ABSTRACT
The aquatic insect communities in relation to habitat area and characteristics at selected sites along Ikpoba River, Benin City were studied from February, 1999 to November, 2000 in order to characterize the insects at the respective sites so as to assess the potential use of aquatic insects for water quality classification in the river. Four sites along the river course which were located at Temboga, bridge area by Benin/Agbor road, abattoir and brewery waste discharge areas were selected and designated stations 1, 2, 3 and 4 respectively, with station 1 as control. The stations were sampled monthly to determine their water physicochemical quality and aquatic insect communities. The results obtained revealed that values of physicochemical parameters like current velocity (30.5 – 48.5 cm Sec –1), pH (5.0 – 7.1), conductivity (12.8 – 77 S cm-1), alkalinity (6.2 – 9.9 mgl-1), cadmium (0.02 – 0.18 mgl-1), calcium (0.3 – 1.2 mgl-1), magnesium (0.4 – 3.0 mgl-1), iron (0.22 – 2.60 mgl-1), manganese (0.01 – 0.05 mgl-1), copper (0.02 – 0.22 mgl-1) and chromium (0.10 – 0.36 mgl-1) were not significantly different (P > 0.05) among the stations. On the other hand, values of transparency (44.0 – 112.0 cm) and dissolved oxygen (2.1 – 5.9 mgl-1) were significantly lower (P < 0.05) in stations 3 and 4 compared to stations 1 and 2, while values of temperature (23.6 – 29.8oC), colour (95.0 – 285.5h), total solids (19.5 – 106.1 mgl-1), nitrate (0 – 3.86mgl-1), phosphate (1.22 – 2.56mgl-1), sulphate (2.0 – 8.8 mgl-1), sodium (1.04 – 4.84 mgl-1) and potassium (0.4 – 7.0 mgl-1) were significantly higher (P < 0.05) in station 4 than the other stations. A survey of the speciation of the aquatic insects revealed that a total of 37 species belonging to 32 families in 7 orders were present at the stations. The orders included Trichoptera, Plecoptera, Odonata, Coleoptera, Ephemeroptera, Hemiptera and Diptera in ascending order of abundance. The taxa composition were 28, 25, 26 and 20 taxa in stations 1, 2, 3 and 4 respectively. The abundance and distribution of the insect species in relation to fluctuations in the physicochemical factors operating within their habitats were studied. The levels of dissolved oxygen, nitrate, phosphate, sulphate, sodium and potassium appeared to be important limiting factors. A brief characterization of the insect fauna related to the various sites was attempted. Seven species were included as characteristic of station 1, with Hydrometra sp showing the highest dominance. Eight species were included in station 2 with no species showing overriding dominance over the others. Eight and 12 species were found to be characteristic of stations 3 and 4 respectively. In these two stations, Chironomus sp was recorded as the dominant species. It was evident from the study that aquatic insect communities appeared to be reliable indicators of environmental conditions. It was therefore advocated that a biotic index utilizing the absence or presence of sensitive aquatic insect species should be used for water quality classification in Ikpoba River.
