Abstract
Influences of nitrate and phosphate concentrations on Microcystis species composition and microcystin concentration were confirmed by the results of Lake Suwa investigation between June and October from 1992 to 2003. Phosphate concentration decreased gradually but nitrate concentration remained constant for 12 years. Nitrate : phosphate concentration ratio varied below 10 between 1992 and 1998, but was more than 10 after 1999. The changes in this ratio indicated that the Microcystis bloom limited factor shifted from nitrate to phosphate since 1999. From 1992 to 1998 under a nitrates-limited condition, toxic species (M. aeruginosa and M. viridis) dominated and a high microcystin concentration was detected. A nontoxic species, M. ichthyoblabe, dominated under a phosphate-limited condition and a low microcystin concentration was detected since 1999. These results suggested that the low phosphate concentration (blow 10μg L-1) shifted from toxic M. aeruginosa and M. viridis to nontoxic M. ichthyoblabe in the Microcystis blooms. Nitrate concentration affected the competition between M. aeruginosa and M. viridis.
