Study On The Relationship Between The Community Structure Of Nekton And Environmental Factors In The Southern Water Of The Yangtze Estuary In Spring And Autumn

Under the background of a large number of dredging operations in the Yangtze Estuary Deepwater Channel Regulation Project,to explore the relationship between the community structure of nekton and environmental factors in the southern waters of the Yangtze Estuary,four stations were set up in the North Trough Channel,the North Port Channel and the South Port Channel of the Yangtze Estuary in April(spring)and September(autumn)in 2020,and the fishery resources of two voyages were investigated using single-bag trawls(minimum mesh 25mm),water samples were collected in layers.By comparing changes in number,weight resource density,relative importance index size,alpha-diversity index,use multi-factor regression analysis,canonical correspondence analysis(Canonical Correlation Analysis,CCA)and based on single-factor standard index method and Nemerow pollution index Water quality analysis using the method,in order to explore the spatio-temporal variation of the community and the influence of environmental factors on the distribution of fish and crustaceans,aiming to provide assistance for the construction of ecological impacts of future waterway projects.A total of 49 species of fish were collected in this study,belonging to 16 orders,25 families and 41 genera,all of which are warm-temperate and warm-water.In terms of seasonal changes in resource density,the total density of tails in spring is higher than that in autumn,and the total density of weight in autumn is higher than that in spring.Combined with the growth cycle of fish,it is in line with the characteristics of most fish species hatching in spring and summer and fattening in autumn and winter.Coilia mystus,Coilia nasus,Collichthys lucidus and Miichthys miiuy were the dominant species in both seasons.The results of multivariate regression analysis showed that in spring,the contribution rate of individual resource density to the total density was the highest,and the contribution rate of the individual resource density of various species to the total density was the most significant;The rate is the highest,but the individual resource density of Harpadon nehereus has the most significant impact.In terms of seasonal changes in fish diversity index,the average values of individual diversity index Shannon-Winner diversity index(H’),Pielou evenness index(J’)and Margalef species richness index(d)in autumn are all significantly higher in spring,but the spring Simpson simplicity index(C)was opposite;in terms of weight diversity,the average values of each index had little difference between the two seasons.A total of 30 species of crustaceans were collected,belonging to 2 orders,17 families and 25 genera.In terms of seasonal changes in resource density,the average individual density in spring is nearly 6 times higher than that in autumn,and the total weight density in spring is nearly 2 times higher than in autumn.Portunus trituberculatus,Palaemon annandalei and Palaemon gravieri were the dominant species in both seasons.In terms of the seasonal variation of crustacean diversity index,the individual,weight diversity index H’,J’,and d average values in spring are higher than those in autumn,but C is the opposite.The salinity temperature of the water surface in the study water area is affected by many dynamic factors,and there are large seasonal changes.The planar distribution map and sequence map of resource density reveal that certain groups have the living habits of warming and salting.CCA analysis showed that in terms of the relationship between fish and environmental factors,spring SSS and p H were significant influencing factors,and Lophius litulon,Lepidotrigla japonica,and Pampus argenteus had higher correlations with SSS,SSS and SST in autumn were the significant influencing factors,and Perciformes and Clupeiformes with migratory habits were more sensitive to temperature than other environmental factors.In terms of the relationship between crustaceans and environmental factors,DO and p H in spring and autumn are significant influencing factors,and species such as Palaemon carinicauda,Scylla paramamosain and Eriocheir sinensis are positively correlated with DO in spring;In autumn,there was a positive correlation between p H and SSS for Metapenaeus joyneri and Parapenaeopsis hardwickii.Hydrological and water quality survey contents include p H,dissolved oxygen(DO),chemical oxygen demand(CODMn),inorganic nitrogen(N),active phosphate(calculated as P),suspended matter,oil,Hydrargyrum(Hg),Cuprum(Cu),plumbum (Pb),cadmium(Cd),zinc(Zn),total chromium(Cr),and arsenic(As),a total of 14indicators.Except for p H and DO,the rest were analyzed and tested in the physical and chemical laboratory according to the GB 17378.7-2007 Marine Monitoring Specification.According to the variation range of the single factor standard index,in spring,p H,DO,N,oil,Cu,Zn,As,Hg,Cd,Cr and Pb are all within the standard,and the standard index exceeding rates of inorganic nitrogen and active phosphate are respectively73.91%,4.35%;in autumn,p H,DO,CODMn,active phosphate,oil,Cu,Zn,As,Hg,Cd,Cr and Pb were all within the standard,and the exceeding rate of inorganic nitrogen was45.00%.Based on the water quality evaluation results of the single-factor standard index method,the evaluation standard refers to the GB 3097-1997 Sea Water Quality Standard for comparison,including 23 water samples in spring,17 samples that were not detected or below the detection limit,and 20 water samples in autumn,23 samples were not detected or were below the detection limit,with a total of 497 evaluation results,all of which met the first-class sea water quality standards: p H,DO,Cu,As,Cd,Cr,and N and active phosphate There are generally different degrees of exceeding the standard.In the change of Nemerow pollution index,in the same season,the Nemerow pollution index of the surface water samples was higher than that of the bottom layer,that is,the surface water quality was worse than that of the bottom layer;in the same channel,the water quality in autumn was better than that in spring.Overall,under the current intensity of dredging operations,there has been no dramatic impact on the water quality and ecosystem function of the Yangtze estuary,but with the enhancement of dredging and the continuous development of the waterway,continuous dynamic monitoring will be required in the future to practise the concept of ecological civilisation and green development and to carry out the ecological protection of the Yangtze estuary to the end.