| Picoplankton(varying from 0.2 to 3μm) including pico-phytoplankton and pico-zooplankton are widely distributed in the oceans all over the world. They are important portions of plankton and play an crucial role in the maintaining of stability of ecosystem in the ocean and the carbon flow and energy flow. Studying of the abundance and diversity of picoplankton in different locations and environment will help in the understanding of the rules of the distribution and the ecological status of picoplankton. And the knowledge of abundance and diversity of picoplankton will help to make a solid foundation for studying the impact of human activities on biological diversity and provide reference data and theoretical foundation for environmental assessment.The North Yellow Sea is a semi-closed sea located among Shandong peninsula, Liaodong peninsula and Chaoxian peninsula. It represents an important type of marine ecosystem of the north temperate zone. There are a few reports about the study of abundance of picoplankton in the North Yellow Sea using FEM. In the present paper, we studied the abundance of picoplankton in the North Yellow Sea by flow cytometry. Two different types of picoplankton( Syn and Euk) were identified in the North Yellow Sea in autumn, but Pro was not detected. Syn abundance ranged from 1.57×103 particles mL-1～2.06×105 particles mL-1 (2.16×104particles mL-1 on average). Euk abundance ranged from 5.25×102 particles mL-1～2.88×104 particles mL-1 (4.59×103particles mL-1 on average). In horizontal distribution, the area of the high Syn abundance exists around the northeast of the North Yellow Sea; the area of the low Syn abundance exists around coastal water. The area of the low Euk abundance exists in the center of the studied area. In vertical distribution, the abundance of Syn in the surface layer and 10m layer is higher than the 30m layer and the bottom layer, and the abundance of Euk in the surface and 10m layers is higher than the 20m, 30m and the bottom layer (P﹤0.01). Daily variations of Syn and Euk have their own characters. The Syn abundance changed greatly in the bottom layer. The highest and lowest Syn abundance both exists in this layer while the maximum is 6 times larger than the minimum. The Syn abundance in other layers does not change greatly. The Euk abundances in the surface and 10m layer both reach the maximum at 16:00.The microeukaryotes in the major waters in the world are diverse. But there is not any reports about the study of microeukaryotic diversity in the North Yellow Sea. In the present paper, we studied the microeukaryotic diversity in the North Yellow Sea by Amplified Ribosomal DNA Restrictive Analysis (ARDRA ) using 18S rDNA gene. 579 positive clones were digested by HaeⅢand 103 Operational Taxonomical Unit (OTUs) were obtained at the sites of B107 and C105, which shows that the microeukaryotes in this area are diverse. After the phylogenetic analysis by Mega 4, the 103 OTUs are found to distribute in Dinoflagellates, Prasinophyceae, Ciliophora, Stramenopiles and uncultured marine microeukaryotes. The distribtion of clones and OTUs in each group of microeukaryotes obtained at the two sites showed the same pattern. The microeukaryotic groups are arranged from the most abundant and diverse to the least abundant and diverse as: uncultured marine microeukaryotes , Ciliophora, Prasinophyceae, Stramenopiles and Dinoflagellates. And the numbers of clones and OTUs of uncultured marine microeukaryotes are more than 50% of the total numbers. The results show that these five groups are the main groups of microeuaryotes in the North Yellow Sea.
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