Interactions between suspended particulate matter and algal cells contributed to the reconstruction of phytoplankton communities in turbulent waters

Suspended particulate matter-biofilm aggregates benefit microcystin removal in turbulent water but trigger toxicity toward Daphnia magna

Suspended particulate matter (SPM) and biofilm are critical in removing contaminants in aquatic environments, but the environmental behavior and ecological toxicity of SPM-biofilm aggregates modulated by turbulence intensities are largely unknown. This study determined the removal pathways of microcystin-LR (MC-LR) by SPM and its biofilm under different turbulence intensities (2.25 × 10-3, 1.01 × 10-2, and 1.80 × 10-2 m2/s3). Then, we evaluated the toxicity of SPM-biofilm aggregates to Daphnia magna. The results revealed that SPM contributed to the adsorption of MC-LR, and the removal of MC-LR can be accelerated with biofilm formation on SPM, with 95.66 % to 97.45 % reduction in MC-LR concentration under the studied turbulence intensities. Higher turbulence intensity triggered more frequent contact of SPM and MC-LR, formed compact but smaller clusters of SPM-biofilm aggregates, and enhanced the abundance of mlrA and mlrB; thus benefiting the adsorption, biosorption, and biodegradation of MC-LR. Furthermore, the SPM-biofilm aggregates formed in turbulent water triggered oxidative stress to Daphnia magna, while a weak lethal toxic effect was identified under moderate turbulence intensity. The results indicate that the toxicity of SPM-biofilm aggregates fail to display a linear relationship with turbulence intensity. These findings offer new perspectives on understanding the environmental behavior and ecological outcomes of SPM and its biofilms in turbulent aquatic environments.

Principles and research progress of physical prevention and control technologies for algae in eutrophic water

The abnormal reproduction of algae in water worldwide is prominent in the context of human interference and global climate change. This study first thoroughly analyzed the effects of physical factors, such as light, temperature, hydrodynamics, and operational strategies, on algal growth and their mechanisms. Physical control techniques are safe and have great potential for preventing abnormal algal blooms in the absence of chemical reagents. The focus was on the principles and possible engineering applications of physical shading, ultrasound, micro-current, and ultraviolet (UV) technologies, in controlling abnormal algal reproduction. Physical shading can inhibit or weaken photosynthesis in algae, thereby inhibiting their growth. Ultrasound mainly affects the physiological and biochemical activities of cells by destroying the cell walls, air cells, and active enzymes. Micro-currents destroy the algal cell structure through direct and indirect oxidation, leading to algal cell death. UV irradiation can damage DNA, causing organisms to be unable to reproduce or algal cells to die directly. This article comprehensively summarizes and analyzes the advantages of physical prevention and control technologies for the abnormal reproduction of algae, providing a scientific basis for future research. In the future, attempts will be made toward appropriately and comprehensively utilizing various physical technologies to control algal blooms. The establishment of an intelligent, comprehensive physical prevention and control system to achieve environmentally friendly, economical, and effective physical prevention and control of algae, such as the South-to-North Water Diversion Project in China, is of great importance for specific waters.

Effects of microplastics on sedimentary geochemical properties and microbial ecosystems combined with hydraulic disturbance

Microplastics (MPs) pollution is widely investigated owing to its potential threats to river ecosystems. However, it remains unclear whether hydraulic disturbance deepens or mitigates the effects of MPs-contaminated sediments on the river environment. Herein, we studied the impact of sediment aggregates, organic matter, and enzyme activity, with emphasis on microbial community structure and function in sediments exposed to MPs (1 %, 5 %, and 10 % w/w) in conjunction with hydraulic disturbance. The experimental results showed that the influence of MPs on the sediment under hydraulic disturbance is more significant than that of static culture, especially for various environmental factors (MWD, MBC, and sucrase activity etc.). The proportions of the >0.05 mm-fraction aggregates increased from 74-76 % to 82-88 % in the sediment throughout the entire disturbance process. It has been found that the disturbance generally promotes the interaction between MPs and sediments. FAPROTAX analysis demonstrated that the disturbance reduced the difference in effects on microbial functional genes between the control group and the MPs-added groups by up to 10 times, suggesting that the effects of disturbance on MPs-contaminated sediments are relatively complex. This work provides new insights into the effects of hydraulic disturbance on physicochemical properties and microbial communities of MPs-contaminated sediment.

Combination of magnetic field and ultraviolet for fouling control in saline wastewater distribution systems

Fouling is a significant challenge for recycling and reusing saline wastewaters for industrial, agricultural or municipal applications. In this study, we propose a novel approach of magnetic field (MaF) and ultraviolet (UV) combined application for fouling mitigation. Results showed, combination of MaF and UV (MaF-UV) significantly decreased the content of biofouling and reduced the complexity of microbial networks, compared to UV and MaF alone treatments. This was due to MaF as pretreatment effectively reduced the water turbidity, improve the influent water quality of UV disinfection and increases UV transmittance, eliminating the adverse impacts of UV scattering and shielding, hence increased the inactivation effectiveness of UV disinfection process. MaF assisted UV also reduced the abundance of UV-resistant bacteria and inhibited the risk of bacterial photoreactivation and dark repair. Meanwhile, MaF-UV drastically reduced the contents of precipitates and particulate fouling by accelerating the transformation rate of CaCO3 crystal from compact calcite to loosen hydrated amorphous CaCO3, and enhancing the flocculation process. These findings demonstrated that MaF-UV is an effective anti-fouling strategy, and provide insights for sustainable application of saline wastewaters.

