Autumn Zooplankton Community Structure In Yangtze River Estuary And Its Adjacent Waters

As the estuary of the Yangtze River,one of the two famous long rivers in China,the Yangtze River Estuary is an ecosystem between freshwater and marine enviroment.With numerous water systems,it flows to the waters off the East China Sea.Due to its unique geographical location,it plays an important role as an ecological barrier like"the kidney of the Earth"and provides strong purification for the waters off the sea.Zooplankton is an important link of material and energy in aquatic system.The study of zooplankton species composition,community structure and environmental control factors in the Yangtze Estuary and its adjacent waters will help to clarify the distribution pattern and change trend of zooplankton in this region.Although there have been some historical studies on zooplankton communities in the Yangtze River Estuary,there is an urgent need for more studies on zooplankton communities with the intensification of global climate change and human activities.In the autumn of 2018,Zooplankton samples were collected from 46 stations by vertical trawling in the Yangtze River Estuary and its adjacent waters.According to the geographical and hydrological characteristics of the Yangtze Estuary,the area is divided into two parts,the coastal area and the estuary area.The former reaches the waters off Qidong of Jiangsu province in the north and Taizhou of Zhejiang Province in the south,starting from 122°E in the east and reaching124°E in the west,including 31 zooplankton sampling stations.The latter mainly consisted of 15stations extending southeast from the north branch of Chongming Island to the sea off Changxing Island.The zooplankton were identified and counted by microscopic examination.The abundance and diversity indexes were calculated and were further analyzed by cluster analysis.The relationship between zooplankton community and environmental factors were explored using redundancy analysis and detrended correspondence analysis.The results would provide scientific data when analyzing the response of the Yangtze River Estuary ecosystem to climate change and human activities.A total of 201 species were identified from 46 stations in the Yangtze River Estuary and its adjacent waters,which were divided into 12 groups.The phyla arthropoda had the largest number of species,accounting for 65.17%.Other phyla with abundant species included Mollusca,Coelenterata,Annelida,etc.A total of 102 species belonging to 5 classes,14 orders,47 families,62 genera were identified in the phylum Arthropoda,among which copepods accounted for the most,accounting for about 63%of Arthropoda.Mollusca phylum,a total of 18 species,belonging to 3 classes,1 order,7 families,9 genera;A total of 14 species of Annelida were identified,belonging to 1 class,2 subclasses,2 orders,8 families and 9 genera.There was a great difference between the number of species in the coastal area and the estuary areas.However,Arthropods are still the largest group of species in the two regions.The diversity index of zooplankton in the coastal area of the Yangtze River Estuary showed the obvious patchy horizontal distribution.And the biodiversity index of plaque distribution,species,species richness index,Shannon-Wiener information index increased gradually from northwest to southeast trend,embodied in the regional distribution of high value in Taizhou sea area,and low regional distribution in Qidong and Chongming offshore,but in the low-value area scattered with A2-6 high value position.The changes of other related indexes were not obvious,but large or small values appeared near stations A1-2,A7-3,A10-7 and A11-5,which fully showed the characteristics of patchy distribution.The planar distribution of most environmental factors in the coastal area of the Yangtze River Estuary presented the strong spatial heterogeneity.The concentration of salinity gradually increased from west to east.Similarly,the distribution of temperature and p H also showed this pattern,but there were patches.The concentrations of nitrate,silicate and phosphate decreased gradually from west to east,but the patchiness of phosphate was more intense.Most of the other environmental factors showed the spatial heterogeneity.The heterogeneity of environmental factors may be related to the unique hydrology changes of the coastal environment of the Yangtze River Estuary,and the environmental heterogeneity affects the zooplankton diversity index.There were 9 dominant zooplankton species in the sea area of the Yangtze River Estuary,including Paracalanus aculeatus,Paracalanus sp.copepodite,Parvocalanus crassirostris,Euchaeta Concinna,Oithona Brevicornis,Oithona similis,Oithona simplex,Oncaea Media,Microsetella norvegica;The highest abundance was 2.23×10~3 ind./m~3,and the other abundance ranged from 4.40×10~2 ind./m~3 to 1.49×10~3 ind./m~3.Based on the redundancy analysis of environmental factors for the dominant species in this region,three environmental factors were found to have a great impact on the dominant species,namely silicate,phosphate and nitrate.Based on the number and abundance of species,the 31 stations were grouped into 9 groups,and the dominant species could be grouped into 13 groups.Therefore,the dominant species could not completely replace all species,and the A8-5 station was separated separately from the two clusters.In the two clustering methods,species number and abundance showed the negative correlation at each station.The biodiversity in the estuary area of the Yangtze River Estuary was relatively simple compared with that sea area,but patchy distribution