# Sulfur Development in the Water-Sediment System of the Algae Accumulation Embry Area in Lake Taihu
## 3 Results and Discussion
### Sulfur Development During the Algae Accumulation Experiment
The generally oxic environment in the water during these three months led to the oxidation of DMS, DMDS, and DMTS and thereby hindered the accumulation of these VSCs in the water. In July, the accumulation and decomposition of the bloomed algae led to an increase in DMS, DMDS, and DMTS. The accumulation of a large amount of algae in the embay area led to the death and decomposition of the algae [43], accompanied with oxygen concentrations decreasing to nearly 0 mg L\({}^{-1}\) (Sections 3.2 and 3.5). Oxygen depletion during this process hindered the further oxidation of DMS, DMDS, and DMTS to dimethyl sulfoxide [28; 33], leading to the accumulation of DMS, DMDS, and DMTS in the water. Therefore, the concentrations of DMS, DMDS, and DMTS increased in July. In addition, the algae accumulation experiment in the laboratory also demonstrated that the concentrations of DMS, DMDS, and DMTS increased quickly with the depletion of oxygen after two days of the algae accumulation. These results indicate that the development of oxygen is crucial for the formation and development of VSCs in the water. The development of oxygen not only influenced the formation and development of VSCs in the water, but also influenced the transformation of sulfur in the sediment. During the algae accumulation and decomposition, the depletion of oxygen in the water decreased the OPD across the  
Figure 6: Development of the characteristics of the water-sediment system during the algae accumulation experiment: (**a**) Volatile sulfur compounds in the water during the algae accumulation; (**b**) volatile sulfur compounds in the control treatment; (**c**) the concentrations of chlorophyll \(a\) (Chl_a_) in the water during the algae accumulation experiment; (**d**) profiles of acid volatile sulfide (AVS) at the end of the algae accumulation experiment; and (**e**) dissolved oxygen (DO) in the water during the algae accumulation experiment. Error bars indicate the standard deviation, n = 3. SWI (Section 3.2), which facilitated the growth of sulfate-reducing bacteria and a reduction in sulfate in the sediment [25; 66]. In addition, the deposition of the accumulated algae on the surface sediments increased the organic matter in the surface sediments, which further intensified the consumption of oxygen across the SWI [67; 68] and catalyzed the sulfate reduction. The oxygen depletion and sulfate reduction during the algae accumulation thereby increased the AVS concentrations in the surface sediments both during the field investigation (Section 3.3) and laboratory algae accumulation experiments (Section 3.5). The OPD was significantly negatively correlated (\(p<0.01\)) to both the VSCs in the water and the AVS in the surface sediments (Figure 7, Table S3). While the VSCs originated from the decomposition of the accumulated algae in the water, the sulfides (S\({}^{2-}\)) originated from the AVS in the sediment [22]. The high AVS in the surface sediments during the algae accumulation led to the release of sulfides (S\({}^{2-}\)) and ferrous iron (Fe\({}^{2+}\)) into the overlying water [40], leading to the black coloration of the water in Lake Taihu. Therefore, most of the algae-induced black coloration in Lake Taihu are likely to occur first at the bottom of the water column and then spread to the whole water column. The formation and development of these reductive sulfocompounds during the algae accumulation are the main causes of the black odorous events in Lake Taihu and of the drinking water crisis in 2007 [31; 32]. In addition, the formation of reductive sulfides might accelerate the release of P from the sediments [14], which aggravates eutrophication and HABs in Lake Taihu. Therefore, the reduction in the accumulated algae amount in similar embay areas is crucial for the eutrophication management in Lake Taihu. The accumulated algae in these bays and littoral areas should be disposed timely through salvage or other measures to reduce further ecological disasters during the algae accumulation.