Colonial Microcystis' biomass affects its shift to diatom aggregates under aeration mixing

The effect of hydrodynamic mixing on controlling Microcystis blooms or changing the algal community to diatom dominance has been widely studied; however, the effects of colonial Microcystis biomass on the development of the algal community are poorly known. Here, in order to study the changes in Microcystis blooms under continuous aeration mixing, an experiment was carried out in a greenhouse with factors of varying biomass of Microcystis and inorganic nitrogen and phosphorus enrichment in summer. There were three chlorophyll a (Chl-a) levels in six treatments: low Chl-a level of 68.4 μg L-1 (treatments L, L-E), medium Chl-a level of 468.7 μg L-1 (treatments M, M-E), and high Chl-a level of 924.1 μg L-1 (treatments H, H-E). Treatments L-E, M-E and H-E were enriched with the same inorganic nitrogen and phosphorus nutrients. During the experiment of 30 days, the concentration of Microcystis and Chl-a decreased, and diatom Nitzschia palea cells appeared in all the treatments, which became dominant in treatments M, M-E, H and H-E, with the highest biomass of 9.41 ± 1.96 mg L-1 Nitzschia in treatment H-E on day 30. The rank order of the biomass of Nitzschia from low to high was (L = L-E) < (M = M-E) < H < H-E (P < 0.05). In addition, Nitzschia cells were aggregates attached to Microcystis colonies in all the treatments. The results showed that the initial biomass of colonial Microcystis affected the algal shift from Microcystis dominance to Nitzschia dominance. However, the enriched inorganic nitrogen and phosphorus was beneficial for the Nitzschia increase in the high biomass treatment alone. The shift from Microcystis dominance to diatom dominance under continuous aeration mixing may be caused by low light conditions as well as the nutrients released from Microcystis decay. Moreover, the aerobic condition caused by aeration mixing maintained the colonial mucilaginous sheath to support the growth of Nitzschia cells in aggregation. This study found for the first time that Microcystis blooms could shift to diatom Nitzschia dominance in aggregates. It provided a method to control and manipulate Microcystis blooms to diatom dominance through continuous aeration mixing to proper biomass of Microcystis colonies. The shift to diatoms dominance would provide more high quality food organisms for aquaculture and be beneficial to the material cycling and energy flowing in food web dynamics.

Current status and prospects of algal bloom early warning technologies: A Review

In recent years, frequent occurrences of algal blooms due to environmental changes have posed significant threats to the environment and human health. This paper analyzes the reasons of algal bloom from the perspective of environmental factors such as nutrients, temperature, light, hydrodynamics factors and others. Various commonly used algal bloom monitoring methods are discussed, including traditional field monitoring methods, remote sensing techniques, molecular biology-based monitoring techniques, and sensor-based real-time monitoring techniques. The advantages and limitations of each method are summarized. Existing algal bloom prediction models, including traditional models and machine learning (ML) models, are introduced. Support Vector Machine (SVM), deep learning (DL), and other ML models are discussed in detail, along with their strengths and weaknesses. Finally, this paper provides an outlook on the future development of algal bloom warning techniques, proposing to combine various monitoring methods and prediction models to establish a multi-level and multi-perspective algal bloom monitoring system, further improving the accuracy and timeliness of early warning, and providing more effective safeguards for environmental protection and human health.

Improved eutrophication model with flow velocity-influence function and application for algal bloom control in a reservoir in East China

Improving hydrodynamic conditions is considered an effective method for facilitating the eutrophication management. However, the effect of hydrodynamic conditions on algal growth has rarely been quantified. In this work, a eutrophication model was developed and flow velocity was introduced into the algae growth kinetic formula to simulate the dynamics of algae growth in a drinking water source reservoir in East China. Based on the previous research and model calibration, the flow velocity-influence function f(v) and its parameters were determined. Accordingly, the optimal flow velocity for the dominant algae growth and critical flow velocity for algal growth inhibition were presented to be 0.055 m/s and 0.200 m/s for the study reservoir. Modeled results considering f(v) agreed with better with observations and reproduced the algal overgrowth process more accurately. The spatial-temporal differences in chlorophyll a (Chl a) concentration distribution during the algal proliferation period were analyzed on the basis of simulation results, which corroborated the significant influence of flow velocity on algal growth. The established model was applied to investigate the effect of improvement in hydrodynamic conditions on algal bloom control in the reservoir, and the scenario simulation of the additional sluice was conducted. Results showed that the additional sluice operation inhibited algal overgrowth effectively, resulting in an average decrease of 24.8%, 3.3%, 43.0%, and 37.5% in modeled Chl a concentration upstream north, upstream south, midstream and downstream, respectively. The established model might serve as a practical tool for eutrophication management in the study reservoir and other water bodies with similar hydrological characteristics and geographical features.

Environmental heterogeneity of a tropical river-to-sea continuum and its relationship with structure and phytoplankton dynamics - Lençóis Maranhenses National Park

β-diversity and functional traits of phytoplankton indicators associated with environmental heterogeneity were investigated as environmental quality descriptors in coastal (CS), estuarine (ES), and limnetic (LS) sectors in a tropical river-to-sea ecosystem. Results showed that environmental heterogeneity was marked by spatial differences, contributing to biological heterogeneity. Sporadic and recurrent blooms were associated with environmental spatiotemporal variations and reflected a reduction in ES α-diversity. Salinity acted as an environmental filter that governed the structure and dynamics of the community. The spatial heterogeneity and high turnover of phytoplankton resulted in reliable bioindicators selection. Colonial, bloom-forming and harmful species were associated with highly suspended particulate matter (SPM) because these species are better adapted to these conditions. Species small in size were associated with high concentrations of silicate and chlorophyll-a in the ES because of the occurrence of diatom recurrent blooms. Most flagellates indicators genera have bloom-forming potential. Integrating morphofunctional with taxonomic approaches enabled detailed observations of environmental filters, supporting the selection of priority species and areas for introducing biodiversity monitoring programs and conservation in tropical ecosystems.

