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

Nitrogen fixation: a poorly understood process along the freshwater-marine continuum

Although N₂ fixation is a major component of the global N cycle and has been extensively studied in open-ocean and terrestrial ecosystems, rates and ecological dynamics remain virtually unknown for the inland and coastal aquatic ecosystems (lakes, wetlands, rivers, streams, estuaries) that connect terrestrial and marine biomes. This is due to the diversity of these habitats, as well as the traditional paradigm that N₂ fixation rates were low to nonexistent, and therefore not important, in these ecosystems. We identify three major research themes to advance understanding of aquatic N₂ fixation: 1) the biological diversity of diazotrophs and variability of N₂ fixation rates, 2) the ecological stoichiometry of N₂ fixation, and 3) the upscaling of N₂ fixation rates from genes to ecosystems. Coordinating research across these areas will advance limnology and oceanography by fully integrating N₂ fixation into ecological dynamics of aquatic ecosystems from local to global scales.

Preferable phosphate sequestration using polymer-supported Mg/Al layered double hydroxide nanosheets

The efficient removal of phosphate from waters is critical to mitigating eutrophication. Recently, layered double hydroxides (LDHs) have been believed to be promising adsorbents for phosphate removal. Nevertheless, the scaled-up application of LDHs is limited by the difficulties of separation, excessive pressure drops, and potential metal leaching. In this study, a millimeter-sized nanocomposite, MgAl-201, was fabricated by impregnating Mg/Al LDH nanosheets into a polystyrene anion exchanger D201. The resulting MgAl-201 combines the inherent affinity of Mg/Al LDH toward phosphate and the excellent hydrodynamic performance of the support material. Benefiting from the shielding effect from the cross-linked polymeric host, MgAl-201 exhibits satisfactory chemical stability in the range of pH 3-11 with a negligible metal release. Adsorption experiments show that MgAl-201 has superb applicability to neutral phosphate-contaminated waters. It reaches adsorption equilibrium within 270 min, and the maximum adsorption capacity calculated by the double Langmuir model is 52.0 mg/g. Meanwhile, MgAl-201 exhibits more preferable adsorption toward phosphate than D201 when coexisting anions are at relatively high levels. FTIR and XPS surveys revealed that two distinct adsorption interactions were involved in phosphate removal, that is, electrostatic interactions from the quaternary ammonium groups bonded on the host and the interlayer exchangeable anions in the encapsulated Mg/Al LDH, and specific inner-sphere complexation from the -OH groups in the Mg/Al LDH layers. For wastewater application, a satisfactory treatable volume of 580 BV was achieved to reduce the effluents from 2.0 mg/L to 0.5 mg/L, which was up to 8 times that of the traditional anion exchanger D201. Furthermore, MgAl-201 could be easily regenerated using the Na₂CO₃-NaCl binary solution and maintained good reusability without significant capacity loss after 5 adsorption-desorption cycles. The results suggest that MgAl-201 is of great application capability for preferable phosphate sequestration in advanced wastewater treatment.

Efficient and sustainable phosphate removal from water by small-sized Al(OH)3 nanocrystals confined in discarded Artemia Cyst-shell: Ultrahigh sorption capacity and rapid sequestration

We reported a new strategy for efficient phosphate removal from wastewaters, it relies on the discarded Artemia Cyst-shell in-situ growth of Al(OH)₃ nanocluster, the charged amino-acids components of skeleton make available for the small size of Al(OH)₃ formation (< 10 nm) with high activity, and the three-dimensional porous structure of discarded matrix provides fast kinetics and efficient Al(OH)₃ nanoparticles utilization. These hybrid adsorbents exhibit ultrahigh capacity (850.5 mg/g) and fast kinetics (~2 min) by recent ten-years (2011-2020) survey, the superior selectivity against various foreign ions, with a distribution coefficient (K_d) as high as 4820 mL/g, the porous structure and fast kinetics also accelerate the phosphate accessibility, yielding a satisfactory capacity of ~3000 L/kg sorbent (Artemia CS-Al) for the application, even varying at high feeding-speeds. The saturated adsorbent can be readily regenerated and reused without decrease in performance, this technology is promising for mitigating the contamination problem of excess phosphate worldwide.

High probability of nitrogen and phosphorus co-limitation occurring in eutrophic lakes

Limnologists and governments have long had an interest in whether nitrogen (N) and/or phosphorous (P) limit algal productivity in lakes. However, the types and importance of anthropogenic and biogeochemical processes of N and P differ with lake trophic status. Here, a global lake dataset (annual average data from 831 lakes) demonstrates that total nitrogen (TN): total phosphorous (TP) ratios declined significantly as lakes become more eutrophic. From oligotrophic to hypereutrophic lakes, the probability of N and P co-limitation significantly increases from 15.0 to 67.0%, while P-only limitation decreases from 77.0 to 22.3%. Furthermore, TN:TP ratios are mainly affected by concentrations of TP (r = -0.699) rather than TN (r = -0.147). These results reveal that lake eutrophication mainly occurs with increasing P rather than N, which shifts lake ecosystems from stoichiometric P limitation toward a higher probability of N and P co-limitation. This study suggests that low N:P stoichiometry and a high probability of N and P co-limitation tend to occur in eutrophic systems.

Water quality evaluation and dissolved organic matter characterization of a tropical hypereutrophic reservoir and its streams treated with Phoslock® and microbial bioremediation Enzilimp®

Worldwide, freshwater environments are impacted by inputs of nutrients and dissolved organic matter from human activities. Yet, the recovery of aquatic systems is usually focused only on nutrient management. In our work, we presented the case of an urban and hypereutrophic environment (Pampulha reservoir, Belo Horizonte, Brazil) that receives discharges from several streams and was treated with lanthanum modified bentonite (Phoslock®) and microbial bioremediation (Enzilimp®). Our goals were to evaluate whether the treatment could improve the water quality and characterize the spatiotemporal variation of dissolved organic matter sources and indices according to absorbance and fluorescence measurements from the reservoir and streams post-application months (2018). In our results, the reservoir showed a relative decrease in its phosphorus concentration compared to data from before the treatment. On the other hand, carbon concentrations reached expressive values in the post-application months following a similar pattern found in the streams. Our data showed that the reservoir's high resistance in its hypereutrophic condition was related to the elevated loading of external inputs coming from the streams. The parallel factor analysis (PARAFAC) identified four main carbon sources, two of them being potential tracers of organic pollution in the Pampulha reservoir and watershed, together with absorbance and fluorescence indices. Our findings suggest that carbon parameters can be essential tools to provide adequate monitoring and optimization of water recovery attempts in complex, polluted environments.

New insights into eutrophication management: Importance of temperature and water residence time

Eutrophication and harmful cyanobacterial blooms threaten water resources all over the world. There is a great controversy about controlling only phosphorus or controlling both nitrogen and phosphorus in the management of lake eutrophication. The primary argument against the dual nutrients control of eutrophication is that nitrogen fixation can compensate the nitrogen deficits. Thus, it is of great necessary to study the factors that can significantly affect the nitrogen fixation. Due to the difference of climate and human influence, the water quality of different lakes (such as water temperature, N:P ratio and water residence time) is also quite different. Numerous studies have reported that the low N:P ratio can intensify the nitrogen fixation capacities. However, the effects of temperature and water residence time on the nitrogen fixation remain unclear. Thus, 30 shallows freshwater lakes in the eastern plain of China were selected to measure dissolved N₂ and Ar concentrations through N₂: Ar method using a membrane inlet mass spectrometer to quantify the nitrogen fixation capacities and investigate whether the temperature and water residence time have a great impact on nitrogen fixation. The results have shown that the short lake water residence time can severely inhibit the nitrogen fixation capacities through inhibiting the growth of nitrogen-fixing cyanobacteria, changing the N:P ratio and resuspending the solids from sediments. Similarly, lakes with low water temperature also have a low nitrogen fixation capacity, suggesting that controlling nitrogen in such lakes is feasible if the growth of cyanobacteria is limited by nitrogen.

Preparation of resin-based composites containing Ce and cationic polymers with abundant promotional affinity sites for phosphate capture

A new type of composite, D301-Ce+, for efficient and selective phosphate removal.

