Comparative Toxicity of Two Chemical Dispersants and Dispersed Oil in Estuarine Organisms

Chemical dispersants can be a useful tool to mitigate oil spills. This study examined potential risks to sensitive estuarine species by comparing the toxicity of two dispersants (Corexit^® EC9500A and Finasol^® OSR 52) individually and in chemically enhanced water-accommodated fractions (CEWAFs) of Louisiana Sweet Crude oil. Acute toxicity thresholds and sublethal biomarker responses were determined in seven species (sheepshead minnow, grass shrimp, mysid, amphipod, polychaete, hard clam, mud snail). Comparing median lethal (LC₅₀) values for the dispersants, Finasol was generally more toxic than Corexit and had greater sublethal toxicity (impaired embryonic hatching, increased lipid peroxidation, decreased acetylcholinesterase activity). The nominal concentration-based mean LC₅₀ for all species tested with Corexit was 150.31 mg/L compared with 43.27 mg/L with Finasol. Comparing the toxicity of the CEWAFs using the nominal concentrations (% CEWAF), Corexit-CEWAFs appeared more toxic than Finasol-CEWAFs; however, when LC₅₀ values were calculated using measured hydrocarbon concentrations, the Finasol-CEWAFs were more toxic. There was greater dispersion efficiency leading to greater hydrocarbon concentrations measured in the Corexit-CEWAF solutions than in equivalent Finasol-CEWAF solutions. The measured concentration-based mean LC₅₀ values for all species tested with Corexit-CEWAF were 261.96 mg/L total extractable hydrocarbons (TEH) and 2.95 mg/L total polycyclic aromatic hydrocarbons (PAH), whereas the mean LC₅₀ values for all species tested with Finasol-CEWAF were 23.19 mg/L TEH and 0.49 mg/L total PAH. Larval life stages were generally more sensitive to dispersants and dispersed oil than adult life stages within a species. These results will help to inform management decisions regarding the use of oil-spill dispersants.

The effects of the contemporary-use insecticide (fipronil) in an estuarine mesocosm

To examine the effects of environmentally realistic fipronil concentrations on estuarine ecosystems, replicated mesocosms containing intact marsh plots and seawater were exposed to three treatments of fipronil (150, 355, and 5000 ng/L) and a Control. Juvenile fish (Cyprinidon variegatus), juvenile clams (Mercenaria mercenaria), oysters (Crassostrea virginica), and grass shrimp (Palaemonetes pugio) were added prior to fipronil in an effort to quantify survival, growth, and the persistence of toxicity during the planned 28-day exposure. Results indicated that there were no fipronil-associated effects on the clams, oysters, or fish. Shrimp were sensitive to the highest two concentrations (40% survival at 355 ng/L and 0% survival at 5000 ng/L). Additionally, the highest fipronil treatment (5000 ng/L) was toxic to shrimp for 6 weeks post dose. These results suggest that fipronil may impact shrimp populations at low concentrations and further use in coastal areas should be carefully assessed.