Factors affecting diversity and succession in phytoplankton communities in the coastal area of Si Racha-Si Chang, Chon Buri province

This study comprises three parts including 1) Diversity and abundance of phytoplankton in relation to the environmental factors; 2) Succession of phytoplankton communities in relation to the environmental factors; and 3) Allelopathic relationship between two dominant phytoplankton. The study site located in the coastal area of Si Racha and Ko Si Chang, Chon Buri province where the first study was conducted from October 2008 to May 2009 in two inter-monsoon seasons (Southwest to Northeast: SW-to-NE and Northeast to Southwest: NE-to-SW) and the Northeast (NE) and the Southwest (SW) monsoons. The results indicated the significant influence of seasonal variaton on environmental parameters affecting phytoplankton diversity and density. The high seawater temperature, dissolved inorganic phosphorus (DIP) and dissolved silicate-silicon (DSi) but low in salinity were observed in the NE-to SW inter-monsoon and the SW monsoon periods. The highest concentratons of DIN, DIP and DSi always found at the stations located in the NE part of Ko Si Chang closed to Bang Pakong river mouth. Consequently, the dominant phytoplankton groups in both inter-monsoons were diatoms while the dominant groups in the NE and the SW monsoons were cyanobacteria with osillatoria contributed to high percentage of total count (84%) in the SW monsoon with the unbalanced DN:DIP ratio = 2:1 (balanced N:P ~ 16.1, Redfield ratio). Furthermore the second study in both the NE monsoon (December 2010 to February 2011) and the SW monsoon (May to October 2011) was conducted. The results showed that there were signifiantly high vaues of DIN, DIN:DIP ratio, DSi and low salinity in the SW monsoon, while t found opposite in the NE monsoon. The dominant groups of phytoplankton in the NE and the SW monsoons were diatoms and cyanobacteria, respetively. It also found that sucession pattern of phytoplankton during the NE monsoon changed from bacteriastrum and chaetoceros in December to oscillatoria in February with the value of DIN:DIP = 90:1. In the early SW monsoon (May), oscillatoria and chaetoceros with a bloom of noctiluca scintillans (DIN : DIP = 24:1) were replaced by the bloom of skeletonema costatum in August (DIN:DIP = 36:1) and oscillatoria-ceratium furca in September and October (DIN:DIP = 33:1 and 20:1, respectively). These successions of phytoplankton related to the variations in DIN and DIP concentratons particularly in the unbalanced DIN:DIP rato. The third study on the allelopathic effect between a dinoflagellate ceratium furca and a diatom chaetoceros curvisetus was also performed using C.curvisetus grown in f/2+Si medium with the addition of 0%, 0.1%, 1%, 10% and 100% filtrate from C.furca. The inhibition of diatom growth was noticed in C.curvisetus grew in 100% filtrate since cell density decreased from the beginning of the experiment. The result from 0.1% filtrate stimulated cell division of C.curvisetus. In conclusion, these studies indicated that factors affecting diversity and succession of phytoplankton were dissolved inorganic nutrients especially in the unbalanced DIN:DIP ratio and the allelopathic relationship between different phytoplankton.