Abstract:
Aggregation is a phenomenon that has been observed following oil spills. The size and number of aggregates can change with specific microbial communities, or the presence of chemical dispersants such as Corexit. In order to assess the impact of oil exposure on the formation of small aggregates (<100um), two mesocosm experiments were conducted during July 2016; GOMOO (Gulf of Mexico Open Ocean) and GOMCOAST (Gulf of Mexico Coastal waters). Oil and dispersant exposure were facilitated using the water accommodated fraction (WAF) of oil, a chemically enhanced WAF (CEWAF, mixture of oil and Corexit) and a dilute CEWAF (DCEWAF). 15mL aliquots were taken every 24h from each mesocosm for immediate analysis on a Z2 dual-threshold Coulter counter (Beckman Coulter). The Coulter counter was outfitted with a 100µm aperture, and particles were counted in three ranges; 10-20µm, 20-50µm and >50µm. The data presented will be particle counts (particles/mL) over time within each size fraction. Estimated oil equivalents are reported in GRIIDC datasets R4.x263.000:0038 and R4.x263.000:0039.
Suggested Citation:
Laura Bretherton. 2018. Particle counts in different size fractions taken during mesocosms GOMOO, GOM open ocean, and GOMCOAST, GOM coastal waters. Distributed by: Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC), Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N7CF9NK6
Methods:
GOMOO Mesocosm methods: Twelve 100L mesocosm tanks were filled with Gulf of Mexico seawater collected from the Flower Gardens National Marine Sanctuary Area (27° 53.4180'N; 94° 2.2020'W) which is located ~120 miles off the coast of Galveston (TX). Four treatments were prepared in triplicate. Control tanks were filled with seawater. 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. Mixing ended 24 hrs after the initial oil addition (Knap et al. 1986; Wade et al. 2017 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 (2 L light /dark bottles, 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. Mixing ended 24 hrs after the initial oil addition. The CEWAF was then introduced into the CEWAF mesocosm tanks and filled to 96 L and mixed. From these CEWAF tanks 13 L was removed for other experiments and analyses (7 L for the DCEWAF mesocosms, 2 L light/dark, 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 (2 L light/dark, 4 L hydrocarbon analyses). To the water in the 12 mesocosms, nutrients were added (final concentration f/20) and the tanks stirred. Banks of lights were placed behind each of the glass mesocosm tanks and a 12:12 light/dark cycle employed. Sampling commenced and defined as time zero. 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. GOMCOAST Mesocosm methods: Twelve 100L mesocosm tanks were filled with Gulf of Mexico seawater collected from the Texas coastline, near TABS buoy R (29° 38.1000'N, 93° 38.5020'W) which is located ~100 miles away from Galveston (TX). Four treatments were prepared in triplicate. Control tanks were filled with seawater. 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. Mixing ended 24 hrs. after the initial oil addition (Knap et al. 1986; Wade et al. 2017in 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 (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. Mixing ended 24 hrs after the initial oil addition. The CEWAF was then introduced into the CEWAF mesocosm tanks and filled to 96 L and mixed. From these CEWAF tanks 13 L was removed for other experiments and analyses (7 L for the DCEWAF mesocosms, 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 (2 L dark/light, 4 L hydrocarbon analyses). To the water in the 12 mesocosms, nutrients were added (final concentration f/20) and the tanks stirred. Banks of lights were placed behind each of the glass mesocosm tanks and a 12:12 light/dark cycle employed. Sampling commenced and defined as time zero. 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. A 20 mL sample was taken from each mesocosm tank every 24 h, and immediately analyzed using a Coulter counter (Beckman-Coulter, Z2) fitted with a 100 µm aperture. Particle concentrations were measured in three size fractions; 5-10 µm, 10-20 µm and 20-50 µm. The particle coincidence at the aperture was monitored to ensure it remained below 5%. Particle coincidence is an instance of more than one particle entering the aperture simultaneously, which can underestimate particle counts. Samples where particle concentrations were high enough to cause particle coincidence of >5% were diluted with filtered seawater (0.2 µm, nylon), and only ever happened in CEWAF tanks. Wade, T. L., S. T. Sweet, J. N. Walpert, J. L. Sericano, J. J. Singer, and N. L. Guinasso Jr. (2011), Evaluation of possible inputs of oil from the Deepwater Horizon spill to the Loop Current and associated eddies in the Gulf of Mexico, in Monitoring and Modeling the Deepwater Horizon Oil Spill: A Record-Breaking Enterprise, Geophys. Monogr. Ser., doi:10.1029/2011GM 001095. Knap, A. H., Burns, K.A., Dawson, R., Ehrhardt, M., and Palmork, K.H. (1986), Dissolved/dispersed Hydrocarbons, Tarballs and the Surface Microlayer: Experiences from an IOC/UNEP Workshop in Bermuda. Mar. Pol. Bull., 17(7): 313-319.
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