Eutrophication of lakes cannot be controlled by reducing nitrogen input: results of a 37-year whole-ecosystem experiment

Efficiency and mechanism of a novel La-based hydrogel designed for controlling lake eutrophication: Insight from phosphorus release characteristics of sediment, sulfur-driven autotrophic denitrification and cyanobacterial bloom response

Reported nutrient passivators often target single-nutrient control and require complex, energy-intensive processes. In this study, we developed a mesoporous network-structured spherical La-based hydrogel for dual nitrogen and phosphorus control. The hydrogel framework, cross-linked by sodium alginate and lanthanum, encapsulates free La³⁺. The preparation process avoids high-energy techniques and produces no waste, with recyclable reagents. Through adsorption tests, cryo-electron microscopy, and surface area and pore size analysis, we found that the mesoporous structure shields internal free La³⁺, preventing rapid release and minimizing waste of La³⁺. The cross-linked La³⁺ in the spherical framework is released by microbial degradation of the polysaccharide skeleton, inducing anaerobic conditions in the sediment and passivating soluble reactive phosphorus (SRP) simultaneously. The G-units in sodium alginate act as a carbon source, enhancing sulfur-nitrogen cycle coupling, denitrification and reducing NO₃⁻ release. Three-month laboratory simulations and nine-month field in-situ experiments demonstrated significant nutrient control. In the field trials, one month after application, the SRP concentration in the water column decreased to 0.006 mg/L (a 75 % reduction), and the NO₃⁻ concentration decreased to 0.15 mg/L (an 87.5 % reduction), with greatly improved water transparency and no cyanobacterial blooms. This study provides a novel design concept for a nutrient passivator that involves clean production, balanced input-output, and full mobilization of microbial metabolism. The properties, mechanisms, and effects of the material were comprehensively explored and verified from various aspects. This material can provide a new option for controlling endogenous pollution in eutrophic lakes.

Linking phosphorus dynamics with hypereutrophic conditions on the millennial scale: The paleolimnology of shallow and subtropical Lake Wauberg, Florida, USA

Eutrophication and harmful algal blooms pose a global challenge to water quality and ecosystem services. Whereas eutrophication has been linked to nutrient additions in conjunction with human activities, much less is known about water quality trends when nutrient additions persist for centuries or millennia. Here, we used paleolimnological techniques to reconstruct eutrophication and cyanobacteria dynamics in Lake Wauberg, FL, USA, a lake that has experienced millennial-scale nutrient additions from natural phosphate geology. We measured photosynthetic pigments, cyanotoxins, and nutrient concentrations on a sediment core spanning the last ~6900 years. Our primary hypothesis is that the long-term total phosphorus (TP) additions caused constant cyanobacteria dominance throughout the entire history of the lake. Focusing on the last 5 ka BP with constant lake conditions, photosynthetic pigments and cyanotoxins demonstrated a strong positive relationship with TP over other nutrients. By dividing TP inputs into three levels, primary producers positively increased with low TP inputs but showed no change under moderate levels. Under high (2.2-3 mg g-1) and extreme (>3 mg g-1) TP sedimentary concentrations over the last 0.3 ka BP, substantial increases in cyanobacteria abundance, rapid production of microcystins (MCs), and a possible shift to N-fixation occurred. These data show that chronic and additive TP inputs can produce asynchronous responses in the primary producer community and MC concentrations with substantial increases occurring at higher TP thresholds. Linking the historic ecological response to TP periods with current limnological conditions could provide new directions in forecasting and managing aquatic ecosystems that experience chronic TP inputs.

Effectively mitigated eutrophication risk by strong biological carbon pump (BCP) effect in karst reservoirs

Karst reservoirs can significantly enhance the effect of biological carbon pump (BCP), a crucial process for carbon sequestration, water purification, and eutrophication mitigation. However, the effects of BCP on the fate of carbon (C), nitrogen (N), and phosphorus (P) and its role in regulating eutrophication within river-reservoir systems, remains insufficiently understood, particularly across different geological settings. We investigated the Hongfeng Reservoir (HFR), a typical karst reservoir, analyzing water chemistry, nutrient concentrations, and stable isotopes of dissolved inorganic carbon (δ13CDIC) and nitrate (δ15N-NO3-) to uncover the underlying mechanisms governing the migration of biogenic elements and the process of eutrophication. Our findings reveal a strong BCP effect in the reservoirs that leads to substantial CO2 and HCO3- uptake via phytoplankton photosynthesis during the warm-wet season, resulting in decreased dissolved inorganic carbon (DIC) concentrations and increased pH in the epilimnion. The δ13CDIC (-4.0 ± 0.5 ‰) values in the epilimnion relatively increased in response to phytoplankton photosynthesis that preferentially absorbs the lighter isotope of 12C. Compared with the inflow, the δ15N-NO3- (7.4 ± 0.2 ‰) in the epilimnion of the reservoir is significantly depleted, with the water predominantly aerobic or oxygen-supersaturated. This suggests that nitrification is the dominant process during the warm-wet season. The high NO3- concentrations (44.3 ± 10.1 μmol/L) indicate a sufficient N supply for biological uptake. The strong BCP effects in the epilimnion convert substantial amounts of DCO2 and nutrients into autochthonous organic matter. The resulting increase in pH further reduces the availability of DCO2. Furthermore, BCP-induced calcium carbonate precipitation enhances P removal through co-precipitation, thereby accelerating nutrient depletion and carbon sequestration, which collectively contribute to the mitigation of eutrophication risks. To assess the broader applicability of these findings, we analyzed data from 129 lakes and reservoirs globally. Our results show that karst reservoirs, with their strong BCP effect, exhibit an average Carlson trophic status index (CTSI) 9.8 % lower than non-karst reservoirs, indicating a reduced risk of eutrophication. These insights offer valuable implications for the management of water resources in karstic reservoirs globally.

Lake restoration techniques: A review of methods and future pathways

The aim of this study was to focus on restoration techniques that have been extensively researched and found to be applicable to aquatic ecosystems and those that are in the conceptual or experimental stage. A description of the method is presented objectively and the factors that may favour or hinder achieving sustainable improvements in water quality are detailed. All the information is tabulated as a compendium of knowledge not only for scientists, but also for water managers and lake users. On the basis of the analysis of the review material, an optimisation of restoration methods involving the implementation of hybrid systems is proposed. The proposed solutions consist of combining restoration techniques in the lake with nature-based solutions in the lakeshore zone (e.g. high-performance buffer zones). This approach makes it possible to reduce external sources of pollution and thus provides an opportunity to optimise existing solutions. This is important from the point of view of the environmental and economic/social effect. Achieving a high environmental quality, improves the health and well-being of society and creates opportunities for personal and social development. Future restoration requires not only proven engineering and biological methods, but also a holistic approach that encourages all sections of society to become actively involved in water monitoring and restoration programmes.

Comparative secretome and proteome analysis unveils the response mechanism in the phosphorus utilization of Alexandrium pacificum

Phosphorus (P) acts as a crucial limiting nutrient for the growth of marine phytoplankton cells and the formation of algal blooms. The dinoflagellate Alexandrium pacificum is known for causing frequent and intense blooms in specific estuarine and coastal regions. In this study, we investigated the growth and physiological transformations under conditions characterized by P-deficiency, NaH2PO4, and ATP. For the first time, an integrated comparative analysis of the secretome and proteome was performed to investigate the global protein expression profile of A. pacificum, with 355 and 2308 differentially expressed proteins (DEPs), respectively. The results demonstrated that P-deficiency led to a reduction in growth and notable decreases in metabolic processes in A. pacificum. In P-deficient and ATP groups, the expression of secretory protein alkaline phosphatase A (PhoA) was increased, while intracellular acid phosphatase (ACP) displayed significant upregulation in P-deficient group, indicating that A. pacificum has evolved multiple organic P utilization strategies to adapt to low-P environments. A. pacificum can utilize the intracellular carbohydrate storage pools via glycolysis and the TCA cycle to replenish Calvin cycle intermediates. However, the growth of the ATP and NaH2PO4 groups showed no significant alteration. These results suggest that A. pacificum possesses distinct adaptive strategies towards P-deficiency in the environment and employs specific mechanisms for utilizing organic P, which may be a crucial factor in the formation of blooms in low inorganic P environments.

Phytoplankton abundance and methane emissions are minimally impacted by environmentally-relevant glyphosate concentrations in small-scale outdoor mesocosms

Glyphosate is one of the most widely applied agrochemicals in North America and can be directly transported via runoff into non-target aquatic habitats. Yet, our understanding of how this herbicide affects aquatic ecosystems is limited; past studies have often focused on single species effects and/or used herbicide concentrations several orders of magnitude higher than what has been reported in contaminated aquatic systems. Further, glyphosate in aquatic systems has the potential to alter greenhouse gas emissions (methane) if it is broken down for phosphate utilization by bacteria under specific environmental conditions (i.e., oxygen, nutrient concentrations). In this study, we assessed the temporal changes in nutrients, phytoplankton, copy number of genes associated with breakdown of glyphosate or production of methane (phnJ, mcrA), and methane concentrations in 12-day mesocosms with amendments of glyphosate, nitrogen, and/or phosphorus. We found glyphosate at environmentally-relevant concentrations (∼4 ug/L) did not confer changes in overall ratios and total concentrations of nutrients, or abundance of any major phytoplankton group (cyanobacteria, diatoms, green algae), methane concentration or flux, or gene copy numbers of phnJ and mcrA. Our results suggest that the relatively low concentrations of glyphosate we used (relative to levels used in toxicological studies) did not cause major changes over short time periods in mesocosms, and that the potential for glyphosate to increase greenhouse gas emissions in aquatic systems requires specific conditions to occur and may not be universal in contaminated systems.

