Seasonal biodiversity assessment of benthic macroinvertebrate of asejire reservoir, Southwest Nigeria

Abstract

The benthic macro-invertebrate fauna of Asejire Reservoir, Southwest Nigeria were investigated for two years (2004-2006) covering both the rainy and dry seasons. The major objective of the study was to provide baseline information on aspects of the biology (taxonomic composition, occurrence, distribution and abundance) of the benthic macro-invertebrates, characteristics of the reservoir sediment. A total of twenty sampling stations were established to represent the various sections and regime of the lake. Sediment samples were collected from each station at bimonthly intervals using an improvised Van-veen grab. Altogether a total of three hundred and twenty composite samples were collected and analysed and assessed. The benthic macro-invertebrates comprised twenty-eight species in the class Insecta (13 taxa), Gastropoda (9 taxa), Bivalvia (2 taxa) while Arachnida, Malacostraca, Hirudinea and Gordiadea were made of one species each. Altogether a total abundance of 364,351 individual macro-invertebratess species were collected from the bottom sediment. The dominant species were Potadomamoerch, P. freethi and Melanoidestubaculata with a population of 119,985, 66,660 and 32,768 individual taxawhile the least occurring taxa were Sphaeriumsp and Hydracarinasp with a population of 550 and 1,350 individual. Considerable close association (p < 0.05) were found among many of the macro-invertebrates species. Species abundance was higher in the dry season (46 ±6.5 organism m -2) than in the rainy season (36 ± 6.0 organism m -2) but the difference between the two seasons was not significant (p > 0.05). More species were found in the littoral than in the open water region of the reservoir and the difference between the two regions was significant (p < 0.05). The differences in the abundance of species in the three reaches were also significant (p < 0.05). In conclusion, on the basis of benthic macro-invertebrate taxa composition and abundance, Asejire Reservoir can be inferred to be rich in fauna composition and therefore fairly clean and unpolluted.