Assessing the spatial and temporal variability and related environmental risks of toxic metals in Lake Asejire, south-western Nigeria

Abstract

Lake Asejire in Nigeria is polluted with untreated effluent from nearby aquacultural, urban, agricultural and industrial zones. The metal laden effluent pose a hazard to the health of fish and fish consumers in the lake. This study aimed to (1) assess the spatial and seasonal variations in the levels of cadmium, cobalt, copper, iron, lead, nickel, mercury and zinc in water, sediments and gills and stomach tissues of mud catfish, Clarias gariepinus and Nile tilapia, Oreochromis niloticus; and (2) evaluate environmental risks posed by heavy metals in Lake Asejire. Potential ecological risks of heavy metals were assessed using the pollution loading, geoaccumulation and potential ecological risk indices. Levels of Fe, Co, Cu, Cd, Pb, Ni and Zn were high in water and sediments and in some cases exceeded the Nigerian Standards for Drinking Water Quality and World Health Organisation threshold values. There were significant ( p < 0.05) seasonal variations in the levels of all metals in water and sediments. Significantly high Fe, Co, Cd, Pb, Ni and Zn levels recorded in gills and stomach tissues of mud catfish and Nile tilapia relate well with metal levels in water and sediments. The geoaccumulation index reflected heavy Cu and Zn contamination in sediments whereas Cd pose serious ecological risks to aquatic organisms in the lake. Accurate assessment of the spatial and temporal variability of heavy metal levels in Lake Asejire is crucial for pollution control and environmental management.