Environmental controls on the conversion of nutrients to chlorophyll in lakes

Anthropogenic inputs of nitrogen and phosphorus to lakes have increased worldwide, causing phytoplankton chlorophyll concentrations to increase at many sites, with negative implications for biodiversity and human usage of lake resources. However, the conversion of nutrients to chlorophyll varies among lakes, hindering effective management actions to improve water quality. Here, using a rich global dataset, we explore how the relationship between chlorophyll-a (Chla) and nitrogen and phosphorus and inferred nutrient limitation is modified by climate, catchment, hydrology and lake characteristics. Phosphorus was the dominant control in oligotrophic/mesotrophic lakes, both nitrogen and phosphorus co-limitations were dominant in (hyper)eutrophic lakes, apart from hypereutrophic shallow lakes, where nitrogen was the main limiting factor. A generalized additive model of Chla vs nutrients identified a sigmoidal-type relationship with clear breakpoints between Chla and nutrients in all depth-dependent lake categories, except for nitrogen in shallow lakes. The model revealed that Secchi depth, as the predominant factor explaining the residuals, followed by the lake thermal region, elevation, and maximum depth. Lake shoreline slope, hydraulic retention time, mean depth, shoreline length, and watershed area were also statistically significant drivers for deep lakes. Surface area was only significant in shallow lakes, as it directly affects surface heating and surface contact with the wind, resulting in non-significant impact of thermal region in shallow lakes. These findings provide new insights into the response of global lake eutrophication and its main drivers, which could assist lake managers and policy-makers in mitigating widespread lake eutrophication.

Mitigation of Cadmium Uptake in Bread Wheat (Triticum aestivum L.) and Durum Wheat (Triticum durum L.) with Natural and Enriched Bentonite Treatments

Soil pollution by heavy metals is a significant issue impacting food security and human health. Cadmium, a toxic metal, contaminates soils via industrial and agricultural activities, posing risks to the food chain. This study aimed to evaluate methods for reducing cadmium bioavailability in bread wheat and durum wheat, crucial crops for human nutrition grown on contaminated soils. A greenhouse experiment was conducted in which soil samples were treated with 3-6% natural bentonite and sodium-enriched bentonite and contaminated with 5 and 10 ppm cadmium. Compared to controls, cadmium bioavailability in bread wheat decreased by 55% with 5 ppm of Cd and by 66% with 10 ppm of Cd when treated with 6% sodium-enriched bentonite. Similarly, in durum wheat, cadmium bioavailability decreased by 55% and 48% at 5 and 10 mg Cd kg-1, respectively. Additionally, 6% natural and enriched bentonite applications increased biomass production in both wheat varieties. Bread wheat dry matter increased by 43.69% with 5 ppm of Cd and natural bentonite, while durum wheat showed an increase of 88.66% with 10 ppm of Cd and enriched bentonite. In bread wheat, the highest B concentration was obtained with 6% NB at 5 and 10 ppm of Cd, with increases of 15.5%, 39.53%, and 16.56% compared to controls; similar increases were seen in durum wheat. Ca concentrations increased with Cd application in control samples, whereas Mn concentrations decreased with Cd and bentonite treatments. The highest Na concentrations in both wheat varieties were recorded at 6% EB, resulting in significant increases (bread wheat: 2434%-4126%; durum wheat: 2763%-3592%) compared to controls. Nutrient stability for Fe, Cu, K, Mg, P, and Zn varied according to Cd dose and bentonite type. The addition of natural and sodium-enriched bentonite effectively reduced cadmium bioavailability in bread and durum wheat, while promoting increased biomass production. These findings suggest that bentonite amendments have potential applications for enhancing crop yields and ensuring food safety in cadmium-contaminated environments.

Macronutrients-availing microbiomes: biodiversity, mechanisms, and biotechnological applications for agricultural sustainability

Nitrogen, phosphorus, and potassium are the three most essential micronutrients which play major roles in plant survivability by being a structural or non-structural component of the cell. Plants acquire these nutrients from soil in the fixed (NO3¯, NH4+) and solubilized forms (K+, H2PO4- and HPO42-). In soil, the fixed and solubilized forms of nutrients are unavailable or available in bare minimum amounts; therefore, agrochemicals were introduced. Agrochemicals, mined from the deposits or chemically prepared, have been widely used in the agricultural farms over the decades for the sake of higher production of the crops. The excessive use of agrochemicals has been found to be deleterious for humans, as well as the environment. In the environment, agrochemical usage resulted in soil acidification, disturbance of microbial ecology, and eutrophication of aquatic and terrestrial ecosystems. A solution to such devastating agro-input was found to be substituted by macronutrients-availing microbiomes. Macronutrients-availing microbiomes solubilize and fix the insoluble form of nutrients and convert them into soluble forms without causing any significant harm to the environment. Microbes convert the insoluble form to the soluble form of macronutrients (nitrogen, phosphorus, and potassium) through different mechanisms such as fixation, solubilization, and chelation. The microbiomes having capability of fixing and solubilizing nutrients contain some specific genes which have been reported in diverse microbial species surviving in different niches. In the present review, the biodiversity, mechanism of action, and genomics of different macronutrients-availing microbiomes are presented.

Multivariable integrated risk and spatiotemporal characteristics assessment for water quality using comprehensive risk index in Jiangsu section of the south-to-north water diversion project, China

Water quality serves as a key indicator for assessing the effectiveness and success of water diversion efforts, yet comprehensive assessments of water pollution risk across interbasin areas remain scarce. Therefore, considering water quality standards, water eutrophication, and drinking water safety, a comprehensive water quality risk inedx (CRI) was developed by integrating the Comprehensive Pollution Index (CPI), Eutrophication Index (TLI), and New Water Quality Index (NWQI). This framework was designed to determine the spatiotemporal risk levels at a spatial scale that crosses large river basins. The Jiangsu section, pivotal water supply source for the eastern route of the South-to-North Water Diversion Project (SNWDP-ER), was selected as the study area. In addition, based on the long-term (2003-2022) water quality monitoring data, multiple statistical methods were used to evaluate the effects of the SNWDP-ER on water quality within the lakes and main reaches of the Jiangsu Section. Variance analysis revealed significant differences in water quality pre- and post-water transfer and the operation of SNWDP-ER has contributed to the improvement of overall water quality. Principal Component Analysis (PCA) identified the predominant parameters affecting water quality was converted from CODMn to NH3-N and TP after water transfer and the main objective of water quality control in the study area is agricultural non-point source pollution. The comprehensive water quality risk assessment identified three groups with distinct risk levels and the impact of water transfer project on the water quality risk of reaches was gradually increasing from upstream to downstream. This study greatly enhanced our understanding of the impacts of large-scale water transfer projects on water quality and proposed a comprehensive framework for assessing spatiotemporal risk levels across extensive river basins.

Combining sequential extractions with bulk and micro X-ray spectroscopy to elucidate iron and phosphorus speciation in sediments of an iron-treated peat lake

In shallow lakes, mobilization of legacy phosphorus (P) from the sediments can be the main cause for persisting eutrophication after reduction of external P input. In-lake remediation measures can be applied to reduce internal P loading and to achieve ecosystem recovery. The eutrophic shallow peat lake Terra Nova (The Netherlands) was treated with iron (Fe) to enhance P retention in the sediment. This treatment, however, intensified seasonal internal P loading. An earlier study suggested that Fe addition led to increased P binding by easily-reducible Fe(III) associated with organic matter (OM), which readily releases P when bottom waters turn hypoxic. In this complementary study, bulk and micro Fe K-edge and P K-edge X-ray absorption spectroscopy and micro-focused X-ray fluorescence spectroscopy were applied to characterize the P hosting Fe(III) pool. Combined with sequential extraction data, the synchrotron X-ray analyses revealed that a continuum of co-precipitates of Fe(III) with calcium, phosphate, manganese and organic carbon within the OM matrix constitutes the reducible Fe(III) pool. The complementary analyses also shed new light on the interpretation of sequential extraction results, demonstrating that pyrite was not quantitatively extracted by nitric acid (HNO3) and that most of the Fe(II) extracted by hydrochloric acid (HCl) originated from phyllosilicate minerals. Formation of an amorphous inorganic-organic co-precipitate upon Fe addition constitutes an effective P sink in the studied peaty sediments. However, the high intrinsic reactivity of this nanoscale co-precipitate and its fine distribution in the OM matrix makes it very susceptible to reductive dissolution, leading to P remobilization under reducing conditions.

Towards sustainable agroecosystems: A life cycle assessment review of soil-biodegradable and traditional plastic mulch films

The increasing use of traditional agricultural plastic mulch films (PMs) has raised significant environmental concerns, prompting the search for sustainable alternatives. Soil-biodegradable mulch films (BDMs) are often proposed as eco-friendly replacements; however, their widespread adoption remains contentious. This review employs a comparative life cycle assessment perspective to evaluate the environmental impact of PMs and BDMs across their production, use, and end-of-life stages, providing strategies to mitigate their impact on agroecosystems. BDMs generally exhibit lower energy use and greenhouse gas emissions than PMs but contribute to greater land-use demands. Reported eutrophication and acidification potentials are less consistent, varying based on feedstock types and the scope of assessment of BDM, as well as the end-of-life management of PM. The environmental burden of both mulch types is influenced by the life cycle stage, polymer composition, farming practices, additives, film thickness, and local climatic conditions. The manufacturing stage is a major contributor to energy use and greenhouse gas emissions for both PMs and BDMs, despite their shared benefits of increasing crop yields. However, post-use impacts are more pronounced for PMs, driven by end-of-life strategy and adsorbed waste content. While starch-based BDMs offer a more sustainable alternative to PMs, uncertainties regarding the residence time of BDM residues in soil (albeit shorter than PM residues) and their effects on soil health, coupled with higher production costs, impede widespread adoption. For BDM end-of-life, soil biodegradation is recommended. Energy and material recovery options are crucial for PM end-of-life, with mechanical recycling preferred, although it requires addressing eutrophication and human toxicity. This review discusses these complexities within specific contexts and provides actionable insights to guide the sustainable integration of mulch films into agricultural practices.

