Causes of large Potamogeton crispus L. population increase in Xuanwu Lake

In July 2005, the first outbreak of cyanobacterial blooms, dominated by Microcystis, occurred in Xuanwu Lake, Nanjing, upon which clay flocculation was adopted to control algal blooms. The cyanobacterial blooms were restrained, after which Potamogeton crispus appeared in November 2005 and spread rapidly in the whole lake. Since then, large populations of P. crispus have occurred in Xuanwu Lake annually in winter for the last 10 years. To determine the reasons for the occurrence of P. crispus populations in Xuanwu Lake during 2005-2006, water quality indices were monitored regularly. The data collected included dissolved oxygen, transparency, pH, total nitrogen, and total phosphorus content. Data analysis indicated that the transparency was improved by 179.5 % after clay flocculation, dissolved oxygen content increased by 24.1 %; total nitrogen and total phosphorus content decreased by 54.1 and 74.5 %, respectively, and pH fell from 9.1 to 8.7, all of which can be attributed to the emergency control measures for the cyanobacterial bloom. Data analysis also indicated that the improved water transparency after clay flocculation was the key factor in turion sprouting and seedling propagation of P. crispus. The ameliorative light intensity and favorable nutrient level all promoted the growth of seedlings of P. crispus and later quick colonization. It is suggested that ecological restoration of macrophytic and algal lakes be conducted by some physical or chemical means to improve transparency, reduce nutrient concentration, and adjust water pH, with the purpose of improving water quality for germination and growth of aquatic plants.