Seasonal changes of two Sargassum species: S. yamamotoi and S. kushimotense (Fucales, Phaeophyceae) at Shibushi Bay, Kagoshima, Japan

The seasonal changes in size of two temperate Sargassum species (Phaeophyceae), S. yamamotoi and S. kushimotense, were studied in Shibushi Bay, Kagoshima, Japan. In addition, the line-transect method was conducted to describe their population characteristics and the vertical distribution of the seaweeds. Both species were found on rocky shores in the upper subtidal zone (1.0 m to 2.5 m in depth), and they were never observed in sand and intertidal zones. Maximum lengths were observed in April with 244.0 cm for S. yamamotoi and 194.6 cm for S. kushimotense. They decreased in size from May through August after the reproductive structures matured. Regenerated shoots and newly grown germlings were observed in September and they increased in size from October through March. The maximum density of each population was observed in April with 20 shoots/square m for the former and 90 main branches/square m for the latter. However, the biomass of each species in that month was almost the same: 1752 g dry weight/square m for the former and 1782 g dry weight/square m for the latter.