Diel studies provide important details of mixing and stratifying events and are particularly important in tropical lakes, where metabolic processes proceed at accelerated rates. In order to study mixing and stratifying events in a black water lake at the Negro River basin, we analyzed diel profiles of temperature and dissolved oxygen measured during the flood and dry periods of the 2001-2005 hydrological cycle. Tupé Lake is a narrow and dendritic black water lake located on the left bank of the Negro River about 25 kilometers from Manaus. It is “a submerged, deeply cut, V-shaped valley” or “Ria” lake, which occupies an area of 66.9 ha. The lake is directly connected to the Negro River for most of the time and is isolated for only a few months during dry periods (November-December). The surface water temperature and oxygen saturation in 24 hours ranged from 29.2 °C and 59.1% (06:00) – 31.6°C and 74.0% (14:00) in flood periods, and it ranged from 28.5°C and 83.4% (06:00) – 31.8°C and 124.5% (14:00) in dry. In the bottom, temperature varied moderately in 24 hours, with permanent 26.9 °C in the flood, and between 27.3 ºC (06:00) - 27.4°C (14:00) in dry. Saturation levels ranged from 0.7% (18:00) – 0.9% (02:00) in flood and 0.8% (14:00) – 2.0% (22:00) in dry. A permanent meromixia was observed and t-student Test and k-means Cluster Analysis confirmed it, indicating statistically significant differences in the water column with a close surface layer between 0-1.5 m and a large bottom layer above from 3 meters.