Differential Effect of Hydroxen Peroxide οn Toxic Cyanobacteria of Hypertrophic Mediterranean Waterbodies

Cyanobacterial blooms have been known since ancient times; however, they are currently increasing globally. Human and ecological health risks posed by harmful cyanobacterial blooms have been recorded around the world. These risks are mainly associated with their ability to affect the ecosystem chain by different mechanisms like the production of cyanotoxins, especially microcystins. Their expansion and their harmful effects have led many researchers to seek techniques and strategies to control them. Among them, hydrogen peroxide could be a promising tool against cyanobacteria and cyanotoxins and it is well-established as an environmentally friendly oxidizing agent because of its rapid decomposition into oxygen and water. The aim of the present study was to evaluate the effect of hydrogen peroxide on phytoplankton from two hypertrophic waterbodies in Greece. The effect of hydrogen peroxide on concentration of microcystins found in the waterbodies was also studied. Treatment with 4 mg/L hydrogen peroxide was applied to water samples originated from the waterbodies and Cyanobacterial composition and biomass, phycocyanin, chlorophyll-a, and intra-cellular and total microcystin concentrations were studied. Cyanobacterial biomass and phycocyanin was reduced significantly after the application of 4 mg/L hydrogen peroxide in water treatment experiments while chlorophytes and extra-cellular microcystin concentrations were increased. Raphidiopsis (Cylindrospermopsis) raciborskii was the most affected cyanobacterial species after treatment of the water of the Karla Reservoir in comparison to Aphanizomenon favaloroi, Planktolyngbya limnetica, and Chroococcus sp. Furthermore, Microcystis aeruginosa was more resistant to the treatment of Pamvotis lake water in comparison with Microcystis wesenbergii and Microcystis panniformis. Our study showed that hydrogen peroxide differentially impacts the members of the phytoplankton community, affecting, thus, its overall efficacy. Different effects of hydrogen peroxide treatment were observed among cyanobacerial genera as well as among cyanobacterial species of the same genus. Different effects could be the result of the different resistance mechanisms of each genus or species to hydrogen peroxide. Hydrogen peroxide could be used as a treatment for the mitigation of cyanobacterial blooms in a waterbody; however, the biotic and abiotic characteristics of the waterbody should be considered.

Mitigation of CyanoHABs Using Phoslock® to Reduce Water Column Phosphorus and Nutrient Release from Sediment

Cyanobacterial blooms can be stimulated by excessive phosphorus (P) input, especially when diazotrophs are the dominant species. A series of mesocosm experiments were conducted in a lake dominated by a cyanobacteria bloom to study the effects of Phoslock^®, a phosphorus adsorbent. The results showed that the addition of Phoslock^® lowered the soluble reactive phosphate (SRP) concentrations in water due to efficient adsorption and mitigated the blooms. Once settled on the sediments, Phoslock^® serves as a barrier to reduce P diffusion from sediments into the overlying waters. In short-term (1 day) incubation experiments, Phoslock^® diminished or reversed SRP effluxes from bottom sediments. At the same time, the upward movement of the oxic-anoxic interface through the sediment column slightly enhanced NH₄⁺ release and depressed N₂ release, suggesting the inhibition of nitrification and denitrification. In a long-term (28 days) experiment, Phoslock^® hindered the P release, reduced the cyanobacterial abundance, and alleviated the bloom-driven enhancements in the pH and oxygen. These results suggest that, through suppression of internal nutrient effluxes, Phoslock^® can be used as an effective control technology to reduce cyanobacteria blooms common to many freshwater systems.

A Framework for Reviewing Silvopastoralism: A New Zealand Hill Country Case Study

Silvopastoral systems can be innovative solutions to agricultural environmental degradation, especially in hilly and mountainous regions. A framework that expresses the holistic nature of silvopastoral systems is required so research directions can be unbiased and informed. This paper presents a novel framework that relates the full range of known silvopastoral outcomes to bio-physical tree attributes, and uses it to generate research priorities for a New Zealand hill country case study. Current research is reviewed and compared for poplar (Populus spp.), the most commonly planted silvopastoral tree in New Zealand hill country, and kānuka (Kunzea spp.), a novel and potentially promising native alternative. The framework highlights the many potential benefits of kānuka, many of which are underappreciated hill country silvopastoral outcomes, and draws attention to the specific outcome research gaps for poplar, despite their widespread use. The framework provides a formalised tool for reviewing and generating research priorities for silvopastoral trees, and provides a clear example of how it can be used to inform research directions in silvopastoral systems, globally.

Efficient reclaiming phosphate from aqueous solution using waste limestone modified sludge biochar: Mechanism and application as soil amendments

A novel limestone-modified biochar derived from sewage sludge was prepared to reclaim phosphorus (P) from aqueous solution, and the potential application of P-laden biochar as soil amendments was also investigated. The limestone-modified biochar demonstrated excellent performance on phosphate recovery from aqueous solution in a wide range of pH (2.0-11.0), with maximum adsorption capacity of the biochar (Limestone/sludge mass ratio of 3:1) up to 231.28 mg P/g, which was 10.7 times that of the original sludge biochar. The adsorption was well described by the pseudo second-order model and Langmuir isotherm model. According to the adsorption thermodynamic parameters, the phosphate adsorption was spontaneous (ΔG⁰ < 0) and endothermic (ΔH⁰ > 0) so that increasing the temperature was beneficial to adsorption. Characterization analysis by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscope-energy dispersive spectrometer (SEM-EDS) proved that electrostatic attraction, surface complexation and brushite (CaHPO₄^.2H₂O) precipitation were the dominant mechanism. The P-laden biochar exhibited an excellent ability to be reused as a new slow-release P fertilizer for soil. Pot experiment results showed that the treatment of P-laden LB 3:1 (P content of 22.8%) addition (1 wt%) significantly promoted Indian Lettuce germination (increasing by 14.4%), plant height (increasing by 18.6%), and dry biomass (53.0%) compared with the control, though it underperformed compared to commercial fertilizer.

Growing phosphorus dilemma: The opportunity from aquatic systems' secondary phosphorus retention capacity

The essential cause of phosphorus scarcity and phosphorus-induced risks, i.e. phosphorus dilemma, mainly lies in current low phosphorus flow efficiency (PFE) in agricultural systems. Improving PFE largely depends on secondary phosphorus retention along the phosphorus flow chain from phosphate mining to terrestrial agricultural systems, to aquatic systems, and ultimately to seabed deposition. Our review found that aquatic systems will have the opportunity and growing capacity to retain seaward secondary phosphorus carried by the runoff, due to its location between land and water systems, its ability of converting secondary phosphorus from both land and aquatic systems into aquatic products, and its rapid expansion with low PFE. However, a knowledge gap exists in secondary phosphorus retention in aquatic systems compared to in terrestrial systems. Although the phosphorus retention literature continues to grow in environmental and agricultural & biological sciences, only 8.8% of the documents are related to aquatic systems with few quantification studies. Based on the literature with phosphorus retention quantification since 1979, we divided the reported phosphorus interceptors into abiotic and biotic groups, further into 7 categories and more subcategories. By 2020, eight categories of interceptors had been reported, increased from only one interceptor in 1979. However, most of them focused on wetlands, only a few studies on aquatic organisms which concentrated in 8 countries before 2000. Thus, it is urgent to emphasize aquatic systems' secondary phosphorus retention capacity and its systemic benefits for a sustainable phosphorus use.

Nitrogen and Phosphorus Diffusion Fluxes: Insight from High-Resolution Technology and Hydrodynamic Modeling

Nitrogen and phosphorus are key elements in controlling eutrophication in the aquatic system. Water and sediment samples were collected from Hongfeng Lake, a seasonally stratified reservoir in southwest China, in winter and summer. Diffusion fluxes of NH4+, NO3−, and labile P in summer using diffusive gradients in thin films technology were 3.4, −37.2, and 0.9 mg m−2 day−1, respectively, based on Fick’s first law. The diffusion flux of labile P was 2.05 mg m−2 day−1 in winter. The contributions fraction of the labile P diffusion flux from sediment to the overlying water were higher in winter than those in summer, because of the relatively lower external input, concentrations and higher diffusion fluxes in winter. After the external input decreased, all of the three diffusion fluxes were lower than the previous record. To understand the influence effect of hydrodynamics, environmental fluid dynamics code modeling was used to simulate the flow and temperature field in winter and summer. Modeling results showed that velocity in summer was higher than that in winter due to concentrated rainfall within the catchment. Moreover, the velocity and temperature in the euphotic zone were higher than that of the hypolimnion in summer. Less variation of velocity and temperature in vertical profile in winter than that in summer was observed, which may be attributable to the high specific heat capacity and the low heat conductivity of water. There was no significant correlation among velocity, hydrochemistry, nitrogen, and phosphorus concentrations. Hydrodynamics, solar radiation, and water depth affect the position of the thermocline, which was consequently to water temperature, hydrochemistry, dissolved nitrogen, and phosphorus concentration. Correlation analysis suggested that the higher bottom velocity and total bed shear may accelerate labile P, NH4+, and NO3− diffusion fluxes. These results provide evidence and suggestions for preventing and controlling reservoir eutrophication and water safety management.

