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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.

Self-made glass reactors were employed to study the occurrence of black water bloom induced by different types of organic matters, to clarify the precursor of volatile organic sulfur compounds (VOSCs), and then to preliminarily study its degradation mechanisms under laboratory-controlled conditions. Our research indicated that provided organic matrix were as high as 1.0 g x L(-1), all organic matters could blacken the lake water regardless of sulfur appearance or not. However, compared with sulfur-free compounds that took more than 13 d to blacken the water, sulfur containing materials could accelerate the occurrence of black color to 7-13 d and increase the water chromaticity to above 410 which causing offensive odor consisted chiefly of DMDS, DMTS and DMTeS. Based on the function of methionine on the production of VOSCs, methionine was identified to be the precursors of VOSCs. Methionine was readily broke down by sulfate-reducing bacteria (SRB) (also other bacteria) (at 95% with the duration of 35 d) to produce hydrogen sulfide, methanethiol, and dominantly dimethylpolysulfides such as DMDS, DMTS and DMTeS. And the occurrence of black color had been advanced from 13 d to 8 d. Methanogenic bacteria slightly inhibited the degradation of methionine and reduced the evolution of sulfide. Therefore, the addition of methanogenic bacteria inhibitor set the formation of black color ahead by 1 d. Methionine was also degraded by nonbiodegradation, but it was a secondary pathway and cannot completely degrade methionine to blacken the water.

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.

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.

The course of intermittent sediment disturbance-sedimentation, that sediment disturbance was for 10 min and succeeding sedimentation was for 1430 min each day over a 17-day period, was simulated to investigate the variation of bioavailable phosphorus (BAP) in overlying water. The results showed that the concentrations of BAP increased obviously at Oh after each disturbance, and the maximum concentration of BAP was reached up to 2.82 mg x L(-1) after 1 d (the first disturbance). However, the BAP gradually decreased with the sedimentation time increase (1 h, 6 h, 24 h). Moreover, at 0, 1, 6, 24 h after disturbance, the BAP gradually declined with the number of sediment disturbance increase, but the BAP at Oh after each disturbance was higher than that at 1 h, 6 h, 24 h. This may be attributed to the immediate release of bioavailable particulate phosphorus ( BAPP), as a result of sediment disturbance. The average BAPP/BAP was up to 95.0% at Oh after disturbance over a 17-day period, but this value gradually decreased with the sedimentation time increase (1 h, 6 h, 24 h). At 0 h, 1 h, 6 h, 24 h after each disturbance, the equilibrium concentrations of total dissolved phosphorus (TDP) were reached after 5 d (0.053, 0.062, 0.051, 0.045 mg x L(-1)), and the percentage of TDP in BAP also decreased gradually. Therefore, it is considered that sediment intermittent disturbance can accelerate the transformation from TDP to PP and hinder the development process of eutrophication in standing water. Sequential fractionation also indicates that the percentage of refractory phosphorus in Tot-P increased from 72.8% (raw sediments) to 77.3% (sediments after disturbance), attributable to the increase of concentrations of occluded Fe/Al-P. It is hopefully suggested an acceleration of transformation of phosphorus from mobile fractions to refractory fractions.