Response of internal phosphorus loading from dredged and inactivated sediment under repeated resuspension in a eutrophic shallow lake

In this study, a laboratory sediment resuspension simulation system (RSS) was used to investigate the effect of wind-induced (5.3 and 8.7 m/s) repeated sediment resuspension on internal phosphorus (P) in sediment treated by dredging and La-modified clay (LMC) based inactivation in a shallow lake. The results indicated that the dredged sediment had a better capability to resist repeated wind disturbance than the LMC-inactivated sediment. The concentration of suspended solids (SS) in the inactivated treatment (70.7 mg/L) was 1.7 times that in the dredged treatment (41.7 mg/L) under moderate wind disturbance, and was similar for the two treatments under strong wind disturbance. Nevertheless, dredging performed better than inactivation in reducing 44 % total phosphorus (TP) in overlying water (43 % reduction by inactivation) and 31 % mobile P in sediment (27 % reduction by inactivation) under moderate wind disturbance (p < 0.01) compared with control treatment. Inactivation performed better in reducing 57 % P in porewater (52 % reduction by dredging) and 81 % P flux (13 % reduction by dredging) (p < 0.01) compared with control treatment. Surprisingly, under strong wind disturbance, LMC inactivation could still reduce 18 % P in porewater and 75 % P flux (p < 0.01), whereas dredging increased 25 % P in porewater and 13 % P flux compared with control treatment (p < 0.01). LMC inactivation can increase the sediment P adsorption capacity and decrease the equilibrium P concentration (EPC₀) when compared with control treatment. The contrasting control effects of the two methods were probably due to the different P buffer mechanisms for the two treated sediment. The wind disturbance-induced sediment P release was controlled by the inactivation of Fe and co-inactivation of Fe and La at the surface of dredged and LMC-inactivated sediments, respectively. The results of this study indicated inactivation can be a better method to control sediment internal P loading with repeated strong wind disturbances in eutrophic lakes.

Separate effect of turbulent pulsation on internal mass transfer in porous biofilms

Turbulence is considered to be the result of coupled time-averaged and pulsating velocities, making it difficult to distinguish the respective effects, and the quantitative effect of turbulent pulsation alone on mass transfer within biofilms has not been discussed in the literature. In this study, we constructed a special oscillating grid biofilm reactor combining Particle Image Velocimetry (PIV) measurements and Computational Fluid Dynamics (CFD) simulations to achieve nearly isotropic turbulence in a designed ambient without time-averaged velocity and shear stress. Subsequently, velocity and contaminant concentration distributions were obtained by solving a mass transfer model with a k-ε turbulence model, combined with measurements of biofilm structure parameters. The results showed that the increase in turbulent pulsation intensity led to a significant stratification of the percolation velocity gradient in biofilms, which enhanced convective mass transfer. The changes of biofilm density and porosity under turbulent pulsation were more strongly correlated with convective mass transfer. When the turbulent intensity (q) increased to 2.50 cm s^-1, the removal rate reached the highest value of 96.93%, accelerating the migration of contaminant concentration and the diffusive mass transfer effect was obvious. In addition, the trend of biofilm thickness under turbulent pulsation was consistent with the change of contaminant concentration distribution, and the correlation between them was greater. In summary, at q of 2.50 cm s^-1, there was a positive effect on both convection and diffusion mechanisms in biofilms, and the contaminant removal rate and biofilm thickness reached the maximum, which was the recommended turbulent pulsation conditions.

Effect of different mixing modes on the abundance of Microcystis in Lake Taihu under green light

Light and hydrodynamic force are important physical factors affecting growth of Microcystis. The most recent study found that green light has good effect in inhibiting growth of Microcystis. To understand the effect of mixing modes on Microcystis under the green light, we investigated the effects of continuous mixing and intermittent mixing on the abundance of Microcystis in Taihu Lake under field conditions. The study results found that abundance of Microcystis in control, intermittent mixing group, and continuous mixing group decreased 76.62%, 40.36%, and 95.18% on day 7 compared with that on day 1 in this experiment. At the end of the experiment, abundance percentages of diatoms and green algae to total phytoplankton abundance were 1.57% and 0.48% in control, 2.32% and 0.67% in intermittent mixing group, and 22.47% and 20.27% in continuous mixing group. The results indicated that continuous mixing favored the removal of Microcystis under green light conditions and was helpful for the growth of green algae and diatoms. The results provide a new approach for the control of Microcystis blooms.

Changes in the water environment and its major driving factors in Poyang Lake from 2016 to 2019, China

The hydrological exchange process between Poyang Lake (PYL), the largest freshwater lake in China, and the Yangtze River leads to drastic changes in water area (WA) and water level (WL), as well as apparent fluctuations in lake nutrients, algal organisms, and trophic level index. This study investigated the current status of the PYL water environment and the influence of hydrological changes on the nutrient status of the floodplain of the lake. Based on monthly measured data from six hydrological stations from 2016 to 2019, it was hypothesized that WA and WL were the key regulators of the spatial and temporal distribution patterns of lake water quality and algal growth, including water temperature, water clarity (Secchi depth [SD]), and nutrient levels. The results revealed that (1) the spatial and temporal distribution characteristics of major nutrients in PYL were influenced by dynamic changes in hydrological characteristics (SD, total nitrogen [TN], and total phosphorus [TP]); (2) the eutrophication level in PYL has been in a steady state in recent years, while the central area has been more prone to the risk of eutrophication (e.g., the peak eutrophication index during Period 1 [January to April] in the water near the Duchang station reached 70); and (3) there were significant correlations among environmental variables, nutrients, and algal organisms, with different spatial and temporal distribution characteristics (p < 0.05), while the changes in WA and WL considerably influenced the water environment in the PYL.

Biogeochemical dynamics of particulate organic phosphorus and its potential environmental implication in a typical "algae-type" eutrophic lake

Organic phosphorus (P_o) plays a very important role in the process of lake eutrophication, but there is still a lack of knowledge about the internal cycle of P_o in suspended particulate matter (SPM) dominated by algal debris. In this study, the characterization of bioavailable P_o by sequential extraction and enzymatic hydrolysis showed that 45% of extracted TP was P_o in SPM of Lake Dianchi, and 43-98% of total P_o in H₂O, NaHCO₃ and NaOH fractions was enzymatically hydrolyzable P_o (EHP, H₂O-EHP: 31-53%). Importantly, labile monoester P was the main organic form (68%) of EHP, and its potential bioavailability was higher than that of diester P and phytate-like P. According to the estimation of P pools in SPM of the whole lake, the total load of P_i plus EHP in the H₂O extract of SPM was 74.9 t and had great potential risk to enhance eutrophication in the lake water environment. Accordingly, reducing the amount of SPM in the water during the algal blooming period is likely to be a necessary measure that can successfully interfere with or block the continuous stress of unhealthy levels of P on the aquatic ecosystem.