Controlled release fertilizer: A review on developments, applications and potential in agriculture

Controlled release fertilizer (CRF) plays a crucial yet necessary part in the sustainable agriculture industry. An alarming rise in call for crop production directly influences the increasing need for synthetically derived fertilizers and pesticides production. The application of CRF has been a gamechanger as an environmentally sustainable pathway to increase crop yields by paving desired phase of plant growth via a direct or indirect mechanism. The mechanism of CRF does not only decreases nutrient dissipation due to volatilization and leaching, but also provides a precisely appropriate nutrient release design that is suitable in the physiological and biochemical aspect of the plant growth. However, CRF is not deployed on larger scale of commercial agriculture practices due to being expensive, has relatively low efficiency in releasing nutrients and its coatings are largely composed of petroleum-based synthetic polymers. Alternatively, there are many polymers derived from renewable and biodegradable sources that can be used as coating material for CRF in the form of bio-nanocomposites. Having said that, there is an apparent gap between the mechanism of the CRFs for promoting plant growth and the prominent role of the nanocomposites especially bio-nanocomposites as coating material for CRF synthesis, thus the importance of nanotechnology application in enhancing the effectiveness of CRF. Therefore, this review attempts to bridge the stated gap and summarizes the comprehensive developments, application mechanisms and future potential of CRF as a fertilizer for crop sustainability.

Internal loop sustains cyanobacterial blooms in eutrophic lakes: Evidence from organic nitrogen and ammonium regeneration

Algal bloom species can live upon internal regenerated ammonium (NH₄⁺) for growth during the nitrogen-limited period. However, the linkages between NH₄⁺ regeneration and phytoplankton biomass and community composition dynamics remain largely unknown. To unravel the interactions between NH₄⁺ regeneration and phytoplankton community, we measured water column NH₄⁺ regeneration rates (REGs) during a continuous phytoplankton growing period and a contrast summer/winter turnover in eutrophic Lake Taihu. Measured REGs were higher in summer than in winter and significantly correlated to total phytoplankton biomass, Cyanophyta biomass and its biomass proportions, and the concentrations of particulate nitrogen and dissolved organic carbon as well as the relative abundance of labile components (proteins and lipids). Random forest regression analyses displayed that variation of REGs were mainly controlled by water temperature and algal-related parameters (including chlorophyll a, total phytoplankton biomass, and Cyanophyta biomass). Partial least squares path model further revealed that algal-related parameters were the direct and significant factors regulating REGs, and contributed to the largest effect of the variance in REGs. Of the algal community, Cyanophyta was the dominant phylum to accelerate REGs. Correspondingly, rapid internal NH₄⁺ turnover may strongly support the persistence of cyanobacterial blooms, thus forming a positive feedback between cyanobacterial blooms and REGs during the nitrogen-limited summer months. We therefore deduced that the internal loop between cyanobacterial blooms and REGs during summer may be a key self-maintenance mechanism of continuous cyanobacterial blooms.

Nitrogen cycling processes and the role of multi-trophic microbiota in dam-induced river-reservoir systems

The nitrogen (N) cycle is one of the most important nutrient cycles in river systems, and it plays an important role in maintaining biogeochemical balance and global climate stability. One of the main ways that humans have altered riverine ecosystems is through the construction of hydropower dams, which have major effects on biogeochemical cycles. Most previous studies examining the effects of damming on N cycling have focused on the whole budget or flux along rivers, and the role of river as N sources or sinks at the global or catchment scale. However, so far there is still lack of comprehensive and systematic summarize on N cycling and the controlling mechanisms in reservoirs affected by dam impoundment. In this review, we firstly summarize N cycling processes along the longitudinal riverine-transition-lacustrine gradient and the vertically stratified epilimnion-thermocline-hypolimnion gradient. Specifically, we highlight the direct and indirect roles of multi-trophic microbiota and their interactions in N cycling and discuss the main factors controlling these biotic processes. In addition, future research directions and challenges in incorporating multi-trophic levels in bioassessment, environmental flow design, as well as reservoir regulation and restoration are summarized. This review will aid future studies of N fluxes along dammed rivers and provide an essential reference for reservoir management to meet ecological needs.

Co-occurring microorganisms regulate the succession of cyanobacterial harmful algal blooms

Cyanobacterial harmful algal blooms (CyanoHABs) have been found to transmit from N2 fixer-dominated to non-N2 fixer-dominated in many freshwater environments when the supply of N decreases. To elucidate the mechanisms underlying such "counter-intuitive" CyanoHAB species succession, metatranscriptomes (biotic data) and water quality-related variables (abiotic data) were analyzed weekly during a bloom season in Harsha Lake, a multipurpose lake that serves as a drinking water source and recreational ground. Our results showed that CyanoHABs in Harsha Lake started with N2-fixing Anabaena in June (ANA stage) when N was high, and transitioned to non-N2-fixing Microcystis- and Planktothrix-dominated in July (MIC-PLA stage) when N became limited (low TN/TP). Meanwhile, the concentrations of cyanotoxins, i.e., microcystins were significantly higher in the MIC-PLA stage. Water quality results revealed that N species (i.e., TN, TN/TP) and water temperature were significantly correlated with cyanobacterial biomass. Expression levels of several C- and N-processing-related cyanobacterial genes were highly predictive of the biomass of their species. More importantly, the biomasses of Microcystis and Planktothrix were also significantly associated with expressions of microbial genes (mostly from heterotrophic bacteria) related to processing organic substrates (alkaline phosphatase, peptidase, carbohydrate-active enzymes) and cyanophage genes. Collectively, our results suggest that besides environmental conditions and inherent traits of specific cyanobacterial species, the development and succession of CyanoHABs are regulated by co-occurring microorganisms. Specifically, the co-occurring microorganisms can alleviate the nutrient limitation of cyanobacteria by remineralizing organic compounds.

Impacts of storm events on chlorophyll-a variations and controlling factors for algal bloom in a river receiving reclaimed water

Harmful algal bloom is prevalent in the reclaimed-water-source (RWS) river caused by the excessive nutrient's inputs. Rainfall water may be the sole nutrient-diluted water source for the RWS river. However, the effects of storm events on the algal bloom in the RWS river are poorly understood. This study presents chlorophyll-a (Chl-a) variations before, during, and after the initial storm events (Pre-storm, In-storm, and Post-storm) at four representative sites with distinct hydraulic conditions in a dam-regulated RWS river system, Beijing. The response of Chl-a to the initial storm events mostly depends on the ecosystem status that caused by the river hydraulic properties. The upstream is more river-like and downstream is more lake-like. In the river-like system, elevated water temperature (WT, increased by 2 %) could support the dominating algae (diatom) growth (Chl-a increased by 130 %) from Pre-storm to In-storm period. In the lake-like system, the dominant algae (blue algae) declined (Chl-a sharply decreased by 96%-99 %) due to the lower WT (decreased by 3%-7%) and increased flow velocities from Pre-storm to In-storm period. During the Post-storm period, the dominant algae break out (Chl-a surged by 20%-319 %) in the lake-like system caused by the recovered WT (increased by 3%-6%) and flow velocity. The occurrence of algal bloom can be predicted by the Random Forest (RF) model based on water quality parameters such as total nitrogen (TN). The thresholds of algal bloom for the Pre-storm, In-storm, and Post-storm periods were identified as 30 μg/L, 10 μg/L, and 10 μg/L, respectively. The two driven factors were WT and nitrate (NO₃-N) for the Pre-storm period and were WT and TN for the In- & Post-storm periods. A higher risk of algal bloom is highlighted during the initial storm events in the RWS river. We propose recommendations for improving water quality in the RWS river systems under the climatic change.

