An Evaluation of Select Test Variables Potentially Affecting Acute Oil Toxicity.

In the wake of the Deepwater Horizon incident (2010) in the Gulf of Mexico, an abundance of research studies have been performed, but the methodologies used have varied making comparisons and replication difficult. In this study, acute toxicity tests with mysids and inland silversides were performed to examine the effect of different variables on test results. The toxicity test variables evaluated in this study included (1) open versus closed static test chambers, (2) natural versus artificial diluent, (3) aerated versus nonaerated test solution, and (4) low versus medium energy water-accommodated (WAF) mixing energies. The use of tests using natural or artificial diluent showed no difference in either toxicity test or analytical chemistry results. Based on median lethal concentrations (LC50) of WAFs of unweathered oil (MASS), mysid tests performed in closed chambers were approximately 41 % lower than LC50 values from open-chamber studies, possibly a result of the presence of low-molecular weight volatile aromatics (i.e., naphthalenes). This research also showed that using a medium-energy WAF (with a 20­25 % vortex) increases the number of chemical components compared with low-energy WAF, thus affecting the composition of the exposure media and increasing toxicity. The comparison of toxic units as a measure of the potential toxicity of fresh and weathered oils showed that weathered oils (e.g., Juniper, CTC) are less toxic than the unweathered MASS oil. In the event of future oil spills, these variables should be considered to ensure that data regarding the potential toxicity and environmental risk are of good quality and reproducible.

Acute aquatic toxicity studies of Gulf of Mexico water samples collected following the Deepwater Horizon incident (May 12, 2010 to December 11, 2010).

The potential for the Deepwater Horizon MC-252 oil incident to affect ecosystems in the Gulf of Mexico (GOM) was evaluated using Americamysis bahia, Menidia beryllina and Vibrio fischeri (Microtox® assay). Organisms were exposed to GOM water samples collected in May-December 2010. Samples were collected where oil was visibly present on the water surface or the presence of hydrocarbons at depth was indicated by fluorescence data or reduced dissolved oxygen. Toxicity tests were conducted using water-accommodated fractions (WAFs), and oil-in-water dispersions (OWDs). Water samples collected from May to June 2010 were used for screening tests, with OWD samples slightly more acutely toxic than WAFs. Water samples collected in July through December 2010 were subjected to definitive acute testing with both species. In A. bahia tests, total PAH concentrations for OWD exposures ranged from non-detect to 23.0 µg L(-1), while WAF exposures ranged from non-detect to 1.88 µg L(-1). Mortality was >20% in five OWD exposures with A. bahia and three of the WAF definitive tests. Total PAH concentrations were lower for M. beryllina tests, ranging from non-detect to 0.64 µg L(-1) and non-detect to 0.17 µg L(-1) for OWD and WAF exposures, respectively. Only tests from two water samples in both the WAFs and OWDs exhibited >20% mortality to M. beryllina. Microtox® assays showed stimulatory and inhibitory responses with no relationship with PAH exposure concentrations. Most mortality in A. bahia and M. beryllina occurred in water samples collected before the well was capped in July 2010 with a clear decline in mortality associated with a decline in total PAH water concentrations.