Effect of light availability on Microcystis aeruginosa blooms in shallow hypereutrophic Lake Kasumigaura

Microcystis aeruginosa causes surface blooms in eutrophic lakes in the tropical and temperate zones. We investigated which factors suppressed blooms of M. aeruginosa in Lake Kasumigaura, a shallow hypereutrophic lake in the temperate zone of Japan, from 1999 to 2007 by quantifying the 16S rRNA gene of M. aeruginosa using real-time PCR. These results showed that M. aeruginosa existed in the lake water throughout the year. The abundance of M. aeruginosa usually increased from March to July and decreased from August to February. The factors determining the maximum density of the year were the overwintering population and the growth rate in summer (in June and July). The overwintering population was affected by abundance during the previous summer. The growth rates of M. aeruginosa in summer were correlated significantly with the extinction coefficient and mean daily photosynthetically active radiation irradiance. The mean daily irradiance in summer from 1999 to 2007 was 77% of that from 1983 to 1986, when blooms of M. aeruginosa occurred. Therefore, the most important factor controlling the growth of M. aeruginosa in Lake Kasumigaura after 1999 appears to be the amount of light supplied to the water column.