The Response of Extracellular Polymeric Substances Production by Phototrophic Biofilms to a Sequential Disturbance Strongly Depends on Environmental Conditions

Phototrophic biofilms are exposed to multiple stressors that can affect them both directly and indirectly. By modifying either the composition of the community or the physiology of the microorganisms, press stressors may indirectly impact the ability of the biofilms to cope with disturbances. Extracellular polymeric substances (EPS) produced by the biofilm are known to play an important role in its resilience to various stresses. The aim of this study was to decipher to what extent slight modifications of environmental conditions could alter the resilience of phototrophic biofilm EPS to a realistic sequential disturbance (4-day copper exposure followed by a 14-day dry period). By using very simplified biofilms with a single algal strain, we focused solely on physiological effects. The biofilms, composed by the non-axenic strains of a green alga (Uronema confervicolum) or a diatom (Nitzschia palea) were grown in artificial channels in six different conditions of light intensity, temperature and phosphorous concentration. EPS quantity (total organic carbon) and quality (ratio protein/polysaccharide, PN/PS) were measured before and at the end of the disturbance, and after a 14-day rewetting period. The diatom biofilm accumulated more biomass at the highest temperature, with lower EPS content and lower PN/PS ratio while green alga biofilm accumulated more biomass at the highest light condition with lower EPS content and lower PN/PS ratio. Temperature, light intensity, and P concentration significantly modified the resistance and/or recovery of EPS quality and quantity, differently for the two biofilms. An increase in light intensity, which had effect neither on the diatom biofilm growth nor on EPS production before disturbance, increased the resistance of EPS quantity and the resilience of EPS quality. These results emphasize the importance of considering the modulation of community resilience ability by environmental conditions, which remains scarce in the literature.

Genome Streamlining, Plasticity, and Metabolic Versatility Distinguish Co-occurring Toxic and Nontoxic Cyanobacterial Strains of Microcoleus

Harmful cyanobacterial bloom occurrences have increased worldwide due to climate change and eutrophication, causing nuisance and animal deaths. Species from the benthic cyanobacterial genus Microcoleus are ubiquitous and form thick mats in freshwater systems, such as rivers, that are sometimes toxic due to the production of potent neurotoxins (anatoxins). Anatoxin-producing (toxic) strains typically coexist with non-anatoxin-producing (nontoxic) strains in mats, although the reason for this is unclear. To determine the genetic mechanisms differentiating toxic and nontoxic Microcoleus, we sequenced and assembled genomes from 11 cultures and compared these to another 31 Microcoleus genomes. Average nucleotide identities (ANI) indicate that toxic and nontoxic strains are distinct species (ANI, <95%), and only 6% of genes are shared across all 42 genomes, suggesting a high level of genetic divergence among Microcoleus strains. Comparative genomics showed substantial genome streamlining in toxic strains and a potential dependency on external sources for thiamine and sucrose. Toxic and nontoxic strains are further differentiated by an additional set of putative nitrate transporter (nitrogen uptake) and cyanophycin (carbon and nitrogen storage) genes, respectively. These genes likely confer distinct competitive advantages based on nutrient availability and suggest nontoxic strains are more robust to nutrient fluctuations. Nontoxic strains also possess twice as many transposable elements, potentially facilitating greater genetic adaptation to environmental changes. Our results offer insights into the divergent evolution of Microcoleus strains and the potential for cooperative and competitive interactions that contribute to the co-occurrence of toxic and nontoxic species within mats. IMPORTANCE Microcoleus autumnalis, and closely related Microcoleus species, compose a geographically widespread group of freshwater benthic cyanobacteria. Canine deaths due to anatoxin-a poisoning, following exposure to toxic proliferations, have been reported globally. While Microcoleus proliferations are on the rise, the mechanisms underpinning competition between, or coexistence of, toxic and nontoxic strains are unknown. This study identifies substantial genetic differences between anatoxin-producing and non-anatoxin-producing strains, pointing to reduced metabolic flexibility in toxic strains, and potential dependence on cohabiting nontoxic strains. Results provide insights into the metabolic and evolutionary differences between toxic and nontoxic Microcoleus, which may assist in predicting and managing aquatic proliferations.

The Effects of Ferric Sulfate (Fe2(SO4)3) on the Removal of Cyanobacteria and Cyanotoxins: A Mesocosm Experiment

Cyanobacterial blooms are a global concern. Chemical coagulants are used in water treatment to remove contaminants from the water column and could potentially be used in lakes and reservoirs. The aims of this study was to: 1) assess the efficiency of ferric sulfate (Fe₂(SO₄)₃) coagulant in removing harmful cyanobacterial cells from lake water with cyanobacterial blooms on a short time scale, 2) determine whether some species of cyanobacteria can be selectively removed, and 3) determine the differential impact of coagulants on intra- and extra-cellular toxins. Our main results are: (i) more than 96% and 51% of total cyanobacterial cells were removed in mesocosms with applied doses of 35 mgFe/L and 20 mgFe/L, respectively. Significant differences in removing total cyanobacterial cells and several dominant cyanobacteria species were observed between the two applied doses; (ii) twelve microcystins, anatotoxin-a (ANA-a), cylindrospermopsin (CYN), anabaenopeptin A (APA) and anabaenopeptin B (APB) were identified. Ferric sulfate effectively removed the total intracellular microcystins (greater than 97% for both applied doses). Significant removal of extracellular toxins was not observed after coagulation with both doses. Indeed, the occasional increase in extracellular toxin concentration may be related to cells lysis during the coagulation process. No significant differential impact of dosages on intra- and extra-cellular toxin removal was observed which could be relevant to source water applications where optimal dosing is difficult to achieve.

Effects of elevated sulfate in eutrophic waters on the internal phosphate release under oxic conditions across the sediment-water interface

Eutrophication in freshwater environments may be enhanced by the elevation of sulfate in waters, through the release of internal phosphorus (P) from anoxic sediments. However, the influence of increasing but modest sulfate concentrations (less than 3000 μM) on P release under oxic conditions across the sediment-water interface (SWI) in eutrophic freshwater is poorly understood. In this study, the profiles of P, iron (Fe), sulfur (S) and physicochemical parameters were measured in a simulated lacustrine system with varying concentrations of sulfate (970-2600 μM) in overlying water. The results indicated that elevated concentrations of sulfate increased the soluble reactive P in overlying waters under oxic conditions across the SWI. A 100 μM increase of sulfate was found to induce a 0.128 mgm^-2d^-1 increase of P flux from surface sediments into overlying waters under oxic conditions. Higher sulfate concentrations in the overlying waters increased the concentrations of labile S(-II) in the deep sediments, due to sulfate penetration and subsequent reduction to S(-II). We also found the fluxes of labile Fe (10.34 to 18.33 mgm^-2d^-1) and P (2.70 to 1.33 mgm^-2d^-1) from deep to surface sediment were both positive and greater than the corresponding fluxes (Fe, 2.2 to 3.51 and P, 2.6 to 0.39 mgm^-2d^-1, respectively) from surface sediment to the overlying water, suggesting that reduction of P-bearing Fe(III)(oxyhydr)oxides in deep anoxic sediment acted as a major source of internal P release. In addition, the upward flux of Fe(II) was significantly lower under higher sulfate conditions, indicating that the Fe(II) flux could be mitigated by formation of Fe(II) sulfides in the deep sediment. Under these conditions, less Fe(II) from deep sediments could be re-oxidized and combine with P in the surface, oxic sediment, thereby reducing the retention capacity for P and leading to higher release of internal P to the water column.

Low diffusive nitrogen loss of urban inland waters with high nitrogen loading

Little is known about the exchange of gaseous nitrogen (N2) with the atmosphere from urban inland waters, which are characterized by low carbon-to‑nitrogen ratios and low nitrogen-to‑phosphorus ratios. Here, we studied diffusive nitrogen loss based on the measurement of dissolved N2 concentrations and related gene abundance of N2 production and fixation in rivers and lakes in the megacity of Beijing, China, between 2018 and 2020. The excess dissolved N2 (△N2) ranged from -51.2 to 56.8 μmol L-1 (average - 0.03 ± 13.8 μmol L-1), and approximately 43% of the river samples and 72% of the lake samples being undersaturated with N2, suggesting that the lakes mainly acted as a role of N2 sink. The N2 removal fraction (△N2/DIN, average 3.5 ± 4.3%) at the sites of rivers with positive △N2 was lower than that in other rivers around the world. The average N2 flux (0.8 ± 23.9 mmol m-2 d-1) in the urban rivers was also lower than that in other rivers. The low carbon-to‑nitrogen ratios in Beijing inland waters are not beneficial for N2 production during denitrification, and low nitrogen-to‑phosphorus ratios potentially favor N2 fixation with a high abundance of the nitrogenase nifH gene in the sediment, resulting in low net N2 production. The traditional paradigm is that rivers constantly lose vast N to the atmosphere via denitrification and anammox, but this study indicates that urban inland rivers emit negligible N even under high nitrogen loading.

