Variation Characteristics of Water Quality and Equilibrium Estimation of Nitrogen and Phosphorus in Aquaculture Water Restored by Eichhirnia carssipes

In order to study the restoration efficiency of aquaculture wastewater by Eichhirnia carssipes, a field experiment was constructed in three ponds with the area of 0.54 hm² in Dacheng eco-agricultural territory of Jinting Town, Suzhou City. Three treatments: control, 5% plant coverage and 10 % plant coverage were set up. During the experiment, fish fry was released to the ponds in April and Eichhirnia carssipes was cultivated in the fences in May. From April to November, water samples were collected to monitor the concentrations of total nitrogen (TN), ammonia nitrogen (NH₄⁺-N), nitrate nitrogen (NO₃^--N), total phosphorus (TP) and suspended solid (SS) every month. The fresh biomass, nitrogen and phosphorus contents of Eichhirnia carssipes were monitored every month. The single weight, nitrogen and phosphorous contents of fish were monitored at fry releasing and fish catching. From July to October, the concentrations of TN, NH₄⁺-N and NO₃^--N in aquaculture water restored by the plant were significantly lower than that of the blank control. With feeding and fish growth, the concentrations of TP and SS in aquaculture water gradually increased. When Eichhirnia carssipes was harvested in July, the concentrations of TP and SS in in aquaculture water restored by the plant significantly decreased. The total yield, single weight and utilization rate of nitrogen and phosphorus of fish in the two ponds restored by Eichhirnia carssipes were higher than those of the control, which indicates that Eichhirnia carssipes cultivation in aquaculture ponds had no negative effect on fish growth. Feeding was the main way of nitrogen and phosphorus input in aquaculture water. Fish production and sediment deposition were the main ways of phosphorus output in aquaculture water and they accounted higher proportion in total nitrogen output of aquaculture water. The removal amount of nitrogen and phosphorus by Eichhirnia carssipes harvesting accounted for 7.41%~13.78% and 5.81%~9.87% of the total output, respectively. Compared with the control, the concentrations of nitrogen and phosphorus in the water body could be controlled by low coverage of Eichhirnia carssipes. However, there is still a risk of water environmental pollution caused by the direct discharge of the restored aquaculture water. Thus it is very necessary to take deep-purifying measures to treat the drainage of the aquaculture pond.