Purification Efficiency of Two Ecotypes of Wetland Plants on Subtropical Eutrophic Lakes in China

The accelerated eutrophication rate of (sub)tropical lakes is a major environmental problem. Constructed wetlands are considered as an effective method to purify water bodies. However, the removal rate of nitrogen (N), phosphorus (P) and other nutrients by wetland vegetation is quite different, and the continuous observation of plants and water ecosystems is required. In this study, the growth curves of two subtropical wetland plants, Phragmites australis and Zizania caduciflora, and their absorption effects on N and P in constructed wetlands were studied. The results showed that the growth curves of the two wetland plants were similar, reaching the biomass peak in July to August, and the cumulative absorption of N and P by plants had the same trend with the change of biomass. The N and P concentration of plants reached the peak in March to April, with higher concentration in Phragmites australis than that of Zizania caduciflora. At the end of the growing season, i.e. around October 20, the accumulated absorption of N and P by plants reached the maximum, which was the optimal time for harvest of subtropical wetland plants. Thereafter, the residues of plant litter entered the water, causing the recovery of N and P concentration in the water body, therefore affected the purification function of wetland. There is a strong correlation between the water purification efficiency and plant growth. With the increase of biomass, N and P accumulation in Phragmites australis and Zizania caduciflora, the N, P content and chemical oxygen demand (COD) in the water of wetland showed a significant decreasing trend. The removal rates of N, P, COD and suspended substance (SS) in the constructed wetlands with Phragmites australis and Zizania caduciflora as the main plants were 95%, 96%, 82% and 86%, respectively. In general, the purification capacity of Phragmites australis is slightly higher than that of Zizania caduciflora and precipitation had positive effects on the pollutants concentration of wetland water. The results provide scientific basis for plant selection and management of subtropical constructed wetlands.