Phytoplankton nutrient use and CO2 dynamics responding to long-term changes in riverine N and P availability

Long-term trends in riverine nutrient availability have rarely been linked to both phytoplankton composition and functioning. To explore how the changing availability of N and P affects not only phytoplankton abundance and composition but also the resource use efficiency of N, P, and CO₂, a 25-year time series of water quality in the lower Han River, Korea, was combined with additional measurements of riverine dissolved organic carbon (DOC) and CO₂. Despite persistent eutrophication, recent decreases in P relative to N have been steep in the lowest reach, increasing the annual mean mass ratio of N to P (N/P) from 24 (1994-2015) to 65 (2016-2018). While Chl a and cyanobacterial abundance exhibited overall positive and inverse relationships with P concentrations and N/P, respectively, severe harmful algal blooms (HABs) concurred with short-term increases in P and temperature. Microcystis often dominated HABs at low N/P that usually favors N-fixing cyanobacteria such as Anabaena. In the middle and lower reaches, phytoplanktonic P use efficiency was typically lower at low N/P. V-shaped relationships between N/P and CO₂ concentrations, together with longitudinal upward shifts in the inverse relationship between Chl a and CO₂, implied that eutrophication-enhanced phytoplankton biomass could turn into a significant source of CO_2. after passing a threshold. The combined results suggest that cyanobacterial dominance co-limited by P availability and temperature can lower planktonic P use efficiency, while enhancing riverine CO₂ emissions at low N/P ratios.

Effects of nFe3O4 capping on phosphorus release from sediments in a eutrophic lake

This study applied the techniques of high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films (DGT) to explore the effects and the behind mechanism for inhibition phosphorus (P) releasing from sediments by nFe₃O₄ capping. The highest decreasing rates of SRP and labile P (i.e., 49% and 47%, respectively) and the decreased flux of SRP showed that nFe₃O₄ capping can successfully control sediment internal P release. Adsorption by Fe(III) hydroxides with the oxidation of Fe(II) was one of the reasons for the decrease of P concentrations in nFe₃O₄ capping sediments. This was supported by the increase of Eh and significant negative correlation between Eh with Fe(II) (soluble and labile Fe(II)) and P (SRP and labile P) and significant positive correlation between Fe(II) and P in sediments by nFe₃O₄ capping. An outer-sphere complex between positively charged nFe₃O₄ surface groups and P formation was the other reason to decrease the concentrations of P in the nFe₃O₄ capping sediments. This was supported by the decrease of pH value in sediments by the capping of nFe₃O₄. This study shows that nFe₃O₄, when used as capping agent, can effectively control the sediment internal P release, which is expected to be used as a potential material for repairing lake eutrophication.

Alterations to sediment nutrient deposition and transport along a six reservoir sequence

Reservoir presence and construction has become commonplace along rivers due to the multitude of ecosystem services they provide. Many services are well recognized, including the effectiveness of sequestering both sediments and sediment-bound nutrients such as silts and phosphorus (P). Reservoirs are also capable of transforming or sequestering significant quantities of nutrients with more complex biogeochemical pathways, like nitrogen (N). Reservoir assessments, independent of inflow-outflow models, have primarily focused on a small number of systems creating a growing need to understand how reservoirs function both individually and as reservoir sequences within large rivers and their watersheds. Models have simulated the overall efficiency and drivers of reservoir nutrient deposition, but few have considered how a sequence of reservoirs alters deposition as an interdependent watershed-sediment-transport-system. In this study, we collected sediment cores from a six-reservoir sequence along a 5th - 6th order stream receiving treated waters from a large metropolitan area in the subtropical southeastern United States. Paleolimnological studies of subtropical reservoirs are underrepresented and are needed to understand the history of reservoir development. Using paleolimnological techniques and a known 30 year flux of riverine nutrient loading from waste water treatment facilities, we compared nutrient deposition to reservoir morphological qualities and primary producer community structure during the past ~50 years. Our findings suggest phosphorus deposition is associated with reservoir order downstream of the primary nutrient source, nitrogen deposition is linked to reservoir water retention time, and N:P is most strongly linked to reservoir surface area and watershed population density. Our results were strongly influenced by a large upstream and metropolitan nutrient source, common in large rivers, but under different conditions of nutrient loading (i.e. nonpoint source), reservoirs may express other nutrient depositional patterns.

Efficient recovery of phosphorus and sulfur from Anaerobic Membrane Bioreactor (AnMBR) permeate using chemical addition of iron and evaluation of its nutrient availability for plant uptake

Anaerobic membrane bioreactors (AnMBRs) represent an emerging environmental biotechnology platform with the potential to simultaneously recover water, energy, and nutrients from concentrated wastewaters. The removal and beneficial capture of nutrients from AnMBR permeate has yet to be fully explored, therefore this study sought to foster iron phosphate recovery through a tertiary coagulation process, as well as characterize the recovered nutrient product (RNP) and assess its net phosphorus release, diffusion, and availability for plant uptake. One of the primary goals of this study was to optimize the dose of the coagulant, ferric chloride, and coagulant aid, aluminum chlorohydrate (ACH), for continuous application to the coagulation-flocculation-sedimentation (CFS) unit of an AnMBR pilot plant treating municipal wastewater, through controlled bench-scale jar tests. Anaerobic systems present unique challenges for nutrient capture, including high, dissolved hydrogen sulfide concentrations, along with settleability issues. The addition of the coagulant aid increases settleability, while enhancing phosphorus removal by up to 20%, decreasing iron demand. Water quality analysis indicated that a variety of factors affect nutrient capture, including the COD (chemical oxygen demand) concentration of the permeate and the limiting coagulant dose. COD >200 mg/L was shown to decrease the phosphorus removal efficiency by up to 15%. A combination of inductively coupled plasma optical emission spectrometer (ICP-OES) elemental analysis, inductively coupled plasma mass spectrometer (ICP-MS) elemental analysis, scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES) spectroscopy analysis was used to characterize the P-rich RNP which revealed a 2.58% w/w phosphorus content and the lack of a well-defined crystalline structure. Detailed studies on resin extractable phosphorus to assess the plant uptake potential also demonstrated that iron-based P-rich RNPs may not be an effective fertilizer product, as they can act as a phosphorus sink in some agricultural systems instead of a source.

Cyanobacteria, Cyanotoxins, and Neurodegenerative Diseases: Dangerous Liaisons

The prevalence of neurodegenerative disease (ND) is increasing, partly owing to extensions in lifespan, with a larger percentage of members living to an older age, but the ND aetiology and pathogenesis are not fully understood, and effective treatments are still lacking. Neurodegenerative diseases such as Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis are generally thought to progress as a consequence of genetic susceptibility and environmental influences. Up to now, several environmental triggers have been associated with NDs, and recent studies suggest that some cyanotoxins, produced by cyanobacteria and acting through a variety of molecular mechanisms, are highly neurotoxic, although their roles in neuropathy and particularly in NDs are still controversial. In this review, we summarize the most relevant and recent evidence that points at cyanotoxins as environmental triggers in NDs development.

Integrating resilience with functional ecosystem measures: A novel paradigm for management decisions under multiple-stressor interplay in freshwater ecosystems

Moving beyond monitoring the state of water quality to understanding how the sensitive ecosystems "respond" to complex interplay of climatic and anthropogenic perturbations, and eventually the mechanisms that underpin alterations leading to transitional shifts is crucial for managing freshwater resources. The multiple disturbance dynamics-a single disturbance as opposed to multiple disturbances for recovery and other atrocities-alter aquatic ecosystem in multiple ways, yet the global models lack representation of key processes and feedbacks, impeding potential management decisions. Here, the procedure we have embarked for what is known about the biogeochemical and ecological functions in freshwaters in context of ecosystem resilience, feedbacks, stressors synergies, and compensatory dynamics, is highly relevant for process-based ecosystem models and for developing a novel paradigm toward potential management decisions. This review advocates the need for a more aggressive approach with improved understanding of changes in key ecosystem processes and mechanistic links thereof, regulating resilience and compensatory dynamics concordant with climate and anthropogenic perturbations across a wide range of spatio-temporal scales. This has relevance contexting climate change and anthropogenic pressures for developing proactive and adaptive management strategies for safeguarding freshwater resources and services they provide.

