Abstract

The vertical and surface water variations of pH and dissolve oxygen (DO₂) concentration were studied for 12 months (March 2005 to February 2006) in Asa lake Ilorin, Nigeria. The mean surface pH values ranged between 7.30 and 8.10, with lower values (7.30 - 7.70) obtained October to December while higher values (8.0/8.10) were obtained in January to July. Vertical distribution of mean pH also ranged from 8.1 (Surface) to 5.20 (Bottom). Similarly the monthly variations of mean DO₂ concentration at the surface waters were between 7.60 - 8.20 mg/l, with lower values in October/November and higher values in March/April. Vertical distribution of DO₂ concentration also ranged from 8.20 (surface) to 2.50 (bottom) mg/l. A fairly defined pH and DO₂ stratification were noted from March to September and also in January/ February, while destratification was noted for these parameters in November and December. However a sharp drop in DO₂concentration usually occurred at the thermocline zone of 5 m depth. This sharp drop also became shifted to 7 m depth in October and November due to thermal mixing condition of the lake around this season, and DO₂ concentration were almost uniform from the surface to 5 m depth (mean = 7.60  mg/l). The effects of this on some of the fish fauna of this water body were discussed. The pH and dissolved DO₂ concentration were positively correlated (0.54). There were no marked variations of the pH and dissolved oxygen concentration along the 3 sampling sites across the lake hence fish species composition along the water body is likely to be less varied.

Key words: Dissolve oxygen, pH, fishes, Asa lake, stratification.