Fate of dissolved inorganic nitrogen in turbulent rivers: The critical role of dissolved oxygen levels

Dissolved inorganic nitrogen (DIN) is considered the main factor that induces eutrophication in water, and is readily influenced by hydrodynamic activities. In this study, a 4-year field investigation of nitrogen dynamics in a turbulent river was conducted, and a laboratory study was performed in the approximately homogeneous turbulence simulation system to investigate potential mechanisms involved in DIN transformation under turbulence. The field investigation revealed that, contrary to NO^-₃ dynamics, the NH⁺₄ concentrations in water were lower in flood seasons than in drought seasons. Further laboratory results demonstrated that limitation of dissolved oxygen (DO) caused inactive nitrification and active denitrification in static river sediment. In contrast, the increased DO levels in turbulent river intensified the mineralization of organic nitrogen in sediment; moreover, ammonification and nitrification were activated, while denitrification was first activated and then depressed. Turbulence therefore decreased NH⁺₄ and NO^-₂ concentrations, but increased NO^-₃ and total DIN concentrations in the overlying water, causing the total DIN to increase from 0.4 mg/L to maximum of 1.0 and 1.7 mg/L at low and high turbulence, respectively. The DIN was maintained at 0.7 and 1.0 mg/L after the 30-day incubation under low and high turbulence intensities (ε) of 3.4 × 10^-4 and 7.4 × 10^-2 m²/s³, respectively. These results highlight the critical role of DO in DIN budgets under hydrodynamic turbulence, and provide new insights into the DIN transport and transformation mechanisms in turbulent rivers.

Interactions between suspended sediments and submerged macrophytes-epiphytic biofilms under water flow in shallow lakes

Suspended sediments (SS) pollution is one of the factors affecting the transfer from turbid water state to clear water state in shallow lakes. However, the interactions between suspended sediments and submerged plants are far from clear. In this study, we investigated the settlement laws of SS in overlying water and its impact on the epiphytic biofilm of Myriophyllum verticillatum and Vallisneria natans under water flow. At least 90% of turbidity can be removed from overlying water, and the decreasing trend of water turbidity fitted the logarithmic decay model in all treatments. The size distribution of SS fit the log-normal distribution model in the first 240 min after SS addition. It should be noted that the main peak particle sizes were lower in treatments with submerged macrophytes (8.71-13.18 μm) than without plants (15.14-19.95 μm). Water flow and SS addition significantly increased the thickness of biofilms attached to M. verticillatum (p < 0.05), but they together significantly reduced the biofilm thickness on V. natans (p < 0.05). SS increased the bacterial α-diversity but decreased eukaryotic one in epiphytic biofilms. However, water flow had a more significant impact on microbial communities (especially eukaryotes) than SS and plant species. The relative abundances of dominant phylum Proteobacteria, class Alphaproteobacteria and Betaproteobacteria, and class Verrucomicrobiae increased in epiphytic biofilms after SS addition. Co-occurrence networks reveal that photosynthetic microbes in epiphytic biofilms played an important role in microbial communities under water flow and SS, and many hub microbes were increased by SS addition but reduced by water flow. These data highlight that SS decline can be predicted by the logarithmic decay model and, SS and water flow can affect the epiphytic-biofilm on submerged macrophytes.

Drivers of Spatiotemporal Eukaryote Plankton Distribution in a Trans-Basin Water Transfer Canal in China

Planktonic eukaryotes are important components of aquatic ecosystems, and analyses of the whole eukaryotic planktonic community composition and function have far-reaching significance for water resource management. We aimed to understand the spatiotemporal variation and drivers of eukaryotic plankton distribution in the Middle Route Project of the South-to-North Water Diversion in Henan Province, China. Specifically, we examined planktonic assemblages and water quality at five stations along the canal and another one located before the dam in March, June, September, and December 2019. High-throughput sequencing revealed that the eukaryotic plankton community was primarily composed of 53 phyla, 200 genera, and 277 species, with Cryptophyta, Ciliophora, and norank_k_Cryptophyta being the dominant phyla. Redundancy analysis of the eukaryotic community and environmental factors showed that five vital factors affecting eukaryotic plankton distribution were oxidation-reduction potential, nitrate nitrogen, pH, total phosphorus, and water flow velocity. Furthermore, the geographical distribution of eukaryotic communities was consistent with the distance decay model. Importantly, environmental selection dominantly shaped the geographical distribution of the eukaryotic community. In summary, our study elucidates the ecological response of planktonic eukaryotes by identifying the diversity and ecological distribution of planktonic eukaryotes in trans-basin diversion channels.

The Inhibition of Microcystin Adsorption by Microplastics in the Presence of Algal Organic Matters

Microplastics (MPs) could act as vectors of synthetic chemicals; however, their influence on the adsorption of chemicals of natural origin (for example, MC-LR and intracellular organic matter (IOM), which could be concomitantly released by toxic Microcystis in water) is less understood. Here, we explored the adsorption of MC-LR by polyethylene (PE), polystyrene (PS), and polymethyl methacrylate (PMMA). The results showed that the MPs could adsorb both MC-LR and IOM, with the adsorption capability uniformly following the order of PS, PE, and PMMA. However, in the presence of IOM, the adsorption of MC-LR by PE, PS, and PMMA was reduced by 22.3%, 22.7% and 5.4%, respectively. This is because the benzene structure and the specific surface area of PS facilitate the adsorption of MC-LR and IOM, while the formation of Π-Π bonds favor its interaction with IOM. Consequently, the competition for binding sites between MC-LR and IOM hindered MC-LR adsorption. The C=O in PMMA benefits its conjunction with hydroxyl and carboxyl in the IOM through hydrogen bonding; thus, the adsorption of MC-LR is also inhibited. These findings highlight that the adsorption of chemicals of natural origin by MPs is likely overestimated in the presence of metabolites from the same biota.

