Influence of algal bloom degradation on nutrient release at the sediment-water interface in Lake Taihu, China

Integration of Membrane Bioreactor and Nanofiltration for the Treatment Process of Real Hospital Wastewater in Ho Chi Minh City, Vietnam

Hospital wastewater contains pharmaceutical residues, chemicals, and pathogens that cause coloration and nourish pathogenic microorganisms. The objective of the study was to evaluate the effectiveness of a medical wastewater treatment system at Military Hospital 175 (Ho Chi Minh City, Vietnam) that combined a membrane bioreactor (MBR) system with nanofiltration (NF). The influent of the system was the wastewater discharged from the operating rooms of the hospital. The system has a capacity of 50 L/day and operates at three organic load rates (OLR) of 0.5, 1.5 and 2.5 kgCOD/m3day (COD: Chemical oxygen demand), in which each load rate operates for 40 days. The results showed that most nutritional criteria generally achieved positive results. Specifically, the average COD removal was shown to be consistently high throughout the three phases at 94%, 93.3%, and 92.7%, respectively. For removal of nitrogen, the system demonstrated efficiencies of 75%, 79%, and 83%, respectively, to three phases. The log removal value (LRV) for Escherichia coli and coliform bacteria were higher than four throughout the study period. The average removal efficiency for color and total iron was approximately 98% and 99%, respectively. The water quality after treatment, especially after NF, meets the Vietnamese standard of grade A. The arrangement in which the MBR preceded NF was also found to limit the amount of soil and solids entering subsequent treatment, which therefore improved the efficiency of NF, as demonstrated by the stability of post-NF transmembrane pressures throughout three cycles renewed by two backwashes.

Denitrification and the controlling factors in Yunnan Plateau Lakes (China): Exploring the role of enhanced internal nitrogen cycling by algal blooms

Denitrification plays an important role in nitrogen (N) removal in freshwater ecosystems. This internal process regulates the fluctuations of N concentration, especially for lakes with high nutrients concentrations and long residence time. Lakes in Yunnan plateau (southwestern China) provide typical cases, while studies in this region have been rare. Therefore, we studied denitrification of two lakes (Lake Dianchi in hypereutrophic state and Lake Erhai in mesotrophic) in this region. We used acetylene inhibition technique to quantify potential denitrification rate (PDR) of these lakes in April and August, 2015 and 2016. PDR of the sediments ranged 0-1.21 μmol/(N·m²·hr), and that of overlying water ranged 0-0.24 μmol/(N·L·hr). Then, we used Least Angle Regression to determine the controlling factors for denitrification. Nutrients controlled PDR from two aspects: providing essential nitrogen sources; and affecting the richness and metabolism of denitrifying bacteria. In April, both aspects limited PDR; while only nitrogen sources limited PDR in August, due to depleted nitrate and enhanced denitrifying bacteria activity. Ammonia was most significant to denitrification, indicating that nitrate from nitrification transported to the bottom of well-mixed lake provide major N source by denitrification. The high PDR and low nitrate concentrate in August were evidence of an enhanced internal N cycling by algal blooms.

Unraveling the sources and fluorescence compositions of dissolved and particulate organic matter (DOM and POM) in Lake Taihu, China

Organic matter (OM), a complex entity with diverse functional groups and molecular sizes, has important effects on aquatic systems. We studied the optical compositions and sources of dissolved organic matter (DOM) and particulate organic matter (POM) in Lake Taihu, a large, shallow and eutrophic lake in China. Significant differences in optical compositions and sources occurred between the POM and DOM. The temporal-spatial distribution of the fluorescence indices suggested that the POM in Lake Taihu was mainly from autochthonous sources, but more exogenous characteristics were shown in POM in the river mouths compared with other regions. The chromophoric DOM in Lake Taihu mainly displayed autochthonous characteristics. The POM-DOM PARAFAC model was used to examine OM optical composition and five components were identified, which contained three protein-like components (C1, C2, and C5), a microbial humic-like component (C3), and a terrestrial humic-like component (C4). The POM was dominated by C5 in summer and autumn and C3 in winter and spring, and the DOM was dominated by protein-like components (C1, C2, and C5) through the entire year. The algae-dominated region had a relative higher contribution of tryptophan-like components of POM compared with the macrophyte-dominated region. A conceptual model based on the theory of "four phases of cyanobacteria bloom development" was proposed to fully describe the relationship between POM-DOM exchanges and cyanobacteria bloom development.

Seasonal variation of potential denitrification rate and enhanced denitrification performance via water-lifting aeration technology in a stratified reservoir-A case study of Zhoucun reservoir

Zhoucun reservoir is one of the major water resources in Zao Zhuang city, northern China. The seasonal distribution of surface sediment denitrification rate and enhanced performance of denitrification via water-lifting aeration technology were explored using the acetylene-inhibition technique. Surface sediment denitrification rates ranged from 2.57 ± 1.32 to 923.90 ± 86.81 nmol N₂/(g dw·h), with the highest rates in November (ANOVA, p < 0.05) and significantly low rates in June, July, and August (ANOVA, p < 0.05), mainly because of the seasonal differences in nitrate concentration, water temperature, and sediment total nitrogen (STN). Meanwhile, the N₂/(N₂+N₂O) ratio (83.44-91.70% for the highest sediment denitrification period) indicated that N₂ accounted for a majority of denitrification. Correlation analysis between various environmental factors and denitrification was conducted, and nitrate concentration, STN, low molecular weight organic carbon, the number of aerobic denitrifying bacteria, and the environmental parameters of oxidation-reduction potential (ORP), pH, electrical conductivity (EC), and chlorophyll a (Chl-a) presented significant relationships during the entire study period. On the basis of the multiple regression model, nitrate and low molecular weight organic carbon concentration were the most influential factors on denitrification variability. Moreover, the denitrification rates of the surface sediment clearly increased, from 5.28 to 13.22 nmol N₂/(g dw·h) to 1117.02-3129.47 nmol N₂/(g dw·h), which were higher than those in the non-operating year. This suggests that the denitrification in the sediment system could be enhanced in situ by water-lifting and aeration technology in the reservoir ecosystem.