Dominant Dolichospermum and microcystin production in Detroit Lake (Oregon, USA)

The excessive growth of harmful cyanobacteria, including Dolichospermum (formerly known as Anabaena), in freshwater bodies has become a pressing global concern. However, detailed information about the role of Dolichospermum in shaping bloom dynamics and producing cyanotoxins is limited. In this study, a bloom event dominated by Dolichospermum spp. at Detroit Lake (Oregon, USA) was examined from 2019 to 2021. In 2019, early summer cyanobacterial community succession reached up to 8.7 % of total phytoplankton abundance. Dolichospermum was the major microcystin (MC)-producing genus, with peak MC levels of 7.34 μg L-1. The presence of MCs was strongly correlated with the abundance of Dolichospermum (r = 0.84, p < 0.05) and MC synthetase gene, mcyE-Ana (r = 0.63, p < 0.05). Metabolic analyses further showed that the presence of nif/pst genes linked to nitrogen and phosphorus metabolism was dominated by Dolichospermum from the bloom onset until September. In addition, the abundance of Dolichospermum was significantly correlated with the abundance of nitrogen-fixing nif-Ana gene (r = 0.62, p < 0.05). As the lake experienced a longer N and P scarcity period (May to September), the N2-fixing Dolichospermum was able to dominate over other non-fixing cyanobacteria present, including Microcystis and Planktothrix. Overall, our results facilitate a better understanding of the organism and will help working toward managing/predicting future blooms.

Population and community ecology: past progress and future directions

Population and community ecology as a science are about 100 years old, and we discuss here our opinion of what approaches have progressed well and which point to possible future directions. The three major threads within population and community ecology are theoretical ecology, statistical tests and models, and experimental ecology. We suggest that our major objective is to understand what factors determine the distribution and abundance of organisms within populations and communities, and we evaluate these threads against this major objective. Theoretical ecology is elegant and compelling and has laid the groundwork for achieving our overall objectives with useful simple models. Statistics and statistical models have contributed informative methods to analyze quantitatively our understanding of distribution and abundance for future research. Population ecology is difficult to carry out in the field, even though we may have all the statistical methods and models needed to achieve results. Community ecology is growing rapidly with much description but less understanding of why changes occur. Biodiversity science cuts across all these subdivisions but rarely digs into the necessary population and community science that might solve conservation problems. Climate change affects all aspects of ecology but to assume that everything in population and community ecology is driven by climate change is oversimplified. We make recommendations on how to advance the field with advice for present and future generations of population and community ecologists.

Nitrogen fixation may not alleviate stoichiometric imbalances that limit primary production in eutrophic lake ecosystems

Ecosystem-scale primary production may be proximately limited by nitrogen (N) but ultimately limited by phosphorus (P) because N2 fixation contributes new N that accumulates relative to P at ecosystem scales. However, the duration needed to transition between proximate N limitation and ultimate P limitation remains unknown for most ecosystems, including lakes. Here we present the results of a fully replicated, multi-annual lake mesocosm experiment that permitted full air-water-sediment interactions that mimicked lake ecosystem ecology. We manipulated N supply relative to P to achieve a gradient of N:P stoichiometry. Despite N2 fixation contributing as much as 80% of reactive N in the low N treatments, phytoplankton biomass in these treatments was not different from the unfertilized controls. This suggests that primary production remained N limited in the lowest N treatments, even when N2 fixation was substantial. Although fixed N inputs reduced the N imbalance relative to P in the low N treatments seasonally, fixed N did not accumulate over multiple years. Additionally, reactive N did not readily accumulate in the high N treatments. Instead, water column stoichiometry was proportional to the experimental N and P additions, suggesting a strong influence from external nutrient loading. Thus, we found no evidence that N accumulation from N2 fixation was sufficient to trigger a transition to ultimate P limitation seasonally or across our 3-year experiment. Rather, our results indicate that proximate N limitation perpetuates in eutrophic lakes, likely due to N export being proportional to its inputs. These findings offer new insight regarding the biogeochemical controls on ecosystem stoichiometry and their influence on the timeframe for proximate N limitation and ultimate P limitation in freshwater, marine, and terrestrial ecosystems.

External nitrogen influxes hinder the efficacy of lanthanum-modified bentonite (LMB) on phosphorus and algae control in shallow lakes

Regulating internal and external phosphorus (P) holds a predominant position in eutrophication management of lakes and other water bodies, with less emphasis on controlling nitrogen (N) due to the presence of N2-fixing cyanobacteria. Nonetheless, external N influxes may stimulate the proliferation of non-N2-fixing cyanobacteria, thereby fostering cyanobacteria blooms during summer seasons. To elucidate the significance of N regulation, a two-factor orthogonal experiment was performed to study the influences of external N input on the efficacy of lanthanum-modified bentonite (LMB), a sediment capping material for P immobilization. At the experimentation ends, the total suspended solids (TSS), organic suspended solids (OSS) concentrations and optical attenuation coefficient (Kd) in the LMB + N treatment were 7.34, 8.65 and 5.20 times higher, respectively, compared to the LMB treatment. The total nitrogen (TN), total phosphorus (TP) and soluble reactive phosphorus (SRP) concentrations in the LMB + N treatment were 3.02, 1.30 and 0.60 times higher, respectively, than those in the LMB treatment. However, TP and SRP in the LMB + N treatment were 46.98% and 54.93% lower, respectively, compared to N treatment. The chlorophyll a (Chl a) concentration of algae in the LMB + N treatment was observed to be 2.86 times higher compared to the LMB treatment, and 17.13% lower compared to the N treatment. The biomass of cyanobacteria accounted for more than 95% of algae in the LMB + N treatment and N treatment. Furthermore, the photosynthetic performance of algae in the N treatment increased significantly, compared to the LMB + N treatment. Our results indicated that external N influxes significantly reduce the efficacy of LMB to control P and algae. Thus, the implementation of more stringent N control policies holds great significance in the eutrophication control.

Dual nitrogen and phosphorus reductions are needed for long-term mitigation of eutrophication and harmful cyanobacterial blooms in the hydrologically-variable San Francisco Bay Delta, CA

Cyanobacterial harmful algal blooms (CyanoHABs) are a major concern for water quality, public health and viability of aquatic ecosystems. Increased inputs of nutrients, i.e., nitrogen (N) and phosphorus (P), are known to amplify the occurrence, severity, and duration of CyanoHABs. There is growing concern that CyanoHABs are proliferating along the freshwater to marine continuum, including throughout estuaries. We assessed the influence of nutrient enrichment on the abundance and composition of CyanoHABs and accompanying phytoplankton communities in the San Francisco Bay Delta (SFBD) estuarine ecosystem, a vital resource for California's water supply, fisheries, and recreation. In situ nutrient addition bioassays were conducted in June and September 2022, at the end of a record three-year drought period, and May and August 2023, an extremely high rainfall and discharge year. Water was collected from two locations in the SFBD recognized for having CyanoHAB issues, Discovery Bay (DB) and the Stockton Channel (STK). Both sites showed the highest proportion of cyanobacteria in the total phytoplankton community biomass during summer months, and this was particularly noticeable at STK. In June 2022, additions of N and N+P were both shown to increase overall phytoplankton biomass in DB and N+P specifically stimulated cyanobacteria. P alone was not stimulatory. In September 2022, NH4 promoted the growth of cyanobacteria faster than NO3, particularly in DB communities. A similar set of responses to N occurred in 2023 in DB, despite major differences in freshwater input between years. In 2022, nutrient additions had no significant stimulatory effects on STK phytoplankton communities, suggesting nutrients were replete throughout the bloom season. However, in 2023 N limitation became more evident in STK, likely due to a dilution effect from the very high freshwater discharge from a record snowpack and reservoir releases, ultimately changing the availability of inorganic N during the CyanoHAB growth period. The combined effect of high flow and nutrient dilution in 2023 was responsible for the reduced CyanoHAB potential. By examining these key differences between seasons in these hydrologically contrasting years, it appears that internal supplies of "legacy P" ensure P availability throughout the summer bloom season regardless of hydrologic variability, while N enrichment plays a key role in stimulating algal production and CyanoHABs under hydrologically variable conditions. Once N was added, P further stimulated biomass production in some cases, indicating potential N+P co-limitation. We conclude that under varying hydrologic conditions, long-term dual N and P input reductions are needed to control eutrophication and CyanoHAB outbreaks throughout the SFBD.