Microplastics benefit bacteria colonization and induce microcystin degradation

Microplastics (MPs) can sorb toxic substances and be colonized by microorganisms. However, the interactions between the adsorbed toxic substances and the MPs biofilm remains inadequately understood. Here, a 37-days microcosm experiment was conducted to investigate the influence of polystyrene microplastics (PS-MPs) on microcystin (MC-LR) behavior in turbulent scenarios. The results revealed that adsorption by PS-MPs was the primary process that led to a quick reduction of aquatic MC-LR concentrations. With the colonization of microorganisms on the PS-MPs, the attached biofilm altered the surface properties of PS-MPs, which enhanced the bio-adsorption of MC-LR. Meanwhile, microcystins degrading bacteria, such as Sphingomonadaceae and Methylophilaceae, inhabited in the biofilm, which facilitated the MC-LR biodegradation; this was also demonstrated by the identified MC-LR degradation products. Thus, the MC-LR concentration in water was constantly decreased, with a maximum removal capability of 35.8% in PS-MPs added groups. In addition, a 25% reduction of MC-LR was recorded in PS-MPs added static water. This suggested that the interaction between PS-MPs, biofilm, and MC-LR may be prevalent in natural waters. Our results indicate MPs as vectors for toxic substances could be a double-edged sword (adsorption and biodegradation), which provides new insights for understanding the ecological risks of microplastics.

Cumulative Effects of Physical, Chemical, and Biological Measures on Algae Growth Inhibition

Measures based on concurrent alterations of an environment’s physical, chemical, and biological factors are commonly adopted to control harmful algal blooms (HABs). It was postulated that the combinations and interactions of multiple measures could exert cumulative effects (as the overall effect may or may not be equal to the additive sum for each measure individually). However, few studies have further assessed whether the cumulative effect is synergistic, additive, or antagonistic. This study proposed a framework to distinguish and quantify the cumulative effects. We also designed an experiment to investigate the cumulative effect of the combined utilization of physical (flow velocity), chemical (copper), and biological (propionamide) measures on algae growth inhibition. The results show that the cumulative effect of physical and chemical measures on algae growth inhibition was antagonistic; the cumulative effect of physical and biological measures was antagonistic; the cumulative effect of chemical and biological measures was synergistic, and the cumulative effect of all the measures together tended to be antagonistic. These results showed that the synergistic interactions between chemical and biological measures produced antagonistic effects when physical measures were added. Through response surface methodology analysis, we also found that the physical factor was the most significant factor affecting the cumulative effect, followed by the chemical factor and then the biological factor. Our results provide a more detailed understanding of the interaction patterns among multiple measures that affect algal growth. Importantly, this understanding can be further integrated into future strategy development to fully exploit the potential of the cumulative effect at its maximum performance.

Continuous hydrodynamic mixing weakens the dominance of Microcystis: evidences from microcosm and lab experiments

Hydrodynamic mixing is one of the important environment factors in determining phytoplankton community compositions. Here the influences of continuous hydrodynamic mixing on abundance, morphology, and dominance of Microcystis were investigated in microcosm and lab experiments. Our research results showed that Cyanophyta contributed 57.16% to the total biomass in control, but Chlorophyta was the dominant group in continuous hydrodynamic mixing (CHM) group, contributing 76.54% to the total biomass in the microcosm experiment. The average number of Microcystis in control was 1.95 folds in CHM group. However, the mean abundance of Scenedesmus quadricauda and Pseudanabaena limnetica in CHM was 2.47 and 2.97 folds in control. In the lab experiment, the average number of Microcystis flos-aquae in control was 2.97 folds in CHM group. The mean size of M. flos-aquae colony in control (34.65 μm) group were significantly bigger than that in the CHM (26.78 μm) group. This research results demonstrated that continuous hydrodynamic mixing weakened the dominance of Microcystis, but was beneficial for the others algae (S. quadricauda and P. limnetica) and is helpful in understanding the effect of hydrodynamic mixing on Microcystis blooms in freshwater ecosystem.

Interference of CDOM in remote sensing of suspended particulate matter (SPM) based on MODIS in the Persian Gulf and Oman Sea

The spatial and temporal variability of suspended particulate matter (SPM) in the Persian Gulf and Oman Sea coastal waters has remained challenging to understand among researchers. Here, for the first time in the region, we parametrized SPM concentration in the study area utilizing derived remote sensing reflectance (R_rs) values from Moderate-resolution Imaging Spectroradiometer (MODIS), using 555 and 667 nm wavelengths. Likewise, the findings showed that the developed optical model based on the optical ratio of R_rs (667)/R_rs (555) was sensitive to the concentration of Chromophoric dissolved organic matter (CDOM) in the seawater, within the visible wavelengths less than 600 nm. Comparing the new estimates of the SPM concentration with in situ measurements by Spearman's Rank correlation for validation revealed that the association between estimated and measured SPM concentration would be considered statistically significant (ρ up to 0.86, p < 0.05). This study increased the average accuracy of the estimates up to 73%.

Impact of barge movement on phytoplankton diversity in a river: A Bayesian risk estimation framework

The adverse effect of barge movement on the river's aquatic ecosystem is of global concern. The phytoplankton community, a bioindicator, is possibly the foremost victim of the barge movement. This study hypothesized phytoplankton diversity loss induced by barge movement in a large river. This article presents a novel risk assessment framework to evaluate the hypothesis-with a goal to uncoupling phytoplankton diversity loss due to barge movement over a spatiotemporal scale. For this purpose, a study was conducted in the Bhagirathi-Hooghly stretch of Inland National Waterway 1 of India. This study has proposed a new index of diversity loss and its inferential framework based on full Bayesian Generalized Linear Mixed Model. The results have diagnosed significant barge-induced impact on the phytoplankton diversity and identified ten most impacted species. The proposed framework has successfully disentangled barge-induced phytoplankton diversity loss from the biological process and predicted a substantive overall risk of phytoplankton loss of 31.44%. Besides, it has uncoupled spatiotemporal differential estimates, suggesting a risk of diversity loss in order of 'During vs After' (38.0%) > 'Before vs After' (30.7%) > 'Before vs During' (24%) barge movement in temporal scale and increasing diversity loss along downstream. Finally, the instant study has highlighted the utility of these results to facilitate better water framework directive for inland waterways.

