Eutrophic Freshwater Lake Research Articles

Nitrogen rather than phosphorus driving the biogeographic patterns of abundant bacterial taxa in a eutrophic plateau lake

Eutrophication of freshwater lakes is an important cause of global water pollution. In this study, the composition and biogeographic distribution of both abundant and rare sedimentary bacterial taxa and their relationship with nutrients were assessed in Erhai Lake, a subtropical plateau lake. Proteobacteria (48.3%) and Nitrospirae (11.7%) dominated the composition of abundant taxa, while the rare taxa were dominated by Proteobacteria (25.8%) and Chloroflexi (14.1%). The abundant bacterial taxa had strong energy metabolism, whereas the rare bacterial taxa had strong xenobiotics biodegradation and metabolism. These results indicated different compositions and functions existed between abundant and rare taxa. Total nitrogen (TN) was the most influential factor shaping the biogeographic patterns of both abundant and rare taxa. Phosphorus was not the deterministic factor, although nitrogen and phosphorus were the main contributors to eutrophication. Total organic carbon and pH also contributed to the biogeographic patterns of both abundant and rare taxa. In the eutrophic plateau lake sediments, abundant taxa, rather than rare taxa, played a dominant role in maintaining the community structure and ecological function of the bacterial community. The TN gradient was an important factor that affected the biogeographic distribution and assembly processes of abundant taxa. This study sheds light on the role of TN in shaping the biogeographic distribution and assembly processes of abundant taxa in eutrophic lakes.

Regime transition Shapes the Composition, Assembly Processes, and Co-occurrence Pattern of Bacterioplankton Community in a Large Eutrophic Freshwater Lake.

At certain nutrient concentrations, shallow freshwater lakes are generally characterized by two contrasting ecological regimes with disparate patterns of biodiversity and biogeochemical cycles: a macrophyte-dominated regime (MDR) and a phytoplankton-dominated regime (PDR). To reveal ecological mechanisms that affect bacterioplankton along the regime shift, Illumina MiSeq sequencing of the 16S rRNA gene combined with a novel network clustering tool (Manta) were used to identify patterns of bacterioplankton community composition across the regime shift in Taihu Lake, China. Marked divergence in the composition and ecological assembly processes of bacterioplankton community was observed under the regime shift. The alpha diversity of the bacterioplankton community consistently and continuously decreased with the regime shift from MDR to PDR, while the beta diversity presents differently. Moreover, as the regime shifted from MDR to PDR, the contribution of deterministic processes (such as environmental selection) to the assembly of bacterioplankton community initially decreased and then increased again as regime shift from MDR to PDR, most likely as a consequence of differences in nutrient concentration. The topological properties, including modularity, transitivity and network diameter, of the bacterioplankton co-occurrence networks changed along the regime shift, and the co-occurrences among species changed in structure and were significantly shaped by the environmental variables along the regime transition from MDR to PDR. The divergent environmental state of the regimes with diverse nutritional status may be the most important factor that contributes to the dissimilarity of bacterioplankton community composition along the regime shift.

Nitrogen and phosphorous loading by aquatic avifauna in a shallow eutrophic freshwater lake

Nitrogen and phosphorous loading by aquatic avifauna in a shallow eutrophic freshwater lake

Phosphorus concentrations into a subtropical lake strongly influence nitrogen accumulation, nitrogen export, and Chl a concentrations

We measured water quality monthly for 22 years in water entering, within, and exiting a 65 km2 shallow polymictic and eutrophic freshwater lake in the northern Gulf of Mexico. Fertilizer use in the watershed is the dominate source of phosphorous (P) going into the lake and controls the lake’s P concentrations, but nitrogen (N) fertilizer use was not related to total nitrogen concentration in the lake. Half of the particulate P entering the lake is trapped within it and there is a net accumulation of N that appears to be from the stimulation of nitrogen fixation. The lake’s concentration of Chlorophyll a (µg Chl a l−1) and increase in N in the lake was directly related to the concentration of P in water entering the lake. Variations in the Chl a concentration within a freshwater lake downstream are also directly related to the annual use of P fertilizer, but not to N fertilizer use. Reducing agriculture-sourced P runoff will lower (but not eliminate) both the frequency of algal blooms within Lac des Allemands and the amount of N delivered to the estuary.

