Study Identifies Oil Carbons Associated with Deepwater Horizon in Terrestrial Birds

A Seaside Sparrow (Ammodramus maritimus) in the salt marshes. (Photo by Andrea Bonisoli Alquati)

A Seaside Sparrow (Ammodramus maritimus) in the salt marshes. (Photo by Andrea Bonisoli Alquati)

Oil residues on the water’s surface in the salt marshes of Barataria Bay, Louisiana. (Photo by Andrea Bonisoli Alquati)

Oil residues on the water’s surface in the salt marshes of Barataria Bay, Louisiana. (Photo by Andrea Bonisoli Alquati)

Scientists analyzed the carbon composition in Seaside Sparrow tissues to learn if oil from the 2010 spill was incorporated into the terrestrial food web. The researchers found reduced radiocarbon and stable carbon concentration levels in the feathers of birds captured at oiled sites compared with birds from non-oiled sites, which is consistent with a fossil oil source. Oil-source fingerprinting of sediments collected from sites where the oil-exposed birds were found was consistent with Macondo 252 reference oil while sediments from non-oiled sites were not. These findings indicate a food web link between exposure to Deepwater Horizon oil and a terrestrial ecosystem. The researchers published their findings in Environmental Research Letters: Incorporation of Deepwater Horizon oil in a terrestrial bird.

Seaside Sparrows are an indicator species of marsh contamination because their diverse diet, which includes terrestrial and marine invertebrates, exposes them to varied contaminants through multiple routes. While past analyses found that Deepwater Horizon oil entered the coastal food web, no known reports have identified the oil in entirely terrestrial organisms. This study’s researchers analyzed ten Seaside Sparrows, year-round residents of Louisiana marshes that require highly specific habitat for foraging and nesting, to trace the ecological fate of oil from the 2010 spill.

Study author Andrea Bonisoli Alquati explained their method, “As a radioactive isotope, C-14 naturally decays, and its amount in any given material halves every 5,730 years. In oil, which is buried underground for millions of years, there is no radiocarbon. So, any time an organism incorporates oil, its levels of radiocarbon are diluted by the incorporation of radiocarbon-free carbon from oil.”

Radiocarbon and stable carbon levels in feathers were significantly lower in birds from the oiled site compared to the control site, which was consistent with oil incorporation. The team also analyzed sediment collected from the upper 5 cm of each site. Sediments from control and oiled sites did not differ in radiocarbon content or stable carbon isotopes. Three of the five sediment samples from the oiled site exhibited eleven out of eleven biomarkers indicative of Deepwater Horizon oil. None of the diagnostic ratios for the five sediment samples from the control site indicated the presence of Deepwater Horizon oil.

These findings show that oil spills in the ocean can contaminate terrestrial and marine ecosystems.  Bonisoli Alquati explained, “Oil exposure is not limited to aquatic organisms or to the ones that live in between the terrestrial and aquatic environments because for example they have aquatic larvae. Even the entirely terrestrial species that depend on those organisms can be exposed to oil and affected by its toxicity. The boundary between different ecosystems is blurred, and many species – as well as many contaminants – travel across that boundary.”

A potential impact of the oil’s direct toxic effect may have affected sparrow reproduction. Data from the same Seaside Sparrow population indicated reduced reproductive success in oil-exposed birds in years immediately following the spill (Burns, et al., 2014). The researchers suggest that future studies characterize how oil-derived stress propagated through the coastal food web using a larger sample size to examine multiple locations from a wider region and explicitly analyze exposure variation due to age and sex.

“We hope that the study will help us think more broadly about the damage by oil to the salt marshes,” said Bonisoli Alquati. “Salt marshes are important for protecting our coasts, including from extreme weather events, like hurricanes. Damaging one species might translate into damage to others, ultimately affecting the health of the ecosystem, and its ability to protect us.”

Data are publicly available through the Gulf of Mexico Research Initiative Information and Data Cooperative (GRIIDC) at doi:10.7266/N7D21VN2.

The study’s authors are A. Bonisoli Alquati, P.C. Stouffer, R.E. Turner, S. Woltmann, and S.S. Taylor.


This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Coastal Waters Consortium II (CWC II).

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

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