Selection of water source for water transfer based on algal growth potential to prevent algal blooms

Water transfer is becoming a popular method for solving the problems of water quality deterioration and water level drawdown in lakes. However, the principle of choosing water sources for water transfer projects has mainly been based on the effects on water quality, which neglects the influence in the variation of phytoplankton community and the risk of algal blooms. In this study, algal growth potential (AGP) test was applied to predict changes in the phytoplankton community caused by water transfer projects. The feasibility of proposed water transfer sources (Baqing River and Jinsha River) was assessed through the changes in both water quality and phytoplankton community in Chenghai Lake, Southwest China. The results showed that the concentration of total nitrogen (TN) and total phosphorus (TP) in Chenghai Lake could be decreased to 0.52 mg/L and 0.02 mg/L respectively with the simulated water transfer source of Jinsha River. The algal cell density could be reduced by 60%, and the phytoplankton community would become relatively stable with the Jinsha River water transfer project, and the dominant species of Anabaena cylindrica evolved into Anabaenopsis arnoldii due to the species competition. However, the risk of algal blooms would be increased after the Baqing River water transfer project even with the improved water quality. Algae gained faster proliferation with the same dominant species in water transfer source. Therefore, water transfer projects should be assessed from not only the variation of water quality but also the risk of algal blooms.

Hydraulic flow direction alters nutrients removal performance and microbial mechanisms in electrolysis-assisted constructed wetlands

In this study, an electrolysis-assisted down-flow constructed wetland (E-DFCW) was successfully established, and achieved simultaneously efficient removal of PO₄^3--P (93.6% ± 3.2%), NO₃^--N (97.1% ± 2.0%) and TN (80.6% ± 5.4%). When compared with electrolysis-assisted up-flow constructed wetland (E-UFCW), E-DFCW allowed significantly lower concentrations of PO₄^3--P, NO₃^--N, total Fe and SO₄^2--S in effluents. In addition, microbial community and functional genes prediction results indicated that hydraulic flow direction significantly altered microbial nitrogen, sulfur and carbon metabolisms in electrolysis-assisted constructed wetlands (E-CWs). Specifically, multi-path denitrification facilitated NO₃^--N reduction in cathodic chamber of E-DFCW, whereas autohydrogenotrophic denitrification might dominate NO₃^--N reduction in cathodic chamber of E-UFCW. More abundant and diverse denitrifiers in cathodic chamber of E-DFCW contributed to enhanced denitrification performance. Overall, this work provides microbial insights into multi-path nitrogen metabolisms in electrolysis-assisted denitrification systems in response to hydraulic flow direction.

Implications of phosphorus partitioning at the suspended particle-water interface for lake eutrophication in China's largest freshwater lake, Poyang Lake

Nutrient partition, especially for phosphorus (P), has been prominently changed that was caused by variation of river-lake relationship during the post-Three Gorges Reservoir and catchment alternations. Changes in proportion of total particulate phosphorus (TPP) and total dissolved phosphorus (TDP) might accelerate lake eutrophication, but limited attention has been paid to P partition over suspended particle (SP) levels. Data analysis showed that SP concentration presented a positive effect on TPP in wet season and soluble reactive phosphorus (SRP) in dry season, indicating seasonal physical and chemical variations. Based on this phenomenon, we proposed a hypothesis that the SP levels would affect TDP and TPP proportions by partition in aqueous-solid. It was found that using the parabola models to fit the sorption relationships of SRP and TDP (R² > 0.6, p < 0.01), the maximum sorption capacity (Q_max) was 64.54 mg/kg and 60.52 mg/kg at 400 mg/L of SP level, respectively. In addition, the partition coefficients (K_P) of TDP and SRP were logarithmically increased with SP levels, indicating that higher SP levels (>400 mg/L) would hinder the sorption process. Furthermore, enhancing turbulence lead to less sorption of SRP and TDP at high SP levels (>800 mg/L). The sorption of SRP and TDP related to the presence of Fe/Al oxy-hydroxides were enriched in the Fe/Al-P fraction (47% of TP). The findings of this study indicated that the low SP levels would increase P bioavailability for alga and is not conducive for lake eutrophication management.

Using flocculation and subsequent biomanipulation to control microcystis blooms: A laboratory study

The frequent occurrence and long-term duration of Microcystis harmful algal blooms (HABs) are of great concern. Chemical flocculation is thought to be an effective way to deal with the HABs, while the application of the flocculants at a high dosage pose potential adverse impacts to the aquatic ecosystems. In this study, an alternative approach is proposed that involves the employment of polyaluminum chloride (PAC) combined with the Daphnia magna (D. magna) to achieve sustainable HABs removal efficiency with an acceptable ecological risk. It was found that under a dense Microcystis HABs (algal density of 1.5 × 10⁷ cells/ml), a PAC dosage of 30 mg/l triggered >95% algae removal, but the released Al³⁺ caused 90% mortality of planktonic D. magna. Reducing the PAC dosage to 15 mg/l resulted in a slightly lower algal removal efficiency (>90%). In addition the reduced PAC dosage benefited the proliferation of the remaining unicellular algal cells, which tended to form a large colony during the 25-day experiment. Incubation of D. magna following flocculation with 15 mg/l PAC effectively grazed the remaining algal cells, meanwhile increasing the D. magna density by approximately 40-folds, and enlarging the body size by 1.37-1.50 times. This result implied that the released Al³⁺ was not detrimental to the D. magna. Flocculation with a reduced dosage is sufficient for colonial and large algal cells mitigation, which creates a window time for the biomanipulation of the residual tiny algae. Hence, the subsequent addition of D. magna triggered the sustainable removal of the HABs cells. The present study provides an environmentally friendly strategy for cleaning up the green tides without obvious detrimental effects on the aquatic ecosystem.

