Species composition, abundance and distribution of phytoplankton in the thermocline layer in the South China Sea, Area III: Western Philippines

Abstract

Phytoplankton density, taxonomic composition and distribution in the thermocline layer were investigated to compare with those in the surface layer of 31 stations in the western Philippines during 17 April – 9 May 1998. The samples were collected from surface, thermocline depth (the beginning of thermocline), and chlorophyll maximum depth. In this study, thermocline depth and chlorophyll maximum depth were included in the thermocline layer. Three hundred and thirty-eight taxa, composed of 2 species of blue green alga, 144 species of diatoms and 168 species of dinoflagellates were identified. The occurrence of the species in each sampling depth were recorded. Phytoplankton densities at chlorophyll maximum depth were mostly highest among the sampling depths observed. The most abundance was 20,683 cells/l found at thermocline depth of station 24 due to the blooms of many diatom species. Oscillatoria (Trichodesmium) erythraea and Chaetoceros lorenzianus were dominant from surface through chlorophyll maximum depth. Seven species of diatoms presented as the dominant species only in the thermocline layer. The toxic dinoflagellates were found in low cell densities. Diversity and evenness indices of phytoplankton at chlorophyll maximum depth were high.