Abstract:
Mesocosm experiments were performed with seawater collected from the Northern Gulf of Mexico (NGOM) and “seeded” with microbial populations collected from Galveston Bay, TX. Four treatments were set up in triplicate (A) Control, (B) water accommodated oil fraction or WAF, (C) water accommodated oil fraction with Corexit 9500 or CEWAF, and (D) a 10-fold diluted equivalent of (C), called DCEWAF.
Methods:
Photos were taken with an iphone 6 to capture the various sizes, shapes and forms of the marine snow which formed. Mesocosm methods – COASTal water with coastal microbial concentrate, COAST. Twelve 100L mesocosm tanks were filled with Gulf of Mexico seawater collected 5 miles off shore from Galveston (TX) that had been pre-treated with a charcoal filter to remove large particles and debris. Plankton (<63 µm) were collected using a net and transferred into polycarbonate bottles. This concentrated plankton mass was introduced to the tanks (2 L to each final volume 83 L) immediately prior to starting the experiments. Four treatments were prepared in triplicate. Control tanks were filled with seawater and plankton mixture. Water accommodated fraction (WAF) of oil was prepared by mixing 25 mL (5 ml ~ every 30 min for 2.5 hrs) of Macondo surrogate oil into 130 L of seawater then mixing for 12 to 24 hrs. (Knap et al. 1986; Knap et al. 2016 in preparation). The WAF was then introduced into the WAF mesocosm tanks and filled to 87 L and mixed. From these WAF tanks 6 L was removed for other experiments and analyses (2L for roller tables, 2 L dark/light, 4 L hydrocarbon analyses). In order to make chemically enhanced water accommodated fraction (CEWAF), Corexit was mixed with oil in a ratio of 1:20 and 25 mL of this mixture (5 ml every 30 min for 2.5 hrs) of surrogate oil plus Corexit was added to 130 L of seawater which was mixed for 8 to 24 hrs prior to being loaded into the mesocosm tanks. The CEWAF was then introduced into the CEWAF mesocosm tanks and filled to 96 L and mixed. From these CEWAF tanks 15 L was removed for other experiments and analyses (9 L for the DCEWAF mesocosms, 2L for roller tables, 2 L dark/light, 4 L hydrocarbon analyses). Diluted CEWAF (DCEWAF) was prepared by mixing 9 L of CEWAF with 78 L of the original seawater for a total volume of 87 L. From these DCEWAF tanks 6 L was removed for other experiments and analyses (2L for roller tables, 2 L dark/light, 4 L hydrocarbon analyses). The estimated oil equivalents (EOE) were determined using Macondo surrogate oil as the calibration standard (Wade et al. 2011) for the fluorescence analyses (Horiba Scientific Aqualog Fluorometer). The EOE mean concentration of the three mesocosms for the control, WAF, DCEWAF and CEWAF at the start of the experiments were 0 mg/L, 0.26 mg/L, 2.74 mg/L and 41.5 mg/L, respectively. The EOE mean concentration of the three mesocosms for the in the control, WAF, DCEWAF and CEWAF after 72 hours were 0 mg/L , 0.06 mg/L, 1.03. and 17.3 mg/L, respectively. Knap, A. H., T. D. Sleeter, R. E. Dodge, S. C. Wyers, H. R. Frith, and S. R. Smith. 1983. The effects of oil spills and dispersants use on corals: A review and multidisciplinary experimental approach. Oil and Petrochemical Pollution 1: 157–169. Wade, T.L., Sweet S.T., Sericano, J.L., N.L. Guinasso Jr., Diercks, A.-R., Highsmith, R.C., Asper, V.L., Joung, D., Shiller, A.M., Lohrenz, S.E. and Joye, S.B. 2011, Analyses of Water Samples from the Deepwater Horizon Oil Spill: Documentation of the Sub-Surface Plume. in Monitoring and Modeling the Deepwater Horizon Oil Spill: A Record-Breaking Enterprise, Geophysical Monograph Series, vol. 195, edited by Y. Liu et al., pp. 77–82, AGU, Washington, D. C., doi:10.1029/2011GM001103
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