Relationship Between Microalgae Community Structure And Environment In Shellfish Culture Area Of The Yalu River Estuary

In recent years,due to the ecological evolution and overloading of shellfish aquaculture waters,filter-feeding shellfish such as clams,scallops and oysters have generally been found to grow slowly,the quality of their flesh has declined and the species is experiencing increased mortality,which limits the sustainable and healthy development of that fishery.A recent study has found that marine ecological evolution and overloading of shellfish have led to a relative shortage of microalgae,which was the food for shellfish,as a result,shellfish immunity was weakened,leading to diseases and even large-scale death.The Yalu River Estuary is an important breeding bases for Ruditapes philippinarum.In last several years,R.philippinarum in this area generally stagnated and the fatness declined,and large-scale deaths began to appear in some breeding areas.In this paper,high-throughput sequencing identification and visual grading technology were used to analyze the relationship between the phytoplankton community structure and various environmental parameters in the Yalu River Estuary from March to December in 2020,and to study the feeding selectivity of R.philippinarum from March to December 2020 to 2021.The results showed that nanophytoplankton——Karlodinium veneficum,dominated the Yalu River Estuary throughout the year in 2020.From the dominant species of microalgae at each grain level,Ostreococcus tauri and Micromonas pusilla accounted for a high proportion of biomass in the picophytoplankton community.K.veneficum,Cryptomonadales sp.and Thalassiosira nordenskioeldii accounted for a high proportion of biomass in the nanophytoplankton community.The proportion of biomass of Alexandrium affine,Gyrodinium dominans and Thalassiosira curviseriata dominated in microphytoplankton.The biomass ratio of picophytoplankton,nanophytoplankton and microphytoplankton were 20:63:17 in spring,30:44:26 in summer,1:38:61 in autumn and 2:45:53 in winter,respectively.Meanwhile,Dinophyta had the greatest proportion of biomass throughout the year,followed by Bacillariophyta,and the occurrence frequency and the proportion of biomass of Dictyochophyta,Chrysophyta,and Cryptophyta were lower.On the spatial dimension(Station average),chemical oxygen demand,water temperature,salinity had a positive correlated on total phytoplankton communities,and seawater depth had a negative correlated.In the time dimension(Monthly average),the environmental factor that significantly controlled the phytoplankton community structure were nitrite and salinity.The results of the study on the feeding selectivity of R.philippinarum showed that the most dominant species in the Yalu River Estuary and in the stomachs of R.philippinarum was K.veneficum.The selectivity index(E)of microalgae community indicated that R.philippinarum avoided consuming Bacillariophyta,Chrysophyta and Cryptophyta throughout the year and preferentially consumed Dinophyta and Chlorophyta.In 2020,the annual average biomass of Dinophyta,Bacillariophyta,Chlorophyta,Dictyochophyta,Cryptophyta and Chrysophyta in the stomach contents of R.philippinarum was 54:14:16:1:10:4;it was 41:12:28:0:1:17 in 2021.The microalgae size-class selectivity index showed that R.philippinarum actively selected nanophytoplankton and avoided picophytoplankton.The annual average biomass ratio of picophytoplankton,nanophytoplankton and microphytoplankton in the stomachs of R.philippinarum was 13:48:39 in 2020;it was 14:66:20 in 2021.The main purpose of this study was to understand the evolution law of the diet microalgae community structure in the shellfish culture area of the Yalu River Estuary,analyze the environmental factors affecting the temporal and spatial changes of microalgae grain size and the selection preference of the microalgae grain size and groups of R.philippinarum in different periods,and provide countermeasures for the sustainable development of the increased culture of R.philippinarum in the Yalu River Estuary.