Comparative toxicity of eight oil dispersants, Louisiana sweet crude oil (LSC), and chemically dispersed LSC to two aquatic test species.

The present study describes the acute toxicity of eight commercial oil dispersants, South Louisiana sweet crude oil (LSC), and chemically dispersed LSC. The approach used consistent test methodologies within a single laboratory in assessing the relative acute toxicity of the eight dispersants, including Corexit 9500A, the predominant dispersant applied during the DeepWater Horizon spill in the Gulf of Mexico. Static acute toxicity tests were performed using two Gulf of Mexico estuarine test species, the mysid shrimp (Americamysis bahia) and the inland silversides (Menidia beryllina). Dispersant-only test solutions were prepared with high-energy mixing, whereas water-accommodated fractions of LSC and chemically dispersed LSC were prepared with moderate energy followed by settling and testing of the aqueous phase. The median lethal concentration (LC50) values for the dispersant-only tests were calculated using nominal concentrations, whereas tests conducted with LSC alone and dispersed LSC were based on measured total petroleum hydrocarbon (TPH) concentrations. For all eight dispersants in both test species, the dispersants alone were less toxic (LC50s: 2.9 to >5,600 µl/L) than the dispersant-LSC mixtures (0.4-13 mg TPH/L). Louisiana sweet crude oil alone had generally similar toxicity to A. bahia (LC50: 2.7 mg TPH/L) and M. beryllina (LC50: 3.5 mg TPH/L) as the dispersant-LSC mixtures. The results of the present study indicate that Corexit 9500A had generally similar toxicity to other available dispersants when tested alone but was generally less toxic as a mixture with LSC.

Multiple regression models for hindcasting and forecasting midsummer hypoxia in the Gulf of Mexico.

A new suite of multiple regression models was developed that describes relationships between the area of bottom water hypoxia along the northern Gulf of Mexico and Mississippi-Atchafalaya River nitrate concentration, total phosphorus (TP) concentration, and discharge. Model input variables were derived from two load estimation methods, the adjusted maximum likelihood estimation (AMLE) and the composite (COMP) method, developed by the U.S. Geological Survey. Variability in midsummer hypoxic area was described by models that incorporated May discharge, May nitrate, and February TP concentrations or their spring (discharge and nitrate) and winter (TP) averages. The regression models predicted the observed hypoxic area within +/-30%, yet model residuals showed an increasing trend with time. An additional model variable, Epoch, which allowed post-1993 observations to have a different intercept than earlier observations, suggested that hypoxic area has been 6450 km2 greater per unit discharge and nutrients since 1993. Model forecasts predicted that a dual 45% reduction in nitrate and TP concentration would likely reduce hypoxic area to approximately 5000 km2, the coastal goal established by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force. However, the COMP load estimation method, which is more accurate than the AMLE method, resulted in a smaller predicted hypoxia response to any given nutrient reduction than models based on the AMLE method. Monte Carlo simulations predicted that five years after an instantaneous 50% nitrate reduction or dual 45% nitrate and TP reduction it would be possible to resolve a significant reduction in hypoxic area. However, if nutrient reduction targets were achieved gradually (e.g., over 10 years), much more than a decade would be required before a significant downward trend in both nutrient concentrations and hypoxic area could be resolved against the large background of interannual variability. The multiple regression models and statistical approaches applied provide improved capabilities for evaluating dual nutrient management strategies to address Gulf hypoxia and a clearer perspective on the strengths and limitations of approaching the problem using regression models.