Effects of riverine suspended particulate matter on the post-dredging increase in internal phosphorus loading across the sediment-water interface.

Dredging is frequently used in the river mouths of eutrophic lakes to reduce internal phosphorus (P) loading from the sediment. However, the accumulation of P-adsorbed suspended particulate matter (SPM) from the inflowing rivers negatively affects the post-dredging sediment-water interface and ultimately increases internal P loading. Here, a 360-d experiment was carried out to investigate the influence of riverine SPM on the efficacy of dredging in reducing internal P loading. SPM was added to dredged and undredged sediments collected from the confluence area of Lake Chaohu. Several parameters related to internal P loading, including oxygen profile, soluble reactive P, and ferrous iron across the sediment-water interface, organic matter, alkaline phosphatase activity, and P fractions, were measured throughout the experimental period. The results showed that the P content (especially mobile P) in the sediment increased to the pre-dredging level with the accumulation of SPM in the dredged sediment. In addition, the P flux across the sediment-water interface increased with the accumulation of SPM. Several characteristics of SPM, including high organic matter content, mobile P, high activity of alkaline phosphatase, and high biological activity, were considered correlated with the post-dredging increase in internal P loading. Overall, this study showed that the heavily contaminated riverine SPM regulates the long-term efficacy of dredging as a nutrient management option in the confluence area. Management is needed to avoid or reduce this phenomenon during dredging projects of this nature.

Effects of riverine suspended particulate matter on post-dredging metal re-contamination across the sediment-water interface.

Environmental dredging is often used in river mouth areas to remove heavy metals. However, following dredging, high levels of metal-adsorbed suspended particulate matter (SPM) originating from polluted inflowing rivers might adversely affect the sediment-water interface (SWI). Here, we conducted a 360-day-long experiment investigating whether the riverine SPM adversely affects dredging outcome in a bay area of Lake Chaohu, China. We found that the heavy metal concentrations in the post-dredging surface sediment increased to pre-dredging levels for all metals studied (As, Cd, Cr, Cu, Ni, Pb, and Zn) after the addition of SPM. In addition, the increased concentrations were mostly detected in the relatively bioavailable non-residual fractions. Of the metals studied, the rate of increase was the greatest for Zn and Cd (482.98% and 261.07%, respectively), mostly in the weak acid extractable fraction. These results were probably due to certain characteristics of SPM (fine grain size, and high concentrations of organic matter and heavy metals) and the good oxic conditions of the SWI. Furthermore, As was the only metal for which we observed an increasing trend of diffusive flux across the SWI. However, the flux was still significantly lower than that measured before dredging. In conclusion, the quantity and character of riverine metal-adsorbed SPM affect metal re-contamination across the post-dredging SWI, and this information should be incorporated into the management schemes of dredging projects dedicated to reducing metal contamination in similar areas.

Heavy Metal Pollution, Fractionation, and Potential Ecological Risks in Sediments from Lake Chaohu (Eastern China) and the Surrounding Rivers.

Heavy metal (Cr, Ni, Cu, Zn, Cd, and Pb) pollution, fractionation, and ecological risks in the sediments of Lake Chaohu (Eastern China), its eleven inflowing rivers and its only outflowing river were studied. An improved BCR (proposed by the European Community Bureau of Reference) sequential extraction procedure was applied to fractionate heavy metals within sediments, a geoaccumulation index was used to assess the extent of heavy metal pollution, and a risk assessment code was applied to evaluate potential ecological risks. Heavy metals in the Shuangqiao and Nanfei Rivers were generally higher than the other studied sites. Of the three Lake Chaohu sites, the highest concentrations were identified in western Chaohu. Heavy metal pollution and ecological risks in the lake's only outflowing river were similar to those in the eastern region of the lake, to which the river is connected. Heavy metal concentrations occurred in the following order: Cd > Zn > Cu > Pb ≈ Ni ≈ Cr. Cr, Ni, and Cu made up the largest proportion of the residual fraction, while Cd was the most prominent metal in the exchangeable and carbonate-included fraction. Cd posed the greatest potential ecological risk; the heavy metals generally posed risks in the following order: Cd > Zn > Cu > Ni > Pb > Cr.

Use of multi-objective dredging for remediation of contaminated sediments: a case study of a typical heavily polluted confluence area in China.

Sediments in confluence areas are typically contaminated by various pollutants that have been transported there by inflowing rivers. In this study, we evaluated the pollution status of a confluence area in Lake Chaohu (China). Both the nutrients and hazardous pollutants were analysed. Most sediment cores showed large variations in nutrient concentrations at depths of 10 to 18 cm. Positive release rates of NH4(+)-N and PO4(3-)-P were detected in sediment cores. Hg and Cd were the most typical problematic metal contaminants encountered, and their contamination levels extended to depths of 20 and 25 cm, respectively. Polycyclic aromatic hydrocarbons (mostly acenaphthene and fluorine) were the primary persistent organic pollutants (POPs) present in sediments, and contamination levels frequently could be detected up to a depth of 16 cm. Simulated dredging operations were implemented in the laboratory, with a dredging depth of 15 cm found to be suitable for nutrient suppression. With the goal of suppressing nutrients release and removing high-risk metals and POPs, a multi-objective dredging plan was developed. This plan subdivides the confluence area into five parts that were treated with different dredging depths. A demonstration area was dredged in the most heavily polluted part, and the observed dredging effects were consistent with those expected on the basis of the plan. Such an approach to dredging might also be useful in other areas in the future.

