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Environmental sediment dredging is one of the most common methods for the remediation of contaminated sediments in lakes; however, debate continues as to whether the effectiveness of dredging methods contributes to this phenomenon. To determine sediment resuspension and nutrient release following dredging with a variety of dredging methods, four dredging treatments at wind speeds of 0–5.2 m/s were simulated in this study, namely suction dredging (SD), grab dredging (GD), ideal dredging with no residual sediments (ID), and non-dredging (ND). Field sediments from suction and grab dredging areas (including post-dredged and non-dredged sediments) of Lake Taihu were used to assess the release abilities of soluble reactive phosphorus (SRP) and ammonia nitrogen (NH 4 + -N) from the sediment-water interface. The effects of residual sediments on nutrient concentrations in water were also evaluated. The results reveal that inhibition of resuspension of particulate matter and nutrients released through sediment dredging decreases with increasing levels of residual sediment. Total suspended particulate matter content in the mean water columns of ID, SD, and GD under wind-induced disturbance (1.7–5.2 m/s) decreased by 67.5%, 56.8%, and 44.3%, respectively; total nitrogen and total phosphorus in ID (SD) treatments were 19.8% (12.9%) and 24.5% (11.2%) lower than that in ND treatment. However, there were ~ 1.6 and 1.5 times higher SRP and NH 4 + -N in the GD treatment compared with the ND treatment at the end of the resuspension experiment (0 m/s). A significant increase in the SRP and NH 4 + -N release rates at the sediment-water interface was also observed in field sediments from a grab dredging area, indicating that GD may pose a short-term risk of nutrient release to the water body. Hence, dredging methods with less residual sediments both during and after dredging improves the dredging quality.

Water-level regime alteration-associated redox fluctuation plays a primary role in governing exchange and transformation of nitrogen (N) in water-level fluctuation zones (WLFZs), while few understanding of how hydrological regimes under reservoir operation affected N cycling across the sediment-water interface (SWI), giving rise to uncertainties in reservoir N nutrient management. Batch microcosm simulation experiments with intact sediment cores from WLFZs of the Three Gorges Reservoir (TGR) were conducted for 24 days to identify holistic flooding-drying process mechanism on N-cycling patterns. Our results showed a distinct transition of N-cycling mode across the SWI, shifting from biological denitrogen loss dominated in initial period of flooding to enhance endogenous N retention. A dramatic source-sink switch of nitrous oxide (N 2 O) occurred in the first 1.5 days during the flooding period. However, combined accelerating migration of NH 4 + -N from sediment to overlying water, and subsequently enhanced transformation of NH 4 + -N to NO 3 − -N formed from flooding to drying rotation, thereby increasing N loading to overlying water. The reason for this investigation could be attributed to intensive N loss through coupled nitrification and denitrification in oxic-anoxic microenvironments after flooding. With oxygen replenishment from atmosphere during drying phase, persistent ammonification of organic N in sediments provided sufficient source of NH 4 + -N for the formation of NO 3 − -N fraction in a more oxic overlying water. Therefore, water-level regime alteration by reservoir operation was capable of weakening N removal from water body and lengthening internal N turnover time across redox-variable SWI. These findings elucidate new understanding of holistic hydrological regime mechanisms on N cycling across SWI and provide insight to biogenic N nutrient management for improving the green credentials of hydroelectric reservoir.