Novel method to immobilize phosphate in lakes using sediment microbial fuel cells

Phosphate pollution in lakes poses an intractable remediation challenge. Accumulated stocks of phosphorus in sediments cause high concentrations in the overlying water despite elimination of external sources. We propose to use sediment microbial fuel cells (SMFCs) for lake remediation by sediment phosphorus immobilization. The hypothesis is that SMFCs can increase sediment redox potential at the top layer, and that such changes will allow the sediment to retain phosphorus as immobile species. This study placed an emphasis on scalability, practicality, and use of low-cost materials. Stainless steel net was selected as electrode material, and modifications were tested: (i) chronoamperometric operation with anode poised at +399 mV (versus standard hydrogen potential); (ii) injection of graphite slurry; and (iii) coating with nickel-carbon matrix. Stainless steel electrodes were implemented in laboratory microcosms (1.3 L) and at field scale in a eutrophic freshwater lake. All tests were carried out in untreated sediment and water from Lake Søllerød, Denmark. Phosphate immobilization was shown at lab scale, with 85% decrease in overlying water using steel electrodes. At field scale maximum phosphate decrease of 94% was achieved in the water body above a 16 m2 stainless steel SMFC electrode. Results are promising and warrant further study, including remediation trials at full scale. Added benefits include degradation of sediment organic matter and pollutants, inhibition of methane and sulfide emission and production of electricity.

Habitat heterogeneity induces regional differences in sediment nitrogen fixation in eutrophic freshwater lake

Biological nitrogen fixation (BNF) in sediments is an important source of bioavailable nitrogen in aquatic systems. However, the effect of habitat change caused by eutrophication on nitrogen fixation within sediments is still unclear. In this study, nitrogen fixation rates and diazotroph diversities in sediments with heterogeneous ecological status in one eutrophic lake were investigated by using an isotope tracer method and sequencing of nitrogen-fixing (nif) genes. The results showed that both nitrogenase activity (NA) and nifH abundance in sediments of blooms area were higher than those in vegetation-dominated habitats. Correlation analysis showed that NA was correlated closely to nifH abundance, dissolved sulfide, and iron. The diazotrophic assemblage contained mainly Proteobacterial sequences belonging to Cluster I and III, and the variations of diazotrophic community could be explained by total nitrogen content, total phosphorus content, organic matters, sulfides, ammonium and iron content. Moreover, the co-occurrence network analysis showed the Alphaproteobacteria shaped the major interactions in diazotrophic community, and sediment properties had stronger effect on diazotrophic community in cyanobacteria-dominated habitat. This study revealed that habitat heterogeneity in eutrophic lakes shaped different succession of BNF in sediments and cyanobacterial blooms significantly improved the nitrogen-fixing activity in sediments, which broadened our understanding of nitrogen cycle and nutrient management in eutrophic freshwater lakes.

Statistical Optimization of Microcystis Growth and Microcystin Production in Natural Phytoplankton Community using Microcosm Bioassays

Eutrophication of freshwater lakes and reservoirs causing toxic cyanobacterial blooms has become a global health concern. Commonly used approaches of statistical modelling have not fully captured the complex response of cyanobacterial biomass and mi- crocystin concentration in response to stochastic variation of ambient conditions. This study applied statistically-based experimental design to screen out critical environmental factors, and to investigate the interactive effects of total nitrogen (TN), total phosphorus (TP), temperature (T) and light intensity (L) on Microcystis (MA) growth and microcystin (MC) production in the natural phytoplankton com- munity. A batch of central composite designed nutrient enrichment bioassays (CCD-NEBs) were conducted in an ambient-controlled microcosm based on pneumatic annular flume system. Second-order polynomial CCD regression models predicting MA density and MC concentration were established and validated. MC concentration was found positively related to MA dominance. The interactive effects of TN-TP and TN-L on both MA density and MC concentration were determined to be more significant than other factors. Ranges of the optimum TN/TP ratio for MA growth and MC production under various environmental conditions were defined as 9.5 ~ 10.2 and 9.9 ~ 10.4, respectively. The ranges of nutrient thresholds for MA density of 2.00 × 104 cells mL-1 and MC concentration of 1.0 μg L-1 under different temperature and light conditions were evaluated, respectively. This study revealed that nutrient thresholds for MA blooming events and cyanotoxins safety guideline were fluctuant under lake-dependant conditions of temperature and light intensity, and the optimum TN/TP mass ratios should be alerted for the eutrophication management of blooming formation and microcystin safety.

