The Benthic Macro-Invertebrate Fauna of Owalla Reservoir, Osun State, Southwest, Nigeria

The benthic macro-invertebrate composition of Owalla Reservoir in Southwest Nigeria was surveyed over two annual cycles (2011 – 2013). The study aimed at providing information on their taxonomic composition, abundance and distribution pattern (both in time and space) of the occurring species in the reservoir. Twenty (20) sampling stations representing the major habitat types and basins were established across the reservoir. Bottom sediments were collected using a Van Veen grab and sieved through a 0.5 mm mesh sieve using the reservoir water. The residues were preserved inside a specimen bottle in 10 % formalin and labeled appropriately for specimen analysis and identification which were carried out in the laboratory using appropriate identification keys. The benthic macro-invertebrates of Owalla Reservoir comprised 18 different species belonging to three major phyla (Arthropoda, Annelida and Mollusca), with a total abundance of 5076 individuals. Melanoides tuberculata was the most dominant species (90 % occurrence) and the most abundant (4128 individuals). Enallagma sp. was the least occurring (10 %) while Physa acuta, Radix natalensis and Mutela sp.were the least abundant (16 individuals each). Most of the organisms did not show significant (p > 0.05) spatial variations and none of them showed significant (p > 0.05) seasonal variations in the reservoir. The benthic macroinvertebrate composition of the reservoir is mainly dominated by taxonomic groups with little tolerance to water pollution. This is an indication of little or no anthropogenic influences on the reservoir’s water quality from activities within the catchment basin and an indication of a healthy water condition.