Purpose The Three Gorges Reservoir (TGR) is the largest water conservation project in the world but suffers from harmful algal blooms (HABs) currently. A large amount of phosphorus (P) has accumulated in the sediment due to the construction of the Three Gorges Dam. Phosphorus release from sediment may provide an important P source for overlying water that further triggers HABs. This study aimed to evaluate the contribution of sediment internal P and reveal the mechanisms controlling sediment P release. Material and methods Chemical sequential extraction approach and diffusive gradients in thin films (DGT) techniques were employed to determine the P fractions in sediments and the vertical distribution of P, iron (Fe), and sulfur (S) at the sediment‒water interface (SWI). Results and discussion Results indicated that the total P content in the sediments of the TGR is high, with a mean content of 1368 mg kg −1 . The P concentration of different fractions in sediments followed the order HCl-P > NaOH-P > BD-P. The averaged P release flux at the SWI was estimated at 0.42 mg m −2  day −1 , suggesting that sediment P release is a potential P source for the overlying water. Significant positive relationships between DGT-P and DGT-Fe concentrations from sampling sites Wushan County (WS), Zigui County (ZG), and Xiangxi River (XX) were observed with correlation coefficients ( R 2 ) of 0.91, 0.39, and 0.29, respectively. Furthermore, DGT-P and DGT-S concentrations were also significantly positively correlated at the sampling sites WS, ZG, and XX, with R 2 of 0.71, 0.87, and 0.50, respectively. Conclusion The internal P load is severe in the TGR. The reductive dissolution of Fe–P is likely one of the main mechanisms causing P release in the sediments. Furthermore, sulfate reduction associated coprecipitation with Fe promotes the release of Fe–P. These results provide important scientific and technical support for the mitigation of internal P pollution in large deep-water reservoirs.

As a recently discovered process of nitrogen cycling, anaerobic ammonium oxidation coupled to ferric iron reduction (Feammox) has attracted more attentions. This study investigated the spatial variation of Feammox in the sediment of different zones of a shallow freshwater lake and the effect of organic matter derived from algae and macrophyte on Feammox process. The potential Feammox rates showed significant differences among sediments from algae-dominated area (ADA), transitional area in the center of the lake (TDA), and macrophyte-dominated area (MDA), and in a descending order, ADA, MDA, and TDA. The potential Feammox rate ranged from 0.14 to 0.34 mg N kg −1 day −1 in the freshwater lake sediment. The potential Feammox rates of the sediment with algae or macrophyte amendment were 12.29% and 15.31% higher than the control test without algae and macrophyte amendment. The addition of algae or macrophyte to the sediment from TDA could improve the amount of HCl-extractable total Fe, Fe(III) reduction rate, and the abundance of FeRB. These results demonstrated that organic matter is one of the key regulators of Feammox process.

PurposeSeasonal hypoxia in water bodies can increase levels of reduced chemical species in the hypolimnion as they are released from anoxic bottom sediments. Water-lifting aeration (WLA) is a technology known to solve this problem by elevating near-sediment dissolved oxygen (DO) and increasing mixing in the water column in a canyon reservoir with deep water (>20 m). The influence of WLA in reservoirs with large surface area and small water depth has not been adequately evaluated. For this purpose, the efficiency of an innovative WLA system was reviewed in Zhoucun Reservoir (ZCR), a mildly eutrophic reservoir with a mean depth of 15 m.MethodsThree years of field experiment was performed in ZCR. Water and sediment samples were collected before and after the operation of the WLA system to explore the causes for the deterioration of water quality in ZCR, and the efficiency of the innovative WLA system installed in 2015. With the use of a microporous aerator and a kinetic energy dissipator, the new-style WLA system is different from WLA studied before.Results and discussionAs a result of thermal stratification and the long duration of anaerobic environment in the hypolimnion, the sediment oxygen consumption rate in ZCR was calculated to be 16.14 mg m−2 h−1, which was much higher than other reservoirs, and the main pollutants in ZCR were ammonia, phosphorus, manganese and sulphide. After the operation of the new WLA system, the anaerobic environment in the bottom water was completely reversed and the DO concentration in the hypolimnion water has been maintained at more than 6 mg L−1. The release of pollutants from sediments, such as ammonia, phosphorus, manganese and sulphide, was inhibited significantly. Ammonia, phosphorus, manganese and sulphide in water decreased by 89%, 84%, 97% and 87%, respectively.ConclusionsThe results of this study demonstrate that employing WLA systems to oxygenate and mix stagnant water bodies is a viable and potentially favourable management strategy for reservoirs with large surface area and relatively small water depth.

