SEASONAL VARIATIONS OF PICOPHYTOPLANKTON, NANOPHYTO-PLANKTON AND MICROPHYTOPLANKTON COMMUNITIES AT SICHANG ISLAND, CHONBURI PROVINCE

Sichang Island is in the eastern part of the Inner Gulf of Thailand where is primarily influenced by two monsoon winds, the northeast monsoon and the southwest monsoon. The effect of this factor lead to changes in phytoplankton community. However, most studies have been focused on microphytoplankton community not pico- and nanophytoplankton due to their small size and study methods. Therefore, seasonal variations of pico-, nano-, and microphytoplankton around Sichang Island were observed. The samplings were conducted at 10 stations around Sichang Island in two monsoon and two intermonsoon periods. Pico- and nanophytoplankton were analyzed by flow cytometry whereas microphytoplankton were analyzed by a compound microscope. Phytoplankton abundance, size-frationated of chlorophyll a and inorganic nutrient concentrations were analyzed. The community structure of pico- nano- and microphytoplankton could be divided into four groups dominated with diatoms and Synechhococcus which were related to the main factors; temperature, salinity, and nutrient concentrations which clearly differed between seasons. Pico- and nanophytoplankton were observed as major contributor to phytoplankton biomass in all seasons except in the northeast monsoon where microphytoplankton was dominated. Flow cytometry resolved 4 groups of picophytoplankton and 5 groups of nanophytoplankton. They were high in density during the northwest monsoon in contrast to microphytoplankton density.