Eutrophication and Water Quality Management of Reservoirs in DongguanCity

Dongguan City in Guangdong Province,located on the Dongjiang River downstream of the Pearl River Delta,has more than 118 reservoirs with a water capacity of 4. 07 × 108m3,but the actual water supply of these reservoirs only accounts for 50% of the total water supply volume due to the limitation of water quality. In this study the eutrophic status of the main drinking water supply reservoirs in Dongguan City were investigated to determine the causes of eutrophication and cyanobacteria blooms in the reservoirs,and provide insights to improve management of water quality. The trophic state,phytoplankton abundance and biomass of 25 reservoirs in Dongguan City were determined and analyzed in wet,normal and dry seasons from 2011 to 2012. The concentrations of total nitrogen and total phosphorus were determined using national standard methods and the concentration of chlorophyll a was evaluated by the acetone extraction method. The results show that five reservoirs were mesotrophic,sixteen were eutrophic,and four were hypereutrophic. The abundance of phytoplankton ranged from 1. 07 × 105 cell /L to 7. 67 ×108cell /L and was higher in the dry season than in the wet season. Cyanobacteria accounted for over half of the total phytoplankton population for 20 of the 25 reservoirs and cyanobacterial blooms occurred in a few reservoirs during dry season. The trophic status of these reservoirs was significantly related to the intensity of anthropogenic disturbance,land use in the catchment area,function of the reservoirs and water replenishment method. Strengthening the supervision and control of external pollution,especially reducing the total loads of phosphorus to the reservoirs,is the effective and ultimate measure for controlling eutrophication in the reservoirs. Proper reservoir operation can alleviate eutrophication of the five reservoirs connected by the Dongjiang River and Reservoir Water Network Project in the near future.