Purpose This study was to explore the environmental driving factors and the assembly mechanisms of aerobic denitrifying bacteria community with different seasons and rarity in the sediments of Baiyangdian Lake. Materials and methods Sediment samples were collected from 16 locations in Baiyangdian Lake. Microbial community composition and diversity were measured by napA gene sequencing. Redundancy analysis (RDA) and Mantel analysis were used to study the environmental factors that drive subcommunities with different rarity in summer and winter. Based on network analysis, the symbiotic relationship of subcommunities with different rarity in summer and winter was discussed. The assembly mechanism of subcommunities with different rarity in summer and winter was investigated by null model analysis. The influence factor of different rarity subcommunity diversity in summer and winter was studied based on structural equation modeling (SEM). Results and discussion We found that the α -diversity of winter was higher than that of summer, and the α -diversity of rare taxa (RT) was higher than that of abundant taxa (AT). Nitrate (NO 3 − -N) was the most critical factor affecting AT in two seasons; temperature (T) and COD Mn were the most critical environmental factors affecting RT in summer and winter, respectively. All taxa were significantly affected by dissolved oxygen (DO) in winter. The network analysis showed that symbiotic relationships were dominant among species, and the interrelationship between groups was closer in winter. RT was more crucial for preserving the stability of the bacterial community’s overall composition and structure, but AT had stronger environmental adaptability. The null model revealed that dispersal limitation predominated in the community assembly of AT and RT. With the increase of rarity, the contribution of undominated and homogeneous dispersal to microbial subcommunity aggregation increases. The SEM revealed a significant detrimental impact of NO 3 − -N and NO 2 − -N on β -diversity of AT and RT in summer ( P  = 0.009, n  = 16 and P  = 0.002, n  = 16) but promoted β -diversity of AT and RT in winter. COD Mn , Mn, pH, and DO had the greatest influence on β -diversity of AT and RT in winter. Conclusion In conclusion, our study enhances the understanding of aerobic denitrifying bacterial community diversity, assemblage patterns, and environmental driving mechanisms, and provides important implications for microbial ecology. Moreover, these results provide strategies for the subsequent screening of aerobic denitrifying bacteria in natural water, which is of great significance for the realization of in situ nitrogen removal in natural water.

Purpose Macroalgae blooms are particularly common in the coastal zone of the Neva Estuary situated in the eastern Gulf of Finland. Accumulation of green opportunistic algae on sediments is associated with a decrease in redox potential, changes in sediment function, and deterioration of the coastal zone. The aims of this study were to assess the role of macroalgae in the spatiotemporal distribution and activity of sediment microbial enzymes involved in biogeochemical cycles; and to evaluate biochemical indicators sensitive to macroalgae dynamics. Methods Samples of both surface sediments and macroalgae were collected at nine sites located in the coastal zone of the eastern Gulf of Finland. Sixteen sampling campaigns were performed from 2014 to 2019. Sediments were analyzed for pH, Eh, total organic carbon, ammonia–nitrogen, and phosphate content. Algal biomass and sediment enzyme activities were measured. To compare sediments at different sites according to the level of algal biomass, the geometric mean of enzyme activities (GMea) index was applied. Results The results revealed shifts in enzyme activity toward higher relative levels of dehydrogenase, catalase, phosphatase, and urease at sites subject to algal bloom. The relationships between algal biomass and enzyme activities were dependent on the seasons. The geometric mean of enzyme activities was a suitable index to incorporate the set of sediment enzyme activities in a single numerical value, which was sensitive to changes in algal biomass. The organic carbon, phosphorus, and nitrogen contents have been identified as factors potentially involved in the regulation of enzyme activity providing additional information to link the amount of macroalgae biomass during blooms, nutrient content, and metabolism of sediment microorganisms. Conclusions This enzymological study demonstrated the important role of macroalgae in the spatiotemporal distribution of sediment microbial enzymes involved in the main biogeochemical cycles. The findings of this study can provide a basis for understanding the impact of macroalgae on biogeochemical processes in sediments, which is important for the monitoring and assessment of the ecological effects of macroalgal blooms in aquatic environments.

