[Diffusive Fluxes and Controls of N2O from Coastal Rivers in Tianjin City].

Rivers are an important emission source of greenhouse gases. To explore the spatial characteristics and influencing factors of N2O emission from the coastal rivers in Tianjin City, six rivers into the Bohai Sea from different land-use types were selected, and the N2O concentrations, saturation, and diffusive fluxes were measured using the headspace-gas chromatography method. The N2O concentration was in supersaturation, and the rivers were the source of atmospheric N2O. The average concentration, saturation, and diffusive fluxes of N2O were (23.85±15.20) nmol·L-1, (309.71±197.38)%, and (27.04±16.46) µmol·(m2·d)-1, with the ranges of 12.70-115.69 nmol·L-1, 164%-1502%, and 9.17-244.79 µmol·(m2·d)-1, respectively. The N2O concentrations and diffusive fluxes of the rivers presented great spatial heterogeneity, with the sewage river (Huangdipai River)>urban river (Haihe River main stream, Jiyun River)>suburban river (Duliujian River, Yongding Xinhe River)>agricultural river (Chaobai Xinhe River). The N2O concentration and diffusion fluxes were significantly correlated with salinity, nutrients, and carbon sources. NO3--N and TP contributed greatly to the diffusive flux differences. N2O production and emission greatly related to the nitrogen cycle process in the Tianjin River, and different forms of nitrogen variously contributed to N2O diffusive fluxes. The salinity gradient had the opposite effect on the N2O emission in urban rivers and drainage rivers. The N2O diffusive fluxes of the sewage river in Tianjin were significantly higher than that of other river types. In the future, due to the development of urbanization and the expansion of urban land, more management measures should focus on the hotspots such as the downstream of wastewater treatment plants of sewage rivers, the estuaries of urban rivers, and the residential gathering areas of suburban rivers to reduce N2O emission.

Dredging method effects on sediment resuspension and nutrient release across the sediment-water interface in Lake Taihu, China.

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 (NH4+-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 NH4+-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 NH4+-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.

[Effects of Exogenous Inputs on Phosphorus Recovery and Transport in Newborn Surface Layers from Sediment Dredging].

The effect of external pollution inputs on phosphorus recovery, transport, and transformation in newborn surface layers from sediment dredging remains unclear. Clarifying this issue is important for the control and management of external pollution loads at the watershed scale, particularly after the implementation of sediment dredging activities. In this study, sediments in Meiliang Bay of Lake Taihu were investigated. In-situ dredging simulation was used to study the transport and transformation of phosphorus at the sediment-water interface, before and after dredging, with either external or non-external particulate matter inputs, and to explore the effect of dredging on phosphorus release as part of internal loading. The results showed that limiting the inputs of external particulate matter and dredging had positive impacts on the control of TP and TN in the sediments. Dredging significantly reduced the content of potentially mobile phosphorus (Mobile-P) in surface sediments. Iron-bound phosphorus (Fe-P) was the first main component of the reduced Mobile-P and Organic phosphorus (Org-P) was the second. The content of Loose-bound phosphorus (Lb-P) was less than 1‰ of the total phosphorus. After 210 days of the experiment, the concentration of PO43--P in the pore water of the dredged treatment was lower than that of the undredged treatment, and this difference was more pronounced without external particulate matter input. Furthermore, the concentration of PO43--P in the pore water of the dredged treatment (without external particulate matter input) was maintained at a low level, while this first increased and then subsequently decreased for the other treatments. The concentrations of PO43--P in pore water were positively correlated with Fe-P in the corresponding sediment layers. Source-sink transition took place between winter and spring, leading to the switch in sediment functioning as a sink to a source. The results indicated that dredging could reduce the release rate of internal phosphorus from sediments. Furthermore, limiting the input of external particulate matter plays an important role in facilitating the control of internal phosphorus loading by dredging.

