The seasonal variation of ph and dissolved oxygen (dO2) concentration in Asa lake Ilorin, Nigeria

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.