原著論文 226 原著論文 Seasonal Variation in Nitrogen and Phosphorus Concentrations in the Mekong River at Vientiane Ⅰ．INTRODUCTION The Mekong River is vital for people in the basin, as it has been fully utilized for various sectors including agriculture, fishery, industry, domestic consumption and transport. Especially, the agricultural and fishery production in the basin significantly depends on not only the water resources of the river but also the nutrients carried by the river (van Zalinge et al., 2004). It is said that for thousands of years the flood-prone areas in the Mekong Delta have received a lot of nutrients as well as sediments every year by flood water, though this story seems not to be clearly proven quantitatively so far (Hagiwara et al ., 2003; Kazama et al., 2003). The floods simultaneously cause damage to public constructions and individual properties in the lower reaches (Kite, 2001). The adverse effects of floods should be mitigated, while flood control without a correct view of the natural nutrient cycle mentioned above might severely alternate the natural ecosystem in the region. In order to seek appropriate measures Toshiaki IIDA 1) Somphone INKHAMSENG 2) Koshi YOSHIDA 3) Shinnosuke ITO 1) 1) Faculty of Agriculture, Yamagata University （1-23 Wakaba, Tsuruoka, Yamagata 997-8555, Japan） 2) Faculty of Engineering, National University of Laos （Tadthong Campus, Vientiane Capital City, Lao PDR） 3) Graduate School of Agricultural and Life Sciences, The University of Tokyo （1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan） 水 文 ・ 水 資 源 学 会 誌 J. Japan Soc. Hydrol. & Water Resour. Vol. 20, No.3, May 2007 pp. 226 - 234 For delineation of current condition and characteristics of the nutrient concentrations in the Mekong River, continuous frequent observation was carried out in the mainstream at Vientiane. The water samples were taken twice a week from April 2002 to December 2005 and were analyzed for nitrogen and phosphorus species. Detailed seasonal variation characteristics which had not been detected by previous monthly observation were revealed by the frequent observation. Overall, the nutrient concentrations were low, possibly owing to lower population density and lower coverage of agricultural land in the upstream of the sampling point. The ammonium-nitrogen concentration, however, showed an increasing trend during the recent decades. The nitrogen concentrations abruptly increased in May, coinciding with the first small rise in the discharge after the low flow period. It is suggested that the first runoff after the dry season took the accumulated nitrogen in the basin and on the river bed to the river, bringing about the abrupt increase in the nitrogen concentrations. After June, the nitrogen concentrations steadily decreased, regardless of the discharge enhancement during the flood season. It is considered that the nitrogen supply after June was diluted by plentiful water during the flood season. On the other hand, the phosphorus concentrations gently increased from April to June and were greatly fluctuated during the flood season. Continuous dilution effects by the plenty of water and occasional larger scale soil erosions during the flood season may account for the variation characteristics of phosphorus concentrations during the flood season. Key words: the Mekong River, nitrogen, phosphorus, concentration, seasonal variation