A critical review of operational strategies for the management of harmful algal blooms (HABs) in inland reservoirs

Innovative multiplex qPCR method for rapid and reliable detection of microcystin-producing genes during harmful algal blooms: Insights from Utah Reservoirs

Cyanobacterial harmful algal blooms (cyanoHABs) have the potential to produce cyanotoxins, which pose significant health risks to both humans and animals. The gold standard methods for monitoring cyanoHABs involve enzyme-linked immunosorbent assay (ELISA), liquid chromatography combined with triple quadrupole mass spectrometry (LC-MS/MS) and manual cell counting under microscopy. However, these techniques, while effective, are costly and time-consuming, which may not be optimal for timely decision-making to safeguard public health. Quantitative polymerase chain reaction (qPCR) offers a complementary approach that serves as an indicator of the potential for toxin production. It provides accurate results with a rapid turnaround time and high throughput capacity, and greater affordability. To assess the reliability of qPCR in predicting toxin production and determining when toxin levels exceed recreational advisory thresholds, we conducted experiments utilizing two DNA extraction methods for qPCR testing: RapidDNA and ClassicDNA. Sampling was conducted across nine water bodies in Utah throughout the recreational season from June 1 to October 31, 2023. We targeted cyanotoxin-encoding genes mainly associated with microcystins, the dominant cyanotoxin reported for these water bodies, for qPCR analysis. Toxin levels were measured using both ELISA and LC-MS/MS with cyanobacteria cell counting conducted as a reference. Out of nine water bodies studied, cyanoHABs were detected in five (i.e., Utah Lake, and Deer Creek, Echo, Schofield, and Pineview Reservoirs). Analysis of the data revealed a significant linear relationship between both the qPCR results of mcyE (associated with microcystin production) obtained from RapidDNA and ClassicDNA methods, and the levels of microcystins measured by ELISA and LC-MS/MS. RapidDNA qPCR methods offer a potential warning tool for indicating toxin production during blooming events, though this method is not suitable for determining risk during the pre-blooming period. Conversely, ClassicDNA methods can be utilized during the pre-blooming period to prepare for potential blooms. These results provide insight into the genetic potential of blooms around the state to produce microcystins. Findings can be implemented in both Recreational Water Quality and Drinking Water programs nationally.

Algae removal and degradation of microcystins by UV-C system: A review

Harmful algal blooms (HABs), driven by eutrophication, are a growing ecological threat, compromising water quality and ecosystem health through the release of toxic microcystins (MCs). These toxins pose significant risks to both aquatic life and human health. Among the emerging solutions, UV-C technology has gained attention for its efficiency in inhibiting algal growth and degrading MCs, offering a cost-effective and environmentally friendly approach with minimal secondary pollution. However, existing studies often overlook key aspects, including the variability in algae sensitivity to UV-C wavelengths, the stability of treatment across diverse aquatic conditions, and the toxicity of degradation byproducts. This review highlights the mechanisms underlying UV-C-based algae removal, explores its potential limitation, such as algal resistance, and compares its efficacy with other remediation methods. Notably, the lack of comprehensive research on wavelength-specific sensitivity and real-world application efficacy represents a significant knowledge gap. Further investigation into these areas is essential to optimize UV-C technology for mitigating HABs and improving water safety in eutrophic environments. PRACTITIONER POINTS: The choice of UV band should be adjusted to the algae species. The UV-C system, with limited studies and applications in natural water bodies, demonstrates instability. Combining UV-C with other technologies substantially enhances the efficiency of algal control. Future research should emphasize strategies to prevent the rapid release of microcystins (MCs) from this system due to cell lysis and extracellular release within a short time frame.

Fate of a toxic Microcystis aeruginosa bloom introduced into a subtropical estuary from a flow-managed canal and management implications