Spatial and Temporal Distribution of Particulate Phosphorus and Their Correlation with Environmental Factors in a Shallow Eutrophic Chinese Lake (Lake Taihu)

Spatial and seasonal variations of particulate phosphorus (PP) in a large shallow, eutrophic Lake Taihu with different ecotypes (including a phytoplankton-dominated zone, lake center zone, estuary zone and macrophyte-dominated zone) were investigated. The results showed that particulate organic phosphorus (POP) was the dominant form of PP (>88.0%). The concentration of POP showed higher levels in the bloom-sensitive northwestern zone (phytoplankton-dominated zone and estuary zone) during warm seasons, phytoplankton blooms and input of exogenous particulate matter were the main sources of POP in the lake water. Based on ³¹P nuclear magnetic resonance (³¹P NMR) analysis, orthophosphate (Ortho-P) was the dominant molecular species of PP and positively correlated with soluble reactive phosphorus (SRP) (p < 0.01). This suggested that the release of Ortho-P from suspended particulate matter (SPM) was the main source of SRP in the lake water. Pyrophosphate (Pyro-P), which is regarded as a highly labile species of P compounds, represented a large fraction of PP, and its significant positive correlations with chlorophyll a (Chl a), indicated that the concentration of Pyro-P could be used as an important indicator for the degree of eutrophication of Lake Taihu. These results proved that PP in lake water was a significant factor supporting lake eutrophication and must be controlled.

Multiple biomarker responses in caged benthic gastropods Bellamya aeruginosa after in situ exposure to Taihu Lake in China

BACKGROUND

Freshwater sediments have been recognized as a long-term sink and potential source for environmental pollutants released into the aquatic ecosystems. In this study, the sediment quality of Taihu Lake, which is susceptible to anthropogenic contamination, was assessed by a combination of chemical analytical and biological end points. Specifically, the snail Bellamya aeruginosa was caged in situ at two locations representing different pollution levels for different exposure times (7, 14 and 21 days). At each of these time points, biochemical parameters, i.e., phase I biotransformation enzymes ethoxyresorufin-O-deethylase (EROD), the antioxidant enzymes superoxide dismutase and catalase, reactive oxygen species, protein carbonyl content and lipid peroxidation, were evaluated in the hepatopancreas of snails. In addition, surface sediments were collected for analysis of contaminants of concern, including inorganic pollutants, organochlorine pesticides, polychlorinated biphenyls and polybrominated diphenyl ethers.

RESULTS

Chemical analyses revealed that sediments from Taihu Lake were contaminated with trace elements and organic pollutants. Concentrations of trace elements (Cu, Ni and As) and organochlorinated pesticides (4,4'-DDE) exceeded their corresponding threshold effect level according to the sediment quality assessment values for freshwater ecosystems in Canada, indicating that adverse biological effects may occur. All biomarkers, except EROD activity, were induced in snails during all exposure times. The integrated biomarker response index (IBR) indicated that during the initial exposure phase (7 days), B. aeruginosa were subjected to significant environmental stress, which diminished during later sampling time points.

CONCLUSIONS

Results showed that IBR correlated well with the levels of environmental contaminants, demonstrating the applicability of this biomonitoring approach to complex environmental exposure scenarios.

Reconstructing clear water state and submersed vegetation on behalf of repeated flocculation with modified soil in an in situ mesocosm experiment in Lake Taihu

The geo-engineering approach of modified soil flocculation has been widely applied to mitigate algal blooms and eutrophication in relatively small lakes. Nevertheless, its potential ecological risks and feasibility should be examined and identified prior to its application in large natural lakes given the multiple functions of these water bodies in human health and welfare. In situ mesocosm experiments on modified soil flocculation were performed in Lake Taihu during summer 2010 and 2011. Chitosan-modified kaolinite (CMK) soil was used to flocculate algal blooms and improve water transparency to facilitate the re-establishment of the submersed macrophyte Vallisneria natans in this shallow eutrophic lake. Moreover, the ecological effects of CMK soil were assessed. Results showed that repeated additions of CMK (0.3g/L for each time) improved water quality in terms of Chl-a, TN, and TP concentrations; TN/TP ratio; turbidity; redox conditions; and nitrification and denitrification activities. These effects lasted for 48days. After the fourth dose of CMK, the biomass of all phytoplankton categories, except for that of Cryptophyta, decreased by >90% (ca. 1-2×10⁶cell/L or 0.38-0.55mg/L of wet weight). Zooplankton biomass markedly decreased after the first CMK addition, and copepods became dominant. These effects, however, did not last for the long term. Most importantly, submersed V. natans was restored successfully when water clarity and quality were improved through repeated CMK flocculation. Nevertheless, the indices of carbohydrate depletion and free amino acid accumulation indicated that the plant experienced physiological stresses. The reestablishment of V. natans reinforced the positive effects of repeated CMK dosing on water quality, and promoted a clear water state. V. natans is recommended for vegetative restoration in shallow eutrophic lakes given its facile transplantation, high stress tolerance, and physiological traits, which can be applied as indices of post-flocculation effects. In summary, the combination of repeated CMK dosing and revegetation of V. natans can feasibly improve water quality and initiate the restoration of a clear water state in shallow eutrophic lakes.

Mechanisms driving phosphorus release during algal blooms based on hourly changes in iron and phosphorus concentrations in sediments

Algal growth causes a drastic change in aquatic conditions over a diel cycle, which may induce sensitive feedback systems in sediments, causing P release. In this study, a microcosm experiment was performed using a suction sampler (Rhizon) to observe changes in soluble reactive phosphorus (SRP) and soluble Fe(II) concentrations in the top 20 mm sediment layer on a 3-h time interval, at different phases of harmful algal bloom (HAB) development. The results showed that the algal blooms prevailed up to 15 days after incubation, after which the process of bloom collapse proceeded until the 70th day. The concentrations of pore-water soluble Fe(II) and SRP increased throughout the incubation period. Compared to day 1, maximum increases of 214% in soluble Fe(II) and 387% in SRP were observed at night during the bloom and collapse periods, respectively. The diffusive fluxes of Fe and P at the sediment-water interface (SWI) generally corresponded to their changes in concentrations. Hourly fluctuation in soluble Fe(II) and SRP concentrations were observed with two distinct concentration peaks occurred at 21:00 p.m. and 06:00 a.m. (or 03:00 a.m.), respectively. These findings suggest that Fe-P coupling mechanisms are responsible for the release of P from sediments. During the collapse period, soluble Fe(II) concentrations were suppressed by the increase of labile S(-II) at night. Meanwhile, SRP concentrations were decoupled from Fe cycling with small fluctuations (<11% RSD) on an hourly timescale, and the decomposition of algae was a dominant source contributing to the release of P from sediments. These results significantly improved the understanding of processes and mechanisms behind the stimulated release of P from sediments during HABs.

