Abstract:

Testing the direct effect of oil or oil+Corexit 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 young Chaetoceros: Rolling table experiment to test the aggregation of diatoms, TEP (transparent exopolymers), POC (particulate organic carbon) as a function of oil and oil+Corexit. Distributed by: Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC), Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N78K7743

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 one experiment of two investigating the effects of oil and oil+Corexit on the formation of diatoms aggregates. 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. In this experiment an exponentially growing Chaetoceros was used, and three treatments analyzed: the control that contained only the culture, the OIL treatment that contained 1 mL oil per L culture and the Oil+Corexit treatment, which additionally contained Corexit. Samples were analysed for cell abundance, particulate organic carbon (POC), PO13C and transparent exopolymer particles (TEP).

Data Parameters and Units:

treatment tank replicate: repl. Calculated: calc. particulate organic carbon = POC, ug/tank particulate organic nitrogen = PON, ug/tank transparent exopolymer particles = TEP, ug Gxeq/ tank, microgram xanthan equivalents/tank aggregates > 1 mm = agg Surrounding seawater = SSW Marine snow = MS Chaetoceros sp. The ratio between POC and PON per weight = C:N Control treatment = Contr Treatment with oil, Macondo oil, the surrogate for the oil spilled during the Deepwater Horizon spill (1 ml oil per Liter) = OIL Treatment with oil and Corexit, Corexit 9500A, the dispersant used predominately during the Deepwater Horizon spill (10µL per mL oil) = oil+Cor-10 Replicates = repl Average = avg Standard deviation = std The fraction of organic carbon due to cells = Cell-C The fraction of organic carbon due to oil or oil + corexit = Oil+cor-C PO13C = 13C signal of particulate organic carbon empty fields in data file refer to no data collection

Methods:

Replicate acrylic roller tanks of 1.16 L volume were filled bubble free. Experiments were incubated in the dark, at 20˚C at an rpm of 2.4 (1 & 2). Experiments were conducted with Macondo surrogate oil and Chaetoceros sp., (isolated Nov. 2014 at 38.70°N, 123.67°W) a common bloom forming diatom. Three types of treatments, each in duplicate, were compared: a control (diatom culture), an oil addition (OIL) and an oil plus Corexit addition (Oil+Cor). To prepare the OIL treatments, one mL of Macondo oil (Marlin Platform, Dorado source oil) was slowly added into the center of the turning tank, using a syringe inserted through the silicon stopper. Corexit 9500A (Clean Seas), either 10 µL or 30 µL, was added for the oil plus Corexit treatments (DOR: 1: 33 or 1: 10). Upon addition of source oil, oil droplets rose to the top, and especially in the absence of Corexit, a slick layer formed, which was not available for aggregation and was not sampled. Table 1a: Treatments of experiment Treatment Control 1.16L Chaetoceros OIL (Chemically undispersed oil) 1.16L Chaetoceros + 1 mL source oil Oil+Cor-10 or Oil+Cor-30 (Corexit-dispersed oil) 1.16L Chaetoceros + 1 mL source oil + 10 µL Corexit 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), and PO13C. Dispersed oil concentrations and partitioning between the aggregate and SSW phase was determined using POC and PO13C data. PO13C of the diatoms, the oil and the Corexit were separately determined (quadruplicates) and a two endmember mass balance calculation used to estimate the relative contributions of (i) cells and (ii) oil or oil+corexit to the particulate organic carbon (POC). Duplicate filters (GF/F) prepared for POC analysis were measured in a CEC44OHA elemental analyzer (Control equipment). The PO13C signature was determined on replicate filters using a Finnigan Delta Plus Advantage. 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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