Toxicologie environnementale et analytique

Effect of Organic Effluents on Water Quality and Benthic Macroinvertebrate Community Structure in Njoro River, Kenya

Abstract

Callen Nyaboke Aera, Charles Mwithali M’Erimba and Kitaka Nzula

Njoro River, which is the primary freshwater source for the urban and rural-based residents within its basin, has been subjected to increasing anthropogenic pressure that threatens its ecological and socio-economic values. An empirical study was conducted in the mid reaches of this river to determine the effect of organic effluents on water quality and benthic macroinvertebrate community structure. Organic pollutants input into the river can lead to immense ecosystem impairment. The effect of organic effluents on water quality was assessed in this study using a combination of physico-chemical parameters and benthic macroinvertebrate community structure. Four sampling sites were chosen corresponding to both diffuse and point source pollution of organic effluents. Dissolved oxygen, conductivity, turbidity and pH were determined in situ. Water samples were collected using 500 ml acid washed bottles before laboratory analysis for Biological Oxygen Demand, nitrogen, phosphates and Total Suspended Solids. Benthic macroinvertebrates were sampled using a kick net (mesh size 250 μm with an effective area 0.2025 m2) during each sampling session and identified using aquatic invertebrates identification keys. Results indicate that effects of organic effluents were more pronounced at downstream sites compared to the upstream ones. One-Way ANOVA indicated a significant difference in dissolved oxygen, conductivity, Biological Oxygen Demand, turbidity, ammonium, nitrates and total phosphorus levels among the sampling sites. Benthic macroinvertebrates species diversity was high in upstream sites with order Ephemeroptera being the most dominant taxa (51.26%) whereas order diptera dominated the downstream sites (42.38%). Among the genus of order diptera identified downstream, genus Polypedilum was dominant. The results from this study form baseline information in biomonitoring the Njoro River which can be adopted at a wider scope to the existing management strategies and policies towards pollution control and abatement in aquatic systems.