Identifying external nutrient reduction requirements and potential in the hypereutrophic Lake Taihu Basin, China

Reducing external nutrient loads is the first step for controlling eutrophication. Here, we identified external nutrient reduction requirements and potential of strategies for achieving reductions to remediate a eutrophic water body, Lake Taihu, China. A mass balance approach based on the entire lake was used to identify nutrient reduction requirements; an empirical export coefficient approach was introduced to estimate the nutrient reduction potential of the overall program on integrated regulation of Taihu Lake Basin (hereafter referred to as the "Guideline"). Reduction requirements included external total nitrogen (TN) and total phosphorus (TP) loads, which should be reduced by 41-55 and 25-50%, respectively, to prevent nutrient accumulation in Lake Taihu and to meet the planned water quality targets. In 2010, which is the most seriously polluted calendar year during the 2008-2014 period, the nutrient reduction requirements were estimated to be 36,819 tons of N and 2442 tons of P, and the potential nutrient reduction strategies would reduce approximately 25,821 tons of N and 3024 tons of P. Since there is a net N remaining in the reduction requirements, it should be the focus and deserves more attention in identifying external nutrient reduction strategies. Moreover, abatement measures outlined in the Guideline with high P reduction potential required large monetary investments. Achieving TP reduction requirement using the cost-effective strategy costs about 80.24 million USD. The design of nutrient reduction strategies should be enacted according to regional and sectoral differences and the cost-effectiveness of abatement measures.

Metatranscriptomics analysis of cyanobacterial aggregates during cyanobacterial bloom period in Lake Taihu, China

Molecular mechanism of interaction between the bloom-forming cyanobacterial species and attached microbios within cyanobacterial aggregates has not been elucidated yet and understanding of which would help to unravel the cyanobacteria bloom-forming mechanism. In this study, we profiled the metabolically active community by high-throughput metatranscriptome sequencing from cyanobacterial aggregates during cyanobacterial bloom period in Lake Taihu, China. A total of 308 million sequences were obtained using the HiSeq 2500 sequencing platform, which provided a great sequence coverage to carry out the in-depth taxonomic classification, functional classification, and metabolic pathway analysis of the cyanobacterial aggregates. The results show that bacteria dominated in cyanobacterial aggregates, accounting for more than 96.66% of total sequences. Microcystis was the most abundant genus, accounted for 26.80% of total assigned sequences at the genus level in cyanobacterial aggregates community; however, Proteobacteria (46.20%) was found to be as the most abundant active bacterial populations at the phylum level. More importantly, nitrogen, phosphonate, and phosphinate metabolism which associated with eutrophication were found in this study. Especially, the enzymes and organisms relating to denitrification and anammox of nitrogen metabolism, which reduced nitrogen concentration by reducing nitrate to nitrogen to inhibit the eutrophication, were first discovered in Lake Taihu during cyanobacterial bloom period. The present study provides a snapshot of metatranscriptome for cyanobacterial aggregates in Lake Taihu and demonstrates that cyanobacterial aggregates could play a key role in the nitrogen cycle in eutrophic water.

Influence of ignited sediments on external phosphorus adsorption and sedimentary phosphorus forms

Phosphorus (P) adsorpted by sediments, when covered by and mixed with ignited sediments from Meiliang Bay in Tai Lake, was analyzed in the laboratory. Potassium dihydrogen phosphate (KH₂PO₄) was added to the parallel experimental units to simulate periodic external P input. Based on the Langmuir model, the sediments after ignition had a greater S_max (maximum P adsorption), a lower equilibrium phosphorus concentration at zero adsorption (EPC₀), and a lower degree of phosphorus saturation in comparison with sediments without ignition. This was confirmed by the variation in the dissolved inorganic phosphorus in the overlying water. When sediments were mixed with or covered by the ignited sediments, 5.985 and 5.978 mg of input P disappeared from the overlying water, respectively. However, when the sediments were mixed with the ignited sediments, 84.18% of the input P was converted to HCl-P, whereas when they were covered by the ignited sediments, sedimentary P was released, mainly from Fe/Al-P (up to 87.50%). This was attributed to differences in the microenvironments where less-intense anaerobic conditions were formed in the mixed sediments than in the sediments covered by the ignited sediments. This suggests that the injection of ignited sediments into existing sediments enhances their P adsorption and retention. It is favor of the control of the eutrophication with a simple technology.

Laboratory assessment of bioleaching of shallow eutrophic sediment by immobilized photosynthetic bacteria

Eutrophic sediment is a serious problem in ecosystem restoration, especially in shallow lake ecosystems. We present a novel bioleaching approach to treat shallow eutrophic sediment with the objective of preventing the release of nitrate, phosphate, and organic compounds from the sediment to the water column, using porous mineral-immobilized photosynthetic bacteria (PSB). Bioactivity of bacteria was maintained during the immobilization process. Immobilized PSB beads were directly deposited on the sediment surface. The deposited PSB utilized pollutants diffused from the sediment as a nutritive matrix for growth. We evaluated the effects of light condition, temperature, initial pH, amount of PSB beads, and frequency of addition of PSB beads for contaminant removal efficiency during bioleaching operations. The presented study indicated that immobilized PSB beads using porous minerals as substrates have considerable application potential in bioremediation of shallow eutrophic lakes.

