Effect of temperature on light-limited growth of the harmful diatom Coscinodiscus wailesii, a causative organism in the bleaching of aquacultured Porphyra thalli

The diatom Coscinodiscus wailesii is a harmful diatom which indirectly causes bleaching of aquacultured Nori (Porphyra thalli) through competitive utilization of nutrients during bloom events. To elucidate the mechanism of the bloom, the effect of temperature on light-limited growth of C. wailesii was examined at 13 irradiance levels (5–350 μmol m−2 s−1) in combination with 4 temperature levels (9.0–20.0 °C). The results showed that all the parameters of growth-irradiance curves such as the maximum growth rate, half saturation constant, threshold value of irradiance and saturation irradiance for growth increased with increasing temperature. On the basis of the relationship between temperature and growth-irradiance curves and seasonal fluctuation of the light environment in Harima-Nada, the population dynamics of C. wailesii during the period from October to March was evaluated using two indices Dt (depth of the threshold irradiance for growth) and D0.5 (depth of growth rate is 0.5 divisions per day). As the vegetative cells of C. wailesii tend to distribute in relatively deeper water layers where the growth is limited by irradiance, the increase in the depth range over which C. wailesii is able to grow or grow at a high growth rate is concluded to be an important factor allowing the development of the blooms.