Seasonal iron‑sulfur interactions and the stimulated phosphorus mobilization in freshwater lake sediments

Sulfur reduction in freshwater ecosystems has previously been considered as negligible because of often very low sulfate concentrations and generally low sulfate reducing capacity in freshwater sediments. In this study, seasonal variations on three types of sediments from central lake, dredged and algae accumulated areas in a eutrophic lake in China, Lake Taihu, were investigated. The high temperature in summer and the accumulation of algae are conducive to the reduction processes in freshwater lake sediments. Iron reduction was observed as the major anaerobic process in all types of sediments, while sulfate reduction was weak in central and dredged lake areas. However, strong sulfate reduction with increase of sulfate reducing bacteria and sulfides generation (119.5 ± 0.2 μmol L−1) was found in surface sediments in algae accumulated areas. Based on the results of Fe reduction rate and the quantity of Fe reducing bacteria, extensive sulfate reduction in algae accumulated sediments inhibited the microbial Fe reduction, and the ΣS2−-mediated chemical Fe reduction (SCIR) dominated instead. Iron was principally stored in the sediments as Fe sulfide compounds, which weakened the rebinding of phosphorus and stimulated phosphorus mobilization. Therefore, attention should be paid to the alteration of Fe cycling and phosphorus mobility caused by the SCIR in algae accumulated sediments and the consequent effects on the eutrophication of freshwater lakes.

Dark accelerates dissolved inorganic phosphorus release of high-density cyanobacteria.

Bloom-forming cyanobacteria dramatically influence nutrient cycling in eutrophic freshwater lakes. The phosphorus (P) assimilation and release of bloom-forming cyanobacteria significantly may also affect the phosphorus source and amounts in water. To understand the phosphorus release process of bloom-forming cyanobacteria below the accumulated surface and sedimentary bloom-forming cyanobacteria, the degradation of bloom-forming cyanobacteria dominated by Microcystis spp. at different cell density in the dark was investigated over a 25-day microcosm experiment. The dissolved inorganic phosphorus (DIP) and dissolved total phosphorus (DTP) contents increased with the increment of cyanobacterial density, and the dark status markedly increased the proportion of DIP in water during the decline period of bloom-forming cyanobacteria. Meanwhile, the process of cyanobacterial apoptosis accompanied by the changes of malondialdehyde (MDA) and phosphatase (AKP) contents, and the increases of superoxide dismutase (SOD) and catalase (CAT) activities of cyanobacteria in the dark, especially in low-density groups (5.23×108 cells L-1), which further affect the physicochemical water parameters. Moreover, the DIP release from high-density cyanobacteria (7.86×107 cells L-1~5.23×108 cells L-1) resulted from the relative abundance of organophosphorus degrading bacteria in the dark. Therefore, the fast decay of cyanobacteria in the dark could accelerate DIP release, the high DIP release amount from accumulated bloom-cyanobacteria provide adequate P quickly for the sustained growth of cyanobacteria.

Dynamic sulfur-iron cycle promoted phosphorus mobilization in sediments driven by the algae decomposition.

