Mesocosm Studies: Polycyclic Aromatic Hydrocarbons (PAHS) Exposure, Concentrations and Removal Rates

Polycyclic Aromatic Hydrocarbons (PAHs) are toxic constituents found in crude oils, which can be removed from the water column through a combination of processes: evaporation, sedimentation, photo-oxidation and/or biodegradation, collectively termed weathering. Marine snow consists of many particles including bacteria, phytoplankton, mineral particles, fecal pellets and aggregates and plays an important role in the process of removing PAHs from the water column through sedimentation and enhanced biodegradation. The microbial community produces exopolymeric substances (EPS) in response to stresses including exposure to petroleum may lead to excess production of marine snow, therefore affecting the biodegredation and transport and fate of PAHs. This research hypothesizes that the PAH removal from the water column is enhanced by microbial activity in the presence of petroleum and petroleum plus Corexit. In this study, mesocosm experiments were used to investigate PAH half-lives when petroleum and dispersants are present. The first four mesocosm experiments were undertaken with water collected from near shore or off-shore locations in the Gulf of Mexico. As part of these studies, oil and oil plus dispersant mixtures known as WAF (water accommodated oil fraction) and CEWAF (chemically enhanced water accommodated fraction) were generated in 130 L baffled recirculation tanks, and ~80 L transferred to the mesocosms. A 1:10 dilution of the CEWAF (DCEWAF) was an additional mesocosm treatment. Control treatments with no oil or dispersant were used for comparison. Concentrated phytoplankton collected from Galveston Bay were added to all mesocosm tanks. In mesocosm 3 (M3) and 4 (M4) f/20 nutrient additions were made. Total scanning fluorescence (TSF) analysis was performed to determine estimate oil equivalents (EOE) concentrations at the start, during and at the end of the experiment. PAH composition and concentration were determined using Gas Chromatography/Mass Spectroscopy (GC/MS). The concentrations of EOE and PAH as well as changes in the PAH composition of the WAF, DCEWAF and CEWAF over time were determined. Biomarker data were measured in selected samples in order to investigate the biodegradation process. The mesocosm experiments were designed to: 1) simulate the conditions of DWH oil spill using WAF, DCEWAF and CEWAF generated from a baffled recirculating system; 2) establish a relationship between EOE measured by TSF that allows for real time oil concentration estimates in mesocosm experiments; 3) compare PAH removal pattern under different biological conditions and 4) examine the impact of Corexit addition in the removal half-lives of PAH, providing additional information for evaluation of future usage of Corexit during marine oil spills