[Distribution Characteristics and Fluxes of Nitrogen and Phosphorus at the Sediment-water Interface of Yuqiao Reservoir].

Yuqiao Reservoir is an important source of drinking water in Tianjin. In recent years, the eutrophication status is becoming more and more serious, but its internal loading and distribution characteristics of nitrogen and phosphorus is not clear. The profiles of nitrogen and phosphorus at the sediment-water interface were analyzed using the Peeper (pore water equilibrium) technique and the spatial distribution was investigated. The existing forms of the released nitrogen and phosphorus were investigated by the static intact sediment cores, and the fluxes of nitrogen and phosphorus at the sediment-water interface were estimated by static incubations with intact sediment cores. The results demonstrated the following. ① The contents of PO43--P, NH4+-N, NO3--N, and NO2--N in sediments were 0.5-6.5, 0.5-10.9, 2.2-16.2, and 0.05-0.6 mg ·kg-1, respectively. The contents of nutrient were lower as depth increased, and horizontal distribution characteristics indicated significant differences. ② The contents of PO43--P and NH4+-N in the interstitial water were much higher than in the overlying water, suggesting that the interstitial water had the potential to diffuse nutrients to the overlying water. The concentrations of PO43--P and NH4+-N in the interstitial water increased rapidly at 0-5 cm and then decreased gradually. ③ PO43--P and NH4+-N diffused from the sediment to the overlying water, and the fluxes of them were 1.1-13.3 mg ·(m2 ·d)-1 and 20.6-250.5 mg ·(m2 ·d)-1, respectively. The exchange fluxes of NO3--N and NO2--N ranged from -20.4 to 33.4 mg ·(m2 ·d)-1 and from -7.4 to 0.4 mg ·(m2 ·d)-1, respectively. PO43--P and NH4+-N were the main nutrients in the sediment released to the overlying water. The fluxes were high in the south and low in the north, and also high in the mouth of the Linhe River and downstream of the reservoir. Compared with similar studies, the fluxes at the sediment-water interface of Yuqiao Reservoir were relatively high, indicating that the sediment was an important source of nutrients for the overlying water in Yuqiao Reservoir.

[Characteristics of N2O Release and Influencing Factors in Grass-type and Algae-type Zones of Taihu Lake During Summer].

Spatial heterogeneity of N2O generation and emissions in multi-ecotype lakes limited the accurate estimation of the N2O fluxes in lakes, but few studies on the characteristics of N2O generation and emissions have been conducted. In this study, N2O flux at the water-gas interface, dissolved N2O concentration in the water column, and N2O flux at the sediment-water interface in typical grass-type and algal-type zones of Taihu Lake were analyzed during summer, and indoor micro-environment experiments were conducted to illustrate the main factors affecting the generation and emissions of N2O. The results showed that the N2O fluxes at the water-gas interface, dissolved N2O concentration, and N2O fluxes at the sediment-water interface of the emergent macrophyte type area was higher than the algae-type area and submerged macrophyte area during the summer., with N2O fluxes at the water-gas interface of (115.807±7.583), (79.768±1.842), and (3.685±0.295) µmol ·(m2 ·h)-1, respectively. The dissolved N2O concentration in the water column were (0.051±0), (0.029±0.001), and (0.018±0) µmol ·L-1, respectively; and the N2O fluxes at the sediment-water interface were (178.275±3.666), (160.685±0.642), and (75.665±1.016) µmol ·(m2 ·h)-1, respectively. The spatial difference could be attributed to dominant plants and the concentration of inorganic nitrogen in the water column. The results of micro-environment experiments showed that nitrate and organic carbon sources could significantly increase the N2O production potential of sediments, the high concentration of NH4+-N in the water column might inhibit the N2O production in sediments, and the production rates of N2O in the sediment increased remarkably when the incubation temperature increased, suggesting that the generation and emissions of N2O were mainly restricted by nitrate, organic carbon, and temperature in summer.