Direct evidence of the algae bloom in eutrophic freshwater lakes on sulfur cycle and the subsequent iron oxide reduction and the iron oxides-bound phosphate (Fe-P) release in sediments is lacking. In this study, microcosms experiment was carried out to investigate the dynamic variations of S, Fe and P species in the water column and sediments as well as the sulfate reducing bacteria (SRB) abundance variation in the sediments during algae decomposition. The sulfate reduction was stimulated by the algae decomposition, which resulted in dramatic sulfate decline, sulfide increase and SRB growth. In addition, large amounts of acid volatile sulfide (AVS), pyrite sulfur (Pyrite-S) and elemental sulfur (S0) accumulated in the sediment. In particular, the contents of sedimentary Fe(II) and Pyrite-S in surface sediments continuously accumulated until the end of the experiment. Moreover, the terminal Fe-P content reduced by 35.4% compared with the initial concentration at high algae density group. These results suggested the irreversible reduction of iron oxides and revealed iron chemical reduction mediated by sulfide during algae decomposition. In addition, the connection of sulfur-iron cycle and the significant promotion of Fe-P mobilization in sediments was established, which should be paid more attention in the eutrophic freshwater ecosystems.

Algal blooms in a freshwater reservoir – A network community detection analysis of potential forcing parameters

Nutrient leakage due to modern agriculture and disposal of untreated urban wastewater results in the eutrophication of freshwater lakes and reservoirs, with elevated levels of cyanobacteria algal blooms often resulting in toxic conditions for animals and humans. A unique dataset of 22 coincident nutrient, phytoplankton, meteorological, and reservoir condition variables were recorded or calculated monthly for Hoover Reservoir near Columbus, Ohio, from February 1999 to December 2005. Network science was used in this study to visualize and differentiate selected nutrient and phytoplankton seasonality in Hoover Reservoir. Nutrient and phytoplankton concentrations in Hoover Reservoir respond independently to a number of external and internal environmental variables, but are also biochemically interdependent within the reservoir ecosystem. A number of forcing parameters can significantly alter seasonal phytoplankton growth and succession patterns. Chief among these are variability in the daily, seasonal, and/or yearly patterns and intensities of precipitation and the resultant surface runoff; duration and intensity of solar radiation; air and resultant water temperatures; and agricultural practices. The phytoplankton population in Hoover Reservoir is ecologically driven, with algal succession and nutrient levels constraining population concentrations. Complemented by selected statistical analyses, this limited network community detection study provides the hydrologist/ecologist with a dynamic and quantitative visualization of the complex hydrometeorological variable and biogeochemical process forcing parameters present in reservoirs and lakes.

Genome analyses provide insights into the evolution and adaptation of the eukaryotic Picophytoplankton Mychonastes homosphaera

BackgroundPicophytoplankton are abundant and can contribute greatly to primary production in eutrophic lakes. Mychonastes species are among the common eukaryotic picophytoplankton in eutrophic lakes. We used third-generation sequencing technology to sequence the whole genome of Mychonastes homosphaera isolated from Lake Chaohu, a eutrophic freshwater lake in China.ResultThe 24.23 Mbp nuclear genome of M.homosphaera, harboring 6649 protein-coding genes, is more compact than the genomes of the closely related Sphaeropleales species. This genome streamlining may be caused by a reduction in gene family number, intergenic size and introns. The genome sequence of M.homosphaera reveals the strategies adopted by this organism for environmental adaptation in the eutrophic lake. Analysis of cultures and the protein complement highlight the metabolic flexibility of M.homosphaera, the genome of which encodes genes involved in light harvesting, carbohydrate metabolism, and nitrogen and microelement metabolism, many of which form functional gene clusters. Reconstruction of the bioenergetic metabolic pathways of M.homosphaera, such as the lipid, starch and isoprenoid pathways, reveals characteristics that make this species suitable for biofuel production.ConclusionThe analysis of the whole genome of M. homosphaera provides insights into the genome streamlining, the high lipid yield, the environmental adaptation and phytoplankton evolution.

