TRIPLE ISOTOPES OF DISSOLVED OXYGEN IN THE MEKONG AND CHAO PRAYA RIVER BASINS, SOUTHEAST ASIA

Triple isotopes of dissolved oxygen (¹⁶O, ¹⁷O, ¹⁸O) and oxygen argon ratios were used to determine ratios of respiration to photosynthesis in large tropical river basins of Southeast Asia. These ratios give estimates of gross production integrated over time. High values of Δ¹⁷O represent systems where production is dominant over gas exchange with the atmosphere. Low values represent systems dominated by gas exchange. We collected water samples at 14 sites in the Chao Praya River basin, Thailand, and Mekong River basin, Thailand and Cambodia, for triple isotope analysis of the dissolved oxygen. Samples were collected during low flow, dry season conditions in March 2004 at two main stem sites and 6 tributaries of the Chao Praya River. Samples were also collected on the main stem Mekong and Tonle Sap Rivers and 4 tributaries to the Mekong River in Thailand. Dissolved oxygen values ranged from 3.53 mg/L to 7.85 mg/L and the rivers were alkaline with pH values ranging 7.2 to 8.6. The Δ¹⁷O of samples ranged in value from +93 per meg in the Yom River to +11 in the Nan River, both tributaries of the Chao Praya River (values reported with respect to air). δ¹⁸O values range from +1.97 per mil in the Tonle Sap River to -9.88 per mil in the Mun River, both large tributaries of the Mekong River (values reported with respect to air). As the oxygen-argon ratio increases, δ¹⁸O becomes increasingly depleted.