Nutrient mass balance of a large riverine reservoir in the context of water residence time variability

The excessive input of nitrogen (N) and phosphorus (P) from anthropogenic activities is the main reason behind the cultural eutrophication and algal blooms in freshwater ecosystems. Here, I present a comprehensive budget of N, P for a large reservoir (Lake Diefenbaker) within a highly cultivated watershed. I constructed a 4-year nutrient budget from 2011 to 2014, using grab samples and daily flow data, and a multi-decadal (1978 through 2014) budgeting to examine the effect of inter-annual variability of water residence time on retention of N and P, and if retention of N and P is affected differently. The 4-year budget showed that the reservoir was a net source of total nitrogen (TN) during 2011 and 2014, but a net sink during 2012 and 2013. This resulted in retention coefficients of - 35% and - 4% in 2011 and 2014, respectively. With respect to the total phosphorus (TP) budget, the reservoir acted as a net sink in all 4 years, with a mean retention coefficient of 87%. Consistent with findings of the 4-year budget, the results of the multi-decadal budgeting showed that the reservoir was a net sink for TP during the period of record with a mean retention of 81% (1583 t/year). Regarding TN, the mean retention was lower (49%, 4836 t/year) and more variable relative to TP over the long term. Unlike TP, the results showed that the retention of TN has been decreasing noticeably since 1978. Overall, the retention of TP in this lake is primarily controlled by in-lake sedimentation and most likely does not change substantially in response to inter-annual variation of hydraulic variables such as water residence time. For TN, the role of sedimentation could be minor in retention process in this reservoir (or similar reservoirs elsewhere), but in-lake biological processes could play a more important role. These findings are useful for understanding the role of larger reservoirs with water residence time of 1-3 years in nutrient retention and how changes in flow and water residence time due to climate variability and water management can influence the nutrient retention efficiency.

Coupled influence of pH and dissolved organic carbon on the immobilization of phosphorus by lanthanum-modified zeolite

Wind-driven waves and currents in shallow lakes frequently trigger the resuspension of sediments in the photic layer, which is characterized with a high pH and high dissolved organic carbon (DOC) concentration. The mechanism of phosphorus-inactivating agents (PIAs) immobilizing phosphorus under the coupled influence of pH and DOC is not clarified, and the applicability of PIAs in eutrophic shallow lakes is thus still doubtful. We found that, under the coupled influence of pH and DOC, the uptake of phosphate by LMZ was affected mainly by pH at low DOC concentrations and by DOC at high DOC concentrations. A high pH (9.3) and high DOC concentration (24.7 mg/L) greatly increased the release of phosphorus from sediment to water. However, the addition of LMZ substantially reduced the P concentrations in water, mainly via capture of dissolved inorganic phosphorus. The results of the reversibility of the adsorption of phosphates and DOC showed that phosphate had much higher affinity than DOC towards LMZ. The phosphate once adsorbed on LMZ was resistant to release when exposed to conditions of either a high pH (9.5), high DOC concentration (250 mg/L) or both; i.e., only <5% of the adsorbed phosphate is releasable. Therefore, we proposed that, to avoid the coupled influence of pH and DOC in the photic layer of eutrophic shallow lakes, LMZ could be applied in multiple low doses in the season when the growth of algae is minimal (a low pH and low DOC concentration).

Comparing Quantitative Methods for Analyzing Sediment DNA Records of Cyanobacteria in Experimental and Reference Lakes

Sediment DNA (sedDNA) analyses are rapidly emerging as powerful tools for the reconstruction of environmental and evolutionary change. While there are an increasing number of studies using molecular genetic approaches to track changes over time, few studies have compared the coherence between quantitative polymerase chain reaction (PCR) methods and metabarcoding techniques. Primer specificity, bioinformatic analyses, and PCR inhibitors in sediments could affect the quantitative data obtained from these approaches. We compared the performance of droplet digital polymerase chain reaction (ddPCR) and high-throughput sequencing (HTS) for the quantification of target genes of cyanobacteria in lake sediments and tested whether the two techniques similarly reveal expected patterns through time. Absolute concentrations of cyanobacterial 16S rRNA genes were compared between ddPCR and HTS using dated sediment cores collected from two experimental (Lake 227, fertilized since 1969 and Lake 223, acidified from 1976 to 1983) and two reference lakes (Lakes 224 and 442) in the Experimental Lakes Area (ELA), Canada. Relative abundances of Microcystis 16S rRNA (MICR) genes were also compared between the two methods. Moderate to strong positive correlations were found between the molecular approaches among all four cores but results from ddPCR were more consistent with the known history of lake manipulations. A 100-fold increase in ddPCR estimates of cyanobacterial gene abundance beginning in ~1968 occurred in Lake 227, in keeping with experimental addition of nutrients and increase in planktonic cyanobacteria. In contrast, no significant rise in cyanobacterial abundance associated with lake fertilization was observed with HTS. Relative abundances of Microcystis between the two techniques showed moderate to strong levels of coherence in top intervals of the sediment cores. Both ddPCR and HTS approaches are suitable for sedDNA analysis, but studies aiming to quantify absolute abundances from complex environments should consider using ddPCR due to its high tolerance to PCR inhibitors.

Analyzing long-term water quality of lakes in Rhode Island and the northeastern United States with an anomaly approach

Addressing anthropogenic impacts on aquatic ecosystems is a focus of lake management. Controlling phosphorus and nitrogen can mitigate these impacts, but determining management effectiveness requires long-term datasets. Recent analysis of the LAke multi-scaled GeOSpatial and temporal database for the Northeast (LAGOS-NE) United States found stable water quality in the northeastern and midwestern United States; however, sub-regional trends may be obscured. We used the University of Rhode Island's Watershed Watch Volunteer Monitoring Program (URIWW) dataset to determine if there were sub-regional (i.e., 3000 km2) water quality trends. URIWW has collected water quality data on Rhode Island lakes and reservoirs for over 25 yr. The LAGOS-NE and URIWW datasets allowed for comparison of water quality trends at regional and sub-regional scales, respectively. We assessed regional (LAGOS-NE) and sub-regional (URIWW) trends with yearly median anomalies calculated on a per-station basis. Sub-regionally, temperature and chlorophyll a increased from 1993 to 2016. Total nitrogen, total phosphorus, and the nitrogen:phosphorus ratio (N:P) were stable. At the regional scale, the LAGOS-NE dataset showed similar trends to prior studies of the LAGOS-NE with chlorophyll a, total nitrogen, and N:P all stable over time. Total phosphorus did show a very slight increase. In short, algal biomass, as measured by chlorophyll a in Rhode Island lakes and reservoirs increased, despite stability in total nitrogen, total phosphorus, and the nitrogen to phosphorus ratio. Additionally, we demonstrated both the value of long-term monitoring programs, like URIWW, for identifying trends in environmental condition, and the utility of site-specific anomalies for analyzing for long-term water quality trends.

Waste to phosphorus: A transdisciplinary solution to P recovery from wastewater based on the TRIZ approach

Phosphorus (P) is a limited yet essential resource. P cannot be replaced, but it can be recovered from waste. We proposed the TRIZ approach (Teoria reszenija izobretatielskich zadacz - Rus., Theory of Inventive Problem Solving - Eng.) to identify a feasible solution. We aimed at minimizing the environmental impact and, by eliminating contradictions, proposed viable technical solutions. P recovery can be more sustainable based on circular economy and 4Rs (reduction, recovery, reuse, and recycling). The TRIZ approach identified sewage sludge (SS) as waste with a large potential for P recovery (up to 90%). Successful selection and application of SS management and P recovery require a transdisciplinary approach to overcome the various socio-economic, environmental, technical, and legal aspects. The review provides an understanding of principles that must be taken to improve understanding of the whole process of P recovery from wastewater while building on the last two decades of research.

Omnivorous Carp (Carassius gibelio) Increase Eutrophication in Part by Preventing Development of Large-Bodied Zooplankton and Submerged Macrophytes

Fish, being an important consumer in aquatic ecosystems, plays a significant role by affecting the key processes of aquatic ecosystems. Omnivorous fish consume a variety of food both from pelagic and benthic habitats and may directly or indirectly affect the plankton community as well as the lake trophic state. We conducted a 72-day outdoor experiment in mesocosms with and without Prussian carp (Carassius auratus) to evaluate the effect of this often-stocked omnivorous fish on the plankton community and water quality. We found that the presence of fish increased the biomass of planktonic algae, total and inorganic suspended solids, leading to decreased light intensity in the water and a lower biomass of benthic algae. Fish also prevented development of submerged macrophytes and the establishment of large-bodied zooplankton. However, the fish did not increase nitrogen concentrations and even was lowered total phosphorus levels, in part due to nutrient storage in the fish. We conclude that stocking of Prussian carp should be avoided, or removed where stocked and abundant, to obtain good ecological quality of shallow lakes, characterized by clear water and high abundance of macrophytes.

