Characteristics Of The Water Environment And Plankton Community Structure In Ponds For Juvenile Carbs Culture Under Sichuan Basin Environmental Conditions

The Chinese mitten crab（Eriocheir sinensis）,also known as the river crab,has become an important branch of freshwater aquaculture in China because of its high economic value and the expanding area and scope of its farming.Kaijiang is located in the Sichuan basin region,and the river crab farming in this region started late,and there is a relative lack of experience in river crab farming compared with the main river crab production areas such as Anhui and Jiangsu,and a lack of research reports related to river crab farming.Therefore,in order to promote the development of river crab farming in this region in an ecological and healthy direction,it is necessary to study the water environment and plankton community structure of river crab farming.To study the characteristics of changes in water environment factors and plankton community structure in juvenile crab culture ponds under watershed conditions,11water quality factors such as dissolved oxygen（DO）,water temperature（T）,and p H,as well as the density,biomass,and diversity of phytoplankton and zooplankton,were sampled and analyzed from May to September 2021 in three juvenile crab culture ponds and their water sources at the Kaijiang Minsheng Fishery Development Co.The impact of juvenile crab farming on the aquatic environment has been scientifically assessed with the view to providing basic data for the development of the river crab farming business in the region.1.Impact of juvenile crab culture on the water environment under basin environmental conditionsDuring the whole culture period,the water temperature（T）of the juvenile crab pond water was significantly higher than that of the water source waters（P<0.05）;the contents of electrical conductivity（EC）,nitrite nitrogen（NO₂^--N,）and nitrate nitrogen（NO₃^--N）were significantly lower than that of the water source waters（P<0.05）;the contents of p H,dissolved oxygen（DO）,ammonia nitrogen（NH₄⁺-N）and total nitrogen（TN）were generally lower than that of the water source waters;soluble phosphate(PO₄^3--P),permanganate index(COD_Mn)and total phosphorus（TP）were lower than those of the water source waters in the early stage and higher than those of the water source waters in the middle and late stage.The water quality indicators of the juvenile crab ponds basically comply with the limits of the Class III standard of the Environmental Quality Standard for Surface Water,except for COD_Mn,TN and TP,indicating that there is some pollution of the water environment by juvenile crab farming under basin environmental conditions.2.Characteristics of zooplankton community structure in juvenile crab culture ponds under basin environmental conditionsA total of 65 species of zooplankton（16 dominant species）were found throughout the culture period,including 36 species of Rotifera（9 dominant species）,15 species of Copepoda（2 dominant species）and 14 species of Cladocera（5 dominant species）.The average densities of rotifera,cladocera and copepoda in juvenile crab ponds ranged from 1685.0～25370.0 ind/L,1.7～34.3 ind/L and 57.7～960.3 ind/L,respectively;the average biomass ranged from 1.087～10.261 mg/L,0.022～0.434 mg/L and1.234～13.738 mg/L,respectively;the mean densities of the water source waters ranged from 30～510 ind/L,0～12 ind/L and 2～166 ind/L,respectively;the mean biomass ranged from 0.001～0.132 mg/L,0～0.316 mg/L and 0.006～1.081 mg/L,respectively.The Shannon-Weiner diversity index（H′）,Pielou evenness index（J）and Margalef richness index（D）of juvenile crab ponds ranged from 1.233～1.994,0.451～0.678 and1.478～2.404,respectively;the water source waters ranged from 0.849～2.171,0.474～0.910 and 0.577～2.306,respectively.The results of RDA ranking showed that water body T,DO,TP,TN and COD_Mnwere the main factors affecting the distribution of zooplankton community.The integrated water quality physicochemical index and biodiversity index can classify the juvenile crab culture ponds and water source waters as medium pollution status.3.Structural characteristics of phytoplankton communities in juvenile crab culture ponds under basin environmental conditionsThroughout the culture period,a total of 137 species of phytoplankton in 7 phylum and 66 genus were found,including 79 species of chlorophyta,22 species of bacillariophyta,17 species of cyanophyta,14 species of euglenopghyta,2 species of both cryptophyta and pyrrophyta and 1 species of chrysophyta;a total of 12 dominant species can be classified into 9 functional groups,SN,LM,N,F,J,X1,D,Y and E.The average density of phytoplankton in the water column of juvenile crab ponds ranged from 2.030×10⁶～44.567×10⁶cell/L,with an average biomass of 1.303～36.709 mg/L;the average density of water source waters ranged from 0.096×10⁶～0.978×10⁶cell/L,with an average biomass of 0.143～1.884 mg/L.In juvenile crab the ranges of variation of H′,J and D were 2.189～3.355,0.586～0.963 and 1.531～2.974,respectively;the ranges of variation in the water source waters were 1.954～3.274,0.681～0.848 and0.697～2.823,respectively.The results of redundancy analysis showed that N、P、DO and COD_Mnwere the main factors affecting the phytoplankton community in the water column of juvenile crab ponds.Combined with the water quality physicochemical indicators and biodiversity index,the juvenile crab breeding ponds and water source waters can be classified as lightly-moderately polluted.Combining these three experiments to evaluate the water environment for juvenile crab farming,the results show that there is some pollution of the water environment by juvenile crab farming under basin environmental conditions.It can be considered to plant composite aquatic plants in the subsequent culture,use micro-ecological preparations regularly to get rid of excessive nutrient salts,and purify the culture tailwater after the culture is finished and wait until all water quality indicators meet the standards before discharging to the outside.