Abstract:

Testing the direct effect of Corexit when added to oil contaminated natural seawater on aggregation of diatoms. Once aggregates had formed samples were collected separately from the surrounding seawater and the marine snow fraction.

Suggested Citation:

Uta Passow. 2017. Impact of Corexit on aggregation of Skeletonema grethae or natural particles in oil contaminated natural seawater, low oil concetrations: Rolling table experiment to test the aggregation of diatoms, TEP (transparent exopolymers), POC (particulate organic carbon) as a function of the presence of Corexit and investigate the change in marine snow over time. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N7WD3XNF

Publications:

Passow, U., Sweet, J., & Quigg, A. (2017). How the dispersant Corexit impacts the formation of sinking marine oil snow. Marine Pollution Bulletin. doi:10.1016/j.marpolbul.2017.08.015

Purpose:

This is an experiment testing the impact of Corexit on aggregation of natural particles or Skeletonema grethae in naturally dispersed oil (NWAF) after the Refugio Oil spill off California May 2015. The NWAF was collected 5/28/2015; 9 days after the spill. Roller tank experiments mimic in situ conditions of the ocean in that continuous sinking of particles is possible (infinity water column). Once aggregates > 1mm (operationally defined) have formed, they are harvested separately from the surrounding seawater, which contains non-aggregates cells or small aggregates. Both fractions are analyzed separately, so that the fraction of material incorporated within aggregates may be calculated. Aggregation of natural particles or of natural particles plus Skeletonema was monitored in two treatments each: NWAF or NWAF+Skel treatments contained natural seawater and where appropriate diatoms that had been contaminated with oil. The NWAF-Cor and the NWAF+Skel+Cor treatments additionally contained Corexit (50 µL per liter sample). Samples were analysed for cell abundance, particulate organic carbon (POC), and transparent exopolymer particles (TEP).

Data Parameters and Units:

Experiment, tank, sample, fraction, replicate, transparent exopolymer particles = TEP (GXeq/tank), particulate organic carbon = POC (ug/tank), C:N – weight ratio of POC to PON, average (avg) cells/tank, standard deviation (std) of 6 replicates cells/tank µg Gum Xanthan equivalent/tank or µg Xeq/tank aggregates > 1 mm = agg. SSW = surrounding seawater NWAF+Skel+Cor-50 = treatment with natural accommodated oil fraction plus Skeletonema plus 50µL Corexit per liter sample

Methods:

Replicate acrylic roller tanks of 1.16 L volume were filled bubble free. Experiments were incubated in the dark, at 15˚C at an rpm of 1.4. Experiments were conducted with oil-contaminated seawater (NWAF) collected by bucket from the surface near Santa Barbara, CA (34˚27 N, 120˚03 W) 9 days after the Refugio pipeline spill. Skeletonema grethae (CCMP 775) was added to replicate treatments prepared for each experiment: The oil-contaminated seawater without and with S. grethae (in early stationary phase) was compared to the respective treatments where 50 µL Corexit per liter was added. The oil-contaminated seawater was siphoned into roller tanks from below the bucket surface. Aggregate appearance and numbers were monitored during all experiments. When sufficient aggregates had formed, all treatments were harvested. Aggregates > 1mm (visually discernable) were manually collected and analyzed. After all aggregates > 1 mm were removed the remainder of the material, termed surrounding seawater (SSW), was subsampled. Both fractions were analyzed for POC, cell abundance, TEP (transparent exopolymer particles). Duplicate filters (GF/F) prepared for POC analysis were measured in a CEC44OHA elemental analyzer (Control equipment). Diatom cells were counted (Olympus CX41) using a hemocytometer; at least 6 subsamples and 200 cells each were counted per sample. Counts were at times only conducted on 1 of the two replicate treatments. TEP concentrations were determined in triplicate using the colorimetric method and are expressed in Gum Xanthan equivalents (GXeq.). Corexit binds to the dye Alcian Blue generating artificially high “TEP” values in the presence of Corexit, thus TEP concentrations can’t be determined in the presence of Corexit. All biomass results (cell abundance, POC, TEP concentration) are normalized per tank to allow budgets and make aggregate and SSW fractions directly comparable.

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