High water temperature tolerance in photosynthetic activity of Zostera japonica Ascherson and 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 micromol photons/square m/s) and ten water temperature conditions (5-35degC). 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 (Psub(maxg)) appeared at 29degC and most seedlings cultured at 30-35degC bleached and withered. At the same time, the light compensation point (Isub(c)) increased only at 30degC during the culture period. As a result, the upper critical water temperature for survival was 29degC 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.