Dual phosphorus and nitrogen nutrient reduction will be more effective than a phosphorus-only reduction in mitigating diatom and cyanobacterial booms in Lake Erie, USA-Canada

Lake Erie, USA-Canada, plays an important ecological and socioeconomic role but has suffered from chronic eutrophication. In particular, Western Lake Erie (WLE) is the site of harmful algal blooms (HABs) which are suspected of being driven by excessive nutrient (phosphorus (P) and nitrogen (N)) inputs. During 2022 and 2023, in-situ nutrient dilution and addition bioassays were conducted at a WLE bloom-impacted location to investigate whether a nutrient reduction regime would be effective in limiting phytoplankton growth during the June diatom-dominated spring blooms and August cyanobacteria-dominated summer blooms. The primary objectives of this experiment were to 1) Determine if a proposed 40% P-alone reduction would effectively reduce phytoplankton growth and mitigate blooms and 2) assess whether reductions in both P and N are more effective in controlling phytoplankton biomass than exclusive reductions in either N or P. Samples were analyzed for nutrient concentrations and growth rate responses for specific algal groups utilizing diagnostic (for major algal groups) photopigments. Results indicated that although both 20% and 40% dilutions led to lower phytoplankton biomass and growth rates, 40% reductions were more effective. Our results support the USA-Canada Great Lakes Water Quality Agreement recommendation of a 40% P reduction, but also indicate that a parallel reduction of N input by 40% would be most effective in controlling bloom magnitudes. Overall, our findings underscore the recommendation that a year-round dual N and P 40% reduction is needed for long-term control of eutrophication and algal blooms, including HABs and diatoms, in Lake Erie.

Microbial nitrogen nutrition links to dissolved organic matter properties in East African lakes

East African lakes, especially soda lakes, are home habitats for massive numbers of wildlife such as flamingos, mammals, and fishes. These lakes are known for their high primary production due to local high temperatures, light intensities, and alkalinity (inorganic carbon). However, these lakes, normally within remote areas, receive low nutrient inputs. Ammonium (NH4+) recycling and/or nitrogen fixation can become the major N supply mechanisms for phytoplankton. However, the driving forces on microbial N nutrition in lakes with minimal anthropogenic disturbance remain poorly understood. Using stable isotope tracer techniques, NH4+ recycling rates were measured in 18 lakes and reservoirs in East Africa (Tanzania and Kenya) during the dry season in early 2020. Three functional genes (nifH, gdh, and ureC) relating to microbial N nutrition were also measured. The regeneration of NH4+ supported up to 71 % of the NH4+ uptake. Positive community biological NH4+ demands (CBAD) for all lakes and reservoirs indicate an obvious N demand from microbial community. Our study provides clear evidence that microbial NH4+ uptake rates linked closely to the dissolved organic matter (DOM) properties (e.g., the absorption coefficient at 254 nm, percents of total fluorescence intensity contributed by microbial humic-like and protein-like components) and that water residence time drives microbial NH4+ recycling by regulating the duration of in-lake DOM processing and influencing algal growth. Phytoplankton, especially those of Cyanophyceae, showed maximum biomass and higher NH4+ recycling rates at a certain range of water residence time (e.g., 5-8 years). However, CBAD showed a decreasing trend with longer water residence time, which may be influenced by changes in the algal community composition (e.g., % Cyanophyceae vs. % Bacillariophyceae). These results indicate that DOM dynamics and the water residence time have the potential to facilitate the understanding of microbial nitrogen supply status in East African lakes.

Sewage-and fertilizer-derived nutrients alter the intensity, diversity, and toxicity of harmful cyanobacterial blooms in eutrophic lakes

Cyanobacterial harmful algal blooms (CHABs) are promoted by excessive nutrient loading and, while fertilizers and sewage are the most prevalent external nutrient sources in most watersheds, the differential effects of these nutrient sources on CHABs are unknown. Here, we tracked CHABs and performed experiments in five distinct lakes across the Northern US including Lake Erie. Fertilizers with ammonium and orthophosphate, membrane (0.2 μm)-filtered sewage (dominated by reduced forms of nitrogen) sand-and membrane-filtered sewage (dominated by nitrate), and an inorganic nutrient solution of ammonium and orthophosphate were used as experimental nutrient sources for CHABs at N-equivalent, environmentally realistic concentrations. Phytoplankton communities were evaluated fluorometrically, microscopically, and via high throughput sequencing of the 16S rRNA gene, and levels of microcystin and the δ15N content of particulate organic nitrogen (δPO15N) were quantified. Fertilizer and both sources of wastewater increased the abundance of cyanobacteria in all experiments across all five lakes (p < 0.05 for all) whereas effects on eukaryotic phytoplankton were limited. Sand-filtered sewage contained less P, organic matter, and ammonium but more nitrate and had a 25% less potent stimulatory effect on cyanobacteria than membrane-filtered sewage, suggesting nitrification may play a role in reducing CHABs. Fertilizer increased microcystin levels and decreased the δPO15N whereas wastewater increased δPO15N (p < 0.05 for all). Microcystis was the genus most consistently promoted by nutrient sources (p < 0.05 in all experiments), followed by Cyanobium (p < 0.05 in 50% of experiments), with increases in Microcystis biomass consistently elicited by membrane-filtered wastewater. Collectively, results demonstrate that differing types of sewage discharge and fertilizers can promote CHAB intensity and toxicity, while concurrently altering CHAB diversity and δPO15N. While membrane-filtered sewage consistently favored Microcystis, the discharge of sewage through sands muted bloom intensity suggesting sand-beds may represent a tool to remove key nutrients and partially mitigate CHABs.

Spatial heterogeneity in sediment phosphorus pools and phosphatase activity in a eutrophic reservoir

Agriculture is necessary for food production, but agricultural inputs of phosphorus (P) to waterways can lead to harmful algal blooms in downstream reservoirs. Some of the P that enters these water bodies can be stored in reservoir sediments and later contribute to internal P loading, supplementing external P loads carried in from rivers. Increased P can lead to harmful algal blooms. However, how P is cycling in the sediment of these water bodies varies spatially and temporally has been relatively unstudied. Our objective was to understand how P concentration and form vary spatiotemporally, as well as how P is processed in the sediment of the reservoir. We sampled 30 locations in both August and October 2018 around Milford Reservoir (Kansas), a man-made eutrophic reservoir with frequent harmful algal blooms. We collected water chemistry samples, field measurements of temperature, dissolved oxygen, and pH, and sediment samples to analyze for P chemical speciation and phosphatase enzyme activity. We show that P release by phosphatase activity was higher under anaerobic and basic conditions, which subsequently affects spatiotemporal variation in sediment P pools. We found that low oxygen positively influenced phosphatase activity and sediment P pools, and may drive high internal P loading and harmful algal blooms in the summer months. This research increased our understanding of P cycling in a reservoir highly impacted by agricultural inputs and contributed to a small but growing body of research on internal P loading in midwestern reservoirs.

Spatially Explicit Impact of Land Use Changes in the Bay Area on Anthropogenic Phosphorus Emissions and Freshwater Eutrophication Potential

Land use changes significantly impact anthropogenic phosphorus (P) emissions, their migration to a water environment, and the formation of freshwater eutrophication potential (FEP), yet the spatiotemporally heterogeneous relationships at the regional scale have been less explored. This study combines land use classification, P-flow modeling, spatial analysis, and cause-effect chain modeling to assess P emissions and P-induced FEP at a fine spatial resolution in Guangdong-Hong Kong-Macao Greater Bay Area and reveals their dynamic responses to land use changes. We find that land conversion from cultivated land to impervious land corresponded to an increase in P emissions of 4.1, 1.8, and 0.5 Gg during 2000-2005, 2005-2010, and 2010-2015 periods, respectively, revealing its dominant but weakening role in the intensification of P emissions especially in less-developed cities. Expansion of aquacultural land gradually became the primary contributor to the increase in both the amount and intensity of P emissions. Land conversions from cultivated land to impervious land and from natural water bodies to aquacultural land led to 35.9% and 25.3% of the increase in FEP, respectively. Our study identifies hotspots for mitigating the environmental pressure from P emissions and provides tailored land management strategies at specific regional development stages and within sensitive areas.

Intensified effect of nitrogen forms on dominant phytoplankton species succession by climate change

Nutrient proportion, light intensity, and temperature affect the succession of dominant phytoplankton species. Despite these insights, this transformation mechanism in highly turbid lakes remains a research gap, especially in response to climate change. To fill this gap, we investigated the mechanism by which multi-environmental factors influence the succession of dominant phytoplankton species in Lake Chagan. This investigation deployed the structural equation model (SEM) and the hydrodynamic-water quality-water ecology mechanism model. Results demonstrated that the dominant phytoplankton species in Lake Chagan transformed from diatom to cyanobacteria during 2012 and 2022. Notably, Microcystis was detected in 2022. SEM revealed the primary environment variables for this succession, including water temperature (Tw), nutrients (total nitrogen (TN), total phosphorus (TP), and ammonia nitrogen (NH4N)), and total suspended solids (TSS). Moreover, this event was not the consequence of zooplankton grazing. An integrated hydrodynamic-water quality-bloom mechanism model was built to explore the mechanism driving phytoplankton succession and its response to climate change. Nutrients determined the phytoplankton biomass and dominant species succession based on various proportions. High NH4N:NO3N ratios favored cyanobacteria and inhibited diatom under high TSS. Additionally, the biomass proportions of diatom (30.77 % vs. 22.28 %) and green (30.56 % vs. 23.30 %) decreased dramatically. In contrast, cyanobacteria abundance remarkably increased (35.78 % to 51.71 %) with the increasing NH4-N:NO3-N ratios. In addition, the proportion of non-nitrogen-fixing cyanobacteria was higher than that of the nitrogen-fixing cyanobacteria counterparts when TN:TP≥20 and NH4N:NO3N ≥ 10. Light-limitation phenotypes also experienced an increase with the rising NH4N:NO3N ratios. Notably, the cyanobacterial biomass reached 3-6 times that in the baseline scenario when the air temperature escalated by 3.0 °C until 2061 under the SSP585 scenario. We highlighted the effect of nitrogen forms on the succession of dominant phytoplankton species. Climate warming will increase nitrogen proportion, providing an insightful reference for controlling cyanobacterial blooms.

