Vegetation and soil properties in restored wetlands near Lake Taihu,China

Riparian wetlands are important components of the lake ecosystem, and they play essential roles in maintaining system health. Remediation of degraded lakeshore wetlands is an essential component of lake restoration. A study was conducted to investigate the restoration of lakeshore wetlands, which were converted to rice fields and then abandoned for 2, 5, 10 and 15 years, near Lake Taihu. Soil samples (0–20 cm and 20–40 cm) were taken and plant species were investigated. The carbon content in the soil had increased significantly, rising from 0.71% to 1.85% between 2 and 15 years. Organic matter accumulation improved soil texture, and water stable aggregate content (>0.25 mm) and soil porosity increased. Total nitrogen in the soil increased from 0.06% to 0.13%, and total Kjeldahl nitrogen increased from 124.4 mg kg⁻¹ to 351.5 mg kg⁻¹. Total phosphorus in the soil increased from 0.045% to 0.071%, and the Olsen-P value increased from 5.13 mg kg⁻¹ to 16.0 mg kg⁻¹. Results showed that phosphorous did not increase as much as nitrogen. In the vegetation restoration process, plant species composition moved towards a natural wetland community, and spatial heterogeneity and landscape diversity increased. The richness of plant biodiversity increased rapidly in the first 2 years, then more slowly in later restoration stages. The wetlands recovery process may be complicated by interactions of biota and soil and hydrological conditions.     

Nutrient removal in constructed microcosm wetlands for treating polluted river water in northern China

River water pollution is increasingly widespread in northern China and can lead to problems with the drinking water for the residents if not properly treated. Constructed wetlands are a promising solution and have become increasingly popular in China. In this study the nutrient removal and plant uptake in constructed microcosm wetlands vegetated with Typha orientalis, Phragmites australis, Scirpus validus and Iris pseudacorus for treating simulated polluted river water in northern China were investigated. The performance of the treatment systems from April to November was assessed. The maximum TN, NH₄-N and TP removal efficiencies were 68%, 93% and 67%, respectively. And the maximum nutrient uptake by plants constituted 51.89% of the N removal and 34.17% of the P removal throughout the trial. S. validus and I. pseudacorus have a higher nutrient uptake capacity and are preferred species from a treatment perspective in constructed wetland in northern China.     

Distribution and bioavailability of heavy metals in different particle-size fractions of sediments in Taihu Lake,China

Abstract

Taihu Lake is one of the most important water sources in the economically developed central-eastern part of China, and metal pollution is a major concern for the lake. The distribution and bioavailability of Cd, Cr, Cu, Pb, Sb and Zn were analysed in undifferentiated bottom sediments and in various particle-size fractions of the sediment from different parts of the lake. The average concentration of total metals in undifferentiated sediments ranged from 0.86 mg kg^-1 (Cd) to 95.45 mg kg^-1 (Zn) for the entire lake, with the highest concentrations in Zhushan Bay. The concentration of heavy metals was higher in extremely fine sands (0.064–0.125 mm) and fine sands (0.125–0.25 mm) than in other fractions. Sequential extractions showed that Cu, Zn and Cd were the most bioavailable accounting for 55.6%, 38.7% and 30.0% of their total concentration, respectively. However, the bioavailable proportion of many metals was not significantly different between grain grades except for Cu and Zn, which were higher in silts (<0.064 mm) than in other grades. Compared with the background values of local soils, the concentration of Zn, Cd, Cu, Pb and Sb was higher, indicating enrichment in the sediment. From ecological safety concerns, Zn, Cd and Cu should be examined closely because of their higher bioavailabilty in the sediment.     

