Vertical distribution and seasonal variation of eelgrass bed in Iwachi Bay, Izu Peninsula, Japan

Abstract

The vertical distribution of an eelgrass bed was surveyed in Iwachi Bay, the Pacific coast of central Japan. Samplings were conducted from May to November in 1977 using SCUBA. Eelgrasses were distributed at the depth of 3 to 11 m. The lower limit of the bed at 12 m is probably due to the low light intensity at that depth relative to the surface. Nevertheless, the plants showed good growth at 7-10 m depth, which is comparatively deeper than other eelgrass habitation in Japan. Highest population density (290 shoots/m2) was observed in September and fresh weight of biomass (888 g/ m2) was highest in July at 7 m depth. The ratio of reproductive shoots to the total shoots was also highest at the aforementioned depth which was about 36% in June. The maximum value of the mean leaf area index was about 3 at 10 m depth in July. It was then considered that the high values of water transparency and solar radiation, and longer sunshine duration compared with the other localities contributed to the growth of eelgrass in deeper waters in Iwachi Bay.