|Other Abstract||Guanting reservoir is the first large reservoir built in China, which is also an important water supply for Beijing. It quitted from water supply system due to water pollution in recent years. Since water shortage problem in Beijing is becoming more and more serious,recovering the water quality and restarting its water supply function becomes a key project of the central government .
In order to assess the water quality and the status of hydrobiological resource in Guanting reservoir, and to protect and recover its water quality, a continuous monthly investigation of water chemistry and hydrobiological resource in it (Beijing region) was conducted in 2007. Evaluations upon water quality and water environment was carried out based on water chemistry, biology and ecological system. Administrative measures were also proposed. The results are as following:
Guanting reservoir (Beijing region) is divided into three parts by Xiangjiao dam, which are lotus Lake, west lake and weishui reservoir, with an area of 2266hm2,the water depth of 1.5～4m, the average temperature of 14.8℃, and SD of 0.2～0.9m, pH of 8.28～11.39. The water is alkalescence, and belongs to carbonate type, magnesium group, Ⅱ.
The main problem for water quality in Guanting reservoir was the unattainment of N、P and cyanobacterial bloom. The average value of TN was 2.22 mg/L, of which, value of west lake was 3.65mg/L, Lotus Lake 1.51 mg/L, weishui reservoir 1.29 mg/L. The average value of TP was 0.26 mg/L,of which, value of west lake was 0.43mg/L, Lotus Lake 0.14 mg/L, weishui reservoir 0.16mg/L. According to the ground water quality standard（GB3838～2002）, West lake belongs to inferior V type, while Lotus lake and weishui reservoir belong to Ⅳ～Ⅴ type.
Eight phylum and 158 genus of phytoplankton were collected.Chlorophyta, Bacillariophyta and Cyanophyta were predominant in species, while Cyanophyta, Bacillariophyta and Chlorophyta were predominant in density. The average density of phytoplankton was 93,000,000inds/L, and the average biomass of it 60mg/L.Values in west lake （115,000,000inds/L～140,000,000inds/L；55 mg/L～78mg/L）was the highest, Lotus lake （85,000,000inds/L；52 mg/L）the second, and weishui reservoir （60,000,000inds/L～90,000,000inds/L；35 mg/L～75 mg/L）the lowest. The highest biomass of phytoplankton was in summer, the second in autumn, then in winter. Microcystis aeruginosa was the most predominant species in Guanting reservoir(Beijing section),with a percentage over 50% of the total number, Which appeared in May to October, particularly in July and August.
There were 100 genera of zooplankton, of which, 40 species protozoa,45 species rotifer, 4 species cladocera, and 11 species copepods. The average density of zooplankton was 3256inds/L, and that of biomass 3.14mg/L.The composition of zooplankton tended to be miniaturization. Number of rotifer accounts for over 80% of the total species, being the predominant population, which was less only in winter. Brachionus angularis, Keratella cochlearis and Polyarthra trigla etc, the typical indicator species in eutrophication water,were the main predominant species in Guanting reservoir.
Forty three species of zoobenthos were found, which belonged to 4 phylum 24 families 42 genus. Among them, 5 species annelid, 27 species arthropod, 10 species mollusk, and 1 species nemathelminthes. The average density was 2062inds/m2, the average biomass 254 g/m2.For horizontal distribution of average density of zoobenthos, the tiptop was in weishui reservoir（5037inds/m2）, then the Lotus lake（720 g/m2），and west lake（221inds/m2）being the lowest. For biomass, the highest value was in Lotus lake（720 g/m2）,then the weishui reservoir（39.80 g/m 2）,then west lake（1.19 g/m2）.For seasonal variation, density and biomass in November and December were the highest, which were relatively lower from June to September. The typical indicator species in eutrophication water, Limnodrilus hoffmeisteri clapareda, Tanypus punctipennis, and Tokunagayusurika akamusi were predominant species.
Twenty seven species of macrophytes were found. There were two dominant species in weishui reservoir, Potamogeton crispus linn and Myriophyllum spicatum , no emergent plants were found. In west lake, there was a few emergent plants, no submergent plants appeared. Submergent plant Potamogeton crispus linn existed in Lotus lake.
Six orders 13 families 26 genera 27 species of fish were collected. Among them, Silver carp and big head carp possessed the most biomass, which were stocked into the reservoir by the reservoir administrative department to treat the eutrophication problem. However, the problem of continuous fish farming in Guanting reservior needs further discussion.
Results of different physiochemical index evaluation and biology assessement were compared. Results of combining phytoplankton,rotifer,zoobenthos, density and biomass of phytoplankton, composition of zooplankton and zoobenthos were more consistent with water chemistry assessment. Evaluation of species diversity index were not often ideal.
Ecological system healthy assessment was employed for the first time in Guanting reservoir, which included 3 elements, covering 11 specific index. The reservoir was evaluated as in the state of sub-health assessed by analytic hierarchy process and fuzzy mathematics comprehensive assessment. In general, the healthy status of ecological system in Guanting reservoir was declining, due to the decrease of the water quantity and the acuity of eutrophication.
The main problem for water quality in Guanting reservoir was the unattainment of N、P and cyanobacterial bloom, which was due to external and internal pollution, small water quantity and lack of water exchange. Overall water quality recovery administrative strategies were suggested.|