The Caloosahatchee Estuary in southwest Florida, USA, is regularly subject to the introduction of toxic Microcystis aeruginosa blooms, often originating from the eutrophic Lake Okeechobee via the C-43 Canal. The focus of this study was to determine the responses of one of these introduced blooms to progressively elevated salinity levels as the bloom water mass moved through the estuary. In the upper estuary, salinities were freshwater, and surface blooms of large colonies of M. aeruginosa were observed, along with peak microcystin toxin concentrations up to 107 μg L-1, all in the particulate fraction. In the mid-estuary, salinity levels increased to 2-6, and surface blooms were again observed, with peak microcystin concentrations up to 259 μg L-1, however, significant levels of extracellular toxin were also observed (i.e., 17.8 μg L-1), suggesting a level of osmotic stress on M. aeruginosa. In the lower estuary, salinities ranged from 6 to 25 and very few viable M. aeruginosa colonies were observed, but significant levels of extracellular microcystin (i.e., 0.5 μg L-1) were present throughout the water column. It is noteworthy that average total microcystin concentrations in the water column (i.e., particulate + extracellular) remained constant throughout the movement of the bloom water mass during its transit through the estuary, revealing the negligible rate of microcystin degradation during the ten-day transit. The results also provide insights into the changes in the distribution of particulate and extracellular microcystin along the salinity gradient, which has implications for management of risks for ecosystem and human health, and how these risks may be affected by management of releases from three water control structures in the C-43 Canal. Discharge rates from the water control structures play major roles in the rate of movement of blooms through the C-43 Canal-Caloosahatchee Estuary ecosystem. The potential implications of discharge regulation for the management of M. aeruginosa in the ecosystem are discussed from the perspectives of blooms of allochthonous and autochthonous origin.

Hydrology and water quality evaluation for potential HABs under future climate scenarios

Harmful algal blooms (HABs) are increasingly a global concern and the issue of all fifty states in the U.S as it poses a threat to human health and aquatic ecosystem. This study aimed to investigate the relationship of HABs with streamflow and water quality parameters and assess the hydrology-based potential future HABs in the Ohio River Basin at Ironton (ORBI) using the Soil and Water Assessment Tool (SWAT). SWAT was calibrated and validated against potential HABs indicators including streamflow, total suspended solids (TSS), and dissolved oxygen (DO) with acceptable accuracies. Twenty-one years (2002-2022) of model simulated data were analyzed to relate the 2015 HABs occurrence in the Ohio River with unique temperature, streamflow, TSS, and DO conditions. Additionally, a future climate model was developed to project these variables for the next two decades (2023-2043) using data from the Canadian Regional Climate Model (CRCM5) for Representative Concentration Pathway (RCP) 4.5. The 2015 HABs formation was found to be associated with a series of high flows contributing to high nutrient transport followed by an extended period of low flows balancing nutrients flushing rate. The projections of average temperature, streamflow, and TSS concentration showed increments of 5%, 15%, and 28%; whereas DO concentration showed a decrement of 8%. Flood frequency analysis was conducted to better understand the HABs probability related to peak flow conditions. For the base condition, results showed 3% probability of peak flow (4550 m3/s) associated with 2015 HABs formation at the Ironton gauge station and 20% probability of the same flow in the next two decades demonstrating an increased risk of HABs and highlighting the necessity of mitigation measure implementations.

Light scattering study of algal floc growth and structure: alum vs. polymeric plant-derived flocculant

The flocculation dynamics of Microcystis aeruginosa algal cultures using alum and aqueous Moringa oleifera seed extracts as flocculants were analyzed through light scattering and fractal analysis. Floc growth in continuously stirred M. aeruginosa suspensions, with cell densities ranging from 200 to 800 μg L-1 chlorophyll a (Chl a), exhibited distinct patterns in fractal dimension (dF) evolution relative to floc size: a smooth, monotonic increase; stochastic increase; and stabilization or leveling off. dF values ranged from 1.3 to 2.6, with floc diameters (D4,3 volume-weighted mean) spanning 30 to 300 μm. Alum (0.1 to 0.4 g L-1) induced fast diffusion-limited flocculation, initially producing lower dF values, which progressively increased due to structural rearrangement at a slower rate. In contrast, at sufficiently high concentrations (0.1 to 0.2 g L-1 BSA equivalent), M. oleifera seed proteins facilitated stable, high dF ≈ 2.0 early on, evidently through patch charge interactions. Flocs formed with alum were prone to shear-induced breakage, limiting both their size and stability, whereas M. oleifera extract produced larger, more stable flocs with greater resilience to shear due to robust particle network formation by the polymer. Both flocculants effectively treated 800 μg L-1 Chl a M. aeruginosa suspensions, but M. oleifera extract demonstrated better performance in terms of floc size at similar mass concentrations. These findings highlight the potential of Moringa seed extract as a sustainable and effective alternative to conventional flocculants like alum, offering insights into their mechanisms and performance in flocculation processes.

