## 4 Discussion
However, compared with the KKs, the maximum values of CH\({}_{4}\) emission in the NNs occurred in September, which may be linked to the emergent macrophytes, \(N\). _nucifera_ Gaertn. On the one hand, numerous studies have indicated that vegetation is the key factor of CH\({}_{4}\) release in wetlands and the high emission is possibly attributed to primary production, which could supply organic matter incorporated into the sediment and induce the production of CH\({}_{4}\) by methanogenesis [16; 17; 21; 43]. Table 1 shows that the average vegetation biomass (798.68 g m\({}^{-2}\)) and SOC content (35.57 g kg\({}^{-1}\)) in the NNs were significantly higher than that of the KKs. The high vegetation biomass indirectly affects the carbon accumulation in sediments, thus contributing to the high SOC content, and should stimulate the production and emission of CH\({}_{4}\), which can lead to maximal CH\({}_{4}\) emission. However, [PERSON] et al. [41] pointed out that the peak in CH\({}_{4}\) emission flux lagged the peak in biomass production by 2-3 weeks. [PERSON] et al. [45] found the rates of highest CH\({}_{4}\) emission from Florida everglades with emergent aquatic vegetation, which is likely attributed to the organic matter incorporated into sediments. On the other hand, numerous examples in the literature have provided evidence that the contribution of plants transported through the aerenchyma to total CH\({}_{4}\) emission in the freshwater wetlands is 80-90% [23; 24; 25]. Most of the CH\({}_{4}\) released from the shallow sediment escaped oxidation and reached the atmosphere, and it was estimated that about 60-80% of the CH\({}_{4}\) emission was from the littoral zones in boreal lakes [21; 22]. In our studies, the contribution of plant-dependent CH\({}_{4}\) emission to the total emission in the NNs was 30-85% (Figure 5a). Hence, the lotus plant zone in the Hong Lake has been shown to emit more CH\({}_{4}\) than that of open water (Figure 5b), which is probably attributed to the supply of fresh organic matter from the lotus plant in the littoral zone and the greater transport of CH\({}_{4}\) through the plant. This implies that the littoral zone vegetation of the lake can be the key factor in regulating the CH\({}_{4}\) emission in the NNs. In general, water depth in wetlands is a major factor affecting the spatial and temporal variation of CH\({}_{4}\) emission flux [21; 46]. Although we did not find a significant linear relationship between the CH\({}_{4}\) emission and water depth, there was a significantly negative correlation between the CH\({}_{4}\) emission fluxes and DO concentrations, but the \(R^{2}\) value was much lower (Figure 7). One study reported that lower DO concentrations in water overlying the sediment in the lakes led to a higher CH\({}_{4}\) production [47], and thus increased the release of the CH\({}_{4}\) in a eutrophic lake [17; 48]. In our study, the average DO concentration (4.5 mg/L) in the NNs was significantly lower than that of the KKs (7.9 mg/L, Figure 3g,h). Some studies have shown that small and shallow lakes emit more CH\({}_{4}\) than larger and deeper lakes, which could be attributed not only to the rich substrate supply from the littoral zone production of organic matter, but also to less time for CH\({}_{4}\) oxidation during passage through water column [14; 40; 49; 50]. This could further explain the higher CH\({}_{4}\) emission fluxes in the NNs, compared to the KKs. Based on our results, the need to consider the CH\({}_{4}\) emission in the littoral region of freshwater was further emphasized, which could influence the sink of carbon (C) in lakes. Therefore, the average CH\({}_{4}\) emissions (12.86 mg C-CH\({}_{4}\) m\({}^{-2}\) h\({}^{-1}\) in the NNs) reported in the present study were higher than the average CO\({}_{2}\) emission (3.78 mg C-CO\({}_{2}\) m\({}^{-2}\) h\({}^{-1}\) with no vegetation growth) in Chinese subtropical lake Donghu, as reported by [PERSON] et al. [33] in the same climate region.