Effects of temperature and irradiance on growth of strains belonging to sevenSkeletonemaspecies isolated from Dokai Bay, southern Japan

The effect of temperature on the growth of Skeletonema ardens, S. costatum sensu stricto, the S. marinoi–dohrnii complex, S. japonicum, S. menzelii, S. pseudocostatum and S. tropicum isolated from Dokai Bay in southern Japan were examined under five to seven different temperatures and an irradiance of 150 µmol m−2 s−1. The effect of irradiance on the growth of the seven Skeletonema species was also examined under a wide range of irradiances ranging from 7–700 µmol m−2 s−1 at 20°C. All Skeletonema species were able to grow at temperatures ranging from 15 to 25°C. Intra-species differences in specific growth rates of four strains for S. menzelii, and five strains for S. ardens, the S. marinoi–dohrnii complex, S. japonicum and S. tropicum were not significant (Kruskal–Wallis test, P > 0.05). Significant inter-species differences in specific growth rates were observed at 10, 15, 25 and 30°C (Kruskal–Wallis test, P < 0.01; Steel–Dwass test, P < 0.01). The S. marinoi–dohrnii complex and S. japonicum grew faster than other species at the lower temperatures of 10 and 15°C, and S. ardens and S. menzelii grew at the highest temperature of 35°C. The maximum specific growth rates (μ max) from growth–irradiance curves ranged from 1.50 to 3.44 d−1. Threshold values of irradiance (I 0) and saturation irradiance (S) for growth ranged from 3.9 to 7.6 µmol m−2 s−1, and from 250 to 740 µmol m−2 s−1, respectively. In Dokai Bay, our results suggested that the occurrence of Skeletonema species should be affected mainly by temperature and less by irradiance. In particular, only two species, the S. marinoi–dohrnii complex and S. japonicum could maintain their populations in the surface water during the cold season, whereas during other periods, all seven species could maintain their population under the strong influence of estuarine circulation, which rapidly flushed the surface water out of Dokai Bay. Temperature and irradiance dependent growth values were in good agreement with their geographical distributions. The S. marinoi–dohrnii complex and S. japonicum were capable of growing in cold regions, but our results suggested that S. ardens and S. menzelii will tend to prefer tropical regions.