The role of periphytes in the shift between macrophyte and phytoplankton dominated systems in a shallow, eutrophic lake (Lake Taihu, China)

Based on experiments of periphyte response to different trophic levels and their impact on macrophyte production, it was found that the periphyte biomass increased with the nutrient concentrations. Increased trophic level and periphyte biomass resulted in decreased macrophyte photosynthesis. It was suggested that the periphytes could cause resilience and hysteresis in the system shifts between macrophyte and phytoplankton domination. Other factors, such as fish farming, storm induced waves and mechanical destruction, and high water levels could be the perturbations during the system shifts, but these are not the key factors. Instead, the nutrient loading and periphyte abundance could determine the shift in lake ecosystem between macrophyte and phytoplankton domination. This finding could theoretically elucidate the mechanism of ecosystem shifts between macrophyte and phytoplankton domination.     

Genetic Diversity of Eukaryotic Microorganisms in Lake Taihu,a Large Shallow Subtropical Lake in China

We investigated the genetic diversity of eukaryotic microorganisms (0.8-20 mum) by sequencing cloned 18S rRNA genes in six genetic libraries constructed from six locations in Lake Taihu, a large shallow subtropical lake in China. Genetic libraries of eukaryotic ribosomal RNA were screened by restriction fragment length polymorphism (RFLP) analysis, and one clone representative of each RFLP pattern was partially sequenced. A total of 528 clones were clustered into 165 RFLP patterns and finally into 131 operational taxonomic unit (OTUs). Phylogenetic analysis revealed that each library included many unique OTUs, as well as members of distantly related phylogenetic groups. A majority of the clones were from alveolates, stramenopiles, cercozoa, cryptophytes, chlorophytes, and fungi, with members of choanoflagellida, euglenida, centroheliozoa, ancyromonadidae, ichthyosporea, and kathablepharid representing a minor fraction of the library. Six OTUs (15 clones) were not related to any known eukaryotic group. Canonical correspondence analysis suggested that the differences in eukaryotic microorganism community composition of in the six regions were partially related to trophic status, sediment resuspension, and top-down regulation by metazooplankton.     

Specific absorption coefficient and the phytoplankton package effect in Lake Taihu,China

In order to investigate variations of absorption and total chlorophyll-a (TChl-a)-specific absorption coefficient of phytoplankton in Lake Taihu, 57 water samples obtained from Lake Taihu during November 8–22, 2007 were used in this study. Package effect and accessory pigments’ influences on the absorption spectra were also examined. Phytoplankton absorption was measured by quality filter technical, and TChl-a concentration was measured by “hot ethanol” method. Results yielded significant variations in phytoplankton absorption and TChl-a-specific absorption coefficient. Phytoplankton absorption coefficient at 675 nm is highly correlated to TChl-a concentration, while absorption at 440 nm is less correlated to TChl-a concentration because of great package effect and accessory pigments’ influence. There was an inverse relationship between a _ph*(λ) and TChl-a concentration. Four types of absorption spectra are identified by normalizing a _ph*(λ) to a _ph*(440). The a _ph*(λ) variation is mainly due to accessory pigments and package effect, whose influence at 675 nm ranges from 83.2% to 28%, with an average of 65.3%. Meanwhile, the wide varying ratio of a _ph*(440) to a _ph*(675) indicates a large variation range in the ratio of accessory pigment to TChl-a concentration. Those findings are significant to estimate Chl-a concentration based on bio-optical model, estimate primary production from remote sensing, and plan further ecological restoration measures for Lake Taihu. Handling editor: Luigi Naselli-Flores     

Phosphorus release from cyanobacterial blooms in Meiliang Bay of Lake Taihu, China