Direct and indirect effects of native plants and herbivores on biotic resistance to alien aquatic plant invasions

Abstract Biotic resistance to alien plant invasions is mainly determined by ecological interactions in two layers of the food web: competition with native plant species and herbivory by native herbivores. While the direct effect of native plants on alien plant performance via competition has been well documented across ecosystems, less is known about the direct and indirect effects of herbivores in providing biotic resistance. Our main aims were to determine whether temperate native aquatic plants and herbivores can provide biotic resistance to plant invasions, understand the underlying mechanisms and search for potential interactive effects of competition and herbivory on invader performance (i.e. growth). We mimicked natural temperate mesotrophic and eutrophic freshwater lakes in mesoscosms, by growing three native submerged plant species in monocultures (Ceratophyllum demersum, Myriophyllum spicatum and Potamogeton perfoliatus) at three competition levels (no, low and high) without and with the native aquatic generalist snail Lymnaea stagnalis. We subsequently simulated an early stage of establishment of the South American highly invasive alien plant species Egeria densa. We found that competition by native plant biomass significantly reduced invader performance but depended on native species identity. Herbivory had no direct negative effect on invader performance as the snails fed mainly on the available filamentous algae, which are commonly found in meso‐ and eutrophic systems, instead of on the plants. However, the consumption of filamentous algae by herbivores indirectly had positive effects on the invader total biomass, thus facilitating the invasion by E. densa. Nonetheless, these indirect effects worked through different pathways depending on the native plant identity. Synthesis. We found evidence for biotic resistance through competition by native plant species. However, we show that herbivores can indirectly facilitate South American plant E. densa invasion promoting its growth through selective feeding on filamentous algae, but this effect depends on the native plant species involved. Our experiment illustrates the important role of indirect interactions to understand the potential of biotic resistance in natural ecosystems.

Spatial and seasonal variability of nitrous oxide in a large freshwater lake in the lower reaches of the Yangtze River, China

Aquatic ecosystems are recognized as a source of N2O in accordance with the flux estimations of rivers and estuaries; however, limited research has been conducted on large lakes. In this study, we report the annual N2O dynamics of a large eutrophic freshwater lake located in the subtropical zone of East China. The dissolved N2O concentrations in Lake Chaohu were observed to be between 8.5 and 92.3 nmol L−1 with emission rates between 0.3 and 53.6 μmol m−2 d−1, exhibiting considerable spatiotemporal variability. The average seasonal N2O concentrations were obtained, with the highest value of 23.4 nmol L−1 found in winter and the lowest value of 12.7 nmol L−1 found in summer. In contrast to the N2O concentrations observed, the highest N2O emission rates occurred during summer, while the lowest emission rates occurred in autumn. The emissions of N2O were substantially high in the western part of the lake, which suffers from serious eutrophication. In addition, the hotspots of N2O emissions have been found around the inflowing mouth of the Nanfei River, which transports large amounts of nutrients into the lake. The results suggest that anthropogenically enhanced nutrient inputs may have a significant role in the production and emission of N2O. However, the negative relationship between the surface water temperature and the N2O concentration suggests that, N2O fluxes might be influenced by other inconspicuous mechanisms. In the future the nitrogen dynamics of water and sediment in the lake should be collated to reveal mechanisms controlling N2O emissions. In summary, Lake Chaohu acts as a source of N2O with its most eutrophic part contributing 54.9% of the total N2O emissions of the whole lake.

Influences of eutrophication on methanogenesis pathways and methanogenic microbial community structures in freshwater lakes

Freshwater lakes, especially eutrophic ones, have become a hotspot of methanogenesis. However, the effects of eutrophication and seasonality on methanogenesis activity and methanogenic microbial community remain unclear. In the current study, for two adjacent lakes at different trophic status, their methanogenesis potential in different seasons was evaluated using incubation experiments. The density, diversity, and community structure of methanogens were analyzed based on the mcrA gene. Correlation analysis and redundancy analysis were carried out to identify the environmental factors driving the variations of methanogenesis potential and methanogen community. The results showed that eutrophication could result in active methanogenesis with relatively high seasonal variance. The methanogenesis variation could be well explained by carbon input in association with algal growth, as well as the change of methanogen population density. With the dominance of Methanomicrobiales in both lakes, the hydrogenotrophic pathway had a major contribution to total methane production. The considerable proportion of Methanomassiliicocales in eutrophic lake implied that methylotrophic methanogenesis might be previously underestimated. These results added new insights towards methanogenesis process in eutrophic freshwater lakes.

