## 3 Results and Discussion
### Analysis of Samples  
The distribution characteristics of DMSs in the surface water of Lake Ulansuhai are shown in Figure 4. All three types of DMSs including DMS, DMDS, and DMIS were detected in the water samples by employing the optimized method developed in this study. The low SD of the testing results indicated the good reproducibility of the analysis. Specifically, DMS and DMDS were detected in all sampling sites in the lake, while DMIS was detected in 3 out of 5 sampling sites. The average concentration of three DMSs in descending order are DMS (3606.2 ng/L) > DMDS (403.4 ng/L) > DMTS (162.1 ng/L). The concentration of DMS in the water body of Lake Ulansuhai ranged from 783.8 to 8093.4 ng/L indicating a relatively severe DMS pollution in the lake water column. However, a clear spatial heterogeneity can be found in the distribution of DMS in the water of Lake Ulansuhai. Higher DMS concentrations were found in the central (W2), north (W3, W4), and west (W5) parts of the lake with large areas of submerged macroplants. The DMDS concentration ranged from 11.6 to 1276.8 ng/L, which also showed great spatial heterogeneity in the lake. Relatively severe DMDS pollution was found in the central (W2) and north (W3 and W4) parts of the lake. The concentration of DMIS was relatively the lowest among the three DMSs which ranged from not detected (W1 and W5) to 309.5 ng/L. The spatial distribution of DMIS was very similar to DMDS in Lake Ulansuhai, with relatively high DMIS concentrations in the central (W2) and north (W3 and W4) parts of the lake. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Dimethyl Sulfides** & \begin{tabular}{l} **Lower Limit of** \\ **Detection** \\ **LOD/(ng/L)** \\ \end{tabular} & \
\begin{tabular}{l} **Lower Limit of** \\ **Quantification** \\ **LOQ/(ng/L)** \\ \end{tabular} &
\begin{tabular}{l} **Based on** \\ **Concentration ng/L** \\ \end{tabular} \\ \hline DMS & 29 & 95 & 200 \\ DMDS & 1.2 & 4.1 & 100 \\ DMIS & 5.0 & 17 & 100 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Detection limit and quantification limit of dimethyl sulfur-containing compounds. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Dimethyl** & \begin{tabular}{l} **Standard** \\ **Concentration/** \\ **(ng/L)** \\ \end{tabular} & \begin{tabular}{l} **Detected** \\ **Concentration/(ng/L)** \\ \end{tabular} & \
\begin{tabular}{l} **Spiked** \\ **Recovery/\%** \\ \end{tabular} &
\begin{tabular}{l} **Relative Standard** \\ **Deviation RSD/\%** \\ \end{tabular} \\ \hline DMS & 100 & 99.1 & 99.07 & 5.18 \\  & 500 & 486.1 & 97.22 & 5.94 \\ \cline{2-5}  & 50 & 48.5 & 96.93 & 3.08 \\ DMDS & 100 & 94.2 & 94.22 & 5.13 \\  & 500 & 496.7 & 99.34 & 6.25 \\ \cline{2-5}  & 50 & 46.7 & 93.39 & 4.79 \\ DMIS & 100 & 91.2 & 91.17 & 5.47 \\  & 500 & 496.3 & 99.25 & 2.56 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Spiked recoveries and relative standard deviations of DMS compounds. Compared with other lakes, the DMSs concentrations in the surface water of Lake Ulansuhai were higher than deep lakes such as Lake Constance [31] and Lake Kinnetet [32] but similar to the shallow eutrophic Lake Yangcheng [11]. It is notable that the DMS, DMDS, and DMTS in the water of Lake Ulansuhai showed a certain degree of pollution for the average concentrations of the three compounds that largely exceeded the olfactory threshold concentrations. The most severe pollution of DMSs were found in the central and the north parts of the lake. Lake Ulansuhai is a typical grassy eutrophic lake. The lake floor is heavily covered with submerged macroplants, mainly _Potamogeton pectinatus_, and this situation is extremely severe in the central and the north parts of the lake. [PERSON] et al. [33] found that the decay and degradation of the debris from dead submerged macroplants under hypoxic or anoxic condition could be the source of the DMSs in freshwater lakes. During this study, the water column was nearly hypoxic (DO < 2 mg/L) in the central and north parts of Lake Ulansuhai.