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Abstract

The surveys were carried out throughout Vietnamese coastal waters, include 4 areas: the Tonkin Gulf coastal waters, the Central coastal waters, the Southeast coastal waters and the Southwest coastal waters. A total of 440 samples were collected from 220 stations during two surveys in the northeast monsoon in 2015, 2016 and 220 samples were collected from 110 stations during one survey in the southwest monsoon in 2017 and one survey in the northeast monsoon in 2018. The results showed that, a total of 548 phytoplankton species were identified, belonging to 128 genera of 81 families, 51 ordes of 7 phyla: Bacillariophyta - 288 species, Pyrrophyta – 217 species, Chlorophyta – 22 species, Cyanobacteria - 14 species, Euglenophyta - 4 species, Silicoflagellata - 2 species and Charophyta - 1 species. The cell abundance of phytoplankton in the northeast monsoon in coastal waters was more in southwest monsoon with the average density of phytoplankton were 142 x 103 cells/l and 48 x 103 cells/l, respectively. In the northeast monsoon, the cell abundance of phytoplankton was maximum in the Tonkin Gulf coastal waters. A bloom of Pseudo-nitzschia spp. group was recorded in coastal waters from Thanh Hoa to Nghe An in the northeast monsoon in 2015 and a bloom of Skeletonema costatum was recorded in coastal waters from Hai Phong to Thanh Hoa in the northeast monsoon in 2018. In the southwest monsoon, the cell abundance of phytoplankton was maximum in the Southeast coastal waters. Some coastal regions were impacted of the discharge of large rivers, aquaculture regions, upwelling regions increased nutritional levels, due to diatoms growed optimally. Diatoms dominated more than 90% density of phytoplankton in coastal waters of Viet Nam.