Eutrophication Evaluation And Plankton Distrubution Research Of Hedi Reservoir

Hedi reservoir was built in 1958 in order to supply the water demands of agriculture irrigation. Also it is used for generating hydroelectricity, drinking water supply and tourism. It is the largest reservoir in southwest coastal area of Guangdong province. The watershed land-use in this area was dominated by agriculture. The pollution generated from industry, agriculture and urban city becomes more serious in recent years and has resulted in the deterioration of water quality in Hedi reservoir.The trophic state and phytoplankton distribution were investigated in five stations of Hedi reservoir in February, July, September and December in 2003. The TN, TP, PO₄-P, NO₃-N, NO₂-N, NH₄-N and Ch1. a were measured and the species and phytoplankton abundance are identified by microscope.The characteristic of physical factors, the distribution of nutrients in different stations and seasons, species and abundance of phytoplankton, the dominant species and the diversity index were studied. The trophic state in Hedi reservoir was evaluated on the basis of TSI and other methods. Suggestion was given on eutrophication monitoring, assessment and prevention in water.The data showed that precipitation and drainage would influence the distribution of nutrients and phytoplankton. There is obvious hyperbolic relationship between precipitation and Secchi Disk depth. The concentration of nutrient decreased gradually from (Shijiao) riverine zone to the dam. Nutrient concentration showed no seasonal distribution pattern as they were influenced mainly by precipitation and water quality of inflow. A total of 88 algal species belonging to seven phyla were observed. The dominant species are Microcystis sp. and Pseudoanabaena sp. in Cyanophyta, and Cyclotella sp. and Melosira sp. in Bacillariophyta. There were higher concentration of chlorophyll a and phytoplankton abundance in the autumn and summer than in the winter and spring. High hyperbolic relationship was found between phytoplankton abundance and chlorophyll a concentration. However, there was no significant co-relationship between nutrients and phytoplankton abundance. TSI_M index and TLI indices showed that Hedi reservoir was mesotrophic, but extreme conditions occurred in spring at Lanshan site (oligo-trophic) and all but Lanshan site in autumn (eutrophic). The eutrophic level was higher in the autumn and summer than in the winter and spring. The environmental capacity of COD_cr, BOD₅, NH₄⁺-N, TP and TN were calculated. Hedi reservoir had high environmental capacity on CODcr and BOD₅, while relatively low capacity on NH₄⁺ - N, TP and TN. As the results, the stress to reduce TP and TN in Hedi reservoir was high.