Changes in shoot density, biomass and leaf area of Zostera caulescens Miki from Summer to Autumn in Funakoshi Bay of the Sanriku Coast, Japan

The world's longest seagrass, Zostera caulescens Miki, is found in Funakoshi Bay along the Sanriku Coast of Honshu Island, Japan. Our study describes the changes in vertical distribution of shoot density, leaf area index (LAI) and biomass of Z. caulescens from summer (flowering season) to autumn (preparing for overwintering) in Funakoshi Bay. The samples were collected from a 0.5×0.5m quadrat at depths of 4.4m, 10.9m, 13.3m, 14.3m and 15.3m in July, and, for October, at 5.4m, 7.4m, 9.2m, 12.9m, and 15.7m. Above- and below-ground biomass, and flowering and vegetative shoot densities decreased with increasing bottom depth in both summer and autumn seasons except at the shallowest sampling depth, while the maximum above-ground biomass in both seasons was 187.3gDW/m^2 at the depth of 7.4m and 213.5gDW/m^2 at the depth of 10.9m, respectively. Densities of vegetative shoots were greater than those of flowering shoots at each sampling depth in both seasons except a bottom depth of 15.3m in July. The maximum length of flowering shoots in autumn (737cm) was greater than in summer (546cm), and the range of shoot lengths in autumn was wider than in summer. Biomass and densities of vegetative shoots at different depths in autumn were higher than those in summer, and Leaf Area Indices (LAI) of flowering and vegetative shoots at different depths in summer were higher than those in autumn. In addition, the mean LAIs of individual flowering and vegetative shoots in summer were greater than those in autumn as well. These results suggest that Z. caulescens has a higher LAI in summer when there is low seawater transparency and after ripening from summer to autumn, it also makes vegetative shoots in preparation for growth during the next spring