Effect of temperature on growth of the cyanobacterium Aphanizomenon flos-aquae in Lake Biwa and Lake Yogo

SUMMARY The water bloom-forming cyanobacterium Aphanizomenon flos-aquae Ralfs ex Bornet et Flahault (Nos-tocales, Cyanophyceae) appeared in Lake Biwa and Lake Yogo in 1999 for the first time. The morphological characteristics were described using natural samples. In contrast to the other water bloom-forming cyanobacteria such as Microcystis and Anabaena in Lake Biwa and Lake Yogo, the small summer population of A. flos-aquae is apt to grow in winter, suggesting the low temperature preference or tolerance of this species. In order to clarify the effect of temperature on the growth, culture experiments were conducted using an axenic strain isolated from Lake Biwa. The strain could grow at above 8°C with an optimum temperature ranging from 23 to 29°C, and survived even at 5°C for at least 25days under low light conditions. Although these results confirmed the ability of the bloom formation during late autumn and winter, it is still unclear why the Aphanizomenon bloom occurred at temperatures of ca 10°C in December and not immediately after the disappearance of Microcystis and/or Anabaena bloom during autumn.