Heavy Metals, Nutrients, Total Hydrocarbons and Zooplankton Community Structure of Osse River, Edo State, Nigeria

An aquatic ecological survey was carried out across Osse River from April, 2013 to September, 2014. The study was aimed at assessing the heavy metals, nutrients and total hydrocarbons in the water and sediment in conjunction with the zooplankton biodiversity. Surface water, sediment and zooplankton samples were collected from four (4) selected stations. Station 1 was chosen as control station upstream, far away from perturbations; while Stations 2, 3 and 4 were chosen at locations of distinct anthropogenic activities. Surface water and bottom sediment samples were analyzed for heavy metals, nutrients and total hydrocarbons using Atomic Absorption Spectrophotometer (AAS) and Gas Chromatograph-Flame Ionization Detector (GC-FID), respectively. Zooplankton specimens were sorted and dissected where necessary under a binocular dissecting microscope (American Optical Corporation, Model 570), while counting and identifications were done with an Olympus Vanox Research Microscope (mag X60) Model 230485. Results showed that the river is an oligotrophic aquatic ecosystem. The significant Varimax rotated matrices of manganese (0.947), copper (0.883) and zinc (0.817) revealed that these parameters were the active components in the water; while that of copper (0.896) was the active component in the sediment. This revealed that essential metals were the active components in both media. This is consistent with the fact that concentrations of manganese in the water at Stations 3 (0.97 mg/l) and Station 4 (1.26 mg/l), and copper at Station 4 (1.05 mg/l) slightly exceeded the regulatory limits. The zooplankton individuals were spatially distributed in the following order: Station 1 (923) > Station 4 (385) > Station 3 (191) > Station 2 (123). The lowest number of zooplankton individuals were recorded at Stations 2 and 3, i.e., locations of highest perturbations where high concentrations of manganese, nickel and THC were recorded in the water, and nickel, lead, copper and THC were recorded in the sediment. The high concentrations of manganese and THC; particularly Ni which was higher than other stations and FEPA limit, can be attributed to oil exploration activities, such as gas flaring, petrochemical production, storage and transit. Perturbation in zooplankton community structure is prognostic of possible impacts on other aquatic biota of economic relevance. There are indications that anthropogenic activities at Osse River are liable to cause severe ecological perturbations in future if not put in constant check. Continuous stringent bio-monitoring study of the aquatic environment is recommended to put the levels of heavy metals, nutrients and total hydrocarbons in constant check