To investigate the relationship between cyanobacterial density and phosphorus release into a lake aquatic environment, in situ experiments with 2.5 L microcosms were conducted in Meiliang Bay, located in the northern part of Lake Taihu, China. The effects of different environmental factors on phosphorus release and the ways changes of water quality indexes are involved in phosphorus release were further examined. It was found that total dissolved phosphorus (TDP) concentration kept to low levels (around 0.488 mg L⁻¹) in the microcosm with the low cyanobacterial density (8.85 × 10⁷ cell L⁻¹) throughout the experimental period, whereas first-order kinetics of TDP release was observed in microcosms with intermediate (7.60 × 10⁸ cell L⁻¹) and high cyanobacterial density (3.65 × 10⁹ cell L⁻¹). Accordingly their TDP release rate constants were both approximately 0.8930 d⁻¹ in the latter two treatments. The dissolved inorganic phosphorus (DIP) concentrations also increased with the increase of cyanobacterial density in 4 days. However, the DIP decreased from 35.52 mg L⁻¹ on day 4 to 6.72 mg L⁻¹ on day 6 in microcosm with the high cyanobacterial density during the experiments. Temperature could remarkably improve phosphorus release, while disturbance and illumination had negative effects on it. In addition, both TDP and DIP concentrations were positively correlated with electronic conductivity, salinity and total dissolved solid, but negatively correlated to chlorophyll-a and cyanobacterial density when cyanobacterial density was more than 7.60 × 108 cell L⁻¹. Thus, more phosphorus can be released from cyanobacterial blooms at higher cyanobacterial densities in Meiliang Bay, which is also determined by high temperature. Higher dissolved phosphorus concentration in cyanobacteria-dominated lakes (regions) is mainly due to the decomposition of cyanobacteria during the outbreak of cyanobacterial bloom in Lake Taihu, especially in Meiliang Bay.     

Characterization of Bacterial Communities Associated with Organic Aggregates in a Large, Shallow, Eutrophic Freshwater Lake (Lake Taihu, China)

Although organic-aggregate-associated bacteria play a pivotal role in microbial food webs and in the cycling of major elements, their community composition and diversity have not been extensively studied, especially in shallow freshwater systems. This study is among the first to explore intra-lake horizontal heterogeneity of organic-aggregate-associated bacterial community composition (OABC) in the large, shallow, and eutrophic Lake Taihu. During November 2006, samples were collected at four locations representing different trophic states and food web structures. Regional variability of OABC and diversity were studied by amplified ribosomal DNA restriction analysis and comparative analysis of four large 16S ribosomal RNA clone libraries. Our results demonstrate that OABC were numerically dominated by members of the β-proteobacteria (19.2–38.6%), Bacteroidetes (3.6–20.0%), and α-proteobacteria (11.5–19.2%) groups. The dominance of the Bacteroidetes group was related to algae-based aggregates. Horizontal heterogeneity of OABC exists within habitats, suggesting that the trophic state of the water and the physicochemical properties of organic aggregates (OA) play a key role. Diverse bacterial communities found on OA were substantially different from free-living ones. Comparative statistical analyses of the habitats of OA-associated bacteria highlight the potential ecological importance of the exchange between OABC and the surrounding planktonic community. Lastly, we found at least 45% of sequences closely related to ones previously found in soils, sludge, sediments, and other habitats. This demonstrates that microorganisms from terrestrial and sediment habitats are an important component of OA. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The bacterioplankton of Lake Taihu,China: abundance,biomass, and production

Abundance, biomass and production of pelagic bacteria were examined over one year at monthly sampling intervals across a trophic profile in Meiliang Bay, Lake Taihu. With the lowest density in the open lake, the bacterial abundance showed a clear trend in relation to trophic status. The carbon content per cell was higher in autumn and winter, and the opposite was true for bacterial biomass. Bacterial ³[H]-TdR and ¹⁴[C]-Leu incorporation rates, cell production, turnover times and carbon production varied during the annual cycle at different sites. The ratio of bacterial production to primary production was high, independently of the method used, indicates that the microbial food web in Lake Taihu is an important component of the total food web of the lake and dominated by external inputs. Electronic supplementary material Supplementary material is available for this article at and is accessible for authorized users     

Distribution Characteristics and Risk Assessments of PAHs in Fish from Lake Taihu,China