The influence of redox potential on phosphorus release from sediments in different water bodies

Anthropogenic activities have significantly enriched P in sediments of many water bodies, with redox potential (Eh) being a key factor in controlling P adsorption or release.This study evaluates the impact of Eh on P release from sediments in the Weiyuan River, Honghu Lake, and Bao'enqiao Reservoir using reactor experiments. P speciation was further analyzed through SEDEX method. Results show that within an Eh range of -300 mV to +230 mV, more P is released from sediments into the water column. The P fractions CDB-P and Fe(II)-P exhibit the most significant changes, especially in reservoir sediments where ΔCDB-P (85.5 mg/kg) and ΔFe(II)-P (80.6 mg/kg) are the highest among the three water bodies, followed by lake sediments. Additionally, after redox oscillation, the EPC0 of lake and reservoir sediments increased to 16.2 and 18.8 times their initial values, respectively, significantly raising the risk of eutrophication.

Molecular characterization on the fractionation of organic phosphorus induced by iron oxide adsorption using ESI-FT-ICR MS

The interaction between organic phosphorus (OP) and iron oxide significantly influences the phosphorus cycle in the natural environment. In shallow lakes, intense oxidation-reduction fluctuations constantly alter the existing form of iron oxides, but little is known about their impact on the adsorption and fractionation of OP molecules. In this study, electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR MS) was used to investigate the fractionation of OP from alkali-extracted sediment induced by crystalline goethite and amorphous ferrihydrite adsorption at a molecular scale. The results showed that ferrihydrite and goethite both exhibited high OP adsorption, and the adsorption amount decreased as the pH increased. The adsorption kinetics matched the pseudo-second-order equation. The ESI-FT-ICR MS analysis showed that 91 P-containing formulas were detected in the alkaline-extracted sediment solution. Ferrihydrite and goethite adsorbed 51 and 24 P-containing formulas, respectively, with adsorption rates of 56.0 % and 26.4 %. Ferrihydrite could adsorb more OP compounds than goethite, but no obvious molecular species selectivity was observed during the adsorption. The P-containing compounds, including unsaturated hydrocarbons-, lignin/carboxyl-rich alicyclic molecule (CRAM)-, tannin-, and carbohydrate-like molecular compounds, were more suitable for iron oxide adsorption. The double bond equivalence (DBE) is a valuable parameter that indicates OP fractionation during adsorption, and P-containing compounds with lower DBE values such as lipid- and protein-like molecular were prone to remain in the solution after adsorption. These research results provide insights into the biogeochemical cycling process of P in the natural environment.

Exploring the influence of aquatic phosphate on Fe floc dynamics in water treatment

The formation of flocs is crucial in the coagulation process of water treatment. However, the nature of ligand exchange on the surface of primary nanoparticles (PNPs) during floc formation requires further investigation to enhance our understanding of the coagulation mechanism. Phosphate (P) is a ubiquitous nutrient ion in aquatic surface water, in this study, the impact of P on floc growth under different pH conditions were investigated. The results revealed that floc growth patterns depended on both P dosage and pH. The mode of ligand exchange between P and in-situ formed ferric hydroxide within a pH range of 5 to 10 was further explored, and remarkable disparities in pH changes induced by P addition were observed. At lower pH levels, OH- release occurred relatively slowly, stabilizing with continued P addition. At neutral pH, OH- release was comparatively higher with P addition, while under alkaline conditions, both the quantity of OH- and its release rate decreased. It was deduced that Fe-OH21/2+ sites function as "active sites," while Fe-OH1/2- sites act as "inert sites" on the surface of PNPs formed during flocculation. These sites are crucial in the interconnections between flocs formed during coagulation and in floc growth. Analyses of Fe PNPs by Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), with and without P addition, revealed that the introduction of P inhibits or interferes with the self-crystallization of Fe PNPs through chemical coordination reactions. The results offer deeper insights into the coagulation mechanism and the transformation of Fe flocs in raw waters containing P during water treatment practices.

Examining contaminant transport hotspots and their predictability across contrasted watersheds

Hydrobiogeochemical processes governing water quantity and quality are highly variable in space and time. Focusing on thirty river locations in Québec, Canada, three water quality hotness indices were used to classify watersheds as contaminant transport hotspots. Concentration and load data for suspended solids (SS), total nitrogen (TN), and total phosphorous (TP) were used to identify transport hotspots, and results were compared across hotness indices with different data requirements. The role of hydroclimatic and physiographic characteristics on the occurrence and temporal persistence of transport hotspots was examined. Results show that the identification of transport hotspots was dependent on both the type of data and the hotness index used. Relationships between temporal and spatial predictors, however, were generally consistent. Annual transport hotspot occurrence was found to be related to temporal characteristics such as the number of dry days, potential evapotranspiration, and snow water equivalent, while hotspot temporal persistence was correlated to landcover characteristics. Stark differences in the identification of SS, TN, and TP transport hotspots were attributed to differences in mobilization processes and provided insights into dominant water and nutrient flowpaths in the studied watersheds. This study highlighted the importance of comparing contaminant dynamics across watersheds even when high-frequency water quality data or discharge data are not available. Characterizing hotspot occurrence and persistence, among hotness indices and water quality parameters, could be useful for watershed managers when identifying problematic watersheds, exploring legacy effects, and establishing a prioritization framework for areas that would benefit from enhanced routine monitoring or targeted mitigation strategies.

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.

Combining effects of submerged macrophytes and lanthanum-modified bentonite on sediment enzyme activity: Evidence from mesocosm study

Lanthanum-modified bentonite (LMB) combined with submerged macrophytes (SM) has been a conventional means of eutrophication management in lakes in recent years, and is one of the most important methods for P removal. However, trends in nutrients and sediment enzymes at the water-sediment interface during this process have not been systematically assessed, and there are still some gaps in how abiotic properties drive changes in enzyme activity. Here, we show changes in aquatic environmental conditions under the action of different ratios of modified bentonite (0, 10%, 20%, and 30%) in combination with SM (Vallisneria natans, Potamogeton lucens, and Hydrilla verticillate) and quantify their effects on sediment enzyme activities. The results showed that the nutrient cycling at the water-sediment interface was facilitated by the combined effect of SM and LMB, which effectively reduced the overlying water nutrient concentration, increased the sediment enzyme activity and enhanced the N cycling process. Partial least squares structural equation model (PLS-SEM) showed that sediment parameters strongly influenced changes in enzyme activity, with NO3-N as the main controlling factors. Our study fills in the process of changing environmental conditions in lake water under geoengineered materials combined with macrophyte measures, especially the changes in biological properties enzyme activities, which contributes to a clearer understanding of nutrient fluxes during the management of eutrophication in lakes.

Microcystis abundance is predictable through ambient bacterial communities: A data-oriented approach

The number of cyanobacterial harmful algal blooms (cyanoHABs) has increased, leading to the widespread development of prediction models for cyanoHABs. Although bacteria interact closely with cyanobacteria and directly affect cyanoHABs occurrence, related modeling studies have rarely utilized microbial community data compared to environmental data such as water quality. In this study, we built a machine learning model, the multilayer perceptron (MLP), for the prediction of Microcystis dynamics using both bacterial community and weekly water quality data from the Daechung Reservoir and Nakdong River, South Korea. The modeling performance, indicated by the R2 value, improved to 0.97 in the model combining bacterial community data with environmental factors, compared to 0.78 in the model using only environmental factors. This underscores the importance of microbial communities in cyanoHABs prediction. Through the post-hoc analysis of the MLP models, we revealed that nitrogen sources played a more critical role than phosphorus sources in Microcystis blooms, whereas the bacterial amplicon sequence variants did not have significant differences in their contribution to each other. Similar to the MLP model results, bacterial data also had higher predictability in multiple linear regression (MLR) than environmental data. In both the MLP and MLR models, Microscillaceae showed the strongest association with Microcystis. This modeling approach provides a better understanding of the interactions between bacteria and cyanoHABs, facilitating the development of more accurate and reliable models for cyanoHABs prediction using ambient bacterial data.

Effect of the ten-year fishing ban on change of phytoplankton community structure: Insights from the Gan River

The Yangtze River is one of the largest riverine ecosystems in the world and is a biodiversity hotspot. In recent years, this river ecosystem has undergone rapid habitat deterioration due to anthropogenic disturbances, leading to a decrease in freshwater biodiversity. Owing to these anthropogenic impacts, the Chinese government imposed a "Ten-year fishing ban" (TYFB) in the Yangtze River and its tributaries. Ecological changes associated with this decision have not been documented to evaluate the level of success. This study evaluates the success of the TYFB by analyzing the changes in phytoplankton communities and comparing them to periods before the TYFB was imposed. A total of 325 phytoplankton species belonging to 7 phyla and 103 genera dominated by Chlorophyceae and Bacillariophyceae were identified. Species diversity according to the Shannon-Wiener ranged between 1.19 and 3.00. The results indicated that phytoplankton diversity increased, while both density and biomass decreased after the TYFB. The health of the aquatic ecosystem appeared to have improved after the TYFB, as indicated by the phytoplankton-based index of biotic integrity. Significant seasonal and spatial differences were found among the number of species, density, and biomass of phytoplankton, where these differences were correlated with pH, water depth, chlorophyll-a, permanganate index, transparency, copper, ammonia nitrogen, and total phosphorus based on redundancy analysis. Results from this study indicate that the TYFB played an important role in the restoration of freshwater ecosystem in the Yangtze River and its tributaries.

