Temporal and spatial variability in macroinvertebrate community structure in relation to environmental variables in Gbako River, Niger State, Nigeria

Abstract

Temporal and spatial variability in macroinvertebrate community structure in relation to environmental variables in Gbako River, Niger State, Nigeria was evaluated monthly for a period of six months using modified kick sampling techniques. Four study stations were selected along the river course (upper reaches of less human impacts through mid-reaches with relative high human impacts to lower reaches of less human impacts), designated as stations 1, 2, 3, and 4. High concentrations of dissolved oxygen, lower nutrient and BOD levels were recorded in stations 1 and 4 while lower concentrations of dissolved oxygen, higher nutrient, conductivity levels, and BOD levels were recorded in stations 2 and 3. There was an abundance of the pollution sensitive taxa such as Ephemeroptera (mayflies), Coleopteran (Gyrinus spp., Dytiscus spp.) and Anisoptera (Gomphus sp., Lestinogomphus, Cordulex spp.) in all the stations, especially the upper and lower reaches, whereas on the other hand, some pollution tolerant species like the Crustaceas, Dipterans, Mollusca (Neritina rubricate, Potadoma sp.) were merely restricted to the middle reaches (stations 2 and 3). Of the total number of individual benthic invertebrates recorded during the entire study, 53% was recorded in the dry season while the remaining 47% was recorded in the wet season. However, this difference was not statistically significant (P > 0.05) when the student t-test (tstat/cal = 0.388), tcrit = 2.447) for the densities and abundances of macroinvertebrates recorded during the two sampling seasons was applied. Canonical Correspondence Analysis (CCA) separated the less impacted from the more impacted sites and also showed that the invertebrate fauna was significantly (P < 0.05) associated with environmental factors of Gbako River. The CCA identified conductivity, depth, flow velocity, dissolved oxygen, biological oxygen demand, and phosphates as important variables structuring the macroinvertebrate assemblages. The higher number of benthic invertebrates recorded in the dry season could be attributed to the unstable nature of the substrates through inputs and influx of storm water during the rainy season months.