Effects of Cassava effluent on Benthic Macroinvertebrate Assemblages in a Tropical Stream in Southern Nigeria

The study of the effect of cassava effluents on macroinvertebrates along downstream reaches of the Orogodo River, the Niger Delta was carried out monthly from January to June 2006. Three study stations were selected along the river course (upstream of the cassava impacted site, the cassava-impacted site, and downstream of the cassava impacted site). The study showed that cassava effluents caused a decrease in dissolved oxygen and pH and an increase in biochemical demand (BOD) and nitrates. Significant differences in these parameters were established among the stations sampled. A post hoc test indicated that station II (the cassava impacted site) was the cause of the observed differences. A total of 55 benthic macroinvertebrate taxa with a mean of 6,116 individuals were collected from the three stations along the river. The analysis showed that the overall density of fauna differed significantly among the stations. Cassava effluents permitted the dominance of oligochaetes and dipterans at station II and this resulted in a decline and total elimination of other benthic macroinvertebrates, which are intolerant of the effects of effluents. These preliminary data suggest that the response of benthic macroinvertebrates is important in the study of impacted aquatic systems and that macroinvertebrates have a great capacity to recover from the cassava effluent impact in terms of taxonomic diversity.