Beyond hypoxia: occurrence and characteristics of black blooms due to the decomposition of the submerged plant Potamogeton crispus in a shallow lake.

Organic matter-induced black blooms (hypoxia and an offensive odor) are a serious ecosystem disasters that have occurred in some large eutrophic shallow lakes in China. In this study, we investigated two separate black blooms that were induced by Potamogeton crispus in Lake Taihu, China. The main physical and chemical characteristics, including color- and odor-related substances, of the black blooms were analyzed. The black blooms were characterized by low dissolved oxygen concentration (close to 0 mg/L), low oxidation-reduction potential, and relatively low pH of overlying water. Notably higher Fe2+ and sigmaS(2-) were found in the black-bloom waters than in waters not affected by black blooms. The black color of the water may be attributable to the high concentration of these elements, as black FeS was considered to be the main substance causing the black color of blooms in freshwater lakes. Volatile organic sulfur compounds, including dimethyl sulfide, dimethyl disulfide, and dimethyl trisulfide, were very abundant in the black-bloom waters. The massive anoxic degradation of dead Potamogeton crispus plants released dimethyl sulfide, dimethyl disulfide, and dimethyl trisulfide, which were the main odor-causing compounds in the black blooms. The black blooms also induced an increase in ammonium nitrogen and soluble reactive phosphorus levels in the overlying waters. This extreme phenomenon not only heavily influenced the original lake ecosystem but also greatly changed the cycling of Fe, S, and nutrients in the water column.

[Impacts of Asian clams (Corbicula fluminea) on lake sediment properties and phosphorus movement].

To examine the impact of Corbicula fluminea on sediment properties and phosphorus dynamics across sediment-water interface in lake, the microcosm experiment was carried out with sediment and lake water from the estuary of Dapu River, a eutrophic area in Taihu Lake. Rhizon samplers were used to acquire pore water, and soluble reactive phosphorus (SRP) flux across sediment-water interface and sediment properties were determined. The activity of C. fluminea destroyed the initial sediment structure, mixed sediment in different depths, increased oxygen penetration depth, sediment water content, and total microbial activity in sediment. The downward movement of overlying water was enhanced by the activity of C. fluminea, which decreased Fe2+ in pore water by oxidation. The production of ferric iron oxyhydroxide adsorbed SRP from pore water and decreased SRP concentration in pore water, and this increased iron bound phosphorus in corresponding sediment. The emergence of C. fluminea accelerated SRP release from sediment to overlying water, and enhanced SRP flux increased with the rise of introduced C. fluminea density. Metabolization of C. fluminea might play an important role in accelerating SRP release.

Distribution characteristics and toxicity assessment of heavy metals in the sediments of Lake Chaohu, China.

The concentrations of Cu, Pb, Zn, Cd, and Ni were measured in surface and core sediments to determine their distribution characteristics and toxicity in the sediments of Lake Chaohu. The results revealed that metal concentrations in the surface sediments had a tendency to increase from the estuarine mouth to the lake center. The distribution characteristics of the five target metals were similar along the sediment profiles at each site. Principal component analysis revealed that all of the measured variables were loaded in the same component, indicating that there was a strong relationship among these measured variables, which was confirmed by the correlation analysis. Two sets of sediment quality guidelines (SQGs): simultaneously extracted metal (SEM) and acid volatile sulfides (AVS) models (including ∑SEM/AVS, ∑SEM-AVS, and ∑SEM-AVS/ f (oc)) and threshold effect level and probable effect level values were used to predict the sediment toxicity. Comparison of the results obtained using these two sets of SQGs revealed that only a small portion of the entire set was identical, while the majority of the results were different and sometimes completely contradictory. These contradictory results would cause a great deal of trouble for environment managers. More accurate and universal SQGs must be developed for environmental researchers and local environmental managers and regulators.

[Analysis and authentication of cpDNA psbA-trnH regions of Dendrobium species of fengdous].

The psbA-trnH regions of Dendrobium species of Fengdous were sequenced by our research group. The psbA-trnH sequences of fifteen Dendrobium species were analyzed with software MEGA 4.0. The results showed that the lengths of sequences varied from 721 to 767 bp. The variable sites were 42 while the informative sites were 11. Genetic distances were calculated using the Kimura 2-parameter model. Genetic distances varied from 0.0013-0.0183 among fifteen species while the average genetic distance was 0.0148. The interspecies differences of psbA-trnH regions were demonstrated. Six indels happened in this fragment, which led to the great difference of sequence lengths among fifteen species. We found that there were no population differences in the psbA-trnH region of various species of Fengdous so far. By using the database of various Dendrobium species of Fengdous and two genetics software, the botanical origin of the inspected species of Fengdous was authenticated successfully by sequencing the psbA-trnH regions. The psbA-trnH region of cpDNA can be used as a candidate marker for authentication of Dendrobium species of Fengdous.