# 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 algae accumulation experiment showed that VSCs in the water increased exponentially after three days (Figure 6a), when the concentrations of Chl\(a\) decreased accordingly (Figure 6c). This might be a result of the death of the accumulated algae. In contrast, VSCs in the control treatment remained stable (Figure 6b). The concentrations of DO in the algae accumulated columns decreased from \(>7\) mg L\({}^{-1}\) to \(<0.1\) mg L\({}^{-1}\) after two days of accumulation and decomposition of the algae (Figure 6e), which supported findings from previous studies [22; 40]. These results indicated that the death and decomposition of algae takes place during large algae accumulations. A previous study by [PERSON] et al. [43] also found that the algae died quickly after two days of accumulation. The decomposition of algal particles led to the sharp depletion of oxygen in the water [22; 40], which in turn accelerated the further decomposition of the algal particles. Similar to the forms of VSCs in the field algae accumulation area in July, the MTL was not noticeably higher than other VOSCs in other seasons. Under the anoxic conditions during the algae decomposition, MTL and H\({}_{2}\)S were produced in large quantities (Figure 6b) through the decomposition of sulfur-containing amino acids [62; 63]. The other forms of VOSCs, including DMS, DMDS, and DMTS, were formed subsequently [64]. The DMS, DMDS, and DMTS could be oxidized to dimethyl sulfoxide, dimethyl sulfone, and other substances in oxic environments. While the algae particles degraded during the algae accumulation, the water column was mostly under anoxic conditions (under 0.1 mg L\({}^{-1}\)), leading to the high concentrations of DMS, DMDS, and DMTS. The algae accumulation and decomposition led not only to an increase in VSCs in the overlying water, but to an increase in AVS in the surface sediments (Figure 6d). At the end of the algae accumulation experiment, the concentration of AVS in the top 1 cm layer of the surface sediments in the algae accumulation treatment columns was 76.90 mg kg\({}^{-1}\) dw, which is much higher than that in the control treatment (35.08 mg kg\({}^{-1}\) dw) (\(p=0.043\)). This was similar to the development of AVS in the surface sediments in July during the field investigations (Section 3.3), indicating that the formation of reductive sulfur occurred in both the water and sediment during the algae accumulation. The depletion of oxygen during the accumulation and decomposition of the algae led to the formation of reductive  
Figure 5: Concentrations of volatile sulfur compounds in water during the field investigation period. Error bars indicate the standard deviation, n = 3.  
sultofocompounds in the water and the reduction of sulfates in the sediment [25]. The reduction of sulfates in the sediment during the algae accumulation led to the increase in AVS in the surface sediments (Figure 6d). Mechanisms for the Formation and Development of Reductive Sulfocompounds in the Water-Sediment System  
VSCs in the water were formed from dimethylsulfoniopropionate (DMSP) [26; 28], which originates from sulfur-containing amino acids [27]. Previous studies revealed that the heightened concentrations of VSCs observed in Lake Taihu are driven by high levels of sulfur-containing amino acids in the algae itself and an increased abundance of the algae during the accumulation season [65]. MTL and H\({}_{2}\)S were formed primarily with the decomposition of sulfur-containing amino acids in the algae [62; 63]. Consequently, dimethyl sulfides (DMS, DMDS, and DMTS) were formed through the methylation and oxidation of MTL [28; 33], while the DMS, DMDS, and DMTS were further oxidized to dimethyl sulfoxide in oxic environments [28; 33]. Therefore, the concentrations of MTL and H\({}_{2}\)S were higher than those of other VSCs in January, April, and October.