Abstract
The blooming of Microcystis aeruginosa Kütz. in the reservoir of Mae Kuang Udomtara dam, Chiang Mai, Thailand was investigated for 18 months during August 1996-January 1998. The water quality in the reservoir classified by trophic level was found to be mesotrophic to eutrophic. The main problem of water quality in the reservoir was the proliferation of phytoplankton, Microcystis aeruginosa Kütz. which secreted microcystin (hepatotoxin). It was found throughout the investigation in large amount during July 1996-January 1997. The factors effecting the proliferation were the amount of soluble reactive phosphorus and the total phosphorus which showed negative correlation with the volume of water in the reservoir. The vertical changes of the biovolume of M. aeruginosa was more obvious at the water surface and decreased at the lower levels. Six types of microcystin were isolated from the samples. Microcystin RR was present in the highest concentration in this investigation followed by microcystin LR, Z-RR, YR, Z-LR and ThtyrR. Toxicity tests with animals showed an LD50 for microcystin RR of 700-800 mg.kg-1 body weight, whilst the value for microcystin LR was 70 mg.kg-1 body weight.
