Abstract:

n recent years, with the rapid growth of economy and population, a large amount of wastewater with high level of N and P has been discharged into the natural waters. It has been speeding up the process of eutrophication of lakes. The Dianchi Lake is in a status of heavy eutrophication, accompanied with frequent occurrence of cyanobacterial blooms. Many efforts have been proposed to improve water quality. Bioremediation with water hyacinths was one of the most efficient approaches to reduce the nutrient load and control cyanobacterial blooms. Partial death of water hyacinth (Eichhornia crassipes) in largescale confineplanted enclosures in Waihai of Dianchi Lake was however observed. Water physicochemical properties, cyanobacterial biomass and the phytopathology of water hyacinth were investigated in areas of normal growth, mild death, and severe death as well as the blank control (area without water hyacinth planted) in August, 2013, in order to find out the causes of death. The results showed that the cyanobacterial biomass in the plots of mild death and severe death ((421±049)×109 cells/L and (75700±1900)×109 cells/L, respectively) were significantly (P＜005) higher than those in the plots of control and normal growth ((014±009)×109 cells/L and (146±011)×109 cells/L, respectively). The same trend was observed for the concentration of NH+4N. The concentration of NH+4N in the plots of mild death and severe death (1144±002 mg/L and 36987±1537 mg/L) were significantly (P＜005) higher than those in the plots of control and normal growth (032±001 mg/L and 034±001 mg/L). The concentration of dissolved oxygen (DO) in the plots of mild death and severe death were only 140±013 mg/L and 030±004 mg/L, respectively, which were significantly (P<005) lower than those in the control and normal growth plots (770±083 mg/L and 680±097 mg/L); The redox potential of the severe death plots (Eh -27970±2970 mv) (Eh) was significantly (P<005) lower than those in the other three plots. In addition, no evidence of plant disease was found based on pathological detection. Hence, the death causes of water hyacinth might be a consequence of cyanobacteria accumulation, followed by algal biomass decomposition which resulted in hypoxia of water with excessively high concentration of NH+4N. These findings could provide references on theory and practice for aquatic ecosystem restoration with water hyacinth. The present research has brought up a topic of using water hyacinth as a cyanobacteria blocker, which will influence the migration of nutrients and the dynamics of nitrogen, phosphorus and carbon cycle in lakes. Further research in this field may provide new strategies of ecological restoration in practice.