Optimum dredging time for inhibition and prevention of algae-induced black blooms in Lake Taihu, China.

Dredging, which is the removal of polluted surface sediments from a water body, is an effective means of preventing the formation of algae-induced black blooms. However, an inappropriate dredging time may contribute to rather than inhibit the formation of black blooms. To determine the optimum dredging time, four treatments were simulated with sediment samples collected from Lake Taihu: dredging in January 2014 (DW), April 2014 (DA), July 2014 (DS), and no dredging (UD). Results showed that typical characteristics associated with black blooms, such as high levels of nutrients (NH4 (+)-N and PO4 (3-)-P), Fe(2+), ∑S(2-) ([HS(-)] + [S(2-)]), and volatile organic sulfur compounds (VOSCs), including dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS), were more effectively suppressed in the water column by DW treatment than by UD treatment and the other two dredging treatments. The highest concentrations of NH4 (+)-N and PO4 (3-)-P in the UD water column were 4.09 and 4.03 times, respectively, those in the DW water column. DMS levels in the UD and DS water columns were significantly higher (p < 0.05) than those in the DW water column, but DMDS and DMTS levels were not significantly different between the treatments. After several months of dredging, surface sediments of the DW and DA treatments were well oxidized, and concentrations of Fe(2+) and ∑S(2-) were lower than those in UD and DS treatments because of material circulation between sediments and the water column. Water content, which is important for the transport of matter to the overlying water, was lower in the dredged sediments than in the undredged sediments. These factors can suppress the release of Fe(2+) and ∑S(2-) into the water column, thereby inhibiting the formation of black blooms. Black coloration occurred in the UD water column on the seventh day, 2 days later, and earlier, respectively, than the DW and DS water columns and almost on the same day as in the DA water column. This phenomenon is mainly attributed to the oxidation of the new sediment surface in the DW and DA water columns, suppressing the release of sulfur into the water column, because of a long incubation period. Thus, dredging in the winter can prevent the formation of black blooms, while dredging in summer may contribute to them.

[Environmental Effect of Substrate Amelioration on Lake: Effects on Phragmites communis Growth and Photosynthetic Fluorescence Characteristics].

Growth of rooted aquatic macrophytes was affected by the nature and composition of lake bottom sediments. Obviously, it has been recognized as an important ecological restoration measure by improving lake substrate and then reestablishing and restoring aquatic macrophytes in order to get rid of the environmental problem of lake. This study simulated five covering thickness to give an insight into the influence of substrate amelioration on Phragmites communis growth and photosynthetic fluorescence characteristics. The results showed that the total biomass, plant height, leaf length and leaf width of Phragmites communis under capping 5 cm were much more significant than those of capping 18 cm (P < 0.01), at the 120 d, the underground: shoot biomass ratio and fine root: underground biomass ratio were also much higher than those of other treatments (P < 0.05), which indicated that capping 18 cm treatment would significantly inhibit the growth of Phragmites communis , but the growth of control group Phragmites communis was slightly constrained by eutrophicated sediment. In addition, as the capping thickness growing, the underground: shoot biomass ratio of the plant would be reduced dramatically, in order to acquire much more nutrients from sediment for plant growing, the underground biomass of Phragmites communis would be preferentially developed, especially, the biomass of fine root. However, Photosystem II (PS II) photochemical efficiency (Fv/Fm), quantum yield (Yield), photochemical quenching (qP), non-photochemical quenching (qN) of Phragmites communis under different treatments had no significant differences (P > 0.05), furthermore, with much greater capping thickness, the photosynthesis structure of PS II would be much easier destroyed, and PS II would be protected by increasing heat dissipating and reducing leaf photosynthetic area and leaf light-captured pigment contents. In terms of the influence of sediment amelioration by soil exchange on the growth and photosynthetic fluorescence characteristics of Phragmites communis, plant growth could be effectively promoted under capping 2 cm and capping 5 cm by increasing the Eh value and nutrient content, whereas plant under capping 18 cm would be much easier adaptive to low-light stress in winter season, of which capping 2 cm treatment was conducive to enhance the initial slope of RLCs (α), maximum electron transport rate (ETRmax) and minimum saturating irradiance (Ek). With regard to the harness of environmental problem of lake, the eutrophication status of lake will be mitigated by using multi-ecological measures to control the internal nutrients content once the external loading was first effectively controlled.

