Toward a Brighter Future: Enhanced Sustainable Methods for Preventing Algal Blooms and Improving Water Quality

: This comprehensive review explores the escalating challenge of nutrient enrichment in aquatic ecosystems, spotlighting the dire ecological threats posed by harmful algal blooms (HABs) and excessive particulate organic matter (POM). Investigating recent advancements in water treatment technologies and management strategies, the study emphasizes the critical need for a multifaceted approach that incorporates physical, chemical, and biological methods to effectively address these issues. By conducting detailed comparative analyses across diverse aquatic environments, it highlights the complexities of mitigating HABs and underscores the importance of environment-specific strategies. The paper advocates for sustainable, innovative solutions and international cooperation to enhance global water quality and ecosystem health. It calls for ongoing advancement, regular monitoring, and comprehensive research to adapt to emerging challenges, thus ensuring the preservation of aquatic biodiversity and the protection of communities reliant on these vital resources. The necessity of integrating technological innovation, ecological understanding, and global cooperation to safeguard aquatic ecosystems for future generations is paramount.


Introduction
The global recognition of aquatic ecosystems' susceptibility to nutrient enrichment highlights a critical environmental concern.Recent studies have broadened the understanding of nutrient inputs' impacts on biological processes across various ecosystems, moving beyond the focus on toxic algal blooms and dead zones in lakes and coastal areas [1][2][3][4].The link between nutrient loading increase and agricultural activity, driven by population growth, is well-established.Even arid regions experience consistent nutrient runoff despite potential decreases in overall runoff due to increased water demand [5][6][7][8].Sources of nutrient elevation include atmospheric deposition, wastewater, and runoff from impermeable surfaces, revealing the complex nature of this challenge [1,9,10].Some papers stressed the essential role of strict environmental regulations in reducing nutrient loads from both direct and indirect sources as a key strategy for ecosystem preservation [11,12].
Harmful algal blooms (HABs) refer to the overgrowth of algae that can have deleterious effects on ecosystems, economy, and health.These blooms can be triggered by a variety of factors, including increased nutrient loads from fertilizers, warmer temperatures, changes in water flow, and the presence of invasive species.Typically, these conditions promote the rapid growth of algae that can produce harmful toxins [24][25][26][27].In freshwater, such as lakes and rivers, HABs often involve blue-green algae (also known as cyanobacteria).One notorious example is the 2014 Toledo bloom in Lake Erie, which left 500,000 residents without drinking water due to the high concentration of microcystins, a group of toxins produced by these cyanobacteria [28][29][30].In marine environments, HABs often involve dinoflagellates like Karenia brevis, which causes Korea and Florida's red tides.These blooms can lead to massive fish kills and can contaminate shellfish, making them dangerous for human consumption [31,32].
While often perceived negatively, algae play a foundational role in aquatic habitats.They serve as the primary producers in aquatic food webs, providing the base for most aquatic life forms.However, when they grow excessively and form blooms, the adverse impacts can be significant.These include the depletion of oxygen in the water, known as hypoxia; the production of toxins harmful to fish, wildlife, and humans; and significant disruptions to aquatic environments and local economies dependent on these waters [33][34][35].In essence, while algae are critical to aquatic ecosystems, their unchecked growth during HABs can cause severe ecological and economic damage [32,36,37].
Researchers examine the nature and impact of particulate organic matter (POM) in aquatic environments, identifying its diverse composition-including bacteria, phytoplankton, protozoa, metazoans, and organic debris-and the factors influencing its deposition and ecological function [38][39][40][41][42]. POM typically ranges in size from 0.5 µm to several millimeters, depending on the source and composition.It is often characterized by a complex chemical nature, including carbohydrates, lipids, proteins, and nucleic acids [43][44][45].The reactivity of POM is influenced by its chemical structure and environmental conditions, which can affect its role in biogeochemical cycles and its interaction with pollutants [46][47][48][49].POM particles can exhibit varying degrees of hydrophobicity, influencing their aggregation and sedimentation behavior.They often form conjugates with metals and organic pollutants, affecting their transport and bioavailability [49,50].POM particles have sedimentation coefficients that reflect their size and density, typically ranging from 1.1 to 1.6 g/cm 3 [51,52].The adverse effects of excessive POM and harmful algal bloom (HAB) levels on nutrient cycling, pollution absorption, and biodiversity are discussed, along with the crucial role of POM removal in maintaining environmental and organismal health [23,[53][54][55][56][57].
Moreover, some papers have studied the role of fine particulate organic matter (FPOM) in ecosystems, highlighting its significance as a food source for filter-feeding animals and its role in exporting organic matter to water bodies affected by HABs.FPOM is generally smaller than 0.5 µm and often consists of decomposed organic materials and microorganisms.Its fine size and high surface area enhance its reactivity and capacity to adsorb contaminants, making it a critical component in nutrient cycling and pollutant transport [58,59].FPOM particles are typically more hydrophilic than larger POM, thus influencing their stability in suspension and interaction with aquatic organisms.Their smaller size and lower density usually result in lower sedimentation coefficients, which affect their settling rates in water columns [18,60].Researchers discuss strategies for POM and HAB mitigation, such as using physical (e.g., filtration and sedimentation), chemical (e.g., coagulants and oxidants), and biological methods (e.g., bioremediation and phytoremediation), and emphasize the careful selection of treatment methods to avoid unintended ecological impacts [18,[61][62][63][64][65][66][67][68][69].
This study conducts a comprehensive review of HAB control technologies for both freshwater and marine ecosystems, aiming to identify and recommend advanced, sustainable methods for removing harmful substances with minimal environmental impact.It reaffirms the importance of addressing the impacts of nutrient enrichment on aquatic ecosystems and calls for the adoption of effective environmental regulations and manage-ment strategies.The need for selecting appropriate remediation methods to improve water quality, conserve biodiversity, and ensure the health and safety of aquatic environments is emphasized.

