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

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. 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.

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.