Seasonal dynamics of Zostera caulescens: relative importance of flowering shoots to net production

Abstract

Zostera caulescens occurs only in limited localities around Japan and Korea, and is listed as a threatened plant species in Japan. We present the first quantitative data on seasonal dynamics of Z. caulescens based on a 13-month field study of a subtidal seagrass bed (4–6 m deep) in Funakoshi Bay, northeastern Japan. We investigated the relative importance of flowering shoots on total net production: these flowering shoots form a canopy structure several meters above the sea bottom. Flowering shoots were observed throughout the year but showed large seasonal variation in density, with a maximum in spring to summer (>30 shoots m −2) and a minimum in winter (3 shoots m −2). The density of vegetative shoots fluctuated between 120 and 238 shoots m −2, but did not show a significant seasonal variation. Age distribution of both flowering and vegetative shoots showed marked seasonal variation, with the peak in recruitment observed during winter to early spring. Leaf plastochrone intervals were 2- to 3-fold shorter in summer than in winter, with an annual average of 15.3 and 18.1 days for flowering and vegetative shoots, respectively. On the basis of these data, the absolute age of the oldest flowering shoot was estimated at 13 months. Biomass and net production of Z. caulescens were highest in summer and lowest in winter. During spring to autumn, flowering shoots constituted more than 70% of both biomass and production, while the contribution of vegetative shoots dominated during winter, when the density of flowering shoots was low. Annual net production was estimated to be 292 and 134 g DW m −2 per year for the aboveground parts of flowering and vegetative shoots, respectively, and 47 g DW m −2 per year for the belowground parts, giving a total of 473 g DW m −2 per year. These findings demonstrated that the flowering shoots of Z. caulescens are important not only for seed production, but also as major photosynthetic parts of the population, helping to achieve high primary production despite their distribution at the deeper parts of the multispecific seagrass bed.