Dynamics of phosphorus composition in suspended particulate matter from a turbid eutrophic shallow lake (Lake Chaohu, China): Implications for phosphorus cycling and management

Particulate phosphorus (P) dominates the total P (TP) content in lacustrine water columns and is a primary source of dissolved P in turbid eutrophic shallow lakes. However, the spatiotemporal variability of P compositions in suspended particulate matter (SPM) remains poorly understood. In this study, we applied chemical extraction and solution ³¹P nuclear magnetic resonance (³¹P NMR) to assess the seasonal variations of SPM P compositions from a shallow turbid lake (Lake Chaohu, China) and its main river tributaries. P fractionation analysis indicated that mobile P (the sum of labile-P, iron-bound P, and organic P) accounted for >60% of the TP in SPM and showed high spatiotemporal variability throughout the year-long field investigation. In most seasons, riverine SPM (in urban rivers or rivers with high flow) contained a higher mobile P content than that of the lake and was therefore a dominant source of lacustrine mobile particulate P. Solution ³¹P NMR identified five types of P compounds in SPM, with highest contributions from orthophosphate. Organic P components and concentrations showed high seasonal variability, and elevated p values occurred during the summer algal bloom. The correlation analysis between organic and inorganic P fractions inferred the possible degradation of organic P into reactive inorganic components of SPM. Consequently, biological or chemical processes would further transform the labile inorganic P into soluble reactive phosphorus, which is readily utilized by lacustrine algae. Our results suggest that the labile forms of P in SPM were highly dynamic and significantly contributed to the eutrophication of the turbid shallow lake.

The Influence of Ship Waves on Sediment Resuspension in the Large Shallow Lake Taihu, China

Sediment resuspension induces endogenous nutrient release in shallow lakes, which has been demonstrated to be associated with eutrophication. In addition to natural wind-driven resuspension, navigable shallow lakes (such as Lake Taihu, China) also experience resuspension from human activities, such as ship waves. Both processes determine the intensity, frequency, and duration of sediment resuspension, and may consequently affect the pattern of cyanobacteria blooms in the lake. In this study, acoustic Doppler Velocimeter (ADV), Optical Backscatter Sensor (OBS), and temperature wave tide gauge (instrument model :RBR duo TD|wave) were placed in an observation platform in the lake to obtain high-frequency flow velocities, suspended sediment concentration (SSC), and wave parameters before, during, and after a cargo ship passed by. We found that the ship wave disturbance intensity is greatly influenced by the draft depth. The movement generated by ship disturbance is primarily horizontal rather than vertical. Compared with the wind-induced wave, the disturbance caused by the ship waves has a high intensity, short duration, and narrow range of influence. The maximum total shear stress under ship disturbance can reach 9~90 times the critical shear stress under a natural state. Therefore, the effect of ship waves on sediment resuspension near the channel of Lake Taihu is much greater than that of wind-induced waves. These findings represent an important step towards understanding the quantitative relationship between ship wave disturbance and sediment resuspension, and lay the foundation for future research in order to understand and control the eutrophication of shallow lakes.

Recommended turbulent energy dissipation rate for biomass and lipid production of Scenedesmus obliquus in an aerated photosynthetic culture system

Effects of turbulent energy dissipation rate (increased from 1.28 × 10^-6 to 1.67 × 10^-5 m² s^-3) on Scenedesmus obliquus biomass and lipid accumulation at different aeration rates (0.3, 0.6, 0.9, 1.2, and 1.5 L min^-1) were investigated. The turbulent energy dissipation rate was calculated by CFD model simulation. When the turbulent energy dissipation rate increased to 7.30 × 10^-6 m² s^-3, the biomass and lipid productivity increased gradually, and finally reached their maximum values of 1.11 × 10⁷ cells mL^-1 and 16.0 mg L^-1 day^-1, respectively. When it exceeded 7.30 × 10^-6 m² s^-3, the biomass and lipid productivity showed a decreasing trend. Therefore, the most favorable turbulent energy dissipation rate for S. obliquus growth and lipid accumulation was 7.30 × 10^-6 m² s^-3.

Variations of suspended particulate concentration and composition in Chinese lakes observed from Sentinel-3A OLCI images

The concentration and composition of suspended particulate matter provide important information for evaluating water quality and understanding the variability in the underwater light field in lakes. In this study, inherent optical property (IOP)-centered algorithms were developed to estimate the concentrations of chlorophyll-a (Chla, [mg/m³]) and suspended particulate matter (SPM, [g/m³]) and the Chla/SPM ratio (an indicator of the suspended particulate composition) of 118 lakes in the middle and lower reaches of the Yangtze and Huai Rivers (MLYHR) of China using Sentinel-3A/OLCI (Ocean and Land Colour Instrument) data collected from August 2016 to July 2018. The mean Chla concentration and Chla/SPM ratio were high in summer and low in winter, while the mean SPM peaked in winter and decreased in summer. The 94 lakes in the Yangtze River basin had a higher mean Chla concentration (30.94 ± 14.84) and Chla/SPM ratio (0.97 × 10^-3 ± 0.60 × 10^-3), but a lower mean SPM (44.87 ± 12.61) than the 24 lakes in the Huai River basin (Chla: 27.35 ± 12.18, Chla/SPM: 0.79 × 10^-3 ± 0.48 × 10^-3, SPM: 47.31 ± 13.40). Regarding the mean values of each lake, Chla and Chla/SPM ratio correlated well with temperature, whereas the wind speed and precipitation had little effect on the variations of suspended particulate matter. Moreover, shipping transportation and sand dredging activities affected the spatial distribution of Chla, SPM, and Chla/SPM in several large lakes (e.g., Lake Poyang and Lake Dongting). Chla/SPM related well with other proxies that express the suspended particulate composition, and had a significant correlation with the Chla-specific absorption coefficient of phytoplankton at 443 nm (a_ph^⁎(443)). The remotely sensed concentration and composition of suspended particulate matter can provide a comprehensive reference for water quality monitoring and expand our knowledge of the trophic status of the lakes.

