Chiang Mai Journal of Science

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The surveying and monitoring of toxic cyanobacteria in 6 raw water resources for water supplies and fisheries, specifically the reservoirs of Mae Kuang Udomtara dam and Mae Ngat Somboonchol dam in Chiang Mai province; Kwan Phayao in Phayao province; Nong Han in Sakon Nakhon province; the reservoir of Lamtakong dam in Nakhon Ratchasima province and Bang Phra reservoir in Chon Buri province was carried out from April 2000 to March 2001. All 6 reservoirs were found to be contaminated by toxic cyanobacteria, Microcystis aeruginosa Ktz. The highest density of toxic cyanobacteria was observed in the reservoir of Mae Kuang Udomtara dam followed by those in Kwan Phayao. There was a small amount of M. aeruginosa in the reservoir of Mae Ngat Somboonchol dam, Nong Han, the reservoir of Lamtakong dam and Bang Phra reservoir. Cell count did not exceed the standard level for raw water used for water supplies in all reservoirs and investigations. For microcystin analysis, microcystin-RR was found in higher amounts in every site, whilst microcystin-LR and -YR were also found in some reservoirs, but in small amounts. Nevertheless, the level of microcystins found in all reservoirs was lower than those determined by the World Health Organization for the standard of raw water used for water supplies. The water quality of all reservoirs was in oligotrophic- mesotrophic status. Based on the standard surface of water quality in Thailand, it was classified to be in the second to third category and was relatively suitable for fisheries and water supplies when treated properly. Eleven other species of toxic cyanobacteria were also found e.g. Cylindrospermopsis raciborskii (Wolosz.) Seenayya & Subba, which showed in higher numbers than other species.
Fish behavior in these water resources, did not show any abnormality. In addition the tissue and intestinal organs of the fish which could have been affected by toxins, were also normal, due to the blooming of M. aeruginosa which occurred in small amounts.

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