Research paper
Benthic foraminiferal morphological response to the 2010 Deepwater Horizon oil spill

https://doi.org/10.1016/j.marmicro.2021.101971Get rights and content

Highlights

  • Benthic foraminifera species exhibit shape changes in response to Deepwater Horizon.

  • The pre and post-Deepwater Horizon length/width ratios are significantly different.

  • Reduced oxygen concentration is likely a long-term driver of shape change.

  • Enhanced organic carbon concentration is likely a short-term driver of shape change.

  • This study has improved our understanding of species responses to oil spills.

Abstract

The 2010 Deepwater Horizon (DWH) oil spill released 4.0 million barrels of petroleum into the Northern Gulf of Mexico (NGoM), causing a substantial Marine Oil Snow, Sedimentation and Flocculent Accumulation (MOSSFA) event. Benthic foraminifera have successfully been used as bioindicators of the event with an 80–93% decline in density coinciding with the oil spill, followed by a recovery period to a steady state within three to five years. Here we present the results of a size and morphological study of four species of benthic foraminifera which remained abundant through the DWH oil spill (Uvigerina peregrina, Brizalina subaenariensis var. mexicana, Bulimina marginata and Cassidulina teretis) to understand how different species were impacted and why such a loss in density occurred. Short-lived radioisotope dating distinguished the pre-DWH and post-DWH depth intervals from 2010 to 2015. The species exhibit shape changes in response to the oil and MOSSFA processes and the pre-DWH length/width ratios of Uvigerina peregrina and Brizalina subaenariensis var. mexicana are significantly different than post-DWH length/width ratios within the cores. Reduced oxygen levels were likely a long-term (3–5 years) driver of this shape change and enhanced organic carbon concentrations were a short-term (1–2 years) driver. This study demonstrated the importance of the benthic foraminiferal morphological response to the DWH oil spill and furthers our understanding of the strategies of taxa that occurred abundantly throughout the record.

Introduction

The Deepwater Horizon (DWH) oil spill was one of the largest in history, releasing 4.0 million barrels of petroleum into the Northern Gulf of Mexico (NGoM) over 87 consecutive days in April 2010 (U.S. DistrictCourt, 2015). Alongside the release of oil, large quantities of chemical dispersant was distributed in an effort to clean-up the oil (Kujawinski et al., 2011). This combination of oil and chemicals released into the marine environment both at the surface and at depth led to sudden environmental and ecological changes (Camilli et al., 2010; Kessler et al., 2011). This included the formation of a substantial Marine Oil Snow, Sedimentation and Flocculent Accumulation (MOSSFA) event (Schwing et al., 2017).

The MOSSFA event was one of the initial biological responses to the oil spill. MOSSFA is caused by oil droplets attracting particles and forming aggregates (Passow et al., 2012), which lose buoyancy (Brooks et al., 2015), thereby, transporting oil to benthic ecosystems (Daly et al., 2016). After DWH, the MOSSFA event led to enhanced sedimentation rates (Brooks et al., 2015), and the associated oxidation of organic material arriving to the seafloor led to decreased levels of dissolved oxygen and reducing conditions on the seafloor (Beyer et al., 2016; Hastings et al., 2016). The MOSSFA also caused a three-fold increase in Polycyclic Aromatic Hydrocarbon (PAH) deposition (Romero et al., 2015). These processes associated with the oil and MOSSFA event impacted a wide variety of species, habitats and ecosystems across the marine and coastal environments (Montagna and Girard, 2020), from micro-organisms such as foraminifera to sperm whales (Ackleh et al., 2012).

Monitoring and assessment of such rapid events is difficult but benthic foraminifera have successfully been used as bioindicators of anthropogenic pollution. This includes heavy metal pollution (Coccioni, 2000), drill cutting disposal (Mojtahid et al., 2006) and oil spills (Morvan et al., 2004; Sabean et al., 2009). This is because they are small, abundant (Yanko et al., 1999; Hallock et al., 2003) and can record changes in the environment over short time scales (Yanko et al., 1999; Coccioni, 2000; Lei et al., 2015).

Benthic ecosystems were severely impacted by DWH with a 80–93% decline of benthic foraminiferal density (Schwing et al., 2015) and a 30–40% decline of species richness and heterogeneity (Schwing and Machain-Castillo, 2020). Benthic foraminifera abundances and assemblages then reached steady state three to five years after the DWH oil spill, indicating ecosystem recovery (Schwing et al., 2018). However, the exact mechanism for this decline remains uncertain and it is unknown whether taxa were influenced by the oil spill despite not becoming locally extinct. Furthermore, although steady state returned within three to five years, the condition of the individuals and whether optimal growth was occurring within foraminifera species was not known. A more in depth understanding of the response of benthic foraminifera was therefore required to fully understand both the nature of the stressors, and whether whole assemblage studies provide an accurate proxy for ecosystem recovery.

This study focuses on the morphological response of four species of benthic foraminifera which remained abundant during the oil spill from 2010 to 2015, in order to examine any immediate effects of the oil spill and subsequent response of the foraminifera over five years following the DWH event. Changes in length, width and shape (length/width) were measured at different depth (time) intervals. Both infaunal and epifaunal (or superficial infaunal) taxa were chosen to allow for comparison, enabling a more detailed view of the benthic foraminifera response at the surface, and at depth.

Section snippets

Field methods

Multi-cores were collected annually from the NGoM since the DWH event, using an Ocean Instruments MC-800 multi-corer by the College of Marine Science at the University of South Florida. This study focused on cores recovered in 2010 (WB 1110), 2011 (WB 0911) and 2015 (WB 0815) from site DHS07 (29 16.207 N, 87 45.469 W) in the NGoM, 83 km from the DWH wellhead and at 399 m water depth (Schwing et al., 2017) (Fig. 1). Site DSH07 was chosen for this study due to its proximity to the DWH oil spill

Length and width measurements

All of the taxa were present throughout the samples, but Brizalina subaenariensis var. mexicana, Uvigerina peregrina and Cassidulina teretis were by far the most abundant through the cores, whereas Bulimina marginata was rare in some samples. Both the length and width values of all taxa show little variation through the cores (Fig. 3, Fig. 4). Uvigerina peregrina has means of 347.37–439.23 μm and 180.45–244.97 μm for the length and width, respectively, across all three cores. B. subaenariensis

Length/width ratios pre-DWH versus post-DWH

Overall, the individual measurements of length and width in all taxa showed no substantial size response to the oil spill (Fig. 3, Fig. 4), which suggests that there was little change in reproduction and growth within these species. It is known that the test size and proloculus size of foraminifera is related to reproduction (Nigam and Rao, 1989). The presence of pollutants can impact chamber production, test size, reproduction and gamete production within foraminifera (Le Cadre and Debenay,

Conclusion

The DWH oil spill was one of the greatest anthropogenically induced environmental disturbances of the 21st century. The MOSSFA event led to reduced oxygen levels in the benthos, enhanced hydrocarbon deposition and reducing conditions on the seafloor. This study furthers our understanding of the benthic foraminiferal response to oil spills by investigating the morphological response of species of benthic foraminifera which remained abundant during and following the DWH oil spill, from 2010 to

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

Data are available at the University of Bristol data repository, data.bris, at https://doi.org/10.5523/bris.13e8unvvw3n8f24dbau7f0bh1y.

Declaration of Competing Interest

None.

Acknowledgements

We would like to thank Gaurav Mehta and Etienne Leroy for their help with data collection.

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    Present address: School of Environment, Geography and Geosciences, University of Portsmouth, Burnaby building, Burnaby Road, Portsmouth, PO1 3QL.

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