Monitoring, simulation and early warning of cyanobacterial harmful algal blooms: An upgraded framework for eutrophic lakes

Cyanobacterial Harmful Algal Bloom (CyanoHAB) is a global aquatic environmental issue, posing considerable eco-environmental challenges in freshwater lakes. Comprehensive monitoring and accurate prediction of CyanoHABs are essential for their scientific management. Nevertheless, traditional satellite-based monitoring and process-oriented prediction methods of CyanoHABs failed to satisfy this demand due to the limited spatiotemporal resolutions of both monitoring data and prediction results. To address this issue, this paper proposes an upgraded framework for comprehensive monitoring and accurate prediction of CyanoHABs. A collaborative CyanoHAB monitoring network was firstly constructed by integrating space, aerial, and ground-based monitoring means. As a result, CyanoHAB conditions were assessed frequently covering the entire lake, its key areas, and core positions. Furthermore, by overcoming technical limitations associated with high-precision simulation of the growth-drift-accumulation process of CyanoHABs, such as the unclear drifting process of CyanoHABs and the mechanism of its coastal accumulation, the multi-scale CyanoHAB prediction was realized interconnecting the entire lake and its nearshore areas. The implemented framework has been applied in Lake Chaohu for over three years. It provided high-frequency and high-spatial-resolution CyanoHAB monitoring, as well as its multi-scale and accurate simulation. The application of this framework in Lake Chaohu had significantly improved the accuracies of CyanoHAB monitoring, simulation, and early warning. This advancement holds significant scientific value and offers potential for CyanoHAB prevention and control in eutrophic lakes.

HABS-BLOCKS©, a Floating, Slow-Release Glucose Source, Promoted the Growth of Heterotrophic Bacteria Relative to Toxic Cyanobacteria in Lake Water Mesocosms

Previously, we demonstrated that the addition of glucose to lake water could alter the composition of the microbial community so that heterotrophic bacteria came to dominate the cyanobacteria. To target the glucose additions to the euphotic zone, a floating, slow-release glucose source, designated HABS-BLOCKS©, was created. HABS-BLOCKS© consist of blocks of pumice stone, vacuum infiltrated with glucose, and covered in layers of soy wax. In this study, the HABS-BLOCKS© were tested in 7-liter mesocosm vessels (n = 4) that received an initial 750 ml of lake water, followed by weekly additions of 500 ml of freshly collected lake water. Three HABS-BLOCKS© were added to each of two replicate mesocosms. For controls, one mesocosm was left untreated and one mesocosm received three "Dummy" HABS-BLOCKS© (contain no glucose). During a ten-week experiment, 25 ml samples were obtained from each mesocosm weekly, which were then filtered, frozen and latter processed for 16S rRNA sequencing. Planktothrix and Cyanobium were the most abundant cyanobacteria in the lake water. Within three weeks of the start of the experiment, the bacterial community in the HABS-BLOCKS© treated mesocosms became dominated by heterotrophic bacteria, e.g., Asticcacaulis, relative to the control mesocosms. Heterotrophic domination in the HABS-BLOCKS© treated mesocosms continued for the rest of the experiment. HABS-BLOCKS© appears to provide a competitive edge for the heterotrophic bacteria that allows them to dominate the toxin-producing cyanobacteria.

Development and application of chitosan-urea cotton adsorbent as biomimicry technology inspired by natural predator-prey relationships

Predator-prey interactions play a crucial role in maintaining ecological balance and possibly provide inspiration for strategies to mitigate environmental changes such as harmful algal blooms (HABs). To this end, this study aims to develop a novel strategy to mitigate HABs based on predator-prey interaction, i.e., Daphnia magna and Microcystis aeruginosa interaction. Bio-compounds (urea and 9-octadecenamide) produced by D. magna when encounter M. aeruginosa, were identified, particularly with urea promoting the aggregation of M. aeruginosa. Then, a novel adsorbent against HABs was synthesized by integrating bio-compounds of urea, and its effectiveness in removing M. aeruginosa was demonstrated. Notably, the adsorbent displayed a high removal efficiency of 99.25 % within 6 h. Our eco-friendly strategy holds promise for controlling HABs, representing the successful application of biomimicry principles.