was still the main feature.The distribution of species number,richness index,Shannon-Weiner information index and evenness were similar,decreasing first and then increasing from northwest to southeast.The high values were concentrated in the corresponding waters of stations C1 and C2,while the low values were distributed in the corresponding waters of stations B5,B6 and B7.Patchy distribution was obvious,mainly in the east and west waters of Changxing Island.The distribution of abundance and purity in this direction increased first and then decreased.The high values were mainly distributed in the waters near B5 station,while the low values were in the waters near B1 and B2.Numerical patches were still obvious,and mainly distributed in northwest and southeast waters of Changxing Island.The planar distribution of environmental factors in the area inside the Yangtze River Estuary presented spatial heterogeneity.Salinity and temperature all presented gradient distribution and the distribution trend was opposite.The concentration of dissolved oxygen,nitrite and phosphate gradually increased from northwest to southeast,but the patchiness was serious.The other factors showed no obvious rules,and the overall distribution showed strong spatial heterogeneity.The nutrient transport in runoff in the estuary area of the Yangtze River Estuary was related to the effect of the open sea,and the heterogeneity of environmental factors led to the distribution of zooplankton patches.The dominant species in estuary area were less than those in the sea area,and there were 6dominant species,which were as follows:Sinocalanus sinensis,Mesocyclops leuckarti,Tisbe furcata,Pseudodiaptomus forbesi,Pseudodiaptomus sp.copepodite,and S.sinensis had the highest abundance at all stations,with a maximum of 1.64×10~2 ind./m~3 and an average of0.61×102 ind./m3.The maximum abundance of other daphnia was 0.15×10~2 ind./m~3-1.53×10~2ind./m~3,and the mean was 0.07×10~2 ind./m~3-0.03×10~2 ind./m~3.The redundancy analysis of 6dominant species and environmental factors showed that chlorophyllα,temperature and phosphate were the main environmental factors affecting the distribution of dominant species,so it was concluded that phosphate was the common environmental factor that had great influence on the distribution of dominant species in the two regions.Based on the clustering analysis of species number and abundance,the stations in this region can be divided into five groups,and the dominant species can be grouped into six groups.In both regions,C8 station was classified as a single category,but the clustering results of the two stations were different,indicating that dominant species could not completely replace all species,but were more aggregated in clustering than those outside the mouth.However,species number and abundance at each station were mostly negatively correlated.Environmental factors of transition transect had distinct effects on species number and abundance.The species composition and abundance of two ecotone transect lines connecting the estuary area and the sea area of the Yangtze River Estuary were correlated with 10 environmental factors.It was found that the distribution of zooplankton from the estuary area to the coastal area was affected by environmental factors.On the two profiles,the variation trend of salinity distribution was very obvious,and the concentration increased gradually with the passage to the offshore sea area.The opposite trend was observed for nitrate,phosphate and silicate;Most of the other salts displayed a patchy distribution trend.The species composition and abundance of zooplankton gradually increased from the estuary to the outside of the mouth.Therefore,the concentration of nitrate,silicate,phosphate and chlorophyllαgradually decreased as the species restriction decreased,and the diversity and abundance of zooplankton gradually increased.It is believed that this is due to a trade-off between phytoplankton and zooplankton.The gradual increase of temperature and salinity promoted the emergence of more exotic species adapted to high temperature and high salinity.In conclusion,the abundance of zooplankton in the Yangtze River Estuary and its adjacent waters was high in autumn 2018.The coastal area had more species richness than than the estuary area,Arthropods accounted for the largest proportion in both areas.The clustering of all species or dominant species in the two regions was different,and the dominant species could not completely reflect the clustering of all species.The characteristics of zooplankton and environmental factors in the plane distribution of the two regions showed the strong spatial heterogeneity.In the process of transition from estuary area to sea area,the species diversity was related to seven environmental factors including salinity,temperature,nitrate,nitrite,phosphate,silicate and chlorophyllαconcentration.The distribution of zooplankton in the Yangtze River estuary and its adjacent waters was determined by multiple environmental factors,and the modes of action of environmental factors were not identical.This study not only updated the basic zooplankton data of the Yangtze Estuary in autumn,but also provided a theoretical basis for the assessment of fishery economic development,hydrological environment and climate change in the region.However,a year-round,higher temporal resolution survey of zooplankton,the important role of zooplankton in the nutrition process,and the spatial coupling between the sediment resting eggs and the water layer zooplankton are needed for the future studies of assessing the spatial and temporal distribution of zooplankton.