Diel Vertical Migration Of Metozoan Zooplankton In The South Branch Of Yangtze Estuary

Estuary is a special section of water,located at the intersection of runoff and tidal current and have the characteristics of freshwater and seawater ecosystems.As an important water source in Shanghai,the Yangtze Estuary has many ecological functions and has become an important support for the development of Shanghai into a world-class city.Hence,the use and protection of water resources have drawn increasing attention.Chongming Island is located to the north of the Yangtze Estuary and is expected to become a world-class ecological island with globally recognized exemplary roles.In recent years,numerous studies have reported the presence of zooplankton in the Yangtze Estuary.However,most of studies focus on community structure and dynamics,the diel vertical migration of zooplankton in the Yangtze Estuary has been seldom studied.Using small-scale sampling,long-term sampling was performed from surface,middle,and bottom layers over a period of 1 year to analyze the migration patterns and types of crustacean zooplankton and their relationship with environmental factors in the Yangtze Estuary.A theoretical basis is provided for the use and protection of water resources in the Yangtze Estuary and for the construction of ecological Chongming Island.The main results are as follows:1.A total of 64 species of zooplankton were identified from June 2017 to May2018 in the Yangtze Estuary,belonging to 36 genera and 22 families.Copepod species were largest in three water layers.The number of rotifer and copepod species decreased from the surface to bottom,while cladocera was largest in the middle layer,followed by surface and bottom.The species similarity index for three water layers was similar in the middle.According to the seasonal variation of average biological density in three water layers,the cladocera density in surface water was significantly higher than that of other water layers in spring and autumn,the middle water density was higher than that of the surface and bottom in summer,and the bottom density was significantly higher than that of the surface and middle in autumn.The copepod density in surface water was significantly higher than that of other water layers in spring and autumn,the middle water density was higher than that of the surface and bottom in winter.The Shannon-Wiener diversity index?H'?,the Pielous evenness index?J'?and the Margalef's richness index?D?ranged from 0.37 to 1.42,0.54 to 1.00,0.75 to 1.55,respectively.The average annual of H',J',D were 0.84±0.26,0.83±0.14,1.19±0.25,respectively.The values of H'and D were highest in surface water,followed by the middle and bottom,which was contrary to J'.According to seasonal variation of biological diversity index,the values of H',J'and D in surface and middle waters peaked in summer and they were lower in winter,the values of D in bottom water peaked in autumn and they were lower in spring.2.There were 10 dominant species of cladocerans in the Yangtze Estuary,among which Diaphanosoma leuchtenbergianum,Moina macrocopa and Bosmina fatalis were mainly distributed in surface layer,Daphnia cucullata and Bosmina coregoni were mainly distributed in middle layer,Moina affinis,D.galeata and Chydorus sphaericus were mainly distributed in bottom layer.The WMD values for the total density of cladoceran in the Yangtze Estuary ranged from 2 to 12 m,and were higher in spring and summer than in autumn and winter,and were higher in the day than at night.In spring,the organisms descended to the bottom layer in the morning and ascended to the surface layer in the afternoon.In summer,the organisms ascended in the morning and descended at night with obvious vertical migration in the afternoon.In autumn,the organisms descended at dusk and ascended at night with no obvious vertical migration in the day.In winter,the organisms vertically migrated with no obvious migratory pattern.In term of migratory types of the dominant species,Chydorus sphaericus,D.galeata and Daphnia carinata descended to the bottom layer in the morning and at night and ascended to the surface layer in the afternoon and early morning.Moina affinis and Bosmina coregoni descended in the day and morning and ascended at dusk.Bosmina fatalis descended in the day and ascended at night.Daphnia cucullata descended in the morning and ascended in the afternoon with no obvious vertical migration at night.Bosmina longirostris,Moina macrocopa,D.leuchtenbergianum had no obvious migratory behaviors.3.There were 7 dominant species of copepods in the Yangtze Estuary,among which S.dorrii,Eucyclops serrulatus and Eucylops macruroides were mainly distributed in surface layer,M.leuckarti was mainly distributed in middle layer,Cyclops vicinus was mainly distributed in bottom layer.The WMD values for the total density of copepods in the Yangtze Estuary ranged from 4 to 10 m,and were higher in spring than that of other seasons,and were higher at night than in the day.In spring,the organisms ascended to surface layer in the day and at night and descended to bottom layer at dusk and in the morning.In summer,the organisms descended in the afternoon and at night and ascended at dusk with no obvious migratory behavior in the morning.In autumn,the organisms descended at dusk and in the morning and ascended at night with no obvious migratory behaviors.In winter,the organisms ascended at midnight and descended in the morning with no obvious migratory behaviors in other times.In term of migratory types of the dominant species,the DVM patterns of S.dorrii and S.inopibus showed a typical vertical migration pattern of descending in the day and ascending at night.Cyclops vicinus descended in the day and at midnight and ascended at night and in the morning.M.leuckarti descended in the morning and ascended in the afternoon with obvious migratory behaviors at night.S.forbesi had no obvious migratory behaviors.The migratory behaviors of E.serrulatus and E.macruroides had no obvious pattern.4.Biological and abiotic factors,including individual size,algae ingestion and predators,affected the diurnal vertical migration of crustaceans in the Yangtze Estuary.Among the abiotic factors,transparency of water,dissolved oxygen,water temperature and total dissolved solids influenced the DVM of crustaceans,whereas p H,electrical conductivity,TN,TP and COD_Mndid have little influence.