Studies On Biogenic Elements And The Distribution And Molecular Diversity Of Phytoplankton Resting Cells In Surface Sediments From The Typical Harbors In The Coastal Areas Of East China

Surface sediments were collected from 52 stations of the four typical sea areas in the coastal areas of East China(Dongshan Bay,Funing Bay,Xiangshan Port and Dafeng Port)in this study.Biogenic elements,including total organic carbon(TOC),total nitrogen(TN),total phosphorus(TP),biogenic silicon(BSi),organic matter(OM),and moisture content(MC),were measured.Sources of organic matter were discussed,and environmental contamination was assessed based on the contents of biogenic elements.The purpose of this study is to provide background environmental information of the typical harbors in the coastal areas of East China.In addition,seven to nine sediment samples were selected in each harbor with the total of 30 sediment samples,and dinoflagellate cysts were analyzed qualitatively and quantitatively based on microscopic observations.Eukaryotic organisms and eukaryotic algae were analyzed by metabarcoding V4 region of the 18 S r DNA sequence.Geographical distributions of dinoflagellate cysts and eukaryotic algae were illustrated,and eutrophication levels and potential for harmful algal blooms were discussed.The results of this study will help to understand the environmental status of the typical harbors in the coastal areas of East China,and to provide the basic information for prevention and controlling harmful algal blooms.The main findings of this study are as followings:(1)The concentrations of TOC,TN,TP,BSi,OM,and MC in surface sediments of Dongshan Bay,Funing Bay,Xiangshan Port and Dafeng Port were in the range of 0.35%-1.52%,0.08%-0.32%,0.020%-0.074%,0.28%-1.85%,1.86%-9.53%,and 19.55%-52.55%,respectively.Within the four sea areas,TN and BSi were higher in Funing Bay,TP was higher in Dongshan Bay,and TOC was higher in Xiangshan Port.(2)TOC was mainly originated from aquatic sources such as phytoplankton based on the molar ratios of biogenic elements.The decomposition and regeneration rates of TOC were much higher than that of BSi.According to the pollution assessment of biogenic elements,Dongshan Bay has been more seriously contaminated,and TN,TP,and TOC reached the Class II pollution standard.TN and TOC in Funing Bay and Xiangshan Port belonged to Class II pollution.Dafeng Bay was slightly contaminated,and only TN exceeded the Class II standard.(3)A total of twenty-one taxa of dinoflagellate cysts were observed under microscopic observation,including seven in Gonyaulacoid group,three in Calcoidinellid group,one in Tuberculodinioid group,three in Gymnodinioid group,and seven in Protoperidinioid group.The species richness of dinoflagellate cysts were 17,19,21,and 20 taxa in Dongshan Bay,Funing Bay,Xiangshan Port,and Dafeng Port,respectively.The abundances of dinoflagellate cysts were low in this study in the range of 48.4-220.1 cysts/D Wt,and the average abundances were 102.0,161.9,96.1,and 73.6 cysts/D Wt in the four sea areas,respectively.Cysts in Protoperidinioid group dominated almost in all samples.(4)Seven taxa of cysts of harmful/toxic dinoflagellate species were observed in this study,including Gonyaulax spinifera,Protoceratium reticulatum,Gyrodinium instriatum,Pheopolykrikos hartmannii,Scrippsiella trochoidea,and two species in Alexandrium.Proportions of cysts of heterotrophic dinoflagellates were higher,with the averages between67.23% and 78.76%.Meanwhile,G:P ratios were lower,between 0.02-0.85.The results suggested the high nutrient levels and primary productivity in the study sea areas.(5)Based on metabarcoding V4 region of 18 S r DNA sequence,1,472,530 DNA sequences and 3,896 OTUs of eukaryotic organisms,including 437,950 sequences and 389 OTUs in 18 classes of 6 phyla of eukaryotic algae were recorded in thirty sediment samples in this study.The sequence abundances and OTU richness of eukaryotic algae were in the order of Funing Bay,Xiangshan Port,Dongshan Bay,and Dafeng Port.Dinophyceae was the first dominant group of eukaryotic algae in Dongshan Bay,Funing Bay and Xiangshan Port.While,Chrysophyceae was the most abundant group in Dafeng Port.(6)Geographical distributions of sequence abundance and OTU richness of eukaryotic algae and sequence of dinoflagellates showed similar pattern,which were higher in offshore area and lower in the coastal area in Dongshan Bay,and decreased from the northwest to the southeast in Funing Bay,and higher in the top bay and lower in the bottom bay in Xiangshan Port.The sequence abundances of eukaryotic algae and the dominant group Chrysophyceae were higher in the west and lower in the east in Dafeng Port,while OTU richness demonstrated by higher in the south and lower in the north.(7)A total of twenty-eight harmful/toxic algal species were recorded in this study based onmetabarcoding,which included seventeen species in Dinophyceae,eight species in Bacillariophyceae,and three species in Raphidophyceae.Within them,many toxic species were detected,including paralytic shellfish poisons(PSP)causative species Gymnodinium impudicum,and Alexandrium catenella,yessotoxins(YTX)causative species Gonyaulax spinifera,and Protoceratium reticulatum,ichthytoxic bloom species Heterosigma akashiwo,Chattonella marina,Fibrocapsa japonica and Heterocapsa neirotundata.These harmful/toxic species mostly distributed in the fishery and coastal areas with intensive human activities.The results suggested the potential risk of the occurrence of harmful algal blooms and algal toxin contamination.Therefore,it is necessary to strengthen management to prevent the occurrences of toxic/harmful algal blooms.(8)Metabarcoding based on high throughput sequencing is a visible and convenient method to study microalgal community,however the accuracy may be affected by many factors,such as the efficiency of extraction of sediment DNA,the differentiation of the diagnostic DNA region,and the coverage at the species level of the database.The results of this study suggested the high diversity of eukaryotic organisms and eukaryotic algae in sediments from the coastal areas of East China.And the diversity of dinoflagellates was much higher than that based on the microscopic observation.Therefore,metabarcoding has the ability to reflect community of eukaryotic algae in the sediment,can be utilized to evaluate the composition of the resting stages of phytoplankton in the sediment,especially for those of the toxic and harmful algal bloom species with more detailed sequence information in the database.However,it would be better to use both methods of metabarcoding and microscopic identification to assess the composition of resting stages in sediments.