PurposeDissimilatory nitrate (NO3−) reduction to ammonium (DNRA) is a significant nitrogen (N) transformation process in estuarine ecosystem. The objectives of this study were to clarify the key biotic and abiotic factors influencing DNRA potential rates in Xiamen Bay.MethodsWe characterized DNRA, anammox, and denitrification rates through 15 N isotope tracer experiments. Then quantitative real-time PCR and high-throughput sequencing analysis were conducted to analyze the abundance and community of DNRA bacteria. Spearman correlation analysis was used to understand the relationships of DNRA rate, nrfA abundance, and community with environmental characteristics.ResultsDenitrification was dominated in dissimilatory NO3− reduction processes in sediments of Xiamen Bay. Bacteroidetes and Proteobacteria were the main phyla of DNRA bacteria and Pelobacter was the dominant genus. Spearman correlation analysis showed that DNRA rate was significantly positively correlated with nitrite (NO2−) and NO3− in interstitial water, total organic carbon (TOC), acid volatile sulfide (AVS), and total nitrogen (TN) in sediment. Besides, nrfA abundance showed positive correlations with DNRA rate, NO2−, TOC, AVS, and TN. Furthermore, AVS and nrfA abundance were positively correlated with Pelobacter.ConclusionDNRA rates were limited by sulfide, organic carbon, abundances of nrfA gene, and DNRA bacteria. Sulfide and organic carbon shaped DNRA bacterial community thereby controlled DNRA rate. Heterotrophic sulfide-producing genus (Pelobacter) prevailed among DNRA bacteria, which might support autotrophic DNRA bacteria. In this way, heterotrophic and autotrophic DNRA bacteria were connected through sulfide-producing genus and acted as a whole functional community.

In this study, the concentrations of Zn, Cr, Ni, Cu, Pb, As, Cd, and Hg in the surface sediments of 94 sites sampled from six water systems in the Lake Taihu Basin in China were measured, and the pollution risks and sources of the metals were identified. The results showed that the mean concentrations of Zn, Cr, Ni, Cu, Pb, As, Cd, and Hg in the riverine surface sediments were 163.6, 102.5, 45.5, 44.7, 37.0, 13.3, 0.5, and 0.1 mg/kg, respectively, higher than the corresponding background values (except for Hg). According to the geoaccumulation index ( I geo ), the Pb, Ni, Zn, Cu, and Cd concentrations in the riverine surface sediments were generally at low levels of pollution. Based on a pollution load index (PLI) evaluation, the Pb, Ni, Zn, and Cu concentrations in the riverine surface sediments were generally at moderate levels of pollution. According to the thresholds of potential ecological risk, the Cd and Hg concentrations in the riverine surface sediments exhibited moderate potential ecological risks. Multivariate statistical analysis indicated that the Pb in the riverine surface sediments primarily originated from domestic sewage, agricultural wastewater discharge, and petroleum combustion; the concentrations of Cr, Ni, and Zn were influenced by the electroplating and alloy manufacturing industries; the concentrations of Cu and As mainly originated from pesticide use and industrial wastewater discharge; and those of Cd and Hg primarily stemmed from industrial wastewater discharge. This research provides information regarding metallic contamination and the possible associated ecological risks to benthic organisms in the surface sediments of river systems and is useful for developing sustainable strategies for environmental pollution control and management in the Lake Taihu Basin.

Purpose Excess nickel (Ni) entering lakes can pose adverse effects on aquatic ecosystems and human health. This study aimed to reveal the spatiotemporal distribution, mobilization kinetics, and potential risk of Ni in sediments of a typical multi-ecological lake, Lake Taihu, China. Methods We conducted seasonal monitoring of the spatial distribution of soluble and labile Ni in sediments using high-resolution dialysis samplers (HR-Peeper) and the diffusive gradient in thin-films technique (DGT), respectively. Results We found that the total Ni concentrations in sediments (mean: 37.56 mg kg −1 ) all exceeded the background value (19.5 mg kg −1 ). The spatial distributions of soluble and labile Ni showed no notable fluctuations along the vertical profiles of sediments in all seasons. The DGT-induced fluxes model implied that there is a partial Ni resupply capacity in the sediment of all three ecological zones, but it is higher in the algal-type zones than in the macrophyte-type and transition zones. Furthermore, an assessment of the ecotoxicological risk found that the risk quotient values for Ni were less than 1 in all sampling seasons, indicating a low ecotoxicological risk of Ni in the sediments of Lake Taihu. Conclusion Our results indicate that the ecological risk in the algal-type lake zones requires special attention. Our findings help towards improving the level of understanding regarding the mobilization process and potential risk of Ni in sediments, which in turn can provide guidance for the prevention and control of sediment Ni pollution in lakes with multiple types of ecological zones.