The combined effects of macrophytes (Vallisneria denseserrulata) and a lanthanum-modified bentonite on water quality of shallow eutrophic lakes: A mesocosm study

Establishment of submerged macrophyte beds and application of chemical phosphorus inactivation are common lake restoration methods for reducing internal phosphorus loading. The two methods operate via different mechanisms and may potentially supplement each other, especially when internal phosphorous loading is continuously high. However, their combined effects have so far not been elucidated. Here, we investigated the combined impact of the submerged macrophyte Vallisneria denseserrulata and a lanthanum-modified bentonite (Phoslock®) on water quality in a 12-week mesocosm experiment. The combined treatment led to stronger improvement of water quality and a more pronounced reduction of porewater soluble reactive phosphorus than each of the two measures. In the combined treatment, total porewater soluble reactive phosphorus in the top 10 cm sediment layers decreased by 78% compared with the control group without Phoslock® and submerged macrophytes. Besides, in the upper 0-1 cm sediment layer, mobile phosphorus was transformed into recalcitrant forms (e.g. the proportion of HCl-P increased to 64%), while in the deeper layers, (hydr)oxides-bound phosphorus species increased 17-28%. Phoslock®, however, reduced the clonal growth of V. denseserrulata by 35% of biomass (dry weight) and 27% of plant density. Our study indicated that Phoslock® and submerged macrophytes may complement each other in the early stage of lake restoration following external nutrient loading reduction in eutrophic lakes, potentially accelerating the restoration process, especially in those lakes where the internal phosphorus loading is high.

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

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

Is the Cyanobacterial Bloom Composition Shifting Due to Climate Forcing or Nutrient Changes? Example of a Shallow Eutrophic Reservoir

Cyanobacterial blooms in eutrophic freshwater is a global threat to the functioning of ecosystems, human health and the economy. Parties responsible for the ecosystems and human health increasingly demand reliable predictions of cyanobacterial development to support necessary decisions. Long-term data series help with identifying environmental drivers of cyanobacterial developments in the context of climatic and anthropogenic pressure. Here, we analyzed 13 years of eutrophication and climatic data of a shallow temperate reservoir showing a high interannual variability of cyanobacterial development and composition, which is a less occurring and/or less described phenomenon compared to recurrant monospecific blooms. While between 2007–2012 Planktothrix agardhii dominated the cyanobacterial community, it shifted towards Microcystis sp. and then Dolichospermum sp. afterwards (2013–2019). The shift to Microcystis sp. dominance was mainly influenced by generally calmer and warmer conditions. The later shift to Dolichospermum sp. was driven by droughts influencing, amongst others, the N-load, as P remained unchanged over the time period. Both, climatic pressure and N-limitation contributed to the high variability of cyanobacterial blooms and may lead to a new equilibrium. The further reduction of P-load in parallel to the decreasing N-load is important to suppress cyanobacterial blooms and ameliorate ecosystem health.

The efficacy of Phoslock® in reducing internal phosphate loading varies with bottom water oxygenation

Eutrophication in lakes and reservoirs has prompted interest in using sediment capping technology to reduce the sediment contribution to internal nutrient loading. One such sediment capping technology is Phoslock®, a lanthanum-embedded clay, which can bind phosphate at the sediment surface and limit its diffusion into the water column. However, in well-oxygenated lakes, naturally occurring iron can bind phosphate by a similar mechanism. We sought to test the efficacy of Phoslock® in limiting phosphate (PO₄ ^3-) fluxes relative to untreated iron-rich lake sediment under conditions of bottom-water oxia and anoxia through laboratory batch core incubations of intact sediment cores from Jordan Lake, a reservoir in central North Carolina. We found that Phoslock® decreased phosphate fluxes relative to the control under anoxic conditions (7.5 ± 9.5 vs. 236 ± 74 µmol PO₄ ^3-•m^-2•d^-1), but provided no benefit relative to the control when the water column was oxygenated (4.5 ± 4.3 vs. 7.0 ± 11.4 µmol PO₄ ^3-•m^-2•d^-1). We also found that Phoslock® itself can act as a source of NH₄ ⁺ to Jordan Lake waters. Applied at recommended levels to the whole lake, Phoslock® addition would result in a pulse increase in water column NH₄ ⁺ concentrations of approximately 2.6 ± 0.8 μM (an increase of 10 to 275% compared to ambient).

Phosphorus recovery from wastewater using pyridine-based ion-exchange resins: Role of impregnated iron oxide nanoparticles and preloaded Lewis acid (Cu2+ )

Inputs of P into receiving water bodies are attracting increasing attention due to the negative effects of eutrophication. Presently available P treatment technologies are unable to achieve strict P discharge limits from wastewater treatment plants (WWTPs) that may be as low as 10 µg/L as P. Moreover, P is a nonrenewable resource and needs to be recycled in a closed-loop process for environmental sustainability. This article provides details of a process where a pyridine-based polymeric ion exchanger is modified with a combination of impregnated hydrated ferric oxide (HFO) nanoparticles and a preloaded Lewis acid (Cu²⁺ ) to effectuate selective P removal from wastewater and its recovery as a solid-phase fertilizer. Three such ion exchangers were studied: DOW-HFO, DOW-Cu, and DOW-HFO-Cu. Each of these materials displays selective phosphate affinity over competing anions chloride and sulfate, and also has the ability to be regenerated upon exhaustion to strip off the P in a concentrated solution. The P in concentrated regenerant can be recovered as struvite, MgNH₄ PO₄ , a slow-release fertilizer, after addition of MgCl₂ and NH₄ Cl. Results of equilibrium and kinetic studies and column experiments with synthetic solutions and a real WWTP effluent are discussed. PRACTITIONER POINTS: Fixed-bed columns with DOW-HFO, DOW-Cu, or DOW-HFO-Cu-can selectively remove phosphorus over competing anions. Fixed-bed columns of above-listed ion exchangers can produce an effluent P < 6 μg/L. DOW-Cu fixed-bed column ran for ≈500 Bed Volumes before breakthrough when fed Dartmouth WWTP secondary effluent. Regeneration of the exhausted DOW-Cu column resulted in ≈90% recovery of the phosphorus. Regenerant solution was used to generate high-purity crystals of magnesium ammonium phosphate, MgNH₄ PO₄ (struvite), a slow-release fertilizer.

Seasonal Distribution of Cyanobacteria in Three Urban Eutrophic Lakes Results from an Epidemic-like Response to Environmental Conditions

Cyanobacterial communities of three co-located eutrophic sandpit lakes were surveyed during 2016 and 2017 over season and depth using high-throughput DNA sequencing of the 16S rRNA gene. All three lakes were stratified except during April 2017 when the lakes were recovering from a strong mixing event. 16S rRNA gene V4 sequences were parsed into operational taxonomic units (OTUs) at 99% sequence identity. After rarefaction of 139 samples to 25,000 sequences per sample, a combined total of 921,529 partial 16S rRNA gene sequences were identified as cyanobacteria. They were binned into 19,588 unique cyanobacterial OTUs. Of these OTUs, 11,303 were Cyanobium. Filamentous Planktothrix contributed 1537 and colonial Microcystis contributed 265. The remaining 6482 OTUs were considered unclassified. For Planktothrix and Microcystis one OTU accounted for greater than 95% of the total sequences for each genus. However, in both cases the non-dominant OTUs clustered with the dominant OTUs by date, lake, and depth. All Planktothrix OTUs and a single Cyanobium OTU were detected below the oxycline. All other Cyanobium and Microcystis OTUs were detected above the oxycline. The distribution of Cyanobium OTUs between lakes and seasons can be explained by an epidemic-like response where individual OTUs clonally rise from a diverse hypolimnion population when conditions are appropriate. The importance of using 99% identity over the more commonly used 97% is discussed with respect to cyanobacterial community structure. The approach described here can provide another valuable tool for assessing cyanobacterial populations and provide greater insight into the controls of cyanobacterial blooms.

Removal of Antibiotics and Nutrients by Vetiver Grass (Chrysopogon zizanioides) from a Plug Flow Reactor Based Constructed Wetland Model

Overuse of antibiotics has resulted in widespread contamination of the environment and triggered antibiotic resistance in pathogenic bacteria. Conventional wastewater treatment plants (WWTPs) are not equipped to remove antibiotics. Effluents from WWTPs are usually the primary source of antibiotics in aquatic environments. There is an urgent need for cost-effective, environment-friendly technologies to address this issue. Along with antibiotics, nutrients (nitrogen and phosphorus) are also present in conventional WWTP effluents at high concentrations, causing environmental problems like eutrophication. In this study, we tested vetiver grass in a plug flow reactor-based constructed wetland model in a greenhouse setup for removing antibiotics ciprofloxacin (CIP) and tetracycline (TTC), and nutrients, N and P, from secondary wastewater effluent. The constructed wetland was designed based on a previous batch reaction kinetics study and reached a steady-state in 7 days. The measured concentrations of antibiotics were generally consistent with the modeling predictions using first-order reaction kinetics. Vetiver grass significantly (p < 0.05) removed 93% and 97% of CIP and TTC (initial concentrations of 10 mg/L), simultaneously with 93% and 84% nitrogen and phosphorus, respectively. Results show that using vetiver grass in constructed wetlands could be a viable green technology for the removal of antibiotics and nutrients from wastewater.

