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The coast is clear, but seabed still contaminated by Gulf spill

The effects of Deepwater Horizon spill continue after the smoke has cleared. US Coast Guard

The catastrophic explosion on the Deepwater Horizon drilling platform allowed five million barrels of oil to gush into the Gulf of Mexico between April and July 2010. Since then, remediation and restoration efforts have helped to remove obvious signs of the disaster and return affected coastal communities to their pre-spill condition. But according to a recent study published in the academic journal PLoS ONE, the benthic, or sea-floor, environment around the disaster area still may require years to make a full recovery.

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.

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 red, while areas with low contamination and high biodiversity (the smallest footprint) were labelled in green.

Map showing the footprint of the Deepwater Horizon oil spill at each of 68 sampling sites visited in late 2010. Gradient runs from green (least affected) through yellow to red (most affected). Paul Montagna et al./PloS ONE

While the red sites tended to be in close proximity (within 3km) of the wellhead, orange and yellow sites - those with moderate footprints - were more variable. This pattern was probably caused by an underwater plume of oil up to 200m thick and 2km wide in some areas that helped distribute contaminants along its path to the southwest of the wellhead. Because of this deep-sea flow of hydrocarbons, some distant habitats were more contaminated, while some closer ones were cleaner than expected.

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.

Map estimating the regional footprint of the Deepwater Horizon oil spill. Green indicates areas least affected by the spill, while red indicates areas most affected. /> A healthy benthic environment is <a href= Paul Montagna et al.
vital for maintaining a robust and productive marine ecosystem. Thus, the current findings could be bad news not just for the benthic creatures living on the Gulf’s sea floor, but for organisms higher up in the water column (and the food chain), as well. This is especially troubling when considered alongside another recent report that at least one species of Gulf fish is exhibiting signs of molecular, genomic, and developmental defects associated with the oil spill.

The biggest remaining question is how long the benthic community will continue to feel the negative effects of the spill. Gulf microorganisms will eventually be able to clean up their habitat, but this process could take a while given the cold temperature and low nutrient levels found in the contaminated area. In fact, given the lengthy recuperation period previously documented for past oil spills in shallower seas, the authors predict that it may take decades to undo the damage caused by the Deepwater Horizon blowout.

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