Abstract
Sargassum forests form important habitats in coastal waters worldwide. Sargassum polycystum C. Agardh is a dominant species consisting of Sargassum forests and distributed widely in the Gulf of Thailand. We studied seasonal variations of S. polycystum on the intertidal reef flats in Samaesarn Island off the northeast coast of the Gulf of Thailand by monthly quadrat sampling, observation of S. polycystum and measurements of environmental variables from January 2014 to December 2015. Percent cover, thallus length, standing crop, percentages of the numbers of immature and mature plants of S. polycystum per 0.25 m² were the maximum during the dry and cold season from November to February. They showed a significant negative correlation with water temperature（p<0.05）and significant positive correlation with DO and phosphate (p<0.05). Percentage of the number of holdfasts, main without stipes of S. polycystum per 0.25 m² , which was the highest in among the numbers of juvenile, immature and mature plants, and main holdfasts without sipes of S. polycystum per 0.25 m² in March 2014 and December 2015, showed a significant and negative correlation with current speed（p<0.05). Plant density, percentage of the number of juvenile plants of S. polycystum were the maximum during the rainy season from May to September. These results indicate that the monsoon drives environmental variables controlling the seasonal pattern of the growth and reproduction of S. polycystum. Its maturation and reproduction occur under a calm sea condition and low water temperature with sufficient solar radiation in January-February at the end of dry season.