[Simulation research on the release of internal nutrients affected by different dredging methods in lake].

A simulated experiment was carried out to study release features of internal source under different sediment dredging methods and the difference between two lake areas in Lake Taihu was also studied. The contaminated sediments were sampled from two sites in Meiliang Bay which were the Inner Bay (A) and the Outer Bay(B). Release rates of phosphorus after ideal dredging and suction dredging are about 20% and 72% of the control and the phosphorus release rate in Inner Bay(A) is about 80% of Outer Bay(B). Release rates of ammonia after ideal dredging and suction dredging are about 40% and 83% of the scallop dredging, but dredging process may even promote the release of ammonia in a short time, the ammonia release rate in Inner Bay(A) is about 150% higher than that in Outer Bay(B). Under the microcosm experiment condition, the ideal dredging method and the suction dredging method may have a better control of internal source in contrast with the scallop dredging. Altogether, sediment dredging may be a useful approach to decrease the release of internal source in the selected sites when the external nutrients are effectively controlled. Consider all kinds of dredging projects, the suction dredging should be the ideal option for sediment dredging in Lake Taihu.

[Influence of dredging on sediment resuspension and phosphorus transfer in lake: a simulation study].

A simulated experiment was conducted to investigate the impacts of sediment dredging on sediment resuspension and phosphorus transfer in the summer and winter seasons under the common wind-wave disturbance, and the contaminated sediment used in this study was from Meiliang Bay, Taihu lake. The result showed that 20 cm dredging could effectively inhibit the sediment resuspension in study area, dredging in winter has a better effect than that in summer, and the higher values of the total suspended solid (TSS) in undredged and dredged water column during the process of wind wave disturbance were 7.0 and 2.2, 24.3 and 6.4 times higher than the initial value in summer and winter simulation respectively. The paired-samples t-test result demonstrated that total phosphorus (TP) and phosphate (PO4(3-)-P) loading positively correlated to TSS content in dredged (P<0.01) and undredged water column (P<0.05), which proved that internal phosphorus fulminating release induced by wind-wave disturbance would significantly increase the TP and PO4(3-)-P loading in the water column. The effect of dredging conducted in summer on the TP and PO4(3)-P loading in the water column was negative, but not for winter dredging (P<0.01). The pore water dissolved reactive phosphorus (DRP) profile at water-sediment interface in summer simulation was also investigated by diffusive gradients in thin films (DGT) technique. Diffusion layer of the DRP profile in undredged sediment was wider than that in dredged sediment. However, the DRP diffusion potential in dredged sediment was greater than that in undredged sediment, showing that dredging can effectively reduce the risk of the DRP potential release in dredged pore water, but also would induce the DRP fulminating release in the short time under hydrodynamic action. Generally, dredging was usually deployed during the summer and the autumn. Considering Taihu Lake is a large, shallow, eutrophic lake and the contaminant distribution is spatially heterogeneous, it is vital to determine the optimal time, depth and scope of dredging.

[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.

[Effects of sediment dredging on benthos community structure and water quality in Zhushan Bay].

