Chiang Mai Journal of Science

       The monitoring of water quality in Ang Kaew Reservoir which is the water used for water supply in Chiang Mai University, was carried out by using phytoplankton as bioindicator including physico-chemical properties from 1995-2002. It was found that the average waer quality in the reservior could be classified as mesotrophic to eutrophic status. In the summer and rainy seasons, the water quality was indicated as mesotrophic to eutrophic whilst in the winter, it was indicated as mesotrophic except from the end of 2001 to the beginning of 2002, the water quality showed hypereutrophic in consequence to the drawing of polluted water from irrigation canal which was added to the reservoir. The large scale blooming of toxic cyanophytes, Anabaena catenula (Kützing) Bornet & Flahault occurred during this period, the algal toxin, microcystin-LR was found in small amounts. The water quality of the reservoir is classified by the standard surface water quality of Thailand, it was in the second to third category which is suitable for household consumption after being properly treated.            One hundred and eighty-six species of phytoplankton in Division Cyanophyta, Chlorophyta, Euglenophyta, Chrysophyta, Pyrrhophyta, and Crytophyta were found. The species which indicated eutrophication to hypereutrophication was Anabaena catenula (Kützing) Bornet & Flahault; the mesotrophication to eutrophication indicator species were Englena acus Ehrenberg, Phacus meson Pochmann, Phacus pleuronectus Müller and Trachelomonas volvocina Ehrenberg, the mesotrophication indicator species were Planktolyngbya limnetica Lemmermann, Cylindrospermopsis raciborskii (Woloszynska) Seenayya and Subba et Raju, Aulacoseira granulata (Ehrenberg) Simonsen and Peridinium inconspiduum Lemmermann.