Advancements in Water Treatment: Focused Strategies for Particulate Organic Matter and Harmful Algae
The systematic classification of water treatment methods is underscored, highlighting the critical need for selectively removing POM and combating HABs.Such classification enhances our understanding of the recent advancements in the field, as outlined in Table 1 [18,23,61,[70][71][72][73][74][75].The review sheds light on worldwide efforts to advance water treatment technologies to tackle POM and HAB challenges [76][77][78].The contributions of the U.S. Environmental Protection Agency (EPA), the National Oceanic and Atmospheric Administration (NOAA), and the practices in countries such as China and those in the OECD demonstrate the effectiveness of diverse monitoring and response strategies [18,[79][80][81].This discussion spans the gamut of water treatment technologies, including physical, chemical, and biological methods, for the effective removal of POM and HABs [82][83][84].A comprehensive review by the EPA in 2023 emphasizes the variety of advanced water treatment approaches, integrating innovative technologies like advanced oxidation processes (AOPs), membrane bioreactors (MBRs), and real-time monitoring systems [85][86][87][88].The crucial role of global collaboration and knowledge exchange in driving innovative solutions to new water treatment challenges is highlighted [89,90].

Physical Methods
Physical methods, such as sedimentation, filtration, and dredging, utilize mechanisms like screens, dissolved air flotation, and filters to capture and isolate contaminants [18,69,91,92].The operational requirements for these methods include regular maintenance to ensure efficiency, considerable energy consumption, and managing the risk to aquatic life due to physical barriers [93,94].Current costs for physical methods can vary widely; for instance, filtration and membrane filtration are generally medium to high cost but offer low environmental impact and high removal efficiency, respectively, despite high maintenance demands and susceptibility to fouling.Byproducts such as sludge and waste solids must be carefully managed to avoid environmental degradation [95][96][97].

Chemical Methods
Chemical methods employ additives like activated carbon, ozone, and various coagulants to destabilize or aggregate particles, which are crucial for contaminant removal.These processes involve specific operational requirements, such as precise dosing and monitoring of chemical levels to prevent environmental harm [98][99][100].The ongoing need for enhancement is driven by challenges like sedimentation inefficiencies and filtration process optimization [101][102][103].Costs for chemical treatments can be high, particularly with ozone and advanced oxidation processes, due to their powerful oxidizing abilities and high operational costs.Byproducts such as sludge from coagulation/flocculation and potential secondary pollutants from ozone treatment necessitate careful management to mitigate environmental impact [104][105][106].