Food Webs and Feedbacks: The Untold Ecological Relevance of Antimicrobial Resistance as Seen in Harmful Algal Blooms

Antimicrobial resistance (AMR) has long been framed as an epidemiological and public health concern. Its impacts on the environment are unclear. Yet, the basis for AMR is altered cell physiology. Just as this affects how microbes interact with antimicrobials, it can also affect how they interact with their own species, other species, and their non-living environment. Moreover, if the microbes are globally notorious for causing landscape-level environmental issues, then these effects could alter biodiversity and ecosystem function on a grand scale. To investigate these possibilities, we compiled peer-reviewed literature from the past 20 years regarding AMR in toxic freshwater cyanobacterial harmful algal blooms (HABs). We examined it for evidence of AMR affecting HAB frequency, severity, or persistence. Although no study within our scope was explicitly designed to address the question, multiple studies reported AMR-associated changes in HAB-forming cyanobacteria (and co-occurring microbes) that pertained directly to HAB timing, toxicity, and phase, as well as to the dynamics of HAB-afflicted aquatic food webs. These findings highlight the potential for AMR to have far-reaching environmental impacts (including the loss of biodiversity and ecosystem function) and bring into focus the importance of confronting complex interrelated issues such as AMR and HABs in concert, with interdisciplinary tools and perspectives.

Changes in Cyanobacterial Phytoplankton Communities in Lake-Water Mesocosms Treated with Either Glucose or Hydrogen Peroxide

When cyanobacterial phytoplankton form harmful cyanobacterial blooms (HCBs), the toxins they produce threaten freshwater ecosystems. Hydrogen peroxide is often used to control HCBs, but it is broadly toxic and dangerous to handle. Previously, we demonstrated that glucose addition to lake water could suppress the abundance of cyanobacteria. In this study, glucose was compared to hydrogen peroxide for the treatment of cyanobacterial phytoplankton communities. The six-week study was conducted in the large mesocosms facility at Grand Valley State University's Annis Water Resources Institute in Michigan. To 1000 L of Muskegon Lake water, glucose was added at either 150 mg or 30 mg glucose/L. Hydrogen peroxide was added at 3 mg/L to two 1000 L mesocosms. And two mesocosms were left untreated as controls. Triplicate 100 mL samples were collected weekly from each mesocosm, which were then filtered and frozen at -80 °C for 16S rRNA amplicon sequencing. The 16S rRNA amplicon sequencing results revealed that hydrogen peroxide treatment quickly reduced the relative abundance of the cyanobacteria compared to the control mesocosms, but the cyanobacteria population returned over the course of the 6-week study. On the other hand, both glucose concentrations caused a rapid proliferation of multiple low abundance proteobacterial and bacteroidotal taxa resulting in notable increases in taxonomic richness over the duration of the study and reducing the relative abundance of cyanobacteria. Although hydrogen peroxide quickly suppressed the cyanobacteria, the population later returned to near starting levels. The glucose suppressed the cyanobacterial phytoplankton apparently by promoting competitive heterotrophic bacteria.

Harmful algal blooms in inland waters

Harmful algal blooms can produce toxins that pose threats to aquatic ecosystems and human health. In this Review, we outline the global trends in harmful algal bloom occurrence and explore the drivers, future trajectories and potential mitigation strategies. Globally, harmful algal bloom occurrence has risen since the 1980s, including a 44% increase from the 2000s to 2010s, especially in Asia and Africa. Enhanced nutrient pollution owing to urbanization, wastewater discharge and agricultural expansion are key drivers of these increases. In contrast, changes have been less substantial in high-income regions such as North America, Europe and Oceania, where policies to mitigate nutrient pollution have stabilized bloom occurrences since the 1970s. However, since the 1990s, climate warming and legacy nutrient pollution have driven a resurgence in toxic algal blooms in some US and European lakes, highlighting the inherent challenges in mitigating harmful blooms in a warming climate. Indeed, advancing research on harmful algal bloom dynamics and projections largely depends on effectively using data from multiple sources to understand environmental interactions and enhance modelling techniques. Integrated monitoring networks across various spatiotemporal scales and data-sharing frameworks are essential for improving harmful algal bloom forecasting and mitigation.

Exploring factors that affect Microcystis abundance in the sacramento san joaquin delta

Cyanobacteria harmful algal blooms (cHABs) are increasing in frequency, intensity and duration in estuaries worldwide. In the upper San Francisco Estuary, also known as the Sacramento San Joaquin Delta (Delta), cHABs have been a topic of concern over the past two decades. In response, managers are urgently working to understand the factors that drive cHABs and identify feasible management options to avert ecological and human health consequences. We used a six year data set to explore relationships between flow parameters, temperature, and Microcystis biovolume to determine the potential for managing large scale hydrodynamic conditions to address Delta cHABs. We also looked at the relationship between Microcystis biovolume and the low salinity zone to see if it could be used as a proxy for residence time, because residence time is positively related to cyanobacteria abundance. We found the low salinity zone is not a useful proxy for residence time in the area of the Delta that experiences the most severe cHABs. Our finding suggest that climatic conditions (i.e., temperature and water year type) have the greatest influence on Microcystis biovolume in the Delta, with higher biovolume during years with lower flow and higher temperatures. Further, there are interannual differences in Microcystis biovolume that cannot be fully explained by flow parameters or temperature, meaning other factors not included in our model may be involved. We conclude that management actions to increase flow may be ineffective at reducing Microcystis to desired levels if water temperatures remain high.