Benthic cyanobacterial detritus mats in lacustrine sediment: Characterization and odorant producing potential

Eutrophic freshwater lake ecosystems are receiving increasing public attention due to a global increase in large-scale harmful cyanobacterial blooms in surface waters. However, the contribution of phytodetritus accumulation in benthic sediments post-bloom remains unclear. In this study, field investigations were performed using microsensors to evaluate benthic phytodetritus mats by measuring TOC/TN ratios, pigments, biodegradable compounds and odorants as descriptive parameters. Results show that the massive amount of phytodetritus trapped by aquatic plants gradually evolved into benthic cyanobacterial detritus mats, which were characterized as anoxic, reductive and low pH. It was confirmed that the occurrence of odorants is more serious in the detritus mats due to decay and decomposition of the accumulated phytodetritus. The mean odorant content in the vegetated zones was 3–52 times higher than that in the unvegetated zones. The dominant odorants were dimethyl trisulfide (DMTS), β-ionone and β-cyclocitral, with mean contents of 52.38 ng·(g·dw)-1, 162.20 ng·(g·dw)-1 and 307.51 ng·(g·dw)-1, respectively, in the sediment. In addition, odorant production appears to be associated with the distribution of biodegradable compounds in the sediment. This is supported by the marked correlation observed between biodegradable compounds and odorants. Multiple regression analysis showed that biodegradable compounds can be used as indicators to predict odorant content in the sediment. It is noteworthy that the odorant trend in the water column and sediment is symmetrical, indicating a risk of diffusion from the sediment to the water column. This study helps to clarifying the contributions of benthic cyanobacterial detritus mats to odorant production in shallow eutrophic lakes. The information provided herein may also be useful for future management of aquatic ecosystems.

An RNA-based quantitative and compositional study of ammonium-oxidizing bacteria and archaea in Lake Taihu, a eutrophic freshwater lake.

Ammonium-oxidizing archaea (AOA) and bacteria (AOB) play crucial roles in ammonium oxidation in freshwater lake sediment. However, previous reports on the predominance of AOA and AOB in the surface sediment of Lake Taihu have been based on DNA levels, detecting the total abundance of microbiota (including inactive cells), and have resulted in numerous contradictory conclusions. Existing RNA-level studies detecting active transcription are very limited. The current study, using RNA-based real-time quantification and clone library analysis, demonstrated that the amoA gene abundance of active AOB was higher than that of active AOA, despite conflicting results at the DNA level. Further exploration revealed a significant positive correlation between the potential nitrification rate (PNR) and the abundance of AOA and AOB at the RNA level, with irregular or contradictory correlation found at the DNA level. Ultimately, using quantitative analysis of RNA levels, we show AOB to be the active dominant contributor to ammonium oxidation. Our investigations also indicated that AOB were more diverse in high-ammonium lake regions, with Nitrosomonas being the active and dominating cluster, but that AOA had an advantage in the low-ammonium lake regions.

Linkages between Macrophyte Functional Traits and Water Quality: Insights from a Study in Freshwater Lakes of Greece

Freshwater ecologists have shown increased interest in assessing biotic responses to environmental change using functional community characteristics. With this article, we investigate the potential of using functional traits of the aquatic plants to assess eutrophication in freshwater lakes. To this end we collected macrophyte and physicochemical data from thirteen lakes in Greece and we applied a trait-based analysis to first identify discrete groups of macrophytes that share common functional traits and then to assess preliminary responses of these groups to water quality gradients. We allocated 11 traits that cover mostly growth form and morphological characteristics to a total of 33 macrophyte species. RLQ and fourth corner analysis were employed to explore potential relationships between species, trait composition and environmental gradients. In addition, a hierarchical cluster analysis was conducted to discriminate groups of plants that share common trait characteristics and then the position of the groups along the environmental gradients was assessed. The results showed total phosphorus, chlorophyll-a, conductivity, pH and Secchi disk depth as main drivers of the environmental gradients. Hierarchical cluster analysis showed a clear separation of macrophyte assemblages with discrete functional characteristics that appeared to associate with different environmental drivers. Thus, rooted submerged plants were related with higher Secchi disk depth, conductivity and alkalinity whereas rooted floating-leaved plants showed a preference for enriched waters with phosphorus and nitrogen. In addition, free-floating plants were related positively with nitrogen and increased pH. Although we did not identify specific trait patterns with environmental drivers, our findings indicate a differentiation of macrophytes based on their functional characteristics along water quality gradients. Overall, the presented results are encouraging for conducting future monitoring studies in lakes focused on the functional plant trait composition, as expanding the current approach to additional lakes and using quantifiable functional characteristics will provide more insight about the potential of trait-based approaches as ecological assessment systems.

