Phenology and environmental characteristics of two Sargassum species (Fucales), S. hemiphyllum and S. glaucescens, from Kagoshima Bay, Japan

The phenology and environmental characteristics of two brown algae, Sargassum hemiphyllum and S. glaucescens (Fucales), were investigated at Yamagawa, Ibusuki, Kagoshima Bay, Japan. Their vertical distributions in the upper subtidal zone were determined using the line transect method. The two species were found on the rocks and cobbles at a depth of 2 m in the upper subtidal zone. S. hemiphyllum (Sh) occurred at a slightly shallower depth than S. glaucescens (Sg). The two species grew well in February as the seawater temperature reached its lowest at 14℃, and Sh reached a maximum length of more than 2 m in May and matured in June. In contrast, Sg matured in July through August with a maximum length of 1.5 m, suggesting that their maturation periods were slightly different. After the maturation of Sh, new primary laterals (branches and leaves) for the next year's growth were already present; however, other parts including branches, leaves, vesicles, and receptacles that had emerged previously were completely withered by the end of summer. As for Sg, whole portions including the rhizoids were withered after their maturation, indicating the disappearance of large Sargassum assemblages from the study site during the summer through early winter.