Wind-induced flow velocity effects on nutrient concentrations at Eastern Bay of Lake Taihu, China

Shallow lakes are highly sensitive to respond internal nutrient loading due to wind-induced flow velocity effects. Wind-induced flow velocity effects on nutrient suspension were investigated at a long narrow bay of large shallow Lake Taihu, the third largest freshwater lake in China. Wind-induced reverse/compensation flow and consistent flow field probabilities at vertical column of the water were measured. The probabilities between the wind field and the flow velocities provided a strong correlation at the surface (80.6%) and the bottom (65.1%) layers of water profile. Vertical flow velocity profile analysis provided the evidence of delay response time to wind field at the bottom layer of lake water. Strong wind field generated by the west (W) and west-north-west (WNW) winds produced displaced water movements in opposite directions to the prevailing flow field. An exponential correlation was observed between the current velocities of the surface and the bottom layers while considering wind speed as a control factor. A linear model was developed to correlate the wind field-induced flow velocity impacts on nutrient concentration at the surface and bottom layers. Results showed that dominant wind directions (ENE, E, and ESE) had a maximum nutrient resuspension contribution (nutrient resuspension potential) of 34.7 and 43.6% at the surface and the bottom profile layers, respectively. Total suspended solids (TSS), total nitrogen (TN), and total phosphorus (TP) average concentrations were 6.38, 1.5, and 0.03 mg/L during our field experiment at Eastern Bay of Lake Taihu. Overall, wind-induced low-to-moderate hydrodynamic disturbances contributed more in nutrient resuspension at Eastern Bay of Lake Taihu. The present study can be used to understand the linkage between wind-induced flow velocities and nutrient concentrations for shallow lakes (with uniform morphology and deep margins) water quality management and to develop further models.

Variation of bacterial communities in water and sediments during the decomposition of Microcystis biomass

The bacterial community composition in water and sediment samples during the decomposition of Microcystis biomass were analyzed using the 454 pyrosequencing technique. We found dramatic shifts in the bacterial community composition of water and sediments after the addition of Microcystis biomass. Among all the detected phyla, only Firmicutes was found to be dominant in both water and sediment samples. The genus Clostridium sensu stricto was the absolutely dominant group in Firmicutes and showed drastic variations with incubation time during the decomposition process. Peak values in relative abundance of Clostridium sensu stricto appeared in the first few days for water and sediment samples. Environmental factors such as pH, dissolved oxygen (DO), and dissolved organic carbon (DOC) in water samples showed drastic variations during the decomposing process, which might be the prominent forces driving the variation of bacterial communities. The abundant genus, Clostridium sensu stricto, were thought to be well adapted to higher DOC and turbidity and lower pH and DO conditions. Compared with the sediment samples, the decomposition of Microcystis biomass had greater influence on the bacterial community composition in water and Clostridium sensu stricto might play important roles in the process of Microcystis biomass decomposition.

Effect of wave-current interactions on sediment resuspension in large shallow Lake Taihu, China

The disturbance of the water-sediment interface by wind-driven currents and waves plays a critical role in sediment resuspension and internal nutrient release in large, shallow lakes. This study analyzed the effects of the interactions between wind-induced currents an1d waves on the driving mechanism of sediment resuspension in Lake Taihu, the third largest freshwater lake in China, using acoustic and optic techniques to collect long-term, high-frequency, synchronous in situ measurements of wind, currents, waves, and suspended solid concentrations (SSCs). The results suggested that water turbidity started to increase at wind speeds of approximately 4 m/s and significantly increased when wind speeds exceeded 6 m/s. In most cases, wind-induced waves were the main energy source for changes in turbidity. Wave-generated shear stress contributed more than 95% to sediment resuspension and that only in weak wind conditions (<4 m/s) did the lake bottom shear stresses generated by currents and waves contributed equally. The relationship between SSC and bottom shear stress generated by wave was established by fitting the observed results. The processes of sediment dynamics were divided into four stages (A through D) according to three shear-stress thresholds. In stage A, SSC remained stable (about 45 mg/L) and τ_w was less than 0.02 N/m². In stage B, the sediment bed was starting to be activated (SSC 45∼60 mg/L) and τ_w was in the range of 0.02∼0.07 N/m². In stage C, a medium amount of sediment was suspended (SSC 60∼150 mg/L) and τ_w ranged from 0.07 to 0.3 N/m². In stage D, large amount of sediment was suspended (SSC 150∼300 mg/L) and τ_w was larger than 0.3 N/m². The findings of this paper reveal the driving mechanism of sediment resuspension, which may further help to evaluate internal nutrient release in large shallow Lake Taihu.

Application of plow-tillage as an innovative technique for eliminating overwintering cyanobacteria in eutrophic lake sediments

Surface sediment in eutrophic lakes is both a destination and a habitat for overwintering cyanobacteria. The resuspension and recovery of viable, overwintering cyanobacteria from the surface sediment during warm spring weather is usually the primary stage of cyanobacterial blooms (CBs) in shallow eutrophic lakes. Therefore, the elimination of overwintering cyanobacteria in sediment is vital to control CBs. In the present study, sediment plow-tillage (PT) was introduced as an innovative technique for eliminating overwintering cyanobacteria in sediments from Lake Chaohu. Four depths of PT (2, 5, 10, and 15 cm) were tested during the 42-day experiment. The results showed that rapid cell death during the first 0-7 d after PT was accompanied by high oxygen uptake rates. The viable cells in deeper sediment died more quickly and at a higher rate after PT. A PT depth of >10 cm effectively eliminated viable cyanobacteria (with a removal rate of 82.8%) from the sediment and prevented their resuspension. The activity of the viable cyanobacteria also decreased quickly as cyanobacteria were eliminated. It appears that the dark, anoxic environment of the deeper sediment after PT was responsible for the elimination of viable cells. Although high release rates of nitrogen and phosphorus were found to accompany the dying and decomposition of cyanobacteria during days 0-7 of the experiment, greater depth of PT was found to decrease nutrient concentrations in the overlying water. In conclusion, we recommend sediment PT as a new technique for eliminating overwintering algae in sediments. However, the release of nutrients from the sediment and the in situ control of CBs in lakes after PT should be further studied.

No enhancement of cyanobacterial bloom biomass decomposition by sediment microbial fuel cell (SMFC) at different temperatures

The sediment microbial fuel cell (SMFC) has potential application to control the degradation of decayed cyanobacterial bloom biomass (CBB) in sediment in eutrophic lakes. In this study, temperatures from 4 to 35 °C were investigated herein as the major impact on SMFC performance in CBB-amended sediment. Under low temperature conditions, the SMFC could still operate, and produced a maximum power density of 4.09 mW m^-2 at 4 °C. Coupled with the high substrate utilization, high output voltage was generated in SMFCs at high temperatures. The application of SMFC affected the anaerobic fermentation progress and was detrimental to the growth of methanogens. At the same time, organic matter of sediments in SMFC became more humified. As a result, the fermentation of CBB was not accelerated with the SMFC application, and the removal efficiency of the total organic matter was inhibited by 5% compared to the control. Thus, SMFC could operate well year round in sediments with a temperature ranging from 4 to 35 °C, and also exhibit practical value by inhibiting quick CBB decomposition in sediments in summer against the pollution of algae organic matter.

