Abstract:

Activities associated with crude oil, natural gas or condensate field could result in several accidents that can lead to massive oil spills. To curb the deleterious effect of the hydrocarbon contents of the crude oil, the chemical clean-up method can be utilized in far and deep offshore environments in Nigeria. Using the dispersants weakens the interfacial tension between oil and water and in the process, bitty droplets of the oil are hatched and entrained within water column. Acute toxicity testing becomes vital as the dispersants have the potentials of causing deleterious effects on resident organisms within the recipient environment. This study aimed to determine the acute toxicity of two oil spill dispersants and the response of the euryhaline-Tilapia guineensis (fish) and benthic-Clibanarius africanus (hermit crab) when exposed to the dispersants. The fish and hermit crab which are standard test organisms for acute toxicity testing in Nigeria as specified by Nigerian Upstream Petroleum Regulatory Commission (NUPRC) were sourced from Nigerian Institute for Oceanography and Marine Research (NIOMR) at Buguma, Rivers State while the dispersants- Corexit 9527 and Finasol OSR were obtained from different licensed oil field chemical stores in Port Harcourt, Rivers State, Nigeria. For the test organisms to adjust to new environment, they were first acclimatized before conducting the range finding test. Based on the results obtained from the range finding test, the definitive test concentration was established at 0.2, 0.4, 0.6, 0.8, 1.0ppt and 2, 4, 6, 8,10ppt for Tilapia guineensis and Clibanarius africanus respectively. Reference experimental group was also instituted using Potassium Chloride at concentrations of 20, 40, 60, 80 and 100ppt. The median lethal concentration (LC50) was calculated from the mortality value using Probit software. Mortality rates increased with increase in concentration of the toxicants and exposure time. The 96hours-LC50 values for Corexit 9527 were 4669mg/l and 181mg/l for Clibanarius africanus and Tilapia guineensis respectively while that of Finasol OSR were 4268mg/l and 188mg/l for Clibanarius africanus and Tilapia guineensis respectively. The study revealed that Tilapia guineensis was more sensitive as the two dispersants were less toxic to hermit crab and more toxic to the fish. Hence, adequate enforcement of regulations on the use of these chemicals should be adhered to in the water column of marine environments and more attention be inclined to biologically derived chemicals (biosurfactants). Keywords: Acute toxicity, Dispersants, Tilapia guineensis, Clibanarius africanus