Species composition, abundance and distribution of phytoplankton in the thermocline layer in the South China Sea, Area IV: Vietnamese waters

Abstract

One hundred and eighty-five samples of phytoplankton were collected from 58 stations in the Vietnamese waters during 30 April – 29 May 1999 to investigated species composition, abundance and distribution in the thermocline layer compared with those in the surface layer. The samples were taken from surface, thermocline depth and chlorophyll maximum depth. In this study, thermocline depth and chlorophyll maximum depth were included in the thermocline layer. Three hundred and fifty- seven taxa, composed of 2 species of blue green alga, 159 species of diatoms and 161 species of dinoflagellates, were identified. The occurrence of species in each layer were recorded. The cell densities at chlorophyll maximum depth were highest among the sampling depths observed in most stations of the survey. Data obtained from the samples collected from surface to 150 m with 25 m interval to study vertical distribution indicates that maximum cell density of the water column presented between 60 – 110 m. The most abundance was 179,386 cells/l found at chlorophyll maximum depth near the Ca Mau Cape due to the bloom of many diatom species. Oscillatoria ( Trichodesmium ) erythraea, Proboscia alata, Pseudosolenia calcar-avis and Thalassionema frauenfeldii were dominant at all sampling depths. Five species of diatoms presented as dominant species only in the thermocline layer. Alexandrium was frequently found at surface through chlorophyll maximum depth in the south. Diversity and evenness indices of all sampling depths were high in the Gulf of Tonkin and decreased through the south of Vietnam. These indices were high in the thermocline layer.