Differences in the Composition of Archaeal Communities in Sediments from Contrasting Zones of Lake Taihu

In shallow lakes, different primary producers might impact the physiochemical characteristics of the sediment and the associated microbial communities. Until now, little was known about the features of sediment Archaea and their variation across different primary producer-dominated ecosystems. Lake Taihu provides a suitable study area with cyanobacteria- and macrophyte-dominated zones co-occurring in one ecosystem. The composition of the sediment archaeal community was assessed using 16S rRNA gene amplicon sequencing technology, based on which the potential variation with respect to the physiochemical characteristics of the sediment was analyzed. Euryarchaeota (30.19% of total archaeal sequences) and Bathyarchaeota (28.00%) were the two most abundant phyla, followed by Crenarchaeota (11.37%), Aigarchaeota (10.24%) and Thaumarchaeota (5.98%). The differences found in the composition of the archaeal communities between the two zones was significant (p = 0.005). Sediment from macrophyte-dominated zones had high TOC and TN content and an abundance of archaeal lineages potentially involved in the degradation of complex organic compounds, such as the order Thermoplasmatales. In the area dominated by Cyanobacteria, archaeal lineages related to sulfur metabolism, for example, Sulfolobales and Desulfurococcales, were significantly enriched. Among Bathyarchaeota, subgroups MCG-6 and MCG-15 were significantly accumulated in the sediment of areas dominated by macrophytes whereas MCG-4 was consistently dominant in both type of sediments. The present study contributes to the knowledge of sediment archaeal communities with different primary producers and their possible biogeochemical functions in sediment habitats.

Effects of sediment and turbulence on alkaline phosphatase activity and photosynthetic activity of phytoplankton in the shallow hyper-eutrophic Lake Taihu, China

Sediments play important roles, as nutrient reservoir, especially in shallow lake ecosystem. The water column of large shallow lakes is often stable but also disturbed by turbulence causing resuspension of sediments. While considerable research has been carried out to investigate the influence of sediment resuspension on nutrient release, fewer studies have been done to understand the contribution of alkaline phosphatase activity (APA) in water as a response to the two conditions (turbulence and stability). Also, effects of the two lake conditions on photosynthetic efficiency of phytoplankton are still poorly understood. This study will evaluate the effect of these two conditions on photosynthetic efficiency and APA. Sediments used in the indoor experiments were collected from Zhushan Bay in Lake Taihu. Turbulence was generated by rotors to simulate the strong wind-induced disturbance in Lake Taihu. Results of the experiments showed that TN and TP in the stable and episodically turbulent conditions were not significantly different, with TN ranging from 1.34 to 1.90 mg/L and TP from 0.08 to 0.18 mg/L. Whereas, the soluble reactive phosphorus in the episodically turbulent condition was significantly higher than in the stable condition. Episodic turbulence could enhance P cycling by resuspending sediment-associated P, which alleviated algal P limitation. In stable conditions, P deficiency induced the production of high APA, which enhanced the availability of P. Although episodic turbulence could also cause increased algal biomass, photosynthetic efficiency of the algae was also affected not only by the nutrients but also by many other factors, especially light availability. Our results suggest that episodic turbulence is an important driver of biogeochemical cycling in large shallow hypertrophic lake ecosystem.

Assessment of changes in potential nutrient limitation in an impounded river after application of lanthanum-modified bentonite

With the advent of phosphorus (P)-adsorbent materials and techniques to address eutrophication in aquatic systems, there is a need to develop interpretive techniques to rapidly assess changes in potential nutrient limitation. In a trial application of the P-adsorbent, lanthanum-modified bentonite (LMB) to an impounded section of the Canning River, Western Australia, a combination of potential P, nitrogen (N) and silicon (Si) nutrient limitation diagrams based on dissolved molar nutrient ratios and actual dissolved nutrient concentrations have been used to interpret trial outcomes. Application of LMB resulted in rapid and effective removal of filterable reactive P (FRP) from the water column and also effectively intercepted FRP released from bottom sediments until the advent of a major unseasonal flood event. A shift from potential N-limitation to potential P-limitation also occurred in surface waters. In the absence of other factors, the reduction in FRP was likely to be sufficient to induce actual nutrient limitation of phytoplankton growth. The outcomes of this experiment underpins the concept that, where possible in the short-term, in managing eutrophication the focus should not be on the limiting nutrient under eutrophic conditions (here N), but the one that can be made limiting most rapidly and cost-effectively (P).

Controls of Sediment Nitrogen Dynamics in Tropical Coastal Lagoons

Sediment denitrification rates seem to be lower in tropical environments than in temperate environments. Using the isotope pairing technique, we measured actual denitrification rates in the sediment of tropical coastal lagoons. To explain the low denitrification rates observed at all study sites (<5 μmol N₂ m^-2 h^-1), we also evaluated potential oxygen (O₂) consumption, potential nitrification, potential denitrification, potential anammox, and estimated dissimilatory nitrate (NO₃^-) reduction to ammonium (NH₄⁺; DNRA) in the sediment. ¹⁵NO₃^- and ¹⁵NH₄⁺ conversion was measured in oxic and anoxic slurries from the sediment surface. Sediment potential O₂ consumption was used as a proxy for overall mineralization activity. Actual denitrification rates and different potential nitrogen (N) oxidation and reduction processes were significantly correlated with potential O₂ consumption. The contribution of potential nitrification to total O₂ consumption decreased from contributing 9% at sites with the lowest sediment mineralization rates to less than 0.1% at sites with the highest rates. NO₃^- reduction switched completely from potential denitrification to estimated DNRA. Ammonium oxidation and nitrite (NO₂^-) reduction by potential anammox contributed up to 3% in sediments with the lowest sediment mineralization rates. The majority of these patterns could be explained by variations in the microbial environments from stable and largely oxic conditions at low sediment mineralization sites to more variable conditions and the prevalences of anaerobic microorganisms at high sediment mineralization sites. Furthermore, the presence of algal and microbial mats on the sediment had a significant effect on all studied processes. We propose a theoretical model based on low and high sediment mineralization rates to explain the growth, activity, and distribution of microorganisms carrying out denitrification and DNRA in sediments that can explain the dominance or coexistence of DNRA and denitrification processes. The results presented here show that the potential activity of anaerobic nitrate-reducing organisms is not dependent on the availability of environmental NO₃^-.

