The effect of crude oil spill at Izombe, Imo State, Nigeria on plankton diversity and abundance

This work was aimed at assessing the effect of crude oil spill on the diversity and abundance of plankton communities of the Njaba River area of the spill. Sampling was carried out in 1999 in nine clusters of stations, three each from upstream, spill-point and downstream transects. Each cluster was made up of three perpendicular sampling points across the river. Standard methods were used in collecting, preserving, identifying and counting the plankton. A total of 60 species of phytoplankton in 55 genera were collected. Phytoplankton constituted 62.9% of abundance and 71.4% of plankton species collected. The MDI for the upstream was comparable to that of the downstream, and both are significantly higher than the MDI for the spill-point (×-test; p < 0.05). In all, 122 zooplankton specimens in 24 species and 23 genera were collected. Zooplankton from the upstream clusters accounted for 81.1% of overall abundance and 95.8% of all the species collected. The more pollutionsensitive species such as the meroplankton were only found upstream and were totally absent in both downstream and spill-point clusters. Index of similarity between upstream and downstream shows 76% similarity for phytoplankton and 81% for zooplankton; but the index of similarity between the spillpoint and either the upstream or downstream was quite low. This indicates there are focalized dislocations in plankton abundance and diversity in the spill-point transects, and evidence of recovery downstream. In conclusion, there was a significant effect of the oil-spill on plankton abundance and diversity causing remediable perturbation beyond the carrying capacity of the spill-point transect.