Sediment quality classification in freshwater lakes predicted by the history of treatment with copper-based aquatic algaecides

Copper-based aquatic algaecides have been widely used in the management of aquatic macrophyte and harmful algal blooms for decades due to their effectiveness and low cost. However, repeated treatment of freshwater lakes results in the accumulation of copper in the sediment, which may adversely affect the ecosystem of the lakes through toxicity to benthic invertebrates and other organisms. Consequently, copper-based aquatic herbicides and algaecides have been frequently subjected to regulations aimed at preventing further ecological deterioration in treated waterbodies. Many states in the US are taking or considering taking an approach that limits or prohibits copper treatment in waterbodies. Freshwater lakes with extensive historical copper treatments typically have significantly higher concentrations of copper in the sediment than untreated lakes. However, the correlation between the extent of the treatments and level of copper accumulation in the sediment has not been quantitatively characterized. In the present study, between 2006 and 2017, copper concentrations were measured in the sediment from selected lakes in New York State with different histories of copper-based aquatic algaecide treatment. Analysis of these data confirmed findings from earlier studies conducted by New York State that lakes with histories of copper treatment have significantly higher copper concentrations in the sediment. It also demonstrated that sediment copper concentrations were significantly higher in the sublittoral zone than the littoral zone in treated lakes. Moreover, a positive correlation was detected between sediment copper concentrations and the total number of treatments with copper-based aquatic algaecide for both littoral and sublittoral zones (p-value = 7.94 × 10-8 and 3.1 × 10-13, respectively). This relationship can be used as a screening tool for regulatory agencies to identify lakes with sediment copper levels possibly higher than the toxicity threshold for aquatic life. Consequently, additional ecological risk assessment may be required before allowing future treatment with copper-based aquatic algaecides. Integr Environ Assess Manag 2024;20:1586-1597. © 2024 SETAC.

Enhancement of adsorption of cyanobacteria, Microcystisa aeruginosaby bacterial-based compounds

In the present study, algicidal bacteria cultivated in an aqueous medium were utilized as a surface modification agent to develop an efficient adsorbent for the removal of Microcystis aeruginosa. The modification considerably enhanced M. aeruginosa cell removal efficiency. Moreover, the introduction of bio-compounds ensured specificity in the removal of M. aeruginosa. Additionally, the cyanotoxin release and acute toxicity tests demonstrated that the adsorption process using the developed adsorbent is environmentally safe. Furthermore, the practical feasibility of the adsorptive removal of M. aeruginosa was confirmed through cell removal tests performed using the developed adsorbent in a scaled-up reactor (50 L and 10 tons). In these tests, the effects of the adsorbent application type, water temperature, and initial cell concentration on the M. aeruginosa removal efficiency were evaluated. The results of this study provide novel insights into the valorization strategy of biological algicides repurposed as adsorbents, and provide practical operational data for effective M. aeruginosa removal in scaled-up conditions.

Molecular identification of Halomonas AZ07 and its multifunctional enzymatic activities to degrade Pyropia yezoensis under high-temperature condition

BACKGROUND

Pyropia yezoensis a commercially important red seaweed species, is susceptible to various microorganisms infections, among which bacterial infections are the most prominent ones. Pyropia yezoensis is often affected by harmful bacterial communities under high temperatures that can lead to its degradation and economic losses. The current study aimed to explore Pyropia yezoensis-associated microbiota and further identify potential isolates, which can degrade Pyropia yezoensis under high-temperature conditions.

METHODS AND RESULTS

The 16S rRNA gene sequencing was used to identify the agarolytic bacterial species. The results showed that Chromohalobacter sp. strain AZ6, Pseudoalteromonas sp. strain AZ, Psychrobacter sp. strain AZ3, Vibrio sp. strain AZ, and Halomonas sp. strain AZ07 exhibited algicidal properties as these strains were more abundant at high temperature (25 °C). Among the five isolated strains, the potential isolate Halomonas sp. strain AZ07 showed high production of agarolytic enzymes, including lipase, protease, cellulase, and amylase. This study confirmed that the isolated strain could produce these four different enzymes. The strain Halomonas AZ07 was co-treated with Pyropia yezoensis cells under two different temperature environments, including 13 °C and 25 °C. The degradation of Pyropia yezoensis occurred at the optimum temperature of 25 °C and effectively degraded their cell wall, proteins, lipids, and carbohydrates.