Will enhanced turbulence in inland waters result in elevated production of autochthonous dissolved organic matter?

Biological activity in lakes is strongly influenced by hydrodynamic conditions, not least turbulence intensity; which increases the encounter rate between plankter and nutrient patches. To investigate whether enhanced turbulence in shallow and eutrophic lakes may result in elevated biological production of autochthonous chromophoric dissolved organic matter (CDOM), a combination of field campaigns and mesocosm experiments was used. Parallel factor analysis identified seven components: four protein-like, one microbial humic-like and two terrestrial humic-like components. During our field campaigns, elevated production of autochthonous CDOM was recorded in open water with higher wind speed and wave height than in inner bays, implying that elevated turbulence resulted in increased production of autochthonous CDOM. Confirming the field campaign results, in the mesocosm experiment enhanced turbulence resulted in a remarkably higher microbial humic-like C1 and tryptophan-like C3 (p<0.01), indicating that higher turbulence may have elevated the production of autochthonous CDOM. This is consistent with the significantly higher mean concentrations of chlorophyll-a (Chl-a) and dissolved organic carbon (DOC) and the enhanced phytoplanktonic alkaline phosphatase activity (PAPA) recorded in the experimental turbulence groups than in the control group (p<0.05). The C:N ratio (from 3.34 to 25.72 with a mean of 13.13±4.08) for the mesocosm CDOM samples further suggested their probable autochthonous origin. Our results have implications for the understanding of CDOM cycling in shallow aquatic ecosystems influenced by wind-induced waves, in which the enhanced turbulence associated with extreme weather conditions may be further stimulated by the predicted global climate change.

Chromophoric dissolved organic matter of black waters in a highly eutrophic Chinese lake: Freshly produced from algal scums?

Field campaigns and an incubation experiment were conducted to evaluate the sources of chromophoric dissolved organic matter (CDOM) in black water spots in highly polluted regions of the Chinese Lake Taihu. A significant positive correlation (p<0.0001) was found between chlorophyll a (Chl-a) and the CDOM absorption coefficient a(350), indicating that algae degradation was likely the primary source of CDOM in black waters. This is supported by our field results that Chl-a, a(350) and the spectral slope ratio (SR) were significantly higher in the black water samples than in the regular samples (p<0.001). Our incubation experiment further substantiated the primary significance of biological CDOM source where a(350) increased with decreasing Chl-a concentrations. After seven days' incubation, a 72.2% decrease and a 74.9% increase were recorded for Chl-a and a(350), respectively, relative to the initial values. Parallel factor analysis identified five fluorescent components. The maximal fluorescence intensity (Fmax) of tryptophan-like C1 and microbial humic-like C3 of black water samples was significantly higher than in the regular water samples (p<0.0005). This is consistent with incubation experiment results showing a rapid increase in Fmax of the two components, emphasizing the priority of the in situ biological CDOM source in black water spots.

Effects of electron acceptors on soluble reactive phosphorus in the overlying water during algal decomposition

Endogenous phosphorus (P) release from sediments is an important factor to cause eutrophication and, hence, algal bloom in lakes in China. Algal decomposition depletes dissolved oxygen (DO) and causes anaerobic conditions and therefore increases P release from sediments. As sediment P release is dependent on the iron (Fe) cycle, electron acceptors (e.g., NO3 (-), SO4 (2-), and Mn(4+)) can be utilized to suppress the reduction of Fe(3+) under anaerobic conditions and, as such, have the potential to impair the release of sediment P. Here, we used a laboratory experiment to test the effects of FeCl3, MnO2, and KNO3 on soluble reactive phosphorus (SRP) concentration and related chemical variables in the overlying water column during algal decomposition at different algal densities. Results showed that algal decomposition significantly depleted DO and thereby increased sediment Fe-bound P release. Compared with the control, addition of FeCl3 significantly decreased water SRP concentration through inhibiting sediment P release. Compared with FeCl3, addition of MnO2 has less potential to suppress sediment P release during algal decomposition. Algal decomposition has the potential for NO3 (-) removal from aquatic ecosystem through denitrification and by that alleviates the suppressing role of NO3 (-) on sediment P release. Our results indicated that FeCl3 and MnO2 could be efficient in reducing sediment P release during algal decomposition, with the strongest effect found for FeCl3; large amounts of NO3 (-) were removed from the aquatic ecosystem through denitrification during algal decomposition. Moreover, the amounts of NO3 (-) removal increased with increasing algal density.

Distinct compositions of free-living, particle-associated and benthic communities of the Roseobacter group in the North Sea

The Roseobacter group is one of the predominant lineages in the marine environment. While most investigations focus on pelagic roseobacters, the distribution and metabolic potential of benthic representatives is less understood. In this study, the diversity of the Roseobacter group was characterized in sediment and water samples along the German/Scandinavian North Sea coast by 16S rRNA gene analysis and cultivation-based methods. Molecular analysis indicated an increasing diversity between communities of the Roseobacter group from the sea surface to the seafloor and revealed distinct compositions of free-living and attached fractions. Culture media containing dimethyl sulfide (DMS), dimethyl sulfonium propionate (DMSP) or dimethyl sulfoxide (DMSO) stimulated growth of roseobacters showing highest most probable numbers (MPN) in DMSO-containing dilutions of surface sediments (2.1 × 10(7) roseobacters cm(-3)). Twenty roseobacters (12 from sediments) were isolated from DMSP- and DMS-containing cultures. Sequences of the isolates represented 0.04% of all Bacteria and 4.7% of all roseobacters in the pyrosequencing dataset from sediments. Growth experiments with the isolate Shimia sp. SK013 indicated that benthic roseobacters are able to switch between aerobic and anaerobic utilization of organic sulfur compounds. This response to changing redox conditions might be an adaptation to specific environmental conditions on particles and in sediments.