Biological Methods
Biological methods focus on leveraging organisms such as zooplankton and bacteria to mitigate harmful algal blooms (HABs), offering an environmentally friendly solution [107][108][109][110][111]. Bacteria can impact HABs by releasing enzymes or toxins that degrade algal cells, while zooplankton directly consume HABs, effectively reducing algal biomass.Real case studies, such as the introduction of copepods in Lake Taihu, China, demonstrate significant reductions in cyanobacteria like Microcystis aeruginosa [112,113].Another study indicated that increased populations of the cladoceran zooplankton reduced the biomass of cyanobacteria, highlighting the potential of zooplankton as a natural control agent for HABs [114,115].Additionally, wetlands play a pivotal role by decomposing particulate organic matter (POMs) and HABs into simpler compounds essential for water purification [18,[116][117][118].Operationally, these methods require conditions that support the growth and activity of beneficial organisms, with low energy demands and minimal environmental disruption [119,120].Costs for biological treatments are generally low, with benefits including biodiversity support and natural pollutant breakdown, though effectiveness can vary, and methods may require longer timescales and specific conditions to achieve optimal results [121][122][123][124].

Comparative Analysis of Harmful Algal Blooms in Freshwater and Marine Ecosystems
This section conducts a comparative analysis of HABs in freshwater and marine environments, identifying specific causative microorganisms, their toxins, and control strategies [80,[125][126][127].Freshwater HABs often involve cyanobacteria and dinoflagellates, with eutrophication, high temperatures, excessive sunlight, and stagnant waters exacerbating these blooms.Key contributors to nutrient excess include fertilizers, domestic wastewater, and industrial residues, which introduce high levels of nitrogen and phosphorus into aquatic systems.Seasonal blooms are particularly significant, with increasing frequencies observed during warmer months and occasionally in non-typical seasons due to climate change and altered precipitation patterns [88,128,129].In contrast, marine HABs involve a wider variety of microorganisms, including cyanobacteria, dinoflagellates, diatoms, etc., with coastal upwelling, nutrient concentration increases, warm temperatures, and decreased salinity as key factors [130][131][132][133].
Toxin production in each ecosystem is discussed, with marine HABs producing toxins like saxitoxins, brevetoxins, and domoic acid, while freshwater HABs generate a range of toxins, including anatoxins, microcystins, saxitoxins, and cylindrospermopsin, posing significant health risks [18].Some invasive species, such as Alexandrium spp.and Karenia spp., have displaced native biota, altering ecosystem dynamics.Studies have shown a global increase in human intoxication incidents caused by HABs, particularly in regions such as North America, Europe, and Southeast Asia.These incidents are often linked to consumption of contaminated seafood or exposure to contaminated water [134][135][136].Various methods for HAB mitigation are proposed, influenced by bloom type and environmental context, including physical, chemical, and biological treatments [76,133].The necessity of an integrative management approach considering water's chemical, physical, and biological aspects is stressed, along with the need for ongoing monitoring and additional research into efficient, environmentally friendly methods tailored to specific blooms [137][138][139].
Table 2 outlines differences in microorganisms, toxins, and mitigation techniques across environments, detailing algal species linked to various poisoning syndromes [140][141][142][143][144][145][146][147][148][149][150][151].The importance of timely monitoring and detection to prevent toxin spread and protect public health and the environment is reiterated [152][153][154].The table also highlights the invasive nature of certain algal species and their impacts on local ecosystems.Lastly, it acknowledges the variability in toxin production across different species within these genera and the existence of other harmful algal genera [155].The need for vigilant, timely monitoring and detection to curb toxin proliferation and protect both public health and ecosystems is reiterated.

Passive Strategies for Managing Harmful Algal Blooms (HABs) in Aquatic Ecosystems
The strategic management of harmful algal blooms (HABs) in both freshwater and marine ecosystems is crucial for maintaining ecological balance.Increasingly, passive management strategies are being applied to manage HABs effectively across various infrastructure types.This review covers various strategies assessed for their effectiveness in controlling HABs, considering the type of infrastructure used.
In marine environments, specific HAB species have been successfully managed using methods such as ballast water treatment (typically in hermetic tanks  [61,63,166].The passive control of marine dinoflagellates through methods like introducing zooplankton or bacteria (open waters), water cycling and filtration (flowing channels and hermetic tanks), or applying binders, hydrogen peroxide, or ozone (open waters and controlled tanks) is also noted [167][168][169].
A detailed assessment of biological methods for HAB control in Table 3 emphasizes economic, ecological, and safety considerations across different infrastructures.This includes evaluating the advantages and disadvantages of using phytoplankton, zooplankton, benthic macroinvertebrates, microorganisms, aquatic plants, fish, and allelochemicals, integrating findings from relevant studies.The exploration of innovative technologies, such as remote sensing, DNA sequencing, and machine learning, for early detection and prevention of HABs is discussed, highlighting their potential for real-time monitoring and prediction across various types of aquatic infrastructure [170][171][172][173][174][175].This review of biological methods for controlling HABs underscores the importance of assessing potential benefits and impacts on native communities and potential toxicity within different infrastructure settings.The optimal use of these techniques may require significant resources and a comprehensive understanding of the local ecosystem to effectively mitigate risks.The adoption of innovative technologies, such as remote sensing, DNA sequencing, and machine learning, offers promising avenues for early detection and prevention of HABs, enabling effective real-time monitoring and predictive capabilities [176,177].The exploration of various management methods, including natural predators, physical barriers, and genetically modified organisms (GMOs), is discussed [72,178], emphasizing the necessity of evaluating their environmental impacts and effects on nontarget organisms before implementation.The debate over genetically modified crops is acknowledged [179][180][181].
Advocating for the continuous improvement of monitoring and management strategies, preceded by a comprehensive assessment of potential risks and benefits within various infrastructure contexts, is crucial for reducing the frequency and severity of HABs.Enhancing public awareness and education on the prevention and management of HABs is highlighted as essential [36,182].