We surveyed the changes of macro-benthos community composition and nutrients concentration in water in Zhushan Bay after it had been dredged 6 months, which aimed to remove the polluted surface sediments. The results showed that the main benthos in the dredged and un-dredged sediments were Limodrilus hoffmeisteri, Pelopia and Bellamya aeruginosa; compared to the un-dredged sediments, the bio-diversity of dredged areas became lower. However, its biomass became higher than that in un-dredged areas. Concentration range changes of TN and TP in overlying water was 1.64-4.45 mg/L and 0.133-0.258 mg/L, respectively. The post-dredged sediments were still in a higher state of nutrients for the higher concentration nutrients in overlying water, macro-benthos were the species that lived in a serious polluted water environment. Using Shannon-Weaver, Simpson, and Goodnight benthic index to evaluate the results show that the dredged area is in the moderately polluted level, but un-dredged area is in the middle-heavily polluted level. According to the benthos fauna surveys and water quality monitoring results, the effective of sediment dredging could play its role only the strict control on the external pollution resources have been made and reduces the effects of polluted water on the sediments.

[Environment effects of algae-caused black spots: impacts on Fe-Mn-S cycles in water-sediment interface].

The driving effects of algal cells settlement in the water-sediment interface on Fe, Mn, S biogeochemistry in laboratory through static cultivation device. Results showed that dissolved oxygen would be exhausted by algae cells in 50 min after the cyanobacteria cells settled to the sediment surface. Soon the water-sediment interface formed the severe anoxia and Fe-Mn oxides and sulfides were deoxidized quickly in the strong reducing environment. The Fe2+, Mn2+ content in interface increased to the summit at the 4th day and their concentrations were 4.40 mg/L and 2.35 mg/L, respectively. When it comes to the end of the experiment, the Fe2+ content had a little reduction and Mn2+ reduced quickly, their concentrations were 3.37 mg/L and 0.97 mg/L at the end of experiment. However, S2- concentration in interface reached the highest at the 2nd day and its content was 0.63 mg/L, and its concentration was only 0.12 mg/L at the end since it has been reduced. The ORP was--150 mV in the sediment surface and indicated that the sediment environment was a strong reducing environment. Phenomenon of algal cells induced black spots in water bodies was the main driving factors on Fe/Mn oxides and sulfides biogeochemistry cycle, and also the extreme anoxia environment would have great harm on the water body's ecology.

[Environment effects of algae-caused black spots: driving effects on the N, P changes in the water-sediment interface].

The impact and driving effect of deposited algal cells in the water-sediment interface on the N, P changes were studied through continuous extracted pore water with home-made static experiment. Results showed that dissolved oxygen in water-sediment interface was depleted in 50 min after algal cells settled. Soon the dead algal cells formed the anoxia and strong reducing environment and the dead cells had a severe anaerobic mineralization in the water-sediment interface, also the water bodies had a intense black and stink phenomenon. PO4(-3) -P, NH4(+) -N concentration in water-sediment interface increased from the 2nd day after added the algal cells to the sediment interface, and its concentration was 4.00 mg/L and 39.45 mg/L, respectively. Its concentration was the 10 fold and 241 fold higher than that the control experiments at the same time (PO4(-3) -P, NH4(+) -N concentration in control experiments was 0.42 mg/L and 0.16 mg/L, respectively). Anaerobic mineralization of dead cells in sediment surface drove the nutrients diffusing upward the overlying water, added the nutrients concentration in water bodies, and it also supplied the nutrient materials for the algal blooms happened again.

[Effects of large-area planting water hyacinth on macro-benthos community structure and biomass].

The effects on macro-benthos and benthos environment of planting 200 hm2 water hyacinth (E. crassipens) in Zhushan Bay, Lake Taihu, were studied during 8-10 months consecutive surveys. Results indicated that average densities of mollusca (the main species were Bellamya aeruginosa) in far-planting, near-planting and planting area were 276.67, 371.11 and 440.00 ind/m2, respectively, and biomass were 373.15, 486.57 and 672.54 g/m2, respectively, showed that average density and biomass of planting area's were higher than those of others. However, the average density and biomass of Oligochaeta (the main species was Limodrilus hoffmeisteri) and Chironomidae in planting area were lower than that of outside planting area. The density and biomass of three dominant species of benthic animal increased quickly during 8-9 months, decreased quickly in October inside and outside water hyacinth planting area. The reason of this phenomenon could be possible that lots of cyanobacteria cells died and consumed dissolve oxygen in proceed decomposing. Algae cells released lots of phosphorus and nitrogen simultaneously, so macro-benthos died in this environment. The indexes of Shannon-Weaver and Simpson indicated that water environment was in moderate polluted state. On the basis of the survey results, the large-area and high-density planting water hyacinth haven't demonstrated a great impact on macrobenthos and benthos environment in short planting time (about 6 months planting time).

