Effects of Oil Pollution on Benthic Macro-invertebrate Communities in Imo River

This study investigated the effects of petroleum hydrocarbon pollution on benthic macro-invertebrate communities in the Imo River, Imo State, South East Nigeria. Benthic macro-invertebrates are widely used as bioindicators of aquatic ecosystem health, and their responses to pollution provide early warning signals for fisheries ecosystem degradation. Four sampling stations were established along the river, with two located downstream of a pipeline pressure station and two in relatively undisturbed upstream sections. Benthic samples were collected using an Ekman grab sampler during wet and dry seasons, and hydrocarbon content of sediments was quantified using gas chromatography. Total hydrocarbon content of sediments at impacted stations averaged 4,820 mg/kg, exceeding safe thresholds by more than threefold. At impacted stations, community composition was dominated by pollution-tolerant oligochaetes and chironomid larvae, while sensitive taxa including ephemeropterans, plecopterans, and trichopterans were absent. Shannon diversity index values at impacted stations (0.74 to 1.02) were significantly lower than at reference stations (1.87 to 2.43). Total macro-invertebrate abundance was paradoxically higher at one impacted station due to a bloom of chironomid larvae, a known pollution indicator. The study confirms that petroleum pollution has severely degraded benthic ecosystem structure in the Imo River and recommends a combination of bioremediation interventions, pollution source control, and regular benthic monitoring as components of an integrated river health management programme. Keywords: oil pollution, benthic macro-invertebrates, Imo River, bioindicators, ecosystem health