Effects of carbon limitation and carbon fertilization on karst lake-reservoir productivity

Nitrogen and phosphorus are universally recognized as limiting elements in the eutrophication processes affecting the majority of the world's lakes, reservoirs, and coastal ecosystems. However, despite extensive research spanning several decades, critical questions in eutrophication science remain unanswered. For example, there is still much to understand about the interactions between carbon limitation and ecosystem stability, and the availability of carbon components adds significant complexity to aquatic resource management. Mounting evidence suggests that aqueous CO2 could be a limiting factor, influencing the structure and succession of aquatic plant communities, especially in karstic lake and reservoir ecosystems. Moreover, the fertilization effect of aqueous CO2 has the potential to enhance carbon sequestration and phosphorus removal. Therefore, it is important to address these uncertainties to achieve multiple positive outcomes, including improved water quality and increased carbon sinks in karst lakes and reservoirs.

The use of coagulant and natural soil to control cyanobacterial blooms in a tropical reservoir

Cyanobacterial blooms have been a growing problem in water bodies and attracted attention from researcher and water companies worldwide. Different treatment methods have been researched and applied either inside water treatment plants or directly into reservoirs. We tested a combination of coagulants, polyaluminium chloride (PAC) and iron(III) chloride (FeCl3), and ballasts, luvisol (LUV) and planosol (PLAN), known as the 'Floc and Sink' technique, to remove positively buoyant cyanobacteria from a tropical reservoir water. Response Surface Methodology (RSM) based on Central Composite Design (CCD) was used to optimize the two reaction variables - coagulant dosage (x1) and ballast dosage (x2) to remove the response variables: chlorophyll-a, turbidity, true color, and organic matter. Results showed that the combination of LUV with PAC effectively reduced the concentration of the response variables, while PLAN was ineffective in removing cyanobacteria when combined to PAC or FeCl3. Furthermore, FeCl3 presented poorer floc formation and lower removal efficiency compared to PAC. This study may contribute to the theoretical and practical knowledge of the algal biomass removal for mitigating eutrophication trough different dosages of coagulants and ballasts.

Longevity and efficacy of lanthanum-based P remediation under changing dissolved oxygen availability in a small eutrophic lake

We set out to study the seasonal variations in porewater phosphorus and lanthanum concentrations in the dated sediment cores from a small eutrophic lake that has been treated with Phoslock, a lanthanum-modified bentonite (LMB) amendment. Three sites were sampled when the hypolimnion was either oxygenated or anoxic: (i) the lake's deepest point, (ii) a littoral site receiving inflows from the catchment, and (iii) a littoral site influenced by nearby septic tanks. Phosphate (PO43--P), lanthanum (La), iron (Fe), dissolved organic carbon (DOC) and sulfate (SO42-) were measured in porewater samples. An inverse diagenetic model was used to quantify fluxes of dissolved elements across the sediment-water interface as well as the net rate of their reactions along the porewater concentration gradients. Results show that porewater P and Fe underwent strong seasonal dynamics, while La did not. P fluxes, 20-fold higher at the deepest site than elsewhere in the basin, were influenced by anoxic conditions in the hypolimnion during summer and winter, suggesting that P mobility remained sensitive to redox fluctuations despite the addition of La. At the deepest site, fluxes of P across the sediment-water interface increased from 1 to 9 × 10-9 μmol cm-2 s-1 between spring and summer, while the rate of P production to the porewater also increased a hundredfold. These increases were concurrent with Fe mobilization. Finally, sediment dating shows that the fraction of P sequestered by La is buried under freshly deposited sediment at a rate of 2-3 mm per year. These results indicate that external P fluxes and erosion control remain crucial to maintain the longevity of the LMB treatment.

Phytoplankton in Deep Lakes of the Dinaric Karst: Functional Biodiversity and Main Ecological Features

Phytoplankton is a polyphyletic group of organisms that responds rapidly to environmental conditions and provides a reliable response to changes, making it a good ecological indicator for water quality monitoring. However, a gradient is almost essential for a reliable relationship between pressure and impact. In a low-gradient environment, ingenuity is required to outsmart the limitations of the commonly used linear relationship. Here, we examine changes in biomass and functional biodiversity by analysing larger data sets (2013-2022) in six ecologically diverse, natural, deep Croatian karst lakes with low nutrient gradients using nonlinear correlation coefficients and multivariate analyses in 209 samples. We found that phytoplankton biomass was most strongly influenced by nutrients, salinity and alkalinity, while light availability and total nitrogen strongly influenced phytoplankton functional biodiversity. An additional analysis of the TN:TP ratio revealed that the oligotrophic Lake Vransko is nitrogen-limited, and lakes Kozjak and Prošće are phosphorus-limited. This further clarified the relationship of phytoplankton to nutrients despite the low gradient. The complex analysis in this study provides a new perspective for predicting changes in the structure and succession of phytoplankton in deep karst lakes for successful management under apparent anthropogenic pressure and climate change.

A Broad-Scale Look at Nutrient Limitation and a Shift toward Co-limitation in United States Lakes

There is a longstanding debate about the role of nitrogen (N) vs phosphorus (P) in limiting primary production in lakes and whether co-nutrient limitation should be considered for managing eutrophication. We evaluated nutrient limitation and eutrophication at a subcontinental scale. Using U.S. Environmental Protection Agency National Lakes Assessment data, we assessed broad-scale patterns in nutrient limitation and compared samples of all surveyed lakes and lakes resurveyed in multiple surveys. We found that N correlated more strongly with productivity in the western U.S., while P correlated more strongly in the eastern U.S. The aggregated subcontinental effect suggests the importance of factors like N-deposition, terrestrial vegetation, underlying geology, and land use for understanding drivers of nutrient dynamics in lakes. Our study showed how patterns can aggregate across subcontinental scales yet still demonstrate considerable variation when more deeply examined on an individual lake level. Overall, we found that nutrient limitation is dynamic over space and time, with most lakes being co-limited. The prevalence of co-limitation also increased from 2007 to 2017. Trophic states within each limitation category varied substantially. Our findings underscore the need for combined N and P reductions to mitigate accelerated eutrophication.

Multidirectional Fate Path Model to Connect Phosphorus Emissions with Freshwater Eutrophication Potential

Excessive anthropogenic phosphorus (P) emissions put constant pressure on aquatic ecosystems. This pressure can be quantified as the freshwater eutrophication potential (FEP) by linking P emissions, P fate in environmental compartments, and the potentially disappeared fraction of species due to increase of P concentrations in freshwater. However, previous fate modeling on global and regional scales is mainly based on the eight-direction algorithm without distinguishing pollution sources. The algorithm fails to characterize the fate paths of point-source emissions via subsurface pipelines and wastewater treatment infrastructure, and exhibits suboptimal performance in accounting for multidirectional paths caused by river bifurcations, especially in flat terrains. Here we aim to improve the fate modeling by incorporating various fate paths and addressing multidirectional scenarios. We also update the P estimates by complementing potential untreated point-source emissions (PSu). The improved method is examined in a rapidly urbanizing area in Taihu Lake Basin, China in 2017 at a spatial resolution of 100 m × 100 m. Results show that the contribution of PSu on FEP (62.6%) is greater than that on P emissions (58.5%). The FEP is more spatially widely distributed with the improved fate modeling, facilitating targeted regulatory strategies tailored to local conditions.

Evaluation of best management practices for mitigating harmful algal blooms risk in an agricultural lake basin using a watershed model integrated with Bayesian Network approach

Lake eutrophication caused by nitrogen and phosphorus has led to frequent harmful algal blooms (HABs), especially under the unknown challenges of climate change, which have seriously damaged human life and property. In this study, a coupled SWAT-Bayesian Network (SWAT-BN) model framework was constructed to elucidate the mechanisms between non-point source nitrogen pollution in agricultural lake watersheds and algal activities. A typical agricultural shallow lake basin, the Taihu Basin (TB), China, was chosen in this study, aiming to investigate the effectiveness of best management practices (BMPs) in controlling HABs risks in TB. By modeling total nitrogen concentration of Taihu Lake from 2007 to 2022 with four BMPs (filter strips, grassed waterway, fertilizer application reduction and no-till agriculture), the results indicated that fertilizer application reduction proved to be the most effective BMP with 0.130 of Harmful Algal Blooms Probability Reduction (HABs-PR) when reducing 40% of fertilizer, followed by filter strips with 0.01 of HABs-PR when 4815ha of filter strips were conducted, while grassed waterway and no-till agriculture showed no significant effect on preventing HABs. Furthermore, the combined practice between 40% fertilizer application reduction and 4815ha filter strips construction showed synergistic effects with HABs-PR increasing to 0.171. Precipitation and temperature data were distorted to model scenarios of extreme events. As a result, the combined approach outperformed any single BMP in terms of robustness under extreme climates. This research provides a watershed-level perspective on HABs risks mitigation and highlights the strategies to address HABs under the influence of climate change.

Functional trait-based phytoplankton biomass and assemblage analyses in the pre-growing season for comprehensive algal bloom risk assessment

Algal bloom (AB) risk assessment is critical for maintaining ecosystem health and human sustainability. Previous AB risk assessments have focused on the potential occurrence of ABs and related factors in the growing season, whereas their hazards, especially in the pre-growing season, have attracted less attention. Here, we performed a comprehensive AB risk assessment, including water trophic levels, phytoplankton biomass, functional trait-based assemblages, and related environmental factors, in the pre-growing season in Dongting Lake, China. Although mesotrophic water and low phytoplankton biomass suggested low AB potential, toxic taxa, which constituted 13.28% of the phytoplankton biomass, indicated non-negligible AB hazards. NH4+ and water temperature were key factors affecting phytoplankton motility and toxicity. Our study establishes a new paradigm for quantitative AB risk assessment, including both potential AB occurrence and hazards. We emphasize the importance of phytoplankton functional traits for early AB warning and NH4+ reduction for AB control in the pre-growing season.