Enhanced Sustainable Strategies for Algal Bloom Prevention and Water Quality Improvement
Recent research underscores the critical importance of prioritizing water quality and ecosystem health, focusing on the sustainable removal of organic matter and the mitigation of harmful algal blooms (HABs) (Table 4).Addressing HABs sustainably is essential not only for ecosystem preservation but also for protecting public health and maintaining economic activities that rely on water bodies.Eliminating various forms of organic matter, including particulates and gases, is vital to mitigate the displacement or replacement of organisms and to preserve biodiversity [21,22,80,183].This study emphasizes the necessity of implementing effective practices to maintain water quality while efficiently removing harmful organisms through environmentally friendly methods, thus reducing the negative impacts on ecosystem health and human activities [184,185].[21,22,80,183].

Areas of Focus Approaches
Water Quality Improvement Focuses on advanced methods for pollutant removal, including physical, chemical, and biological techniques.It also mentions developing diagnostic tests for cyanotoxins and using high-resolution data for site restoration and nutrient interception.

Algal Bloom Mitigation
Highlights integrated approaches, such as drainage water recycling and assessing wetland plants for nutrient capture.Monitoring and modeling specific lakes for HAB reduction and evaluating climate change's impact on nutrient runoff are also covered.

Decision Criteria
Discusses the consideration of contamination type, resource availability, environmental impact, cost-effectiveness, and scalability in projects, thus supporting statewide water quality and HAB management efforts.

Biocontrol and Health Impacts
Reviews research on the susceptibility to HAB toxins and toxic microplastics, focusing on direct and indirect human health impacts, highlighting the need to mitigate adverse health effects on communities and ecosystems.

Socioeconomic Consequences of HABs
New research addresses broader socioeconomic effects on communities and industries, with projects aimed at visualizing water quality trends in the Lake Erie watershed to enhance understanding and communication of these impacts.

Ecologically Sustainable Practices
Emphasizes sustainable and ecological solutions like nutrient capture in wetlands and best practices for nutrient control, including research on water-carbon-nutrient coupling for climate-resilient production.

Areas of Focus Approaches
Policy and Governance Supports policy development and implementation based on research, evaluating water treatment technologies and fostering collaboration among universities and state agencies for adaptive governance, aiming to sustain water quality and ecosystem health.

Innovative and Emerging Solutions
Incorporates predictive models for climate change effects on nutrient runoff and explores enhanced cyanotoxin removal methods, underscoring the importance of emerging technologies in water quality and HAB management.

Community Involvement and Awareness
Focuses on increasing public awareness and involvement through projects that make water quality data more accessible and understandable, encouraging community engagement in addressing water quality issues, thus supporting proactive management strategies.
The effectiveness of physical methods such as filtration, sedimentation, and skimming in removing harmful organisms is well-established.However, the use of chemicals like chlorine, ozone, and hydrogen peroxide should be approached cautiously due to their potential adverse effects on both ecosystems and human health [186,187].UV disinfection is highlighted as an eco-friendly alternative, eradicating harmful organisms without introducing additional chemicals into the water, thereby minimizing ecological disruption and maintaining biodiversity [188].The introduction of live organisms for algae control requires careful consideration to avoid unintended consequences that could displace native species and disrupt ecosystem balance [21].
Achieving success in natural environments is complex, influenced by factors such as water temperature, nutrient levels, and the presence of competing organisms, all of which affect ecosystem health and biodiversity [17,189].Harmful algal blooms (HABs) have severe consequences, including water quality degradation, oxygen depletion, ecosystem damage, reduced biodiversity, unpleasant odors, and toxin production, which directly affect both environmental and human health [79].Recovering and removing flocculated or settled organic matter from water is crucial to prevent further ecological harm and to protect biodiversity [100,184].Sustainable approaches to managing water quality, public health, and ecosystem health are critical, with a strong emphasis on evaluating potential impacts to avoid undesirable outcomes that could harm ecosystems and human activities [190][191][192][193].
Given the severe impact of HABs on water quality and ecosystem health, the paper reiterates the imperative for adopting sustainable solutions to safeguard water quality, protect biodiversity, and ensure the resilience of ecosystems against future challenges.