Experimental study of wind-induced sediment suspension and nutrient release in Meiliang Bay of Lake Taihu, China

Wind-induced sediment suspension and nutrient release is an important source for shallow eutrophic lakes. This work studies the quantitative relationship between wind speed and sediment release rate in Meiliang Bay of Lake Taihu, China, using field observations and indoor simulations. Natural wind, water flow, and water quality conditions were synchronously monitored to establish the relationship between wind speed and sediment release rate. Sediment suspension processes under different wind speeds were also simulated in a specially designed reactor. We then established the relationship between natural wind speed and indoor-simulated sediment release rate through hydrodynamic conditions (expressed using water shear stress). The indoor experiment was a supplement to the field observations. The results showed that (1) the critical wind speeds at which sediment became suspended and demonstrated maximum suspension were approximately 3-4 and 8 m s(-1), respectively; (2) the relationship between wind speed and suspended sediment (SS), total nitrogen (TN), and total phosphorous (TP) release rate could be expressed by exponential functions (SS release rate: y = 1.287e(0.177x) , R (2) = 0.981; TN release rate: y =  7.55e(0.363x), R (2) = 0.981; TP release rate: y = 0.381e(0.186x), R (2) = 0.945); and (3) the critical (wind speed, 8 m s(-1)) release rates of SS, TN, and TP were 1000, 5.8, and 2.2 g m(-2) day(-1), while the maximum (wind speed, 16 m s(-1)) rates were 5000, 24.7, and 5.4 g m(-2) day(-1), respectively.

Long-Term Satellite Observations of Microcystin Concentrations in Lake Taihu during Cyanobacterial Bloom Periods

Microcystins (MCs) produced by cyanobacteria pose a serious threat to public health. Intelligence on MCs distributions in freshwater is therefore critical for environmental agencies, water authorities, and public health organizations. We developed and validated an empirical model to quantify MCs in Lake Taihu during cyanobacterial bloom periods using the atmospherically Rayleigh-corrected moderate resolution imaging spectroradiometer (MODIS-Aqua) (Rrc) products and in situ data by means of chlorophyll a concentrations (Chla). First, robust relationships were constructed between MCs and Chla (r = 0.91; p < 0.001; t-test) and between Chla and a spectral index derived from Rrc (r = -0.86; p < 0.05; t-test). Then, a regional algorithm to analyze MCs in Lake Taihu was constructed by combining the two relationships. The model was validated and then applied to an 11-year series of MODIS-Aqua data to investigate the spatial and temporal distributions of MCs. MCs in the lake were markedly variable both spatially and temporally. Cyanobacterial bloom scums, temperature, wind, and light conditions probably affected the temporal and spatial distribution of MCs in Lake Taihu. The findings demonstrate that remote sensing reconnaissance in conjunction with in situ monitoring can greatly aid MCs assessment in freshwater.

Distribution characteristics of phosphorus in the sediments and overlying water of Poyang lake

Phosphorus (P) is a key indicator of the aquatic organism growth and eutrophication in lakes. The distribution and speciation of P and its release characteristics from sediments were investigated by analyzing sediment and water samples collected during high flow and low flow periods. Results showed that the average concentrations (ranges) of total phosphorus (TP) in the surface and deep water were 0.06 mg L^-1 (0.03–0.13 mg L^-1) and 0.15 mg L^-1 (0.06–0.33 mg L^-1), respectively, while the average concentration (range) of TP in sediments was 709.17 mg kg^-1 (544.76–932.11 mg kg^-1). The concentrations of TP and different forms of P varied spatially in the surface sediments, displaying a decreasing trend from south to north. P also varied topographically from estuarine areas to lake areas. The vertical distribution of TP and different forms of P were observed to decrease as depth increased. The P concentrations during the low flow period were higher than those during the high flow period. Inorganic phosphorus (IP) was the dominant form of P, accounting for 61%–82% of TP. The concentration of bioavailable phosphorus in sediments was relatively large, indicating a high risk of release to overlying water. The simulation experiment of P release from sediments showed that the release was relatively fast in the first 0-5 min and then decreased to a plateau after 1 hr. Approximately 84–89% of the maximum amount of P was released during the first hour.

Using enriched stable isotope technique to study Cu bioaccumulation and bioavailability in Corbicula fluminea from Taihu Lake, China

In this study, we measured trace metals (Cd, Cr, Cu, Ni, Pb, and Zn) in water and sediment from representative sites of Taihu Lake, with focus on the analysis of trace metal accumulation in Corbicula fluminea (bivalve). The results showed that the quality of water in Taihu Lake was generally good and the correlation was not found between Cu bioaccumulation in C. fluminea and the concentration in water and sediment. Thus, using the stable isotope tracer method, we studied Cu uptake from the water phase, the assimilation of Cu from the food phase, and the efflux of Cu in vivo by C. fluminea. The result revealed that this species exhibited a relatively lower efflux rate constant of Cu compared with other zoobenthos species. Using a simple bioenergetics-based kinetic model, Cu concentrations in the C. fluminea were calculated with the measured efflux rate. We put forward a novel method, which was taking the influence of biological kinetic on metal bioaccumulation into account to explain the field survey data.

Oxygen and phosphorus dynamics in freshwater sediment after the deposition of flocculated cyanobacteria and the role of tubificid worms

Flocculation is a promising method for controlling harmful algal blooms; however, little is known about the effects of algae deposition by flocculation on benthic oxygen (O2) and nutrient dynamics. In this study, we aimed to investigate the influence of cyanobacteria flocculation deposition on benthic O2 and phosphorus (P) dynamics and the role of tubificid worms in the process. Chitosan and sediment particles were used to flocculate and deposit cyanobacteria cells onto lake sediment. The impulse deposition of algal flocculation degraded the deposited algal cells, which decreased the O2 penetration depth in sediment and increased the O2 uptake rate. Algae deposition also increased the soluble reactive P (SRP) in pore water and loosely adsorbed P in sediment, and changed SRP flux. Tubificid worms transported algal cells deeper into the sediment, mitigated their degradation, and altered the O2 penetration depth, but not the O2 uptake rate. Tubificid worms enhanced the increase in pore-water SRP and loosely adsorbed P in sediment. Therefore, the deposition of algal flocculation modifies the benthic O2 and P dynamics, and tubificid worms can mitigate or enhance some of these processes.

