Performance of wetland applied to ecological remediation of abandoned fish ponds: A case study in Gonghu Bay, Tai lake

To restore the abandoned fish ponds to "near natural" state, the wetland restoration was carried out in Gonghu Bay lakeside, and its long-term performance of controlling external load was studied for 5 years. The findings showed that water quality and biodiversity had been improved dramatically after the preliminary transformation. The concentrations of permanganate index (CODMn), total nitrogen (TN), and total phosphorus (TP) obviously decreased from 12.91 mg L-1 to 4.32 mg L-1, from 3.46 mg L-1 to 1.42 mg L-1, and from 0.27 mg L-1 to 0.04 mg L-1, respectively. The proportion of Cyanophyta was effectively reduced from 31.82% to 18.89%, and favored the growth of diatoms (31.82%-37.78%) to be the dominant algae species. Aquatic plant species and coverage gradually increased from 16 to 56 and from 5% to 60%, respectively. An in-deep monitoring done for 5 years (2013-2017) showed that the wetland achieved a satisfactory removal efficiency of 58.95% for TN, 64.60% for TP, and up to 77.83% for chlorophyll-a. Besides, three pollution scenarios, such as stormwater runoff, algal bloom, and continuous water transfer, were selected to explore the tolerance of the wetland to the suddenly increased pollution loads. The results dedicated that even if the inlet load was up to 1.0 × 105 m3 d-1, the removal rate coefficients of wetland for chlorophyll-a, TP, and TN were 0.135-0.239 d-1, 0.041-0.112 d-1, and 0.030-0.109 d-1, respectively, which were equivalent to the well-running wetlands. This study confirmed that the wetland was not only a promising ecological remediation technique to contaminated abandoned fish ponds, but also could withstand high pollution load, which had the prospect of sustainable utilization.