Prediction of stream nitrogen and phosphorus concentrations from high-frequency sensors using Random Forests Regression

Stream nutrient concentrations exhibit marked temporal variation due to hydrology and other factors such as the seasonality of biological processes. Many water quality monitoring programs sample too infrequently (i.e., weekly or monthly) to fully characterize lotic nutrient conditions and to accurately estimate nutrient loadings. A popular solution to this problem is the surrogate-regression approach, a method by which nutrient concentrations are estimated from related parameters (e.g., conductivity or turbidity) that can easily be measured in situ at high frequency using sensors. However, stream water quality data often exhibit skewed distributions, nonlinear relationships, and multicollinearity, all of which can be problematic for linear-regression models. Here, we use a flexible and robust machine learning technique, Random Forests Regression (RFR), to estimate stream nitrogen (N) and phosphorus (P) concentrations from sensor data within a forested, mountainous drainage area in upstate New York. When compared to actual nutrient data from samples tested in the laboratory, this approach explained much of the variation in nitrate (89%), total N (85%), particulate P (76%), and total P (74%). The models were less accurate for total soluble P (47%) and soluble reactive P (32%), though concentrations of these latter parameters were in a relatively low range. Although soil moisture and fluorescent dissolved organic matter are not commonly used as surrogates in nutrient-regression models, they were important predictors in this study. We conclude that RFR shows great promise as a tool for modeling instantaneous stream nutrient concentrations from high-frequency sensor data, and encourage others to evaluate this approach for supplementing traditional (laboratory-determined) nutrient datasets.

Primacy of ecological engineering tools for combating eutrophication: An ecohydrological assessment pathway

Eutrophication of freshwater bodies causes loss of earth's biological resources and aggravates climate change, thus assuming major environmental concern. Both endogenous and exogenous nutrient enrichment are responsible for eutrophication. Numerous monitoring and management studies conducted worldwide have resulted high-level technological innovations. These studies cumulatively uphold the significance of ecohydrological and ecological engineering approaches. However, holistic and insightful reviews with feasible recommendations of such huge academic outputs are rather scanty. Therefore, our main objective was to introduce a new perspective of eutrophication as an ecohydrological component; to discover all possibilities of monitoring and restoration of eutrophic water bodies. Furthermore, the present study critically analyzes various methods of treatment of eutrophication (physical, biological, chemical, and eco-engineering). Comprehensive volume of literature has been surveyed using search engines like Scopus, Google Scholar, PubMed, ScienceDirect etc. Meaningful keywords were used to obtain reliable information on methods of ecohydrological assessment in relation to eutrophication of freshwater bodies. According to our survey, ecohydrological research is diversified into conceptual knowledge (37.2%), assessment (32.6%), climate change (9.3%), algae/cyanotoxins (7%), engineering and restoration (7%), modelling (4.6%) and biodiversity (2.3%), in the instant decade (2010-2020). We have identified a clear trend of transition of restoration methods from traditional towards modern techniques over time. Moreover, this review recognizes a pool of biophysicochemical and ecological engineering techniques, which are very effective in regard to time, cost, and labor and have immense scopes of modification for improved results. This work focuses on the importance of ecohydrology and eco-engineering tools for restoration of eutrophic water bodies for the first time. We have highlighted how these approaches have emerged as one of the best suitable and sustainable water resource conservation routes in the present era.

Review of characterization, factors, impacts, and solutions of Lake eutrophication: lesson for lake Tana, Ethiopia

Lake eutrophication and water quality deterioration have become a major environmental problem in urban areas and fertilized basins in developing countries across the world. This paper reviews the characterization, driving factors, and impacts of lake eutrophication as well as the mechanism of preventing and recovering lake eutrophication with case studies of eutrophic lakes across the world including Lake Tana, Ethiopia. In most waterbodies including lakes and reservoirs, total phosphorus concentration, chlorophyll a concentration, and Secchi disk visibility in association with species composition are the common criteria to classify lakes and reservoir as oligotrophic, mesotrophic, and eutrophic. Nutrient-rich runoff from cultivated land and industrialized and urbanized cities concentrated in phosphorus are the critical factors that drove eutrophication in water bodies. Among others, controlling external loading of nutrient, ecological, and mechanical methods were found to be common mechanisms to prevent and recover lake eutrophication. Avoiding the factors that are under human control, i.e., a reduction of external loading of nutrients especially targeted on phosphorus reduction into the water basins, relocates sewage, industrial and domestic waste discharges to be lined out of the catchment of the lake. Furthermore, motivating the community to use less phosphorus-containing fertilizers and promoting phosphorus-free detergents are suggested solutions to sustainably prevent and reduce eutrophication in the long run. These could be some possible measures to safeguard endangered Lake Tana of Ethiopia.

Combined impact of organic matter, phosphorus, nitrate, and ammonia nitrogen on the process of blackwater

Blackwater events are frequently reported over the world and become a serious environmental problem. However, the mechanisms of blackwater occurrence are not fully understood yet. This study simulated the process of blackwater with the combined pollution in an orthogonal experiment, which had 4 factors (TOC, TP, NH4+-N, and NO3--N) and 4 levels (None, Low, Middle, and High). Results showed that the process of water condition changes was divided into two parts, which were "exogenous" and "algae-derived" blackwater, and the influence of four different pollutants on the occurrence of the blackwater was ranked as follows: TOC > TP > NO3--N > NH4+-N. With the increase of organic matter addition, the anaerobic condition in water was prolonged and the concentration of Fe2+ had a significant increase. In addition, under the None phosphorus condition, the descent rates of DO and COD in the water were reduced, and the algae bloom was obviously deferred. Moreover, the addition of organic matter or phosphorus changed the microbial community structure and led to different water processes. Particularly, only on the condition of the high content of TOC and phosphorus, the diversity of sulfate-reducing bacteria (e.g., Pseudomonas, Paludibacter, and Bacteroides) increased significantly, which accounted for 51.4%, causing the significant production of S2- in the water. Water's lack of phosphorus showed a low rate of decomposition of organic matter, which might be the result of a considerable increase in the abundance of aerobic Trichococcus and Malikia. This study shows that organic matter and phosphorus have synergistic effect on blackwater occurrence. In the treatment of blackwater, the exogenous pollutant control should reduce the discharge of organic pollutants, and endogenous control should focus on phosphorus abatement and reduce nitrogen control.

Phosphorous Supply to a Eutrophic Artificial Lake: Sedimentary versus Groundwater Sources

The eutrophic Lake Eichbaumsee, a ~1 km long and 280 m wide (maximum water depth 16 m) dredging lake southeast of Hamburg (Germany), has been treated for water quality improvements using various techniques (i.e., aeration plants, removal of dissolved phosphorous by aluminum phosphorous precipitation, and by Bentophos® (Phoslock Environmental Technologies, Sydney, Australia), adsorption) during the past ~15 years. Despite these treatments, no long-term improvement of the water quality has been observed and the lake water phosphorous content has continued to increase by e.g., ~670 kg phosphorous between autumn 2014 and autumn 2019. As no creeks or rivers drain into the lake and hydrological groundwater models do not suggest any major groundwater discharge into the lake, sources of phosphorous (and other nutrients) are unknown. We investigated the phosphorous fluxes from sediment pore water and from groundwater in the water body of the lake. Sediment pore water was extracted from sediment cores recovered by divers in August 2018 and February 2019. Diffusive phosphorous fluxes from pore water were calculated based on phosphorus gradients. Stable water isotopes (δ2H, δ18O) were measured in the lake water, in interstitial waters in the banks surrounding the lake, in the Elbe River, and in three groundwater wells close to the lake. Stable isotope (δ2H, δ18O) water mass balance models were used to compute water inflow/outflow to/from the lake. Our results revealed pore-water borne phosphorous fluxes between 0.2 mg/m2/d and 1.9 mg/m2/d. Assuming that the measured phosphorous fluxes are temporarily and spatially representative for the whole lake, about 11 kg/a to 110 kg/a of phosphorous is released from sediments. This amount is lower than the observed lake water phosphorous increase of ~344 kg between April 2018 and November 2018. Water stable isotope (δ2H, δ18O) compositions indicate a water exchange between an aquifer and the lake water. Based on stable isotope mass balances we estimated an inflow of phosphorous from the aquifer to the lake of between ~150 kg/a and ~390 kg/a. This result suggests that groundwater-borne phosphorous is a significant phosphorous source for the Eichbaumsee and highlights the importance of groundwater for lake water phosphorous balances.