CONCLUSION

The successful cultivation of Pyropia yezoensis in coastal farm environments is dependent on specific temperature and environmental factors, and lower temperatures have been observed to be particularly beneficial for the survival and growth of Pyropia yezoensis. The temperature below 13 °C was confirmed to be the best niche for the symbiotic relationship of microbiota associated with Pyropia yezoensis for its growth, development, and production.

Daily process and key characteristics of phytoplankton bloom during a low-water level period in a large subtropical reservoir bay

Reservoirs, heavily influenced by artificial management, often harbor phytoplankton assemblages dominated by cyanobacteria or dinoflagellates, triggering significant changes in aquatic ecosystems. However, due to limited sampling frequency and insufficient attention to species composition, the bloom processes and key characteristics of phytoplankton community structure have not been systematically elucidated. During the low-water level period when blooms are most likely to occur (June to September) in a tributary bay of the Three Gorges Reservoir, daily sampling was conducted to investigate phytoplankton community composition, identify significant environmental factors, and evaluate important structure characteristics of phytoplankton community. The results showed that Microcystis aeruginosa maintained a clear dominance for almost a month in stage 1, with low Shannon and evenness but a high dominance index. Phytoplankton total density and biomass decreased drastically in stage 2, but Microcystis aeruginosa still accounted for some proportion. The highest Shannon and evenness but the lowest dominance index occurred in stage 3. Peridiniopsis niei occurred massively in stage 4, but its dominant advantages lasted only one to two days. NH4-N was responsible for the dominance of Microcystis aeruginosa, while TP and PO4-P was responsible for the dominance of Peridiniopsis niei; however, precipitation contributed to their drastic decrease or disappearance to some extent. The TN : TP ratio could be considered as an important indicator to determine whether Microcystis aeruginosa or Peridiniopsis niei dominated the phytoplankton community. Throughout the study period, physiochemical factors explained more variation in phytoplankton data than meteorological and hydrological factors. Pairwise comparisons revealed an increase in average β diversity with stage progression, with higher β diversities based on abundance data than those based on presence/absence data. Repl had a greater effect on β diversity differences based on presence/absence data, whereas RichDiff had a greater effect on β diversity differences based on species abundance data. Co-occurrence networks for stage 1 showed the most complex structure, followed by stage 4, while the network for stage 3 was relatively sparse, although the overall community division remained compact. This study provides a useful attempt to explore the status and changes in phytoplankton community structure during the bloom process through high-resolution investigation.

Chlorophyll-a unveiled: unlocking reservoir insights through remote sensing in a subtropical reservoir

Effective water resources management and monitoring are essential amid increasing challenges posed by population growth, industrialization, urbanization, and climate change. Earth observation techniques offer promising opportunities to enhance water resources management and support informed decision-making. This study utilizes Landsat-8 OLI and Sentinel-2 MSI satellite data to estimate chlorophyl-a (chl-a) concentrations in the Nandoni reservoir, Thohoyandou, South Africa. The study estimated chl-a concentrations using random forest models with spectral bands only, spectral indices only (blue difference absorption (BDA), fluorescence line height in the violet region (FLH_violet), and normalized difference chlorophyll index (NDCI)), and combined spectral bands and spectral indices. The results showed that the models using spectral bands from both Landsat-8 OLI and Sentinel-2 MSI performed comparably. The model using Sentinel-2 MSI had a higher accuracy of estimating chl-a when spectral bands alone were used. Sentinel-2 MSI's additional red-edge spectral bands provided a notable advantage in capturing subtle variations in chl-a concentrations. Lastly, the -chl-a concentration was higher at the edges of the Nandoni reservoir and closer to the reservoir wall. The findings of this study are crucial for improving the management of water reservoirs, enabling proactive decision-making, and supporting sustainable water resource management practices. Ultimately, this research contributes to the broader understanding of the application of earth observation techniques for water resources management, providing valuable information for policymakers and water authorities.

