Application Of Physicochemical And Plankton Index In Water Quality Evaluation Of Eriocheir Sinesis Ecological Culture Ponds In Gehu Lake Area Of Yixing City

Crab（Eriocheir sinensis）is one of the famous and special aquaculture species in our country.It is widely concerned because of its delicious taste and rich nutrition.In recent years,the ecological culture model of crab based on the theory of"strengthening decomposers,promoting producers,and reforming consumers"has shown certain effects in purifying water quality and protecting environment.However,due to the concept of unilateral pursuit of economic benefits,there are still problems in the farming industry of crab,such as excess of nutrients,discharge of toxic and harmful substances,and damage to the surrounding water environment.Gehu Lake is the second largest freshwater lake in southern Jiangsu Province,which belongs to Taihu Lake Basin.Since 2005,the government had taken great efforts to remove the net enclosure,and surrounding areas gradually transformed the net enclosure culture into pond culture,meanwhile explored culture mode which is suitable for local development.Among them,areas around Gehu Lake in Yixing City developed the ecological culture model of crab,which had the characteristics of"high density,high input and high yield".The high financial rewards brought a large number of residual baits,metabolites and drugs.If the crab pond water was polluted,it could be discharged into the outside waters through water exchange,and finally flew into Gehu Lake,polluting Gehu Lake water.These water pollution problems had put forward an urgent demand for monitoring and managing the water quality of crab culture ponds.Water quality evaluation is a kind of water quality monitoring method that describes the quality and utilization value of water quantitatively or qualitatively according to the evaluation index,evaluation standard and evaluation method,which usually uses the physical and chemical index or biological index as basis.In order to explore the water quality of crab ecological culture ponds in Gehu Lake area of Yixing City and its influence in surrounding water environment,this study took the crab ecological culture ponds and water source in Gehu Fish Farm,Gaocheng Town,Yixing City as the research objects,collected water and plankton samples regularly,obtained and analyzed relevant physicochemical index and biological index.According to related water quality evaluation standards,gray correlation analysis,plankton diversity index method and phytoplankton functional group method was used to analyze and evaluate the quality of ponds and water source,and the results were compared to evaluate the ecological benefits of crab culture,in order to make the development of crab ecological culture more sustainable.1.Physicochemical characteristics of water in Eriocheir sinesis ecological culture ponds in Gehu Lake area of Yixing CityWater quality monitoring had been conducted in three ponds and water source at Gehu Fish Farm,Gaocheng Town,Yixing City,Jiangsu Province.Water quality was evaluated using the method of grey correlation analysis.The results showed that throughout the farming cycle,the differences of T,DO and p H between ponds and water source were not significant.Levels of TN,NH₃-N,NO₂^--N and NO₃^--N of pond waters were generally lower than those in the water source,and levels of TP,PO₄^3--P and COD_Mn of ponds were generally higher than those in the water source.Water quality can be improved in farming ponds due to the absorption and transformation of various nitrogen compounds by water plants.2.Characteristics of phytoplankton functional groups of Eriocheir sinesis ecological culture ponds in Gehu Lake area of Yixing CitySamples were taken from three ponds and water source,then analysis was conducted at Gehu Fish Farm,Gaocheng Town,Yixing City,Jiangsu Province.The results showed that 113 species of phytoplankton from 7 phyla were observed in 3 ponds and water source,and the average density and biomass of phytoplankton in ponds were higher than those in water source.There were a total of 11 dominant phytoplankton species in ponds,which belonged to eight functional groups of W₀,X1,J,B,D,MP,M and L₀.The suitable habitats of these functional groups were mainly eutrophic or turgid shallow water bodies,indicating that the ponds were in a state of eutrophication.RDA sequencing analysis pointed out that T,DO and COD_Mn affected the dominant functional groups of phytoplankton in ponds most.3.Structural characteristics of metazooplankton community of Eriocheir sinesis ecological culture ponds in Gehu Lake area of Yixing CitySamples were taken from three ponds and water source,then species identification and quantitative analysis of the metazooplankton were conducted.We used the biodiversity index to evaluate the water quality.A total of 49 metazooplankton species were found,including 22 species of Rotifera,15 species of Cladocera,and 12 species of Copepoda.Brachionus urceus and Brachionus calyciflorus of Rotifera,Ceriodaphnia setosa,Ceriodaphnia pulchella and Moina rectirostris of Cladocera,Nauplii,Microcyclops varicans and Ectocyclops phaleratus of Copepoda are dominant species in crab culture ponds.Rotifera occupy the largest density,but the biomass is the smallest,while the density of Cladocera is the smallest,but they occupy the largest biomass,followed by the density and biomass of Copepoda.RDA ordination analysis shows that T,TN,PO₄^3--P and COD_Mnare the main influencing factors of the metazooplankton community structure in ponds.4.Evaluation of water quality of Eriocheir sinesis ecological culture ponds in Gehu Lake area of Yixing CityAccording to the physical and chemical indicators,the frequencies that the water quality grade was better than the class III of water quality standard in ponds and water source were 76.7%and 50%respectively.The main indicators which were higher than the class III of water quality standard in three ponds and water source were both TN,TP and COD_Mn.According to the phytoplankton indicators,both ponds and water source were inthe state of light-medium pollution,and the water quality in the early and late stages of the ponds was better than that of the water source.According to the metazooplankton indicators,both ponds and water source were in the state of medium pollution,and the water quality of ponds was not significantly better than that of water source.According to the results of water quality evaluation based on physicochemical and plankton indicators,it could be seen that the crab culture mode in this region was prone to the deterioration of water quality after the middle stage of breeding.Therefore,it was suggested to pay attention to the regulation and management of water quality after the middle stage of breeding,and explore the suitable stocking density for this region.At the same time,the use of single water quality evaluation method may be inaccurate,if the physicochemical and biological evaluation method were combined,the real situation of water quality could be better reflected.