Studies Describe how Oil Gets into Marine Snow, Improving Oil Fate Predictions

Researchers use green light to help them look at marine oil snow aggregates in a laboratory setting. Photo Credit: Julia Sweet

Researchers use green light to help them look at marine oil snow aggregates in a laboratory setting. Photo Credit: Julia Sweet

Scientists conducted rolling table experiments to improve our understanding of how marine oil snow forms and to provide input parameters for models that predict oil transport via sinking marine snow. The results show that oil incorporates into marine snow primarily by aggregated phytoplankton that 1) trap whole residue oil droplets, introducing mainly insoluble n-alkanes and high molecular weight fractions into marine snow; and 2) had oil stuck on them via adsorption, introducing mainly soluble and low molecular weight fractions into marine snow. Following oil exposure, phytoplankton trapped oil almost immediately, but adsorption took more than a day longer. The maximum carrying capacity of phytoplankton aggregates for dispersed oil is ~40% of the organic carbon and their sinking velocity was slightly faster compared to their non-oil-containing counterparts. The presence of COREXIT increased insoluble n-alkanes and high molecular weight oil fractions in marine oil snow, suggesting that sedimented marine oil snow may also contain higher amounts of these oil fractions.

The authors published their findings in two journals, Marine Chemistry Partitioning of oil compounds into marine oil snow: Insights into prevailing mechanisms and dispersant effect and Marine Ecology Press Series Incorporation of oil into diatom aggregates.

The sedimentation of oil via sinking marine snow (an aggregation of phytoplankton, feces, and other marine debris particles) during and after Deepwater Horizon (known as MOSSFA or Marine Oil Snow Sedimentation and Flocculent Accumulation) was an important mechanism for its vertical transport (Study Summarizes Current Knowledge on Marine Oil Snow During and After Deepwater Horizon).

“We knew from sediment trap data that there is a clear correlation between the sinking of algae cells and oil but did not understand the mechanisms,” explained study author Uta Passow, who is a member of several research teams studying marine oil snow. Of the two processes they identified for how oil becomes incorporated into marine snow, Passow said that oil trapping is more significant than oil adsorbing.

Before the team’s experiments, they thought that aggregates containing oil would sink more slowly than equally-sized aggregates without oil. “Interestingly, the sinking velocity was not reduced in aggregates containing oil compared to those with no oil, although oil itself is lighter than seawater and does not sink,” said Passow. “We think that the oil allows for cells to be more tightly packed in the presence of oil. If oil allows for more cells to fit into an aggregate of a specific size, then the cells that are heavier than seawater can compensate for the lighter oil and the aggregate as a whole will sink as fast or potentially even faster than those without oil.”

The team did not observe any effects of oil or dispersant on the degree of phytoplankton aggregation, which contrasts with some of their earlier research (Study Explains Pathways for Oiled Marine Snow Formation and Study Explores Complex Dispersant Effects on Marine Oil Snow Formation), partly because high phytoplankton concentrations led to maxima aggregation rates independent of oil or COREXIT. The presence of COREXIT, which increases oil droplet size and concentration, will lead to increased incorporation of oil into aggregates. Additionally, because COREXIT enhances the dissolution rates of oil compounds, thus decreasing their concentration in oil droplets, the oil incorporated into marine snow by trapping is likely enriched with high molecular weight fraction compounds.

“The chemical composition of these oil droplets differs from that of the spilled crude oil because of weathering; therefore, the composition of the oil incorporated into sinking aggregates depends on their history, but non-soluble and non-volatile compounds usually make the main contribution.”

In agreement with this study’s results that suggest preferential sedimentation of high molecular weight oil fractions, there were observations of these oil fractions in different northern Gulf regions during and after Deepwater Horizon (Study Demonstrates Sinking Marine Particles Help Remove PAHs from Water Column, Chemical Study Shows Different Patterns of Oil Weathering, and Hydrocarbons in Deep-Sea Sediments following the 2010 Deepwater Horizon Blowout in the Northeast Gulf of Mexico).

The toxicity of high molecular weight oil fractions incorporated into marine snow may help explain detrimental effects on benthic organisms (Study Reveals Oil Spill Changed Oxygen Conditions in Gulf Sediment and Study Documents Initial Impacts and Recovery of Benthic Foraminifera after Deepwater Horizon).

Authors for the study in Marine Chemistry are Marisa A. Wirth, Uta Passow, Jenny Jeschek, Ines Hand, and Detlef E. Schulz-Bull. Data is available at the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at DOI 10.7266/N7ZP44KS, DOI 10.7266/N7TX3CTM, and DOI 10.7266/N7Q52N25.

Authors for the study in Marine Ecology Press Series are Uta Passow, Julia Sweet, Simone Francis, Chen Xu, Anusha Dissanayake, Y.-Y. Lin, Peter Santschi, and Antonietta Quigg. Data is available at the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at DOI 10.7266/N78W3BP3, DOI 10.7266/N70K26Z2, DOI 10.7266/N77D2SHQ, DOI 10.7266/N73N21SC, DOI 10.7266/N7ZW1JBJ, and DOI 10.7266/N780517B.

By Nilde Maggie Dannreuther. Contact maggied@ngi.msstate.edu with questions or comments.

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This research was made possible in part by grants from the Gulf of Mexico Research Initiative (GoMRI) to the Aggregation and Degradation of Dispersants and Oil by Microbial Exopolymers (ADDOMEx) consortium, the Aggregation and Degradation of Dispersants and Oil by Microbial Exopolymers 2 (ADDOMEx 2) consortium, the Ecosystem Impacts of Oil and Gas Inputs to the Gulf-2 (ECOGIG-2) consortium, and to the University of Georgia for the project Oil-Marine Snow-Mineral Aggregate Interactions and Sedimentation during the 2010 Deepwater Horizon Oil Spill. Other support included the Multi Partner Oil Research, Canada and the Leibniz Institute for Baltic Sea Research.

The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.

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