Improving water quality and balance of nitrogen and phosphorus in Dianchi lake based on ecological restoration with Eichhornia crassipes

Abstract: In order to study the improvement of water quality by confined growth of Eichhornia crassipes and the fate of nitrogen and phosphorus in Caohai of Dianchi Lake, a pilot project with large-scale cultivation of the macrophyte, which was aimed to remove contaminants and alleviate eutrophication in Caohai, was carried out from 2011 to 2013. The project consisted of 3 procedures as follows. 1) Seedlings of E. crassipes were cultivated in a man-made fence made of bamboos and nets. 2) After the rapid growth and propagation of E. crassipes, the macrophyte was harvested utilizing specially designed harvestors. 3) The harvested biomass of macrophyte was crushed and squeezed, the squeeze slag was processed to make organic fertilizer or silage feed, and the squeeze juice was processed to make biogas. During the project, the concentrations of TN (total nitrogen) and TP (total phosphorus) in water at river inlets and Caohai were monitored every month. Plant samples of E. crassipes were collected in the period of cultivation and in the period of harvest to assay the N and P contents in the plant tissue. The coverage area of E. crassipes planted in Caohai was determined by satellite remote sensing technology, and the biomass of per unit area was measured by electronic balance in 1 m2 quadrates at 9 different sites of the plant population. Then, the total biomass of the macrophyte was calculated from the average biomass of per unit area multiplied by the coverage area. Based on the above experiment data, the balance of TN and TP loads in the lake was estimated. The results showed that the average concentrations of TN and TP in the water decreased by 51.04% and 40.00%, respectively, from 14.48 and 0.60 mg/L before E. crassipes cultivation (from 2006 to 2010) to 7.09 and 0.36 mg/L after E. crassipes cultivation (from 2011 to 2013). The decreasing trend of TN and TP average concentrations from the lake inlets to the outlet of Caohai was observed, with the reduction of 9.74 mg/L TN and 0.40 mg/L TP. The balance of TN and TP loads in Caohai demonstrated that the assimilation of N and P by E. crassipes was one of important ways to remove N from the lake, and meanwhile most of the TN load into the lake (40.31%-59.04%) was removed through the nitrification/denitrification and sedimentation, and only a small proportion of TN input remained in the water as detected in the outlet water of the lake. However, the loss of TP via various ways was higher than the TP input into the lake, and 40.44%-116.56% of TP load into the lake was assimilated by E. crassipes, which indicated the P in sediment released into the lake water in the year of 2011. These results indicated that the large-scale confined growth of E. crassipes could play a large role in water quality improvement of Caohai, and the assimilation by E. crassipes was an important way to remove TN and the main way to remove TP from the water. All the above results indicate that the large-scale cultivation of E. crassipes to remove N and P in eutrophic waters is practicable.