Simarmata, Asmika HSihotang, M. Siagian and C2016-03-222016-03-222016-03-22978-979-792-437-9wahyu sari yenihttp://repository.unri.ac.id/xmlui/handle/123456789/8108Oxygen concentration in the water is significantly affected by environmental condition. In this research, oxygen concentration in the lacustrine and transition zones of the Koto Panjang reservoir is studied from June to September 2013, it was coincidence with the minimum water level. A post facto survey method was applied and there were 4 sampling sites, in the Lacustrine 1 (L1), Lakustrine 2 (L2), Transition 1 (T1) and Transition 2 (T2). The vertical sampling points were decided based on the brightness of the water (in the surface, 3m, 6 m, 12 m depth and above the sediment in the bottom). The number of vertical sampling sites was depend on the depth of the water. Results shown that the DO concentration in the surface of the lacustrine zone was 6.14 to 8.26 mg/l and in the transition zone was 7.38-8.6 mg/l. In the bottom of the lacustrine zone was 0 to 0.41 mg/l and that of the transition zone was 0.41-3.29 mg/l. The vertical profile of the Dissolved Oxygen in both stations shown that the highest decrement rate of the oxygen occur from 2 secchi depth (3 m) to 4 secchi depth (6 m). In the lacustrine zone, oxygen concentration drop into critical level (< 2 mg/l) in 12 m depth and in this research, the oxic depth was 7.0 m in L1 and 8.0 m in L2. Whereas in T1: 6.2 m and 8.0 m in T2. During the minimum water level, the oxic depth decrease due to decrement of water volume that lead into increment of organic matter loaded.enKoto Panjang damlacustrinetransitionoxygenUR-Proceedings