Study Of Current State And Fish Culture Manipulation On Nanwan Reservoir Eutrophication

Nanwan reservoir locates southwest of Xinyang city, the maximum storage capacityof Nanwan reservoir is 16.3×10⁸m³, and there is 5.33×10³ hectare water surface can beused for aquaculture, which was one of the first group backbone works of the HuaiheRiver constructed after the Founding of the People's Republic of China. Except forirrigation, this reservoir has other functions such as preventing and controlling flood,generating electricity, aquaculture and tour. So it is very important for the developmentof Xinyang. Furthermore Nanwan reservoir undertakes the important task to supplydomestic water to the 30,0000 residents in the city. So the water quality of Nanwanreservoir is closely linked to the living and health of the residents. For the past few years,along with the fast development of industry, tourism and the coastwise tea and chestnutplant production, vast amount of organic maters and nutrient salts come into the waterbody, thus the water quality was affected seriously. Up to now there was no systemicstudy on the eutrophic status of Nanwan reservoir. So investigating the eutrophic statusand analyzing its course of formation have great significance to the improvement of thereservoir and insuring the safety of drinking water. In order to evaluate the eutrophicationstate of Nanwan reservoir, the physical and chemical indexes of the water body, speciescomposition and biomass of phytoplankton in Nanwan reservoir were investigated.Modified Carlson index (TSI_M), synthetic trophic level .index, species diversity indexesand dominant species of phytoplankton in Nanwan reservoir were adopted in theeutrophication assessment. The effects of changing the ratio between silver carp and bighead carp put in the reservoir on water quality and plankton were also investigated.1. Physical and chemical properties and eutrophication state assessment of waterbody in Nanwan reservoirWater quality indexes including chlorophyll a concentration, total nitrogenconcentration, total phosphorous concentration, dissolved oxygen concentration, BOD,COD_Mn, pH, temperature and transparency etc. were measured in this study. Themodified Carlson index (TSI_M) of Nanwan reservoir was 59.03, indicating that the waterbody arrived eutrophication. The synthetic trophic level index of Nanwan reservoir was54.75, indicating that the water body arrived light eutropher.2. Phytoplankton community composition and assessment in Nanwan reservoir The biomass and species composition of phytoplankton in Nanwan reservoir wereinvestigated, subsequently the species diversity indexes of phytoplankton were calculatedto evaluate the eutrophication status of Nanwan reservoir from biologic aspect. Theaverage phytoplankton biomass of Nanwan reservoir was 4.337 mg/L. The dominantalgae of Nanwan reservoir was Microcystis which was a genus of algae indicating watereutrophication. The Shannon-Weiner index of Nanwan reservoir was 3.45, the Pielouindex was 1.15, and the Simpson's index was 0.84, respectively. Evaluation based on thespecies composition, dominant species, biomass and species diversity index ofphytoplankton indicated that the trophic state of Nanwan reservoir was betweenmesotrophication and eutrophication.3. Effects of regulating fish raising pattern on water quality and plankton ofNanwan reservoirInvestigation was done to study the effect of changing the ratio of silver carp tobighead carp from 20: 80 to 40: 60(the biomass of silver carp was increased from4.373g/m³ to 8.747g/m³, the biomass of bighead carp was decreased from 17.494g/m³ to13.120g/m³). The results indicated that changing the silver carp to bighead carp ratioreduced the concentration of COD_Mn, BOD₅ and chlorophyll a and increased the waterbody transparency significantly, thus improved water quality on a certain degree.Therefore the eutrophycation status of the water body showed a reducing trend.Furthermore, although the regulation had little effect on the density and biomass ofzooplankton, it reduced the density and biomass of phytoplankton significantly. Also, theregulation reduced the ratio of Cyanophyta in the algae colony and its biomass greatlyand avoided the breakout of the water bloom of these alages.