Characteristics And Changes Of Phytoplankton Community Structure In Litopenaeus Vannamei Aquaculture Ponds And Outer River

Litopenaeus vannamei is the main breed of prawn aquaculture in our country.It has the advantages of strong fertility,strong disease resistance,and short growth cycle,so it can be widely cultivated in the world.However,the accumulation of organic matter in the process of aquaculture will cause endogenous pollution of aquaculture water,and with the increase of aquaculture production,tailwater discharge also has a certain impact on the surrounding water environment.Phytoplankton is the main primary producer of aquatic environment,which can accurately reflect the change of water quality and is often used as indicator organisms to monitor and evaluate water quality.Therefore,in order to understand the dynamic changes of phytoplankton community structure in Litopenaeus vannaeus aquaculture pond and its differences with surrounding water,as well as the correlation between phytoplankton community structure and environmental factors,this study tested the water environmental factors and phytoplankton in the greenhouse ponds,open-air ponds and outer river（water source river）of a litopenaeus Vannaeus aquaculture farm in Fengxian district of Shanghai.The phytoplankton diversity was analyzed,and the water quality of each water body was preliminarily judged by the diversity index evaluation method,and the correlation between environmental factors and phytoplankton community structure was studied.The main results are as follows:1.A total of 47 genera belonging to 7 phylum were detected during the breeding period of the greenhouse ponds（from March to May）,among which,Chlorophyta and bacillariophyta are 24 and 12 genera,respectively.It accounted for 76.6%of the total phytoplankton genera in the greenhouse ponds;The total density of phytoplankton ranged from 6.35×10³ to 4.05×10⁷ind./L,and increased with time;The dominant genera are mainly Bacillariophyta and Chlorophyta,among which chlorella of Bacillariophyta is the absolute dominant genus with the dominance degree of more than 0.80;The variation range of Shannon-winner diversity index（H’）was 0.69～3.62,Pielou evenness index（J）varied from 0.16 to 0.8 and Margalf richness index（D）ranged from0.55 to 2.05 during the survey period.According to the comprehensive analysis of various indexes,the water body of the greenhouse aquaculture ponds is inα-pollution state。The results of correlation analysis showed that the density of phytoplankton was positively correlated with suspended solids（SS）,total nitrogen（TN）and dissolved oxygen（DO）;The density of dominant genera was mainly affected by T.H’index was negatively correlated with SS,TN,and DO.J index was negatively correlated with SS,TN,TP,T,and DO.D index was negatively correlated with AP.2.A total of 67 genera belonging to 7 phylum were detected during the breeding period of the open-air ponds（from May to October）,among which,32 genera belong to Chlorophyta,20 genera belong to Bacillariophyta,and 7 genera belong to Cyanophyta;The total density of phytoplankton ranged from 3.13×10⁵ to3.48×10⁷ind./L,and fluctuated and increased with time;The common dominant genera were Cyclotella,Scenedesmus,Cryptomonas,Microcystis and Nitzschia,and Microcystis of Cyanobacteria exploded in large numbers in the late culture period;The H index varies from 1.25 to 4.00,J index varies from 0.49 to 0.82,and D index varies from 1.07 to 2.43 during the survey of the open pond.Based on the evaluation results of each index,the water quality of the open-air ponds is between light pollution and medium pollution.The correlation analysis showed that there was a significant positive correlation between the density of phytoplankton and TN,TP and chemical oxygen demand（COD）in open-air ponds.COD,TP and TAN are the main environmental factors affecting the density of dominant genera in open-air ponds.H’,D and J indexes were significantly correlated with Cladophora,T and AP.3.A total of 56 genera belonging to 7 phylum were detected during the survey of the outer river（from March to October）,among which,Chlorophyta,Bacillariophyta and Cyanophyta dominate.The total density of phytoplankton ranged from 7.75×10³ to4.91×10⁵ind./L,and the density was stable.The most common dominant genera are Chlorella,Scenedesmus and Cryptomonas,among which Cryptomonas and Chlorella are dominant genera in the whole process and have a greater advantage.H’index ranged from 1.16 to 3.80.The J index ranged from 0.34 to 0.93.D index ranged from 0.44 to2.00.According to the evaluation results of all indexes,the water quality of the outer river is between light pollution and medium pollution.The correlation analysis showed that the density of phytoplankton in the outer river was mainly affected by the total number of rotifers and colonies.RDA results showed that there was no significant correlation between the density of dominant genus and environmental factors.H’index has no significant correlation with all environmental factors,J index has significant correlation with TN,NO₂-N and NO₃-N,and D index has significant correlation with TN,TP,T,NO₂-N,NO₃-N and AP.4.The composition of phytoplankton genera and dominant genera in both greenhouse ponds and open-air ponds were similar to that in the outer river,but the algae density in both greenhouse ponds and open-air ponds was significantly higher than that in the outer river.The water quality driving factors that lead to the difference of phytoplankton structure between cultured water and outer river may be COD,DO,NO₃-N and AP.Water quality factors（TP,TN,DO,etc.）are the main factors affecting phytoplankton phylum density in greenhouse ponds and open-air ponds disturbed by human activities,while the phytoplankton phylum density in the outer river that were not disturbed by culture behavior was mainly affected by biological factors（rotifers and total number of colonies）.The innovation of this paper lies in the comparative analysis of phytoplankton community structure in cultured and non-cultured water bodies.The differences in the structure of phytoplankton community between prawn culture system under artificial intervention and natural ecosystem,as well as the effects of aquaculture production on pond ecosystem and water quality were studied,so as to provide management thinking for green aquaculture of prawn.