisheries products and feed produced from agriculture. This P input to the fisheries sector is found to average 28,506 t P.y^-1 based on the past ten-year records. Of this total, P input from captures accounts for 76%; while, 11% represents aquatic feeds from agriculture and animal manures. About 13% is obtained from the imports of fishery products. Coastal and freshwater aquacultures are found to be P consumers because their feeds are almost all produced from agricultural crops grown inland. Moreover, these activities cause most of P losses, approximately 10,188 t P·y^-1, which account for 89% of the total P loss from the fisheries sector. Overall, P in the fisheries sector is found to mobilize through three channels: (a) 44% is consumed within the country; (b) about 16% is exported; and, (c) 40% is lost from the ecosystem. Based on the results of this work it is recommended that future research be directed on ways to minimize P loss and maximize P recycle in Thailand's fisheries sector as to enhance its food security and curtail water pollution.

Characterizing phosphorus removal from polluted urban river water by steel slags in a vertical flow constructed wetland

Phosphorus (P) removal in constructed wetlands (CWs) is often low unless special substrates with high sorption capacities are used. However, the use of special substrates in vertical flow (VF) CWs has not been proved to enhance P sorption. Thus, two VF wetlands were designed to evaluate the potential for enhanced P removal from polluted urban river water, one with slag as substrate and the other as a control with gravel as substrate. Findings from batch experiments showed P sorption capacities of 3.15 gP/kg and 0.81 gP/kg, respectively, for steel slag and gravel. Different organic matter fractions played different roles in P sorption, the effects of which were significant only at high concentrations. Over a 220 days' operation, the VF-slag removed 76.0% of the influent total phosphorus (TP) at 0.159 g/m(2)·d and PO4-P of 70.9% at 0.063 g/m(2)·d, whereas the VF-gravel removed 65.0% at 0.136 g/m(2)·d and 48.6% at 0.040 g/m(2)·d, respectively. Therefore, the merit of using a steel slag substrate in VF wetlands can be significant for the removal of PO4-P.