Eutrophication of a Pumped Water Storage Reservoir in South China

Abstract

Dajinshan reservoir, built in 1974 for the purpose of supplying drinking water, is a pumped water storage tropical reservoir. With a small volume of 10 × 10⁶ m³, the stored water is insufficient to meet water consumption. Most of the stored water is pumped from a polluted river into the reservoir at the dam in the dry season. The specific pattern of importing and exporting water leads to an abnormal variation in water level that is high in the dry season and low in the wet season. In order to understand the trophic state dynamics of this special reservoir, a high-frequency sampling with an interval of 15 days was conducted for analysis of water quality and phytoplankton abundance at four sites in 2005. As the water abstraction actually functions as a point source of pollutants, the dynamics of nutrients and chlorophyll a concentration reveal a pattern that follows the water level. Total phosphorus and total nitrogen concentrations ranged from 0.016 to 0.086 mg/L and 0.5 to 2.0 mg/L, respectively, with a maximum in the dry season. The mean weighed trophic state index, combining TN, TP, Chlorophyll a and Secchi depth, was between 30 and 55, indicating that the reservoir was mesotrophic in the wet season and eutrophic in the dry season. The water quality exhibited a visible vertical gradient near the dam, with higher concentrations of total nitrogen and total phosphorus near the bottom. This vertical gradient implies that internal loading may be important for eutrophication. Phytoplankton abundance had a seasonal variation similar to that of nutrients. It ranged from 0.86 × 10⁶ to 106.27 × 10⁶ cells/L, with peaks in April and in November, respectively. The phytoplankton was dominated by diatoms and Cyanobacteria in the mixing period (winter and early spring), and by Cyanobacteria during stratification only, at which time they constituted 90% of the total abundance, however. Compared with an early survey in 1999 and 2000, water quality shows a rapid deterioration. In order to maintain high water quality for drinking water, measures against eutrophication are urgently required. Dredging of the sediment and reduction of nutrients in the abstracted water are two priority strategies.