The concentrations of PAHs in four species of fish (Common carp, Crucian carp, Bighead carp, and Topmouth culter) from Lake Taihu were tested, and the human health risks of PAHs by fish consumption were evaluated. Results showed that concentrations of PAHs in fish from Lake Taihu were 52.5–247.6 ng/g wet weight (ww), and the BaP equivalent concentrations of total PAHs (B[a]P_eq) were 0.2–0.6 ng/g ww, which were less than the screening value of 2.6 ng/g wet for human consumption. The concentration sequences of PAHs in fish from Lake Taihu from high to low were Bighead carp > Crucian carp > Common carp > Topmouth culter. The human health risk level of PAHs by fish consumption was 5.8 ± 2.5 × 10^?6, which was less than the maximum acceptable risk level of 1 × 10^?5 for human health set by the U.S. Environmental Protection Agency. The tissue residue guideline (TRG) of PAHs for protecting aquatic wildlife was 1.3 mg/kg diet ww, which was higher than the concentrations of PAHs in fish from Lake Taihu. The results indicated that fish consumption from Lake Taihu would not cause health risk or harmful effects on wildlife that consume aquatic biota.     

Phylogenetic Inference of Colony Isolates Comprising Seasonal Microcystis Blooms in Lake Taihu, China

Blooms of the toxin-producing cyanobacterium, Microcystis spp., are an increasingly prevalent water quality problem and health hazard worldwide. China's third largest lake, Lake Taihu, has been experiencing progressively more severe Microcystis blooms over the past three decades. In 2009 and 2010, individual Microcystis colonies, consisting of four different morphospecies, were isolated and genotyped using a whole-cell multiplex PCR assay. The 16S-23S rDNA-ITS sequences were aligned based on Bayesian inference and indicated that one morphospecies was genetically unique (Microcystis wesenbergii) and three were indistinguishable (Microcystis aeruginosa, Microcystis flos-aquae, and Microcystis ichthyoblabe). Microcystin (mcyB) genes were detected intermittently in two of the morphospecies while the other two morphospecies lacked the mcyB gene in all samples. Water temperature was found to influence bloom formation and morphotype prevalence, and chlorophyll a and temperature were positively and significantly correlated with microcystin concentration. Cooler water temperatures promoted toxigenic strains of Microcystis. Wind appeared to influence the distribution of morphotypes across the lake, with M. aeruginosa and M. ichthyoblabe being more susceptible to wind stress than M. wesenbergii and M. flos-aquae. The results of this study indicated that the blooms were composed of a variety of Microcystis morphospecies, with more genotypes observed than can be attributed to individual morphotypes. We conclude that morphology is not a reliable indicator of toxigenicity in Lake Taihu, and caution should be exercised when the M. aeruginosa morphotype is present because it is capable of producing MC-LR, the most toxic microcystin isoform.     

Ecological Risk Assessment of Organochlorine Pesticides in Surface Waters of Lake Taihu,China

The residues of organochlorine pesticides (OCPs) in surface waters from Meiliang Bay, Gonghu Bay, and Xukou Bay of Lake Taihu, China, were investigated, and their ecological risks were assessed using the risk quotient method and probabilistic risk assessment. Environmental concentrations of OCPs in surface water of these bays were relatively lower compared with other rivers or lakes in China. Calculation of risk quotient associated with taxonomic groups indicates moderate ecological risks from OCPs for crustaceans and insects in these bays, while the ecological risks were low for fish and negligible for phytoplankton. The ecological risk quotients associated with individual OCPs were lower than 0.01 in these bays, suggesting a negligible risk to aquatic organisms. Ecological risk from α-HCH was relatively lower compared with DDTs, endosulfans, and γ-HCH. The combined ecological risks were evaluated using probabilistic risk assessment for only eight OCPs owing to a lack of available toxicity data for β-HCH and δ-HCH. The percentage of species with the potential to be at risk from mixture of OCPs was lower than the criteria of 5% in each bay, indicating that the combined ecological risks were acceptable.     

