Crude oil plus dispersant: always a boon or bane?

The toxicities of a Nigerian brand of crude oil (Forcados Light), a newly approved dispersant for use in Nigerian ecosystems (Biosolve), and their mixtures, based on ratios 9:1, 6:1 and 4:1 (v/v), were evaluated against the juvenile stage of prawn, Macrobrachium vollenhovenii, in laboratory bioassays. On the basis of the derived toxicity indices, crude oil with 96-h LC50 value of 0.28 ml/L was found to be about six times more toxic than the dispersant (96-h LC50 1.9 ml/L) when acting alone against M. vollenhovenii. Toxicity evaluations of the mixtures of crude oil and dispersant meant to simulate the environmental control settings of crude oil spillages in aquatic ecosystems revealed that effects of the crude oil/dispersant mixtures varied, depending largely upon the proportion of addition of the mixture components. The interactions between mixture of crude oil and dispersant at the test ratios of 9:1 and 4:1 were found to conform with the model of synergism (RTU=1.2 and 2.1, respectively), while the interactions between the mixture prepared based on ratio 6:1 conformed with the model of antagonism (RTU=0.16), based on the concentration-addition model. Furthermore, the mixtures prepared based on ratios 9:1 and 6:1 were found to be less toxic than crude oil when acting singly against M. vollenhovenii while the mixture prepared based on ratio 4:1 was found to have similar toxicity with crude oil when acting singly, based on the derived synergistic ratio values. The importance of results obtained from the joint-action tests in setting effective and environmentally safe dispersal ratios is discussed.