Physical drivers of Noctiluca scintillans (Dinophyceae) blooms outbreak in the northern Taiwan Strait: A numerical study

The red Noctiluca scintillans (RNS) blooms often break out near Pingtan Island, in the northern Taiwan Strait from April to June. It is essential to gain insights into their formation mechanism to predict and provide early warnings for these blooms. Previous studies and observations showed that RNS blooms are the most likely to occur when winds are weak and shifting in direction. To explore this phenomenon further, we employed a high-resolution coastal model to investigate the hydrodynamics influencing RNS blooms around Pingtan Island from April to June 2022. The model results revealed that seawater exhibited weak circulation but strong stratification during RNS blooms. Residence time were examined through numerical experiments by releasing passive neutrally buoyant particles in three bays of Pingtan Island. The results showed a significantly longer residence time during RNS blooms, indicating reduced flushing capabilities within the bays, which could give RNS a stable environment to multiply and aggregate. This hydrodynamic condition provided a favorable basis for RNS blooms breakout near Pingtan Island. The shifts and weakening of the prevailing northeast wind contributed substantially to weakening the flow field around Pingtan Island and played a crucial role in creating the hydrodynamics conducive to RNS blooms. Our study offers fresh insights into the mechanisms underpinning RNS blooms formation near Pingtan Island, providing a vital framework for forecasting RNS blooms in this region.

A modified stratification index method to assess reservoir water quality trends

Harmful algal blooms (HABs) have been a harm to reservoir health for decades and it is believed that as climate changes and temperatures rise, frequency and severity of HABs are likely to increase as well. Understanding the relationships between physical factors in a reservoir and bloom trends could be key for keeping rising blooms at bay. A modified stability index based on temperature dependent water density at multiple depths was adapted into a code to process high frequency reservoir monitoring data. Metrics and visual tools were developed to use this stability index to objectively analyze how stratification - including intensity, start date, and turnover point - and water quality characteristics are changing, and how they are likely to change in the coming decades. This code was then used to analyze the relationships between physical and environmental reservoir characteristics, a generated stratification index, and algal bloom behavior for several United States Army Corps of Engineers (USACE) reservoirs, with specific focus on Shenango reservoir. Surface temperature, maximum temperature difference, and depth were found to correspond with strength of stratification. The calculated stratification index showed significant strength of correlation to algae when compared to other commonly collected reservoir parameters. Bettering our understanding of when stratification is occurring within reservoirs, as well as when blooms are occurring, will lead to more informed management decisions and better HAB control. For Shenango reservoir, it was determined that a hydrodynamic management strategy that maintains stability (horizontal flushing, epilimnetic withdrawals) during peak periods, while removing epilimnetic biomass and reducing nutrient availability, would be the most practical management strategy for bloom reduction.

Biological and Chemical Approaches for Controlling Harmful Microcystis Blooms

The proliferation of harmful cyanobacterial blooms dominated by Microcystis aeruginosa has become an increasingly serious problem in freshwater ecosystems due to climate change and eutrophication. Microcystis-blooms in freshwater generate compounds with unpleasant odors, reduce the levels of dissolved O2, and excrete microcystins into aquatic ecosystems, potentially harming various organisms, including humans. Various chemical and biological approaches have thus been developed to mitigate the impact of the blooms, though issues such as secondary pollution and high economic costs have not been adequately addressed. Red clays and H2O2 are conventional treatment methods that have been employed worldwide for the mitigation of the blooms, while novel approaches, such as the use of plant or microbial metabolites and antagonistic bacteria, have also recently been proposed. Many of these methods rely on the generation of reactive oxygen species, the inhibition of photosynthesis, and/or the disruption of cellular membranes as their mechanisms of action, which may also negatively impact other freshwater microbiota. Nevertheless, the underlying molecular mechanisms of anticyanobacterial chemicals and antagonistic bacteria remain unclear. This review thus discusses both conventional and innovative approaches for the management of M. aeruginosa in freshwater bodies.

Prediction of hydrological and water quality data based on granular-ball rough set and k-nearest neighbor analysis

Hydrological and water quality datasets usually encompass a large number of characteristic variables, but not all of these significantly influence analytical outcomes. Therefore, by wisely selecting feature variables with rich information content and removing redundant features, it not only can the analysis efficiency be improved, but the model complexity can also be simplified. This paper considers introducing the granular-ball rough set algorithm for feature variable selection and combining it with the k-nearest neighbor method and back propagation network to analyze hydrological and water quality data, thus promoting overall and fused inspection. The results of hydrological water quality data analysis show that the proposed method produces better results compared to using a standalone k-nearest neighbor regressor.

