These conclusions were based on analyses of deep-sea sediment samples collected at 68 sites throughout the Gulf. The sampling locations, which were visited in autumn 2010, were selected because they radiate outward along a contamination gradient stretching away from the wellhead from which the crude oil flowed. The nearest sampling points were less than 1km from the wellhead, while the farthest were 125km away.
Emergency crews respond to the Deepwater Horizon explosion and spill. Image courtesy of the U.S. Coast Guard.
At each site, researchers characterised the physical properties of the benthic sediment and tested for signs of oil and other contaminants. They also quantified the abundance and diversity of benthic organisms commonly used as bioindicators: larger bottom-dwelling creatures known as macrofauna, and smaller species called meiofauna. A statistical technique called a principal component analysis (PCA) was then employed to look for relationships between these ecological variables.
The analysis revealed a clear link between contaminants associated with drilling (especially barium, polycyclic aromatic hydrocarbons, and total petroleum hydrocarbons) and the diversity of benthic creatures. Specifically, areas with more contaminants had very high abundances of nematodes relative to copepods, but low overall diversities of both nematodes and copepods. The driver of these results is unclear, but the researchers hypothesise that the influx of organic material from the oil spill might have led to a bacterial bloom. This would have been advantageous to bacteria-eating nematodes, but could have been harmful to copepods and nematodes with other diets.
To get a clearer picture of how these patterns were affected by each sampling site’s proximity to the wellhead, the researchers used a colour-coding scheme to label each site. Areas that had high contamination and low biodiversity (eg, those where the spill’s footprint was biggest) were marked in green, while areas with low contamination and high biodiversity (the smallest footprint) were labelled in red.
The final step of the study was to increase the extent of the colour-coding and produce a map estimating the footprint across the entire region, rather than just at the 68 sampled points. This was achieved using an interpolation technique known as kriging, which allows researchers to extrapolate from the data they have collected to estimate values at other, non-sampled, sites. The result is a map that covers over 70,000km2 of benthic habitat, of which 167km2 is classified as moderately contaminated and another 24km2 of which is classified as severely contaminated.
Montagna, P.A., Baguely, J.G., Cooksey, C., Hartwell, I., Hyde, L.J., Hyland, J.L., Kalke, R.D., Kracker, L.M., Reuscher, M., and Rhodes, A.C.E. 2013. Deep-see benthic footprint of the Deepwater Horizon blowout. PLoS ONE 8(8): e70540.