Reasons for the Occurrence of Potamogeton crispus Population and Harvesting Impact on the water environment in Xuanwu Lake

To define the reasons for the occurrence of Potamogeton crispus population and its harvesting impact on the water environment in Xuanwu Lake of Nanjing during 2005-2006, different lake zones were monitored regularly to collect data of water quality indexes, including the dissolved oxygen, transparency, total nitrogen and total phosphorus. The data analysis indicated that the transparency was improved by 179.5 percent during the Potamogeton crispus occurrence period, the dissolved oxygen content increased by 24.1 percent, total nitrogen and total phosphorus contents decreased by 54.1 percent and 74.5 percent, respectively, and pH fell to 8.7 from 9.1, all of which can be attributed to the emergency control of cyanobacterial bloom. The water quality improvement was the primary reason for the Potamogeton crispus germination and propagation. The massive harvest of the Potamogeton crispus population in a short term during its growing period directly triggered the decrease of dissolved oxygen content by 42.1 percent, and the decrease of transparency by 51.5 percent (P0.05). The harvest took some nitrogen and phosphorus from the water body but the differences were not significant (P0.05), and the total phosphorus concentration rose after the harvest. It was suggested to take some physical or chemical approach in ecological restoration of grass/algae lakes to improve the transparency, adjust the water pH, and reduce the nutrient concentration, with the purpose of water quality improvement for germination and growth of aquatic plant, thereby making it possible for the aquatic plant to survive and propagate. The aquatic plants were suggested to be harvested gradually in different eco-management stages, which took the nutrients slowly and steadily from the water body. This approach could also improve the water quality and restore and reconstruct the aquatic ecosystems at the same time.