Metazooplankton Community Structure And The Influencing Factors In Litopenaeus Vannamei Aquaculture Ponds And Outer Rivers

Litopenaeus vannamei,with its resistance to stress and disease,fast growth rate,convenient transportation of live shrimp,and significant economic benefits of aquaculture,is the main shrimp species cultured in China.The quality of aquaculture water environment directly affects the growth of shrimp.Metazooplankton is an important indicator of water environment quality assessment due to its species composition,dominant species,density,biomass,and other community structure characteristics as determined by water temperature,dissolved oxygen,nitrogen,phosphorus,and other environmental factors sensitive to changes in the water environment.In order to understand the effects of Litopenaeus vannamei culture on the community structure of metazooplankton in ponds,this study was conducted to monitor ater quality indicators,phytoplankton,and metazooplankton communities in a shrimp farm and its outer rivers in Fengxian District,Shanghai in 2021.A comparative analysis of the differences in community structure of metazooplankton in artificial aquaculture ponds and outer rivers around natural water body were compared,and the relationship between the community structure and environmental factors was explored,It is expected to provide scientific basis for the healthy culture of Litopenaeus vannamei.The following conclusions are drawn:1.A comparison of four surveys（March-May）of the Litopenaeus vannamei greenhouse aquaculture ponds and outer rivers revealed that:（1）A total of 28 species of metazooplankton,10 species of dominant species,and 1species of common dominant species,Brachionus calyciflorus,were identified in greenhouse aquaculture ponds and outer rivers.Jaccard similarity analysis shows that the species composition is at a medium level in general,with small rotifers as the main species,and less large cladoceras and copepods.The number of dominant species in greenhouse aquaculture pond is lower than that in outer river,and the density of metazooplankton[（2616.5±553.9）ind./L]and biomass[（10.56±1.88）mg/L]in greenhouse aquaculture ponds were significantly higher than density[（885.8±336.6）ind./L]and biomass[（3.25±0.53）mg/L]in outer rivers.（2）The mean value of Shannon-Wiener biodiversity index（H’）in greenhouse aquaculture ponds and outer rivers was 1.544±0.109 and 1.801±0.116,respectively;The mean value of Gleason-Margalef species richness index（D）was 1.299±0.114 and1.185±0.080,respectively,which showed thatα-Moderate pollution.The comprehensive nutritional status index（TLI）showed that the greenhouse aquaculture ponds and the outer rivers are generally at a moderate eutrophication level.The Suspended Solids（SS）content in greenhouse aquaculture ponds and the density of Cyanophyta,Bacillariophyta and Chrysophyta in phytoplankton were obviously higher than those in outer rivers.The Water Temperature（WT）,Dissolved Oxygen（DO）,Permanganate index（COD）content and the density of Chlorophyta in phytoplankton were greatly higher than those in outer rivers.（3）Pearson correlation analysis showed that the structure of metazooplankton community in greenhouse aquaculture pond was significantly correlated with Total Nitrogen（TN）,NO₂^--N,SS,DO and Phytoplankton（Cryptophyta,Chlorophyta,Bacillariophyta and Euglenophyta）,while the distribution of metazooplankton community in outer river was significantly correlated with WT,p H,Chlorophylla（Chl.a）,Active Phosphorus（AP）,NO₃^--N,NO₂^--N and Phytoplankton（Chlorophyta,Bacillariophyta,Cryptophyta,Euglenophyta and Pyrrophyta）.The redundancy analysis showed that NO₂^--N and NO₃^--N were the key factors affecting the structure of metazooplankton community in greenhouse aquaculture ponds,while Phytoplankton（Cryptoplankton）and NO₃^--N were the main environmental factors affecting the changes of metazooplankton community in outer rivers.2.A comparison of 17 surveys（May-October）of the Litopenaeus vannamei open auqaculture ponds and outer rivers found that:（1）A total of 34 species of metazooplankton,7 species of dominant species and 3species of common dominant species were identified in open aquaculture ponds and outer rivers,including Brachionus calyciflorus,Polyarthra trigla and Mesocyclops leuckarti.Jaccard similarity analysis showed that the species composition was at a medium level in general.The mean value of H’in open aquaculture ponds and outer rivers was1.543±0.065 and 1.553±0.050,respectively,which were shown asα-Moderate pollution;The mean value of D was 0.789±0.045 and 0.749±0.046,respectively,showing severe pollution.（2）In general,the density,biomass,dominant species and abundance index of metazooplankton in open aquaculture ponds are higher than those in outer rivers.TLI showed that the open aquaculture pond was generally at a moderate eutrophication level,while the outer river was at a mild eutrophication level.The contents of TN and TAN,the densities of Phytoplankton（Cyanophyta and Pyrrophyta）in open aquaculture ponds were obviously higher than those in outer rivers,the contents of DO,p H,TP,COD and the densities of Phytoplankton（Chlorophyta,Bacillariophyta,Cryptophyta,Euglenophyta and Chrysophyta）were greatly higher than those in outer rivers.（3）Pearson correlation analysis showed that the structure of metazooplankton community in open aquaculture ponds was significantly correlated with TN,TP,Chl.a,COD and Phytoplankton（Xanthophyta）,while the distribution of metazooplankton community in outer rivers was significantly correlated with Chl.a,DO,SS,NO₃^--N and Phytoplankton（Cryptophyta,Euglenophyta and Cyanophyta）.The redundancy analysis showed that Chl.a and TP were the key factors that affect the structure of metazooplankton community in open aquaculture ponds,while Phytoplankton（Cryptoplankton）,DO and TAN were the main environmental factors that affect the changes of metazooplankton community in outer rivers.3.According to the research results in 2021,the changes in the structure of metazooplankton community in the shrimp ponds of Litopenaeus vannamei,which are disturbed by human activities,are mainly affected by water quality factors,of which the greenhouse aquaculture ponds are mainly affected by NO₂^--N and NO₃^--N,and the open aquaculture ponds are mainly affected by Chl.a and TP.Therefore,the main physical and chemical factors of the aquaculture water body are scientifically controlled to maintain the water quality of the shrimp ponds in a good growth environment,it is conducive to the healthy growth of shrimp,increases shrimp production,and improves economic efficiency.However,the community structure of metazooplankton in outer river around the natural water body that is not disturbed by aquaculture behavior is mainly affected by biological factors（Cryptoplankton in phytoplankton）and water quality factors（DO,NO₃^--N,TAN）.