[Effects of black spots of dead-cyanobacterial mats on Fe-S-P cycling in sediments of Zhushan Bay, Lake Taihu].

The purpose of the paper was to study the biogeochemical response of Fe-S-P in sediments in an extremely conditions (DO = 0.14 mg x L(-1), Eh = -89.3 mV), which was caused by the dead-cyanobacterial mats. The results showed that organic matter concentration in surficial sediment was higher, and also the Org-P concentration in surface sediments increased 72 mg x kg(-1) than those of natural sediments for the dead-cyanobacterial cells sedimentation on the sediment. The phosphorus bound Fe was dissolved in lower Eh and anoxia in water and it increased the content of activated Fe and PO4(3-) -P in sediments, also the concentration of Fe-P in sediment was lower than that of natural sediments. The concentration of PO4(3-) -P in interstitial water in sediments was higher than that of natural sediments, it would release to the overlying water and could be helpful to the next algae blooms outbreak. Also the activated Fe2+ concentration increased in the sediment in reducing environment (the Dithio-Fe increased 30 micromol than that the natural sediments); moreover, the increased Fe2+ caused the crystalline iron oxides to transform to the amorphous iron oxides. The anaerobic conditions of the water caused the AVS concentration increased and formed massive sulfides, including the production of H2S, which would escape to the atmosphere, but it would be harmful to water column ecological health; the sulfides also increased in surficial sediment for the more production of AVS (the sulfides in dead-cyanobacterial mats district was 50 micromol x g(-1) higher than natural water column).

[Algal-flocculation removal by modified sediment of Taihu Lake in wind action].

Removal effects of cyanobcateria algal cells in chitosan-mediated in-situ-sediment in the sediment resuspension was studied in the laboratory. The research simulated the sediment suspension through quantitative simulated the middle-grade wave of lake Taihu, which usually experienced, by using the Y-type sediment resuspended generator. The results showed that the blue-green algal's removal effect is 93.55% and 99.19% as the dosage adding of chitosan and sediment were (0.100 + 0.200) g x L(-1) and (0.150 + 0.200) g x L(-1), respectively. The removal rate of turbidity of the water body reached 78.60% after still 30 min, in which the chitosan adding dosage was 0.150 g x L(-1); the removal rate of turbidity achieved 93.88% after 8 h of water body still. Furthermore, adding the chitosan could decrease the PO4(3-) -P concentration of water body in a short term. Preliminary results showed that the chitosan which adding dosage was 0.15 g x L(-1) could effectively remove the cyanobacteria cells in middle-grade wave situation; and also indicates using the chitosan-mediated sediment to flocculate the algal bloom of the Taihu Lake has a better application prospect. Contrast study shows that the quantitative simulation method of hydrodynamic intensity and the height of water has the obvious advantage to determine the dosage of chitosan in algal-flocculation removal.

[Effects of sediment resuspension on aqueous nutrient loading in grass type zone of Lake Taihu].