Temporal Variation and Co-occurrence Patterns of Bacterial Communities in Eutrophic Lake Taihu,China

To understand the long-term and local variations of bacteria under the influence of annually re-occurred water bloom, bacterial community composition (BCC) was investigated monthly for 3 years (2009–2011) at four different sites located across Lake Taihu. The bacterial community composition was analyzed by 16S rRNA gene clone libraries and terminal restriction fragment length polymorphism. Co-occurrence patterns among bacterial taxa and environmental variables were determined through network analysis. Overall, strong seasonal variation patterns of BCC were observed whilst the spatial variations of BCC were slight in the long-term observation. However, core species bacteria persisted throughout the annual variations. Network analysis showed that the highly connected operational taxonomic units in bacteria-environment network included both the numerically dominant taxa and some functional groups with low abundance, such as Methylophilaceae and Nitrospira. Co-occurrence networks further revealed that the correlations of bacteria-bacteria could be more critical than those between environment and bacteria in structuring microbial communities, and would be a crucial driving factor of BCC in Lake Taihu.     

Ecological Risk Assessment of DDT Accumulation in Aquatic Organisms of Taihu Lake,China

DDT (Dichlorophenyltrichloroethane) is a toxic, ubiquitous, and persistent bioaccumulative pollutant in the global environment. Although its use as an insecticide has been banned in China since 1983, residual DDT levels in the Taihu Lake are evident. Aimed at the protection of fish resources, three species of high-value fish in the Taihu Lake (Protosalanx hylocranius (Abbott), Salangichthys tangkahkeii (Wu), and Coilia nasus Temminck et Schlegel) were selected and an ecological risk assessment was used to estimate the DDT threats to fish populations. Based on a food web model, a bioaccumulation model was used to estimate the DDT concentration in zooplankton, benthos, and fish and the Monte Carlo (MC) simulation method was used to analyze the uncertainty in the bioaccumulation process. Then the chronic toxicity dose-response relationship of DDT on the Protosalanx hyalocranius (Abbott) population, estimated by ICE and ACE software, was used to calculate the mortality rates in different fish stages with the DDT concentrations. Last, the demographic modeling (Leslie matrix) was used to assess the ecological risk of DDT damage on the Protosalanx hyalocranius (Abbott) population. The results show that the estimated DDT concentrations in benthos and zooplankton were 0.14–16.56 μ g/kg, 2.15–99.30 μ g/kg, respectively. The fish DDT concentration results are a series of increasing curves and the maximal values reach 266, 101, and 211 μ g/kg for Protosalanx hylocranius (Abbott), Salangichthys tangkahkeii (Wu), and Coilia nasus Temminck et Schlegel, respectively. In the process of ecological risk quantization, a 4.06–7.25% reduction in the biomass of the Protosalanx hyalocranius (Abbott) population was estimated in 10,000 MC imulations and 4.21%, 4.89%, and 5.69% reductions were estimated in 75%, 50%, and 25% probability, respectively.     

Nutrient limitation in Ellis Fjord,eastern Antarctica

The occurrence of high chlorophyll-a concentrations (up to 11.6gl^–1) at shallow depths within Ellis Fjord, eastern Antarctica, during spring and summer, coincided with the development of a stratified water column. Macronutrient concentrations during periods of high chlorophyll-a levels were very low. Phosphate concentrations decreased to 0.2 M, nitrate to 0.4 M and silicate to 3.9 M. High rates of nutrient depletion early in the season can be explained by strong sea-ice algal mat development. An SiNP uptake ratio of 25.513.81 indicates a strong demand for silicate. Minimum silicate levels were below those necessary for maximum diatom growth and probably contributed to the successional shift from diatoms to phytoflagellates.     

Nutrient limitation in Crater Lake,Oregon

Experiments were carried out to determine what nutrient (or nutrients) was primarily responsible for limiting phytoplankton productivity in ultraoligotrophic Crater Lake. The experiments included in situ and laboratory nutrient addition bioassays utilizing the natural phytoplankton community, Selenastrum capricornutum bottle assays, photosynthetic responses, photosynthetic carbon metabolism, and response of dark uptake of ¹⁴CO₂ with the addition of NH ₄ ⁺ . The results suggested that a trace metal(s) or its availability was the primary factor limiting the epilimnetic phytoplankton productivity. Nitrogen was extremely low, and quickly became limiting with the addition of trace metals and a chelator. Iron is the most likely candidate as the limiting nutrient. Trace metals and nitrogen are also both important in limiting phytoplankton at 100 m, a depth where biologically mediated turnover of nutrients seems to be more important.     