Principal component analysis to assess the efficiency and mechanism for enhanced coagulation of natural algae-laden water using a novel dual coagulant system

A novel dual coagulant system of polyaluminum chloride sulfate (PACS) and polydiallyldimethylammonium chloride (PDADMAC) was used to treat natural algae-laden water from Meiliang Gulf, Lake Taihu. PACS (Aln(OH)mCl3n-m-2k(SO4)k) has a mass ratio of 10 %, a SO4 (2-)/Al3 (+) mole ratio of 0.0664, and an OH/Al mole ratio of 2. The PDADMAC ([C8H16NCl]m) has a MW which ranges from 5 × 10(5) to 20 × 10(5) Da. The variations of contaminants in water samples during treatments were estimated in the form of principal component analysis (PCA) factor scores and conventional variables (turbidity, DOC, etc.). Parallel factor analysis determined four chromophoric dissolved organic matters (CDOM) components, and PCA identified four integrated principle factors. PCA factor 1 had significant correlations with chlorophyll-a (r=0.718), protein-like CDOM C1 (0.689), and C2 (0.756). Factor 2 correlated with UV254 (0.672), humic-like CDOM component C3 (0.716), and C4 (0.758). Factors 3 and 4 had correlations with NH3-N (0.748) and T-P (0.769), respectively. The variations of PCA factors scores revealed that PACS contributed less aluminum dissolution than PAC to obtain equivalent removal efficiency of contaminants. This might be due to the high cationic charge and pre-hydrolyzation of PACS. Compared with PACS coagulation (20 mg L(-1)), the removal of PCA factors 1, 2, and 4 increased 45, 33, and 12 %, respectively, in combined PACS-PDADMAC treatment (0.8 mg L(-1) +20 mg L(-1)). Since PAC contained more Al (0.053 g/1 g) than PACS (0.028 g/1 g), the results indicated that PACS contributed less Al dissolution into the water to obtain equivalent removal efficiency.

Dynamics of cyanobacterial bloom formation during short-term hydrodynamic fluctuation in a large shallow, eutrophic, and wind-exposed Lake Taihu, China

Short-term hydrodynamic fluctuations caused by extreme weather events are expected to increase worldwide because of global climate change, and such fluctuations can strongly influence cyanobacterial blooms. In this study, the cyanobacterial bloom disappearance and reappearance in Lake Taihu, China, in response to short-term hydrodynamic fluctuations, was investigated by field sampling, long-term ecological records, high-frequency sensors and MODIS satellite images. The horizontal drift caused by the dominant easterly wind during the phytoplankton growth season was mainly responsible for cyanobacterial biomass accumulation in the western and northern regions of the lake and subsequent bloom formation over relatively long time scales. The cyanobacterial bloom changed slowly under calm or gentle wind conditions. In contrast, the short-term bloom events within a day were mainly caused by entrainment and disentrainment of cyanobacterial colonies by wind-induced hydrodynamics. Observation of a westerly event in Lake Taihu revealed that when the 30 min mean wind speed (flow speed) exceeded the threshold value of 6 m/s (5.7 cm/s), cyanobacteria in colonies were entrained by the wind-induced hydrodynamics. Subsequently, the vertical migration of cyanobacterial colonies was controlled by hydrodynamics, resulting in thorough mixing of algal biomass throughout the water depth and the eventual disappearance of surface blooms. Moreover, the intense mixing can also increase the chance for forming larger and more cyanobacterial colonies, namely, aggregation. Subsequently, when the hydrodynamics became weak, the cyanobacterial colonies continuously float upward without effective buoyancy regulation, and cause cyanobacterial bloom explosive expansion after the westerly. Furthermore, the results of this study indicate that the strong wind happening frequently during April and October can be an important cause of the formation and expansion of cyanobacterial blooms in Lake Taihu.

Effects of environmental factors on nutrients release at sediment-water interface and assessment of trophic status for a typical shallow lake, northwest China

Surface sediment and water samples were collected from Daihai Lake to study the biogeochemical characteristics of nitrogen and phosphorus, to estimate the loads of these nutrients, and to assess their effects on water quality. The contents and spatial distributions of total phosphorus (TP), total nitrogen (TN), and different nitrogen forms in sediments were analyzed. The results showed that concentrations of TN and TP in surface sediments ranged from 0.27 to 1.78 g/kg and from 558.31 to 891.29 mg/kg, respectively. Ratios of C:N ranged between 8.2 and 12.1, which indicated that nitrogen accumulated came mainly from terrestrial source. Ratios of N:P in all sampling sites were below 10, which indicated that N was the limiting nutrient for algal growth in this lake. Effects of environment factors on the release of nitrogen and phosphorus in lake sediments were also determined; high pH values could encourage the release of nitrogen and phosphorus. Modified Carlson's trophic state index (TSI(M)) and comprehensive trophic state index (TSI(C)) were applied to ascertain the trophic classification of the studied lake, and the values of TSI(M) and TSI(C) ranged from 53.72 to 70.61 and from 47.73 to 53.67, respectively, which indicated that the Daihai Lake was in the stage of hypereutropher.

A study on influential factors of high-phosphorus wastewater treated by electrocoagulation-ultrasound

A combined treatment of electrocoagulation and ultrasound was proposed to solve some problems which exist in the phosphorus removal processes in fine chemical industry. The intermittently discharged wastewater has the features of high initial phosphorus concentration and wide initial pH variation. The electrocoagulation-ultrasound effective performance for the removal of phosphorus was investigated. The results obtained from synthetic wastewater showed that the total phosphorus (TP) decreased from 86 to about 0.4 mg/L, and the removal efficiency reached about 99.6 %, when ultrasound was applied to the electrocoagulation cell under the optimum working conditions in 10 min. Comparatively, the TP removal efficiency of electrocoagulation group was 81.3 % and the ultrasound group has almost no change. Therefore, we can conclude that the electrocoagulation and ultrasound synergistic effect can effectively degrade high-phosphorus wastewater. We have discussed the impact of various parameters on the electrocoagulation-ultrasound based on the phosphorus removal efficiency. The results obtained from synthetic wastewater showed that the optimum working pH was found to be 6, allowing the effluent to be met the emission standards without pH adjustment. An increased current enhanced the speed of treatment significance, but higher current (>40 mA/cm(2)) enhanced ultrasonic cavitation effect causing flocculation ineffective. In addition, it was found that the optimum ultrasonic power was 4 W/cm(2) and the frequency was 20 kHz. The best ultrasound intervention and ultrasonic irradiation time were processed with electrocoagulation simultaneously. The results indicated that the electrocoagulation-ultrasound could be utilized as an attractive technique for removal of phosphate in the real wastewater.