Atmospheric reduced nitrogen: Sources, transformations, effects, and management

Human activities have increased atmospheric emissions and deposition of oxidized and reduced forms of nitrogen, but emission control programs have largely focused on oxidized nitrogen. As a result, in many regions of the world emissions of oxidized nitrogen are decreasing while emissions of reduced nitrogen are increasing. Emissions of reduced nitrogen largely originate from livestock waste and fertilizer application, with contributions from transportation sources in urban areas. Observations suggest a discrepancy between trends in emissions and deposition of reduced nitrogen in the U.S., likely due to an underestimate in emissions. In the atmosphere, ammonia reacts with oxides of sulfur and nitrogen to form fine particulate matter that impairs health and visibility and affects climate forcings. Recent reductions in emissions of sulfur and nitrogen oxides have limited partitioning with ammonia, decreasing long-range transport. Continuing research is needed to improve understanding of how shifting emissions alter formation of secondary particulates and patterns of transport and deposition of reactive nitrogen. Satellite remote sensing has potential for monitoring atmospheric concentrations and emissions of ammonia, but there remains a need to maintain and strengthen ground-based measurements and continue development of chemical transport models. Elevated nitrogen deposition has decreased plant and soil microbial biodiversity and altered the biogeochemical function of terrestrial, freshwater, and coastal ecosystems. Further study is needed on differential effects of oxidized versus reduced nitrogen and pathways and timescales of ecosystem recovery from elevated nitrogen deposition. Decreases in deposition of reduced nitrogen could alleviate exceedances of critical loads for terrestrial and freshwater indicators in many U.S. areas. The U.S. Environmental Protection Agency should consider using critical loads as a basis for setting standards to protect public welfare and ecosystems. The U.S. and other countries might look to European experience for approaches to control emissions of reduced nitrogen from agricultural and transportation sectors.Implications: In this Critical Review we synthesize research on effects, air emissions, environmental transformations, and management of reduced forms of nitrogen. Emissions of reduced nitrogen affect human health, the structure and function of ecosystems, and climatic forcings. While emissions of oxidized forms of nitrogen are regulated in the U.S., controls on reduced forms are largely absent. Decreases in emissions of sulfur and nitrogen oxides coupled with increases in ammonia are shifting the gas-particle partitioning of ammonia and decreasing long-range atmospheric transport of reduced nitrogen. Effort is needed to understand, monitor, and manage emissions of reduced nitrogen in a changing environment.

Root-mediated acidification, phosphatase activity and the phosphorus-cycling microbial community enhance phosphorus mobilization in the rhizosphere of wetland plants

Rhizoremediation of wetland plants is an environmentally friendly strategy for sediment phosphorous (P) removal, the basic underlying principle of which is the complex interactions between roots and microorganisms. This study investigated the immobilization and mobilization mechanisms of P in the rhizosphere of wetland plants using high-resolution spatial visualization techniques and metagenomic sequencing. Two-dimensional visualization of the spatial distribution of P, iron (Fe) and manganese (Mn) indicated that the sequestration of Fe-oxides rather than Mn-oxides caused the depletion of labile P, resulting in an increase in the Fe-adsorbed P fraction. Plants altered the rhizospheric environments and P-cycling microbial community to mobilize low-availability P from sediments. Mineral P solubilization and organic P mineralization were enhanced by local acidification and increased phosphatase activity, respectively. Microbial P mobilization also increased with increasing relative abundances of P solubilization and mineralization genes (gcd and phnW) and decreasing P transportation genes (ugpA, ugpB, and pit) genes in the rhizosphere. These processes led to the remobilization of 10.04 % of inorganic P, and 15.23 % of organic P, in the rhizosphere during the incubation period. However, the resupply of P via the above processes did not compensate for the depletion of rhizospheric P via root uptake and mineral sequestration. Our results provide novel insights into the mechanisms of rhizospheric P cycling, which will help to inform future phytoremediation strategies.

Dissolved organic carbon spurs bacterial-algal competition and phosphorus-paucity adaptation: Boosting Microcystis' phosphorus uptake capacity

Dissolved organic carbon (DOC) can alter the availability of background nutrients by affecting the proliferation of heterotrophic bacteria, which exerts a notable influence on algal growth and metabolism. However, the mechanism of how allochthonous DOC (aDOC) precipitates shifts in bacterial-algal interactions and modulates the occurrence of cyanobacteria blooms remains inadequately elucidated. Therefore, this study investigated the relationship between bacteria and algae under aDOC stimulation. We found that excess aDOC triggered the breakdown and reestablishment of the equilibrium between Microcystis and heterotrophic bacteria. The rapid proliferation of heterotrophic bacteria led to a dramatic decrease in soluble phosphorus and thereby resulted in the inhibition of the Microcystis growth. When the available DOC was depleted, the rapid death of heterotrophic bacteria released large amounts of dissolved phosphorus, which provided sufficient nutrients for the recovery of Microcystis. Notably, Microcystis rejuvenated and showed higher cell density compared to the carbon-absent group. This phenomenon can be ascribed that Microcystis regulated the compositions of extracellular polymeric substances (EPS) and the expression of relevant proteins to adapt to a nutrient-limited environment. Using time of flight secondary ion mass spectrometry (TOF-SIM) and proteomic analysis, we observed an enhancement of the signal of organic matter and metal ions associated with P complexation in EPS. Moreover, Microcystis upregulated proteins related to organic phosphorus transformation to increase the availability of phosphorus in various forms. In summary, this study emphasized the role of DOC in algal blooms, revealing the underestimated enhancement of Microcystis nutrient utilization through DOC-induced heterotrophic competition and providing valuable insights into eutrophication management and control.

Causal impact analysis of weir opening on cyanobacterial blooms and water quality in the Yeongsan River, Korea: A bayesian structural time-series analysis and median difference test

The construction of weirs in Korea's Four Major Rivers Project has led to an increase in cyanobacterial blooms, posing environmental challenges. To address this, the government began opening weirs in 2017. However, interpreting experimental results has proven to be complex due to the multifaceted nature of blooms. This study aimed to assess the impact of opening the Juksan Weir on cyanobacterial blooms and water quality in the Yeongsan River. Using a median difference test (MDT) and causal impact analysis (CIA) with Bayesian structural time-series (BSTS) models, changes in cyanobacterial cell density (Cyano) and chlorophyll a concentration (Chl-a) before (January 2013 to June 2017) and after (July 2017 to December 2021) the weir-opening event were analyzed. The MDT revealed no significant change in Cyano post-weir opening (p = 0.267), but Chl-a significantly increased by 48.1 % (p < 0.01). As a result of CIA, Cyano decreased, albeit statistically insignificantly (p = 0.454), while Chl-a increased by 59.0 % (p < 0.01). These findings contradict the expectation that Cyano decrease due to the increased flow velocity resulting from weir opening. The absence of changes in Cyano and the increase in Chl-a can be attributed to several factors, including the constrained and inadequate duration of full weir opening combined with conducive conditions for the proliferation of other algae such as diatoms and green algae. These findings suggest that the effectiveness of weir opening in controlling Cyano may have been compromised by factors influencing the overall aquatic ecosystem dynamics. Further analysis revealed that factors such as elevated water temperatures (≥ 30 °C) and reduced flow rates (< 37 m3/s) contributed to the flourishing of cyanobacteria, whose concentrations exceeded 10,000 cells/mL. In analyzing causal relationships in environmental management, especially when there are complex causal interactions, the application of MDT and CIA provides valuable insights.

Exploring correlations between microplastics, microorganisms, and water quality in an urban drinking water source

The microplastic pollution in freshwater system is gradually becoming more severe, which has led to increasing attention on the distribution and potential harmful effects of microplastics. Moreover, microplastics may have an impact on river ecology and pose risks to ecosystems. Therefore, it is important to reveal this process. This study aimed to explore correlations between microplastics and free-living microorganisms in an urban drinking water source of Xiangjiang River by using multivariate statistical analysis. The results indicated that the abundance of microplastics (size 50 μm to 5 mm) in surface water and sediments ranged from 0.72 to 18.6 (mean ± SD: 7.32 ± 2.36) items L-1 and 26.3-302 (150 ± 75.6) items kg-1 dry weight (dw), respectively, suggesting potential microplastic pollution despite the protected status as a drinking water source. Higher microplastic abundances were observed in urban areas and the downstream of wastewater plants, with mostly granular shape, transparent and black color as well as 50-100 μm in size. The multivariate statistical analysis presented that the abundance of microplastics is not significantly correlated with water indicators, due to the complexity of the abundance data. The water indicators showed an obvious correlation with microplastics in colors of transparent and black, and smaller sizes of 50-100 μm. This is also true for microplastics and microorganisms in water and sediment. Proteobacteria was the main prokaryote in water and sediments, being positively correlated with 50-100 μm microplastics; while Chloroplastida was the dominated eukaryotes, presenting a weak correlation with smaller-size microplastics. Overall, when considering the properties of microplastics such as shape, color and size, the potential correlations with water indicators and microorganisms were more evident than abundance. This study provides new insights into the multivariate statistical analysis, explaining the potential correlations among microplastic properties, microorganisms and environmental factors in a river system.