Seasonal succession and spatial distribution of bacterial community structure in a eutrophic freshwater Lake, Lake Taihu

In aquatic ecosystems, both phytoplankton and bacteria play pivotal roles. Based on 16S rRNA gene sequencing, considerable research focused on phytoplankton colony attached and free-living bacteria has revealed the close relationship between them, and indicated that the entire bacterial community mediates crucial biogeochemical processes in aquatic ecosystems. However, our understanding of their distribution patterns and response to environmental factors remains poor. Besides, picocyanobacteria, which were omitted from attached bacteria analysis, were reported to be important in cyanobacterial blooms. To explore the spatiotemporal variation of the entire bacterial community with their driving environmental factors and detect the relationships among them, we collected 61 water samples spanning one year and the entire Lake Taihu regions for surveying the entire bacterial community. Our results indicated: 1) seasonal variation of the bacterial community composition was stronger than spatial variation due to the clearly seasonal variation of Microcystis, Synechococcus (pico-cyanobacteria) and other bacteria (Actinomycetales, Pirellulaceae and Sphingobacteriaceae); 2) the spatial distribution of the bacterial community showed that different phyla were dominant in different regions; 3) the bacterial co-occurrence networks varied seasonally and were dominated by Microcystis, ACK-M1, Chthoniobacteraceae, Synechococcus, Pirellulaceae and Pelagibacteraceae; 4) phytoplankton density, chlorophyll a, water temperature and total nitrogen were the major factors that drove the spatiotemporal variation of bacterial community composition. This study revealed the seasonal succession and spatial distribution of the entire bacterial community in Lake Taihu, providing new insights into the relationship between water bloom-forming cyanobacterial species and other bacteria, and their response to environmental factors in eutrophic freshwater ecosystem.

Toxicological and ecotoxicological evaluation of the water quality in a large and eutrophic freshwater lake of China

Effect-based methods (EBMs) are recommended as holistic approach for diagnosis and monitoring of water quality; however, the application of EBMs is still scare in China. In the present study, water quality of the freshwater lake Taihu (China) was investigated by EBMs. Different types of water samples were collected from three bays of the lake during 2015, 2016 and 2017. A battery of seven effect-based bioassays, including both specific and non-specific toxicity assays, was used. The bioassay battery was recently suggested based on joint activities of the EU project SOLUTIONS and the NORMAN network on emerging pollutants and is also under discussion for being implemented into monitoring activities in the context of the European Water Framework Directive (WFD). Adverse effects were observed towards the primary producer, primary consumer and fish, indicating the potential ecotoxicity of water in Taihu Lake. Mutagenic and estrogenic effects were found in the Ames fluctuation assay and ERα CALUX (Chemically Activated Luciferase Gene-eXpression) assay, respectively, highlighting the potential risks on human health. Algal growth inhibition and mutagenic effects can be observed during each of the three years. Acute toxicity towards Daphnia magna and estrogen receptor agonistic effects were found in at least one of the samples collected in 2016 and 2017, but not in 2015. The endpoints for fish toxicity in the Danio rerio fish embryo test included both lethal and additionally several sublethal effects (only for samples from 2017) and were not compared between years. Algal growth inhibition, fish embryo toxicity, mutagenic effect and estrogenicity were observed in each of the three bays, while Daphnia acute toxicity was only found in Zhushan Bay. Taking together, this study provides a big picture on the water quality of Taihu Lake. The battery of effect-based tools is promising to be a routine for water quality monitoring in China.