Assessment and Characterization of the Benthic Fauna in Omoku (Orashi) and Ikiri (Sombreiro) Rivers in Rivers State, Nigeria

Abstract

Introduction: Freshwater ecosystems are highly productive and essential for biodiversity and human needs, providing services such as water supply, fisheries, and nutrient cycling. Benthic fauna serves as important bioindicators because their diversity and abundance reflect changes in water quality, pollution levels, and habitat conditions. Aim: The study aim at assessing the characteristics of benthic fauna in Omoku (Orashi) and Ikiri (Sombreiro) Rivers in Rivers State, Nigeria. Study Design: Observational and descriptive study design were adopted. This design involved noting, recording and describing data from the two rivers that were earmarked for the study. Methodology: Three replicates of sediment samples were collected from five stations in 12 months (February 2023 – January 2024). The sediment samples were transferred to a linen sieve in order to remove the excess fine sediment; the residue sediment was transferred into a wide-mouth bucket. Samples were preserved with 10% formalin and stained with Eosin stain to enable coloration of the organisms in the sediment for easy sorting and identification. Result: Total abundance of macro-benthic organisms recorded stood at 3444 from 18 taxa (Omoku River having1914 individuals and Ikiri River, 1530 individuals). The most abundant organisms were Notomastustetericeus, 999(29.0%); Chironomus sp., 709(20.6%) and Marphysa sp., 644(18.7%). Calculating Species Richness (Taxa_S), Omoku River comprised of 13 taxa while Ikiri River had 9 taxa. Dominance Index (D):Omoku River (0.19) and Ikiri River (0.18). Simpson’s Diversity Index (1-D) of Omoku River was 0.81 and that of Ikiri River, 0.82. The Shannon-Wiener Index (H), Omoku River: 1.97 and Ikiri River: 1.86. For Evenness (e^H/S) computation: - Omoku River (0.51) and Ikiri River (0.71). Margalef Richness Index showed that Omoku River is 1.72 and Ikiri River is 1.09. Conclusion: The study shows that the ecological health of the Omoku (Orashi) and Ikiri (Sombreiro) Rivers varies from moderately affected to fairly stable, attributed to human activities. Conclusively, the study underscores the pressing necessity for sustainable management practices, encompassing pollution mitigation, oversight of industrial and agricultural effluents, and the preservation of riparian ecosystems.