Variation Characteristics Of Water Environment And Plankton Community In Ponds Of Juvenile Mitten Crab Eriocheir Sinensis During Molting

Chinese mitten crab,Eriocheir sinensis,has become a pillar of the aquatic industry in China.The life cycle of Chinese mitten crab is divided into zoea,megalopa,early juvenile crab,juvenile crab,and adult crab.Juvenile E.sinensis farming is the middle link of Eriocheir sinensis farming,that is,megalopa farming into the juvenile crab.This stage is particularly important in the whole farming stage in the link position.One of the key growth points for juvenile E.sinensis is molting.The present study investigated the water environment and plankton community of Eriocheir sinensis farming ponds at the Songjiang District,Shanghai,during every molting period from May to October 2018.Investigation items include T,dissolved oxygen?DO?,p H,electrical conductivity?EC?,permanganate index(COD_Mn),NH₄⁺-N,NO₂^--N,NO₃^--N,PO₄^3--P,total nitrogen?TN?,total phosphorus?TP?,chlorophyll-a?Chl-a?,phytoplankton and zooplankton.To explore the change characteristics of the ponds water environment and the changes of zooplankton and phytoplankton community structure,density,biomass,and diversity during the molting period of juvenile E.sinensis.Through the summary of the experiment,the environmental health of the ponds water in the ecological farming pond of juvenile E.sinensis was evaluated scientifically.To provide practical guidance for improving and popularizing the ecological farming mode of Eriocheir sinensis,the ecological environment investigation of the water body is considered while the juvenile E.sinensis is cultivated.1.Variation characteristics of water environment in ponds of juvenile mitten crab Eriocheir sinensis during moltingResults show that the E.sinensis larva grows up to the juvenile stage after 11 times of molting and 160 days cultivation.In this period,the average temperature was27.0±2.9?,and the total effective accumulated temperature was 3085.3?·d.Along with the increased time of the juvenile E.sinensis molting,the effective accumulated temperature increased exponentially(y=42.166e^0.2644x,R²=0.9629),and the increase of cumulative effective accumulated temperature followed a polynomial equation?y=33.059x²-98.244x+160.05,R²=0.9992?.The concentration of dissolved oxygen?DO?in the juvenile E.sinensis farming ponds was higher than that of source water at the early stage.Overall,the concentrations of NH₄⁺-N,NO₂^--N,and NO₃^--N of the juvenile E.sinensis farming ponds were lower than that of the source water.The concentration of PO₄^3--P of the juvenile E.sinensis farming ponds was lower than that of source water in the high-temperature season.However,the concentration of COD_Mn of the juvenile E.sinensis ponds was higher than that of source water during the whole farming cycle.In addition to TN,TP,and COD_Mn,the water quality index of ponds water and source water are met the fishery water quality standards and the third-grade surface water standard.2.Variation characteristics of zooplankton community in ponds of juvenile mitten crab Eriocheir sinensis during moltingA total of 60 species of zooplankton were identified during the experiment,belonging to 13 families 31 genera:25 species of Rotifera,20 species of Cladocera,15species of Copepoda,and 15 dominant species.The average density of the juvenile E.sinensis farming ponds Rotifera,Cladocera and Copepoda were 68—975 ind/L,30—353.5 ind/L,and 58.5—413 ind/L.The density variation ranges of source water Rotifera,Cladocera,and Copepoda were 45—627 ind/L,18—261 ind/L and 39—333 ind/L.The average biomass ranges of the juvenile E.sinensis farming ponds Rotifera,Cladocera,and Copepoda were 0.012—0.781 mg/L,2.055—24.190 mg/L,3.549—17.871 mg/L.The biomass ranges of source water Rotifera,Cladocera,and Copepoda were 0.007—0.535 mg/L,0.870—19.140 mg/L,and 2.643—13.869 mg/L.The average variation ranges of Shannon-wiener diversity index,Pielou evenness index,and Margalef richness index of zooplankton in juvenile E.sinensis farming ponds were 3.040—4.100,0.708—0.859 and 2.615—5.192.The variation ranges of Shannon-wiener diversity index,Pielou evenness index,and Margalef richness index of source water zooplankton were 3.149—4.154,0.729—0.927,2.963—4.771.Redundancy analysis?RDA?indicated that the sequencing axes 1 and 2 are the most correlated with EC and NO₂^--N,with correlation coefficients of 0.8196 and-0.3555.DO,COD_Mn,and NH₄⁺-N were also important factors.The water quality of the juvenile E.sinensis farming ponds and source water can be classified as the light pollution stage by combining the physicochemical index of water quality and the diversity index pollution degree index of zooplankton.3.Variation characteristics of phytoplankton community in ponds of juvenile mitten crab Eriocheir sinensis during moltingA total of 92 species of phytoplankton were identified during the experiment,belonging to 8 phyla 48 genera:41 species of Chlorophyta,16 species of Bacillariophyta,14 species of Cyanophyta,12 species of Euglenophyta,four species of Pyrrophyta,three species of Cryptophyta,one species of Chrysophyta,one species of Xanthophyta,and 13 dominant species.According to the phytoplankton functional groups classification,the dominant phytoplankton species can be divided into ten functional groups:Lo,TC,H1,C,D,MP,W2,Wo,X1,and J.The average density and biomass of phytoplankton in the juvenile E.sinensis farming ponds ranged from14.659×10⁶—40.221×10⁶ cell·L^-1 and 15.639—35.463 mg·L^-1.The density and biomass of phytoplankton in source water ranged from 11.528×10⁶—29.976×10⁶cell·L^-1 and 11.744—24.097 mg·L^-1.The average variation ranges of Shannon-wiener diversity index,Pielou evenness index,and Margalef richness index of phytoplankton in juvenile E.sinensis farming ponds were 3.674—4.851,0.644—0.799,and 2.879—3.970.The variation ranges of Shannon-wiener diversity index,Pielou evenness index,and Margalef richness index of phytoplankton in source water were 3.647—4.822,0.654—0.810,and 2.395—3.566.Redundancy analysis?RDA?indicated that the sequencing axes 1 and 2 are the most correlated with T and DO,with correlation coefficients of-0.6014 and 0.2407.NH₄⁺-N,NO₂^--N,and PO₄^3--P were also important factors.The water quality of the juvenile E.sinensis farming ponds and source water can be classified as the light pollution to medium pollution stage by combining the physicochemical index of water quality and the diversity index pollution degree index of phytoplankton.The above studies indicated that the water quality indexes in the juvenile E.sinensis ecological farming area were normal,and the plankton community structure was complex.Alternanthera philoxeroides grows well;the density is reasonable;the ecosystem is relatively stable.The current ponds water environment conditions can meet the needs of normal growth and molting of juvenile E.sinensis.