Chiang Mai Journal of Science

      Distribution of toxic algae and water quality in Kwan Phayao, Phayao province were carried out for 18 month during April 1999-September 2000. Seven species of Cyanophyceae: Microcystis aeruginosa Kütz., Microcystis wesenborgii Kom., Cylindrospermopsis raciborskii (Wolosz) Seenayya @ Subba, Anabaena catenula (Kg.) Born. et Flah., Anabaena aphanizomenoides Forti., Anabaena sp. and Oscillatoria perornata Skuja were found. The factors which positively correlated with the blooming of two species of two species of Microcystis spp were the amount of totoal iron and water temperature. M.aeruginosa and M. wesenbergii were two dominant species and showed the highest number 44,368 and 26,111 cell/ml respectively in April 1999 were 25.3% and 41.6% of total biovolume respectively after that the cells decreased throughout the investigation. C. raciborskii was the dominant species and showed the hightest biovolume 92.6% of total biovolume. It showed positive correlation with pH in September 2000 and still continually increased after the study period. The another toxic algae were found in small amount. The study were found 3 microcystins and showed that microcystin-RR was dominant, the amount was between 0.56-59.3 ng/l but microcystin-YR and microcystin-LR were minute. However, the microcystins were not showed correlation with Microcystis spp. Aulacoseira granulata showed the positive correlation with turbidity and present throughout the investigation. The water quality in Kwan Phayao classified by trophic level was found to be oligotrophic-mesotrophic status to mesotrophic status. However, by the standard surface water quality of Thailand, Kwan Phayao was classfied in the 2-3 category as relativity clean for household consumption after properly treated.