The influence mechanism of water level operation on algal blooms in canyon reservoirs and bloom prevention

The water level operation of reservoirs affects the spatiotemporal patterns of water quality, light-heat, hydrodynamics and phytoplankton, which have implications for algal bloom prevention. However, the theoretical analysis and practical applications of related research are limited. Based on prototype observations and numerical modeling, data on algae, water level operation and environmental factors in the Zipingpu Reservoir from April and September in 2015 to 2017 and 2020 to 2022 were collected. An in-depth analysis of the causal mechanisms between algal blooms and water level operation was performed, and prevention strategies with practical application assessments were developed. Water level operation control in the reservoir from April to September can be divided into five stages (falling-rising-oscillating-falling-rising), with algal blooms occurring only in the second stage. The rising water level with inflow into the middle layers shapes a closed-loop circulation in the surface waters. This distributes the nutrients that were trapped in the surface layer during the first stage, helping algae avoid to phosphorus limitation and thrive in the closed loop circulation, leading to algal blooms (chlorophyll-a exceeding 10 mg/m3). There is a significant positive correlation (p < 0.05) between algal blooms and the rapid rise in water levels in the second stage, occurring within a span of three days. To contain the algal bloom, a water level operation limit of rising waters on the third day after a two-day consecutive rise in water level was examined. This was found to be effective after its practical application to the case reservoir in 2022, with chlorophyll-a concentrations consistently below 10 mg/m3. This study unveils the mechanisms through which water level operation affects algal blooms and presents a successful case of bloom prevention. Furthermore, it serves as a valuable reference for the management of canyon reservoirs.

Research status and prospects of organic photocatalysts in algal inhibition and sterilization: a review

An increasing amount of sewage has been discharged into water bodies in the progression of industrialization and urbanization, causing serious water pollution. Meanwhile, the increase of nutrients in the water induces water eutrophication and rapid growth of algae. Photocatalysis is a common technique for algal inhibition and sterilization. To improve the utilization of visible light and the conversion efficiency of solar energy, more organic photocatalytic materials have been gradually developed. In addition to ultraviolet light, partial infrared light and visible light could also be used by organic photocatalysts compared with inorganic photocatalysts. Simultaneously, organic photocatalysts also exhibit favorable stability. Most organic photocatalysts can maintain a high degradation rate for algae and bacteria after several cycles. There are various organic semiconductors, mainly including small organic molecules, such as perylene diimide (PDI), porphyrin (TCPP), and new carbon materials (fullerene (C60), graphene (GO), and carbon nanotubes (CNT)), and large organic polymers, such as graphite phase carbon nitride (g-C3N4), polypyrrole (PPy), polythiophene (PTH), polyaniline (PANI), and polyimide (PI). In this review, the classification and synthesis methods of organic photocatalytic materials were elucidated. It was demonstrated that the full visible spectral response (400-750 nm) could be stimulated by modifying organic photocatalysts. Moreover, some problems were summarized based on the research status related to algae and bacteria, and corresponding suggestions were also provided for the development of organic photocatalytic materials.

Vertical distribution rules and factors influencing phytoplankton in front of a drinking water reservoir outlet

The phenomenon of algal blooms caused by the excessive proliferation of phytoplankton in drinking water reservoirs is becoming increasingly frequent, seriously endangering water quality, ecosystems, water safety, and people's health. Thus, there is urgent need to conduct research on the distribution rules and factors influencing phytoplankton in drinking water reservoirs. Given that the outflows from reservoirs usually come from the middle and lower layers of the water column and the current studies on phytoplankton in drinking water reservoirs are usually carried out on the surface, an 8-month monitoring of vertical phytoplankton and the corresponding influencing factors in front of the outlet in a drinking water reservoir was conducted. Based on the monitoring results, the distribution rules of phytoplankton and the associated factors were analyzed. The results showed that phytoplankton biomass significantly decreased with increasing water depth, but the biomass near the outlet (40 m depth) still reached the WHO level 2 warning threshold for algal blooms multiple times. During the monitoring period, Cyanophyta, Chlorophyta and Bacillariophyta dominated. The selected multisource environmental factors explained 60.5 % of the spatiotemporal changes in phytoplankton, with thermal intensity (water temperature and thermal stratification intensity) being the driving factor. Meanwhile, excessive TN and TP provided necessary conditions for the growth of phytoplankton. Based on influencing factors, reducing upstream nutrient inflows and thermal stratification intensity are recommended as measures to prevent and control algal blooms. This study provides insights into the vertical distribution rules and factors influencing phytoplankton in a drinking water reservoir, which can provide a reference for the management of drinking water reservoirs and the prevention and control of algal blooms.