Lethal and sublethal effects of oil and chemical dispersant on the ctenophore Mnemiopsis leidyi
University of Delaware
Characterization of both the lethal and sublethal effects of oil and chemical dispersants in key species is important for developing an understanding of the impacts of oil spills and their associated mitigation techniques on coastal ecosystems. This study established the lethal levels of COREXIT® 9500A chemical dispersant, MC-252 weathered crude oil (WAF), and MC-252 weathered crude oil dispersed by COREXIT 9500A (CEWAF) for the trophically important ctenophore Mnemiopsis leidyi at both 15°C and 23°C. M. leidyi was highly sensitive to COREXIT 9500A at both temperatures (15°C: 24 hr LC50 = 6.91 mg/L; 23°C: 24 hr LC50 = 8.26 mg/L), as well as to oil solutions. There was an apparent increase in toxicity of dispersed oil compared to crude oil by loading concentration (15°C: CEWAF 48 LC50= 6.7 mg/L; WAF 48 LC50= 29.49 mg/L), but this difference was less pronounced when the total petroleum hydrocarbons of the solutions were considered (15°C: CEWAF 48 LC50= 242.07 μg/L; WAF 48 LC50 = 143.14 μg/L). Sublethal assays for glutathione-s-transferase (GST) activity, routine respiration rate, and bioluminescence were conducted on individuals surviving 24 hr exposure to various concentrations of the test solutions, the former at 15°C only and the latter two at both 15°C and 23°C. GST activity increased significantly in low COREXIT 9500A solutions at 15°C, as did weight specific respiration rate in the high dispersant and CEWAF treatments, suggesting a metabolic detoxification response to the dispersant containing solutions. The total light response from mechanically and chemically stimulated bioluminescence decreased with exposure to the WAF and CEWAF of MC-252 weathered crude oil at both temperatures, and changes in intracellular calcium and photoprotein availability were indicated. This study demonstrates that M. leidyi is susceptible to the toxic effects of oil and chemical dispersant upon both lethal and sublethal exposure, with the addition of chemical dispersant resulting in increased toxicity compared to oil alone. The sensitivity of M. leidyi to oil and dispersant could have ecosystem wide effects in the event of a coastal spill.