Chiang Mai Journal of Science

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Determination of  intracellular and extracellular microcystin concentrations in Mae Kuang Udomtara Reservoir, Chiang Mai Province has been carried out during December 2004 to August 2005. Every two months water samples were collected at the deepest point of  the reservoir every five meters down from the water surface to the bottom. Physico-chemical parameters of  water such as pH, conductivity, nitrate-nitrogen, ammonium-nitrogen, soluble reactive phosphorus and total iron were also measured. During the sampling period, relatively high density of Microcystis aeruginosa Kützing floating colonies have been found at the water surface. Microcystin-RR and -LR in filtered water samples (extracellular) were extracted by C₁₈ Solid Phase Extraction (SPE) and analyzed by HPLC-UV. Extracellular microcystin-RR was found in only one water sample collected in June 2005 at 5 meters depth with a value of  2.0 µg.l^-1 , while concentrations of  extracellular microcystin-LR were in a range of  0.05-6.5 µg.l^-1 Most of  the concentrations of microcystin . detected were higher than the guideline values recommended by WHO (1.0 µg.l^-1 of microcystin-LR equivalents for long term exposure via drinking water). However, there was no clearly evidence of  any harmful to living organisms in the reservoir during the period of sample collection. Intracellular microcystins were extracted from freeze-dried samples of Microcystis aeruginosa on the filter papers by 70% methanol, centrifuged and analysed by HPLC-UV. Concentrations of  intracellular microcystin-RR and -LR were 0.2-975.0 and 0.5-859.5 ng.g^-1 dried weight, respectively. Concentrations of intracellular microcystin -RR and -LR as well as those of extracellular microcystin -RR and -LR were positively correlated (p<0.01). Values obtained from physico-chemical measurement showed no correlation to the concentrations of  both intra- and extracellular microcystins.

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