Embryotoxicity of mixtures of weathered crude oil collected from the Gulf of Mexico and Corexit 9500 in mallard ducks (Anas platyrhynchos)

Dispersants are applied to marine crude oil spills to enhance microbial degradation and reduce impacts of crude oils on ecosystems. In summer 2010, the dispersant Corexit 9500 was applied to crude oil in the Gulf of Mexico. The co-occurrence of the Deepwater Horizon oil spill with nesting efforts of birds in the Gulf region may have resulted in exposure of adult birds, and subsequently bird eggs, to combinations of crude oil and Corexit 9500. The objective of this study was to examine the embryotoxicity of 50:1 and 10:1 mixtures of weathered crude oil collected from the Gulf of Mexico and Corexit 9500 applied to mallard duck eggs. Combinations of weathered crude oil and Corexit 9500 were applied to eggshells of mallard ducks via paintbrush in varying masses ranging from 0.1 to 59.9 mg and 0.1 to 44.9 mg for 50:1 and 10:1 mixtures, respectively. Conservatively derived median lethal applications for 50:1 and 10:1 mixtures of weathered crude oil and Corexit 9500 were 21.3 ± 4.9 mg/egg (321.8 μg/g egg) and 33.1 ± 11.8 mg/egg (517.0 μg/g egg), respectively. Spleen mass of hatchlings exposed to the 50:1 mixture was the only physiological measure significantly different from controls of both mixtures. Results indicated that decreasing ratios of dispersant relative to weathered crude oil decreased toxicity to mallard embryos. In comparison to treatments of eggs with weathered crude oil alone, toxicity increased when the oil to dispersant ratio was 50:1, but decreased with the mixture that contained more dispersant (10:1).

► We examined toxicity of mixtures of weathered oil and Corexit 9500 in bird embryos.
► Increasing amount of Corexit 9500 relative to weathered oil decreased toxicity.
► The 50:1 mixture was less toxic than weathered crude oil alone.
► The 10:1 mixture was slightly more toxic than weathered crude oil alone.