Conclusions and Future Direction
This research highlights the critical vulnerability of aquatic ecosystems to nutrient enrichment, emphasizing the severe ecological threats posed by harmful algal blooms (HABs) and excessive particulate organic matter (POM) [194][195][196].It delves into the latest advancements in water treatment technologies and management strategies, advocating for a comprehensive approach that integrates physical, chemical, and biological methods to effectively tackle these issues [197][198][199].
Through detailed comparative analysis, this study reveals the complexities involved in mitigating HABs across various aquatic environments, underscoring the importance of strategies tailored to specific environmental and biological factors [200,201].Future studies must focus on sustainable, innovative solutions that are both environmentally friendly and cost-effective.Methods such as nutrient reduction, biomanipulation, and the use of targeted, minimally disruptive algicides are highlighted [202,203].
This study calls for enhancing water quality and ecosystem health on a global scale through ongoing advancements, regular monitoring, and extensive research.It advocates for developing efficient, cost-effective, and environmentally friendly water treatment technologies, with a focus on refining the selectivity and efficiency of treatment methods to target contaminants without harming aquatic ecosystems [204][205][206][207][208][209][210][211][212][213].International coop-eration is essential, as demonstrated by events like the "PICES GlobalHAB International Workshop on HAB Control Solutions in Marine and Estuarine Waters," held in Seattle, Washington, in 2023 [214].Key challenges include ensuring cost-effectiveness at large scales, obtaining public acceptance and regulatory approval, and securing funding for widespread implementation.
Innovations in material science, the incorporation of molecular biology techniques like CRISPR-Cas for managing toxin-producing algal species, and leveraging data science and remote sensing technologies for early detection and monitoring are critical areas for future progress [215][216][217][218]. Collaborative international research efforts are essential to address the transboundary nature of water pollution and ecosystem degradation [219,220].Such collaborations can help standardize monitoring and treatment methodologies, exchange best practices, and align regulatory frameworks to manage nutrient loads and prevent HABs across different regions [221][222][223].
Future research should also consider the socioeconomic aspects of water quality management, integrating the valuation of ecosystem services into policy-making.This promotes strategies that balance environmental conservation with economic development and social well-being [224][225][226].
In summary, the confluence of interdisciplinary research, technological innovation, and international collaboration is vital for protecting aquatic ecosystems, preserving biodiversity, and ensuring the sustainability of water resources for future generations.Effective methods to control HABs must be environmentally friendly and cost-effective, while focusing on comprehensive strategies that combine prevention, early detection, and targeted treatment approaches.
), UV irradiation (open waters or controlled environments), biomanipulation (open waters and flowing channels), and the introduction of ciliates or flagellates (open waters).Both marine and freshwater systems have benefited from strategies like nutrient reduction (open waters and flowing channels), physical removal (open pools and flowing channels), and biomanipulation (open waters and controlled tanks), with their effectiveness supported by recent research [156-160].In freshwater environments, strategies to mitigate HABs caused by species such as Anabaena, Aphanizomenon, and Microcystis include filtration (flowing channels and hermetic tanks), sedimentation (open pools and flowing channels), chemical treatments (e.g., copper sulfate in open waters and flowing channels), aeration (open pools and hermetic tanks), and nutrient management (open waters and flowing channels) [80,161,162].Efforts to control Oscillatoria with copper sulfate treatments (open waters) and sediment removal (open pools) are well-documented [163-165].The management of freshwater Cyanobacteria includes methods such as filtration (flowing channels and hermetic tanks), sedimentation (open pools), chemical treatments (open waters), aeration (open pools), nutrient management (open waters and flowing channels), and UV light exposure (open waters and controlled environments)

Table 4 .
Comprehensive Insights into Sustainable Water Quality Enhancement and Algal Bloom Mitigation