The effects of turbulence on phytoplankton and implications for energy transfer with an integrated water quality-ecosystem model in a shallow lake

Turbulence has significant influences on the growth rate and community structure of phytoplankton in large shallow lakes. Phytoplankton in moving water may be influenced by turbulence and nutrient concentration gradients on a short time scale. To assess this issue, our research used an ensemble water quality and ecological model by internally coupling the three-dimensional hydrodynamic model, the Environmental Fluid Dynamics Code (EFDC), and the one-dimensional ecosystem model, PCLake. The results showed that turbulence dramatically inhibited phytoplankton growth, while nutrients had the opposite effect. In addition, turbulence was the key factor contributing to phytoplankton growth. However, the effects of turbulence on phytoplankton correlated with nutrient concentrations. For lower nutrient concentrations, phytoplankton growth was controlled by nutrients. Logistic regression models were established with the modeled chlorophyll a, total nitrogen (TN), total phosphorus (TP) and turbulent kinetic energy (K_e). The results also showed that turbulence could improve nutrient uptake by phytoplankton, especially at low nutrient levels. The effects of turbulence on phytoplankton may imply that energy transfer occurs between water turbulence and phytoplankton. Our study will provide insight into management and remediation strategies of ecosystems based on energy processes in the future.

Physiological characteristics and toxin production of Microcystis aeruginosa (Cyanobacterium) in response to DOM in anaerobic digestion effluent

The ecological implications of livestock production intensification have received sustained attention across the globe. Anaerobic digestion is the main process for livestock waste treatment. However, the ecological consequences of dissolved organic matter originating from anaerobic digestion (AD-DOM) in eutrophic water bodies remain elusive. In this study, the physiological responses of a bloom-forming cyanobacterium, Microcystis aeruginosa, to AD-DOM were investigated. Moreover, the composition of AD-DOM was identified by using thermochemolysis followed by gas chromatography-mass spectrometry (GC-MS) analysis. The growth of M. aeruginosa FACHB905 was not sensitive to low levels (0.625-1.25%, V/V) of AD-DOM but was inhibited by high levels (2.5-5%, V/V) of AD-DOM, resulting from photoinhibition damage to photosystem II (PSII). The main target of AD-DOM in PSII was the electron accepting side (ψ₀) or the electron donor side (φ P₀), depending on time variables. The reactive oxygen species (ROS) level showed a positive correlation with AD-DOM addition; however, it was higher than that of the control for 3.75-5% AD-DOM on the 6th day. The intracellular microcystin contents (including MC-LR and Dha⁷-MC-LR) decreased in response to AD-DOM addition, but extracellular microcystin increased after 6 days of exposure. In addition, GC-MS detection showed that AD-DOM is mainly composed of lignin-derived aromatic compounds, alkanes/alkene, nitrogen-containing compounds, and sterols. The results presented in this study suggested that AD-DOM released from the livestock industry may play a subtle role in affecting harmful algal blooms through level-dependent variables. In addition, the ecological consequences of microcystin released by toxin-producing species under AD-DOM stress are still worth considering.

The role of turbulence in internal phosphorus release: Turbulence intensity matters

Hydrodynamic fluctuations can trigger sediment suspension concomitantly with internal phosphorus release, while the interactive effect of turbulence mixing and sediment suspension on the regulation of phosphorus dynamics is in need of deep understanding. This study addressed the changes in total phosphorus (TP), phosphate (PO₄^3--P) and suspended sediment (SS) in the overlying water, and measured the profile of dissolved oxygen (DO), Fe(II) and soluble reactive phosphorus (SRP) across the sediment-water interface in the simulated environmental turbulence scenario, For a turbulence intensity (ε) of 3.6 × 10^-3 m²/s³, the SRP flux increased hence PO₄^3--P showed a 36.36% increase relative to its initial level. Although ε of 1.3 × 10^-2 m²/s³ benefited the delivery of oxygen from the bulk aqueous phase to the upper sediment which can trigger the formation of Fe oxides and hydroxides, the turbulence-induced phosphorus diffusion from the sediment exceeded its inactivation and resulted in a large SRP flux. However, a protion of the released PO₄^3--P can be immobilized through SS adsorption and biotic (likely cyanobacteria) assimilation. Higher turbulence intensities (ε of 3.3 × 10^-2 and 7.4 × 10^-2 m²/s³) led to an approximately 40-fold increase in TP concentration and a significant increase in sediment suspension, which contributed to the immobilization of a majority of the phosphate through adsorption; thus, the PO₄^3--P concentrations in the overlying water displayed 47.75% and 41.67% decline, respectively. This study also confirmed the sequential phosphorus buffer mechanisms associated with increasing turbulence intensities. With an ε of 3.6 × 10^-3 m²/s³, bounding to Fe ion had a significant impact on phosphorus inactivation but with an ε of 7.4 × 10^-2 m²/s³, the main immobilization mechanism is switched to phosphorus adsorption from the large quantity of suspended sediment.

Strong turbulence benefits toxic and colonial cyanobacteria in water: A potential way of climate change impact on the expansion of Harmful Algal Blooms

Extreme natural events such as typhoons can amplify the effect of hydrodynamics on the lake ecosystems. Here we presented data on the effect of typhoons on algal cell size based on field observation. Then turbulence simulation systems were used to decipher the response of natural phytoplankton communities to a range of turbulence regimes (linked to typhoon-induced turbulence intensity) under laboratory conditions. Turbulence intensities of 6.17 × 10^-3, 1.10 × 10^-2 and 1.80 × 10^-2 m²/s³ benefited algal growth and triggered abrupt switches from unicellular Chlorella dominated to colonial Microcystis dominance, and the abundance of colonial algae depended on the turbulence intensity. Under the influence of elevated turbulence, Microcystis dominated biomass increased by 2.60-6.58 times compared with that of Chlorella. At a given phytoplankton density and community composition, we observed a significant increase in extracellular microcystins (MCs) and a 47.5-fold increase in intracellular MCs with intensified turbulent mixing, suggesting that the damage of algal cells concomitantly the stimulation of toxin-producing Microcystis. Our results confirmed that the formation of large colonial algal cells, enhancement of the succession of algal species, and most importantly, the induction of toxin-producing Microcystis, were the active adaption strategy when phytoplankton were impacted by strong turbulence. The result implies that the ongoing climates changes and typhoon events are likely to contribute to undesirable outcomes concerning phytoplankton populations.