Oscillations of algal cell quota: Considering two-stage phosphate uptake kinetics

Elucidating the mechanism of effect of phosphate (PO43-) uptake on the growth of algal cells helps understand the frequent outbreaks of algal blooms caused by eutrophication. In this study, we develop a comprehensive mathematical model that incorporates two stages of PO43- uptake and accounts for transport time delay. The model parameter values are determined by fitting experimental data of Prorocentrum donghaiense and the model is validated using experimental data of Karenia mikimotoi. The numerical results demonstrate that the model successfully captures the general characteristics of algal growth and PO43- uptake under PO43- sufficient conditions. Significantly, the experimental and mathematical findings suggest that the time delay associated with the transfer of PO43- from the surface-adsorbed PO43- (Ps) pool to the intracellular PO43- (Pi) pool may serve as a physiologically plausible mechanism leading to oscillations of algal cell quota. These results have important implications for resource managers, enabling them to predict and deepen their understanding of harmful algal blooms.

Intensification of harmful cyanobacterial blooms in a eutrophic, temperate lake caused by nitrogen, temperature, and CO2

Warmer temperatures can significantly increase the intensity of cyanobacterial harmful algal blooms (CHABs) in eutrophic freshwater ecosystems. However, few studies have examined the effects of CO2 enrichment in tandem with elevated temperature and/or nutrients on cyanobacterial taxa in freshwater ecosystems. Here, we observed changes in the biomass of cyanobacteria, nutrients, pH, and carbonate chemistry over a two-year period in a shallow, eutrophic freshwater lake and performed experiments to examine the effects and co-effects of CO2, temperature, and nutrient enrichment on cyanobacterial and N2-fixing (diazotrophic) communities assessed via high throughput sequencing of the 16S rRNA and nifH genes, respectively. During both years, there were significant CHABs (50-500 μg cyanobacterial chlorophyll-a L-1) and lake CO2 levels were undersaturated (≤300 μatm pCO2). NH4+ significantly increased the net growth rates of cyanobacteria as well as the biomass of the diazotrophic cyanobacterial order Nostocales under elevated and ambient CO2 conditions. In a fall experiment, the N2 fixation rates of Nostocales were significantly higher when populations were enriched with CO2 and P, relative to CO2-enriched populations that were not amended with P. During a summer experiment, N2 fixation rates increased significantly under N and CO2 - enriched conditions relative to N-enriched and ambient CO2 conditions. Nostocales dominated the diazotrophic communities of both experiments, achieving the highest relative abundance under CO2-enriched conditions when N was added in the first experiment and when CO2 and temperature were elevated in the second experiment, when N2 fixation rates also increased significantly. Collectively, this study indicates that N promotes cyanobacterial blooms including those formed by Dolichospermum and that the biomass and N2 fixation rates of diazotrophic cyanobacterial taxa may benefit from enhanced CO2 levels in eutrophic lakes.

Coagulation as an effective method for cyanobacterial bloom control: A review

Eutrophication, the over-enrichment with nutrients, for example, nitrogen and phosphorus, of ponds, reservoirs and lakes, is an urgent water quality issue. The most notorious symptom of eutrophication is a massive proliferation of cyanobacteria, which cause aquatic organism death, impair ecosystem and harm human health. The method considered to be most effective to counteract eutrophication is to reduce external nutrient inputs. However, merely controlling external nutrient load is insufficient to mitigate eutrophication. Consequently, a rapid diminishing of cyanobacterial blooms is relied on in-lake intervention, which may encompass a great variety of different approaches. Coagulation/flocculation is the most used and important water purification unit. Since cyanobacterial cells generally carry negative charges, coagulants are added to water to neutralize the negative charges on the surface of cyanobacteria, causing them to destabilize and precipitate. Most of cyanobacteria and their metabolites can be removed simultaneously. However, when cyanobacterial density is high, sticky secretions distribute outside cells because of the small size of cyanobacteria. The sticky secretions are easily to form complex colloids with coagulants, making it difficult for cyanobacteria to destabilize and resulting in unsatisfactory treatment effects of coagulation on cyanobacteria. Therefore, various coagulants and coagulation methods were developed. In this paper, the focus is on the coagulation of cyanobacteria as a promising tool to manage eutrophication. Basic principles, applications, pros and cons of chemical, physical and biological coagulation are reviewed. In addition, the application of coagulation in water treatment is discussed. It is the aim of this review article to provide a significant reference for large-scale governance of cyanobacterial blooms. PRACTITIONER POINTS: Flocculation was a promising tool for controlling cyanobacteria blooms. Basic principles of four kinds of flocculation methods were elucidated. Flocculant was important in the flocculation process.

Anoxygenic phototroph of the Chloroflexota uses a type I reaction centre

Scientific exploration of phototrophic bacteria over nearly 200 years has revealed large phylogenetic gaps between known phototrophic groups that limit understanding of how phototrophy evolved and diversified1,2. Here, through Boreal Shield lake water incubations, we cultivated an anoxygenic phototrophic bacterium from a previously unknown order within the Chloroflexota phylum that represents a highly novel transition form in the evolution of photosynthesis. Unlike all other known phototrophs, this bacterium uses a type I reaction centre (RCI) for light energy conversion yet belongs to the same bacterial phylum as organisms that use a type II reaction centre (RCII) for phototrophy. Using physiological, phylogenomic and environmental metatranscriptomic data, we demonstrate active RCI-utilizing metabolism by the strain alongside usage of chlorosomes3 and bacteriochlorophylls4 related to those of RCII-utilizing Chloroflexota members. Despite using different reaction centres, our phylogenomic data provide strong evidence that RCI-utilizing and RCII-utilizing Chloroflexia members inherited phototrophy from a most recent common phototrophic ancestor. The Chloroflexota phylum preserves an evolutionary record of the use of contrasting phototrophic modes among genetically related bacteria, giving new context for exploring the diversification of phototrophy on Earth.

Species-specific functional trait responses of canopy-forming and rosette-forming macrophytes to nitrogen loading: Implications for water-sediment interactions

Globally intensified lake eutrophication, attributed to excessive anthropogenic nitrogen loading, emerges as a significant driver of submerged vegetation degradation. Consequently, the impact of nitrogen on the decline of submerged macrophytes has received increasing attention. However, a functional trait-based approach to exploring the response of submerged macrophytes to nitrogen loading and its environmental feedback mechanism was unclear. Our study utilized two different growth forms of submerged macrophytes (canopy-forming Myriophyllum spicatum, and rosette-forming Vallisneria natans) to established "submerged macrophytes-water-sediment" microcosms. We assessed the influence of nitrogen loading, across four targeted total nitrogen concentrations (original control, 2, 5, 10 mg/L), on plant traits, water parameters, sediment properties, enzyme activities, and microbial characteristics. Our findings revealed that high nitrogen (10 mg/L) adversely impacted the relative growth rate of fresh biomass and total chlorophyll content in canopy-forming M. spicatum, while the chlorophyll a/b and free amino acid content increased. On the contrary, the growth and photosynthetic traits of resource-conservative V. natans were not affected by nitrogen loading. Functional traits (growth, photosynthetic, and stoichiometric) of M. spicatum but not V. natans exhibited significant correlations with environmental variables. Nitrogen loading significantly increased the concentration of nitrogen components in overlying water and pore water. The presence of submerged macrophytes significantly reduced the ammonia nitrogen and total nitrogen both in overlying water and pore water, and decreased total organic carbon in pore water. Nitrogen loading significantly inhibited sediment extracellular enzyme activities, but the planting of submerged macrophytes mitigated their negative effects. Furthermore, rhizosphere bacterial interactions were less compact compared to bare control, while eukaryotic communities exhibited increased complexity and connectivity. Path modeling indicated that submerged macrophytes mitigated the direct effects of nitrogen loading on overlying water and amplified the indirect effects on pore water, while also attenuating the direct negative effects of pore water on extracellular enzymes. The findings indicated that the restoration of submerged vegetation can mitigate eutrophication resulting from increased nitrogen loading through species-specific changes in functional traits and direct or indirect feedback mechanisms in the water-sediment system.

Microcystis genotypes in a tropical freshwater lake: Discovery of novel MIB-producing Microcystis with potentially unique synthesis pathway

Harmful algal blooms (HABs) are a threat to freshwater systems over the world due to the production of hepatotoxins like microcystin (MC), and nuisance taste and odour (T&O) compounds like 2-methylisoborneol (MIB). While MCs are known to cause detrimental effects to both water quality and human health, MIB is only reported to cause aesthetical problems. In this study, we investigated a tropical, urban lake that was experiencing persistent MC and MIB events. Although it was dominated by Microcystis blooms, analysis revealed that the toxigenic Microcystis were not the only species driving the MC concentrations. Additionally, there was also a lack of causative species for the MIB events. Through isolation, we have identified three toxigenic Microcystis found to produce four different variants of MCs, and two novel non-toxigenic Microcystis that were capable of producing MIB. The ability to produce MIB had never been previously reported for this species. Compared to other major producers such as Planktothricoides sp. and Streptomyces sp., the MIB synthase genes of our Microcystis sp. strains were partial, illustrating the possibility of unique synthesis pathways. The Microcystis sp. strains were found to produce about 2.77-5.22 fg MIB cell-1, with a majority of the contents (70-80 %) existing in the extracellular phase. Correlation analysis of field study indicated that phosphorus limitation may have an indirect effect on non-toxigenic Microcystis abundance and proportion by influencing the toxigenic genotype, suggesting that current measures to control HABs may favour the proliferation of the non-toxigenic Microcystis. The potential for Microcystis sp. to produce MIB through unique synthesis pathway, coupled with the potential dominance of non-toxigenic genotypes in Microcystis blooms, signals the possibility that non-toxigenic Microcystis should be monitored as well.