## 2 Materials and Methods
### CH\({}_{4}\) Measurements  
The floating opaque chambers were used to measure CH\({}_{4}\) emission from the open water and _N. nucifera_ Gaertn. containers. CH\({}_{4}\) gases were sampled from April to December in 2021, and from April to October in 2022, and gas samples were collected once per month on a clear day between 8:30 and 11:30 a.m. (Beijing standard time, GMT + 8 h), with sampling carried out twice in September 2021 and June 2022, and three times in September 2022. The chamber was composed of acrylic organic glass (40 cm height \(\times\) 35 cm diameter) [28]. Air inside the chambers was circulated with battery-driven fans during the measurement to ensure the gas samples were well mixed. In addition, the top of the chamber was set up with a thermometer sensor, while the open end of the chamber was fitted with a cystosepiment and tyre as floating equipment (Figure 2a). During the measurement, three chambers at each site were placed upside down at a distance of 50 to 100 cm apart on the water surface, and the open end of the chamber remained approximately 10 cm below the water surface. Generally, gas samples of the chamber were collected 0, 5, 10, and 15 min after enclosure using a 60 mL polypropylene syringe attached to a three-way stopcock. Subsequently, all gas samples were immediately injected into evacuated bags composed of polymer film and aluminum foil (Hoompo Inc., Ningbo, Zhejiang province, China), then taken to the laboratory for determination within three days. The hanging opaque chambers were used to measured plant-dependent CH\({}_{4}\) emission in the NNs site (Figure 2b). The chamber was composed of acrylic organic glass (30 cm height \(\times\) 15 cm radius). The stem and leaf of the lotus above the water surface were enclosed in the chamber, and the end of the chamber was sealed by two half rounds of acrylic glass plates. In addition, the center of the acrylic glass plate had a small round hole (\(\sim\)3 mm) for passing through the stem of the plant, and the edge of the hole was sealed by plasticine when the plant was sealed in the chamber. Meanwhile, the bottom edge of the chamber was closed with transparent tape to guard against gas leakage. The inner part of the chamber was also set up with the thermometer sensor and fans, its function being similar to the floating opaque chambers. Before sampling, the stem and leaf of the plant were closed in the hanging opaque chamber; subsequently, three chambers were hung on the six bamboo poles (Figure 2b). Gas samples were immediately collected 0, 10, 20, and 30 min after chamber closure using a 60 mL polypropylene syringe attached to a three-way stopcock. Similarly, the gas samples were also injected into evacuated bags and transferred to the lab, where they were tested with the CH\({}_{4}\) concentration. The gas samples for the  
Figure 1: The study site in Hong Lake in the Yangtze River. plant-dependent CH\({}_{4}\) emission in the NNs site were collected twice, namely, on 25 August and 21 September in 2022. The CH\({}_{4}\) concentrations of all gas samples were determined by gas chromatography (Agilent, 7890A, GC system, Agilent Co., Wilmington, DE, USA) equipped with a flame ionization detector. In the study, about 20 mL of each gas sample was extracted from the gas bags to test the CH\({}_{4}\) concentration. We then assessed the flux value: 0.70 and above of the absolute coefficient \(R^{2}\) values were 83%, 58% of which was 0.90 and above, and these dates were considered valid.