Assessment of the concentrations of some heavy metals and their effects on the macroinvertebrate composition in Igun southwestern Nigeria, using reference site approach

Abstract

A survey was conducted from September 2014 to February 2015 to determine the resident aquatic macroinvertebrates and selected heavy metals of Igun and Osu Reservoirs, in Atakumosa West Local Government Area of Osun State, southwestern Nigeria. This was with a view to detecting the effects of gold mining activities on Igun Reservoir using Osu Reservoir as a reference site. Sampling for heavy metals and macroinvertebrate fauna was carried out fortnightly for six months. The collected aquatic macroinvertebrates were preserved in 70% ethanol and identified using appropriate identification keys. Water sample were collected for the determination of heavy metals which include, Cadmium (Cd), Gold (Au), Lead (Pb), Zinc (Zn) and Manganese (Mn), using (Atomic Absorption Spectroscopy) A total of 136 macroinvertebrate specimens representing 17 species in 12 families were collected from Igun Reservoir. Class Gastropoda had the highest number of specimens 61(44.85%), representing five species in three families (Planorbidae, Thiaridae and Physidae). A total of 408 macroinvertebrate specimens representing 32 species in 16 families were collected from Osu Reservoir. Ephemeroptera, represented by six species from two families (Caenidae and Baetidae) had the highest number of specimens, 136 (33.33%). Simpson Diversity Index (1-D) for Igun and Osu Reservoirs were 0.84 and 0.89 respectively. Cd correlated with Mn (r = 0.82) while Pb correlated with Mn (r = 0.62). Pb also negatively correlated with the abundance of Odonata (r = -0.77) in Igun Reservoir while Cd correlated negatively with the abundance of Coleoptera (r = -0.58) in Osu Reservoir. The concentration of zinc was within the WHO permissible limits for freshwater while other heavy metals were above the limits in both reservoirs.