DO,pH, and Eh microprofiles in cyanobacterial granules from Lake Taihu under different environmental conditions

To understand characteristics of cyanobacterial granules from Lake Taihu, dissolved oxygen (DO), pH, and redox potential (Eh) microelectrodes were used to investigate physiological responses within these granules under different irradiance, temperature, and external pH levels. DO and pH levels increased with rising irradiance, while the Eh had an opposite trend. High light combined with high temperature decreased photosynthesis of the cyanobacterial granules. DO diffused from the surrounding water to the granules at low irradiance; however, DO began to diffuse from the granules to the water at high irradiance owing to increased photosynthesis. Dynamic changes of DO, pH, and Eh levels existed within the cyanobacterial granules under light–dark cycles. High DO levels within intercellular space of the cyanobacterial granules are another important buoyancy regulation mechanism. An external initial pH affected photosynthesis of the cyanobacteria in the granules. DO and pH levels of the granules in slightly alkaline solution (pH 8–9) were higher than those in highly alkaline solution (pH 10). Such physical and chemical characteristics within cyanobacterial granules in eutrophic water allowed them to outcompete other aquatic algae. The characterization of the physiological microenvironment within these cyanobacterial granules provides a new research approach to a better bloom management.     

Assessing changes in macrophyte assemblages with salinity in non-riverine wetlands: A Bayesian approach

Bayesian modeling techniques (which accounted for imperfect detection) were used to assess changes in macrophyte assemblages in 58 wetlands along a typical salinity gradient in Western Victoria, Australia. By incorporating detectability into our predictions, an unbiased estimate was made of the relationship between salinity and both individual species occupancy and the expected number of species. When compared to the freshest wetlands, macrophyte species number was predicted to decrease by as much as 60–70% at conductivities of around 6.0 mS cm⁻¹ (11% seawater), a value often considered the upper salinity tolerance for many freshwater aquatic plants. The model also predicted a 40–50% drop in species number at conductivities of around 1.5 mS cm⁻¹ (3% seawater). It was also found that 25 out of 76 freshwater species were unlikely to occur at conductivities above 1.0 mS cm⁻¹. Consequently, secondary salinisation of fresh non-riverine wetlands is highly likely to markedly and negatively impact upon non-riverine wetland macrophyte assemblages.     

Modeling impacts of Yangtze River water transfer on water ages in Lake Taihu, China

To improve water quality and alleviate eutrophication in Lake Taihu, the third largest freshwater lake in China, a Yangtze River water transfer project was initiated in 2002 to bring water from the Yangtze River to Lake Taihu to dilute and divert pollutants out of the lake. We used a three-dimensional numerical model, Environmental Fluid Dynamics Code, to study the impacts of water transfer on the transport of dissolved substances in the lake by using the concept of water age. In particular, the influences of inflow tributaries and wind forcing on water age were investigated. Model results showed that the effect of water transfer on transport processes in the lake is strongly influenced by hydrodynamic conditions induced by wind and inflow/outflow tributaries. During the simulation year (2005), the water ages in Lake Taihu were highly variable both spatially and temporally, with a mean of approximately 130 days in summer and 230 days in the other seasons. Southeasterly winds—dominant in the summer—could improve the quality of water by reducing the water age in the eastern areas of the lake, which are used as a drinking water source, and in Meiliang Bay, the most polluted bay. In terms of dilution, the most efficient flow rate for transferred water was predicted to be approximately 100 m³/s. The spatial distribution of water ages showed that water transfer may preferentially enhance exchanges in some areas of the lake unless nutrient concentrations in the transferred water are reduced to a reasonable level. This study provides useful information for a better understanding of the complex hydrodynamic and mass transport processes in the lake, which is important for developing and implementing effective ecological restoration strategies in the region.