Internal nutrient loading caused by sediment resuspension is becoming a key issue in studying water eutrophication of shallow lakes. A Y-shape apparatus was used to simulate sediment resuspension and sedimentation process under hydrodynamic conditions in situ in grass type zone of Lake Taihu, and effects on aqueous nutrient loading were investigated. The results indicated that, in the light and moderate wind processes, content of ammonia nitrogen and phosphate had remarkably reduced with the increase of the amount of suspended sediments in water column, with the maximal change of -0.140 g x m(-2) and -1.59 mg x m(-2) at the sediment-water interface for the two nutrients respectively, and in the strong wind process, concentration of aqueous phosphate had a significant increase, with the maximal flux of 0.81 mg x m(-2) at the sediment-water interface, while concentration of aqueous ammonia nitrogen showed a small decrease. And in the later sedimentation process after wind, content of aqueous ammonia nitrogen was lower than that before the light and moderate wind processes, but approximated to that before the strong wind process. Content of aqueous phosphate was close to that before the light and moderate wind processes, but had a marked increase compared to that before the strong wind condition, with the maximal increment of 1.36 mg x m(-2). Consequently, sediment resuspension and sedimentation processes impact nutrients loading of overlaying water evidently. Comparison of the results with those from non-grass type zone in Lake Taihu illustrates that the presence of aquatic macrophytes played some roles in reducing the release of nutrients during the sediment resuspension process.

[Estimation of releasing fluxes of sediment nitrogen and phosphorus in Fubao Bay in Dianchi Lake].

NH4(+)-N and PO4(3-)-P release from the sediment of Fubao Bay in Dianchi Lake were simulated using static incubations of intact sediment cores. The results showed that the releasing rates of sediment NH4(+)-N and PO4(3-)-P were 22.941-163.117 mg x (m2 x d)(-1) and 0.90-2.06 mg x (m2 x d)(-1), respectively, with large variation in different sampling areas. The lower releasing rates were associated with declined concentration gradients between sediment pore water and overlying water,which might be related to the high input of sewages in the bay. Using peeper (in situ dialysis membrane samplers) method, concentration gradients of NH4(+)-N and PO4(3-)-P were obtained at the interstitial water-overlying water profiles. Based on calculations using the Fick's First Law,the releasing rates of sediment NH4(+)-N and PO4(3-)-P were estimated at 2.85-81.96 mg x (m2 x d)(-1) and 0.118-0.265 mg x (m2 x d)(-1), respectively. Comparison of the two methods using to estimate the fluxes of sediment NH4(+)-N and PO4(3-)-P, the NH4(+)-N and PO4(3-)-P flux from the calculation using Fick's First Law was lower than that using static incubations. For the whole bay,the area-weighted releasing fluxes of NH4(+)-N and PO4(3-)-P from static incubations were estimated to be (49.9 +/- 8.8) t x a(-1) and (0.79 +/- 0.53) t x a(-1), respectively.

Effects of hydrodynamics processes on phosphorus fluxes from sediment in large, shallow Taihu Lake.

The turnover of phosphorus (P) in lake sediments, a major cause of eutrophication and subsequent deterioration of water quality, is in need of deep understanding. In this study, effects of resuspension on P release were studied in cylindrical microcosms with Y-shape apparatus. The results indicated that there was a positive correlation between flux of suspended substance across sediment-water interface (F(SS)) and the wind speed, and an increasing F(SS) during each wind process followed by a steady state. The maximal F(SS) under light, moderate, and strong wind conditions were 299.9 +/- 41.1, 573.4 +/- 61.7, and 2093.8 +/- 215.7 g/m2, respectively. However, flux of P across sediment-water interface (F(P)) did not follow a similar pattern as F(SS) responding to wind intensity, which increased and reached the maximum in initial 120 min for light wind, then decreased gradually, with maximal flux of 9.4 +/- 1.9 mg/m2. A rapid increase of F(P) at the first 30 min was observed under moderate wind, with maximal flux of 11.2 +/- 0.6 mg/m2. Surprisingly, strong wind caused less F(P) than under light and moderate wind conditions with maximal flux of 3.5 +/- 0.9 mg/m2. F(SS) in water column declined obviously during the sedimentation process after winds, but F(P) varied with wind regime. No obvious difference was detected on F(P) after 8 h sedimentation process, compared with the initial value, which means little redundant P left in the water column after winds.