High water temperature tolerance in photosynthetic activity of Zostera japonica Ascherson & Graebner seedlings from Ago Bay, Mie Prefecture, central Japan

Photosynthetic activity of Zostera japonica seedlings was measured using a gas volumeter at 0 and 6 days in culture under eight light (0-800 mu mol photons/m(2)/s) and ten water temperature conditions (5-35A degrees C). Seedlings from Ago Bay, Mie Prefecture were cultured in incubators accurately controlled at each test temperature for 1 week. After 1 week, maximum gross photosynthesis (P (maxg)) appeared at 29A degrees C and most seedlings cultured at 30-35A degrees C bleached and withered. At the same time, the light compensation point (I (c)) increased only at 30A degrees C during the culture period. As a result, the upper critical water temperature for survival was 29A degrees C in Z. japonica seedlings, which agrees well with that for the southern boundary of Z. japonica around Japanese coast. It is necessary to monitor this species around this boundary as a bio-indicator for seawater warming.