Anatomy of an ecological catastrophe: what to expect in the deep Gulf of Mexico

The oil spill in the Gulf of Mexico is now over a month in duration and continues to worsen. Estimates of the total oil flowing from the damage were estimated initially at 5,000 barrels per day. Now the 26,000 estimate by FSU professor Ian MacDonald looks more plausible. Moreover, the greatest impact of the current spill may occur below, not above, the ocean surface. But the impacts of oil drilling to the deep sea began long before the first oil leaked from the damaged well.

Our scant prior knowledge of the deep sea in its natural state, combined with the technical challenges we face in understanding this remote habitat, mean we may never really know what change to expect or even how to detect it when it does.  What we do know is that compared to shallow water, recovery rates in the deep-sea will be lower due to longer lifespans, slower growth/reproduction rates, and overall lower density and population sizes of organisms.

Below is a brief summary of how off-shore oil drilling and the oil spill will impact the deep.

Impact Types

Drill Cuttings are the actual material, water-based muds, removed from the borehole from the drilling operation.  To this mud barium sulphate, potassium chloride, bentonite, caustic soda, citric acid, soda ash and polymers are added for stabilizing and weighting. Drilling muds and drill cuttings can be contaminated by a variety of heavy metals such as mercury, chromium, cadmium, lead and barium.  The initial drill cuttings are deposited on the seabed. Subsequent material is discharged at the surface of the oil rig but will eventually deposit on the seafloor. This is likely to be the most significant and regular impact on the deep sea.  In just 30 years in the North Sea 1.5 million tons of drill cuttings were left on the seafloor.

Produced water is a mixture of formation water and injected water. Oil and gas reservoirs have a natural water layer (formation water) that lies under the hydrocarbons.   In addition, water is injected into the oil-bearing formation to maintain pressure for oil production. Produced water, along with the trace heavy metals, radionuclides, sulphates, treatment chemicals, produced solids and hydrocarbons, is discharged into the ocean.

Sand contaminated with oil is also produced during oil production and exploration and often deposited at sea.

Drains will carry any contaminated water from the oil platform into the water surrounding.

Seabed engineering in the form of positioning anchors and other submerged structures will both mechanically damage the seafloor and cause sediment plumes.  Both impacts will smother organisms.

And of course massive and catastrophic oil spills that result in billions of barrels of spilled oil and half million gallons of the chemical dispersant Corexit 9500.

Results of Impacts

The impacts above will lead to the smothering of organisms, organic enrichment or reduction, exposure to toxins, etc.  From oil drilling, these effects are local occurring in the near vicinity (<6km) of the operations with the greatest toxicity within 1km of the platform. Near oil platforms or in areas of high oil platform density, the total amount of seafloor organisms is known to be drastically reduced.  The types of deep-sea species found near oil derricks also greatly differ from those found in natural and undisturbed habitats. Microbial and copepod communities exposed to crude oil or occurring with the vicinity of oil platforms also exhibit greatly reduced genetic diversity. Pollution throughout marine communities is associated with a loss of biodiversity both in terms of species richness and evenness. Although natural sedimentation occurs, the type, amount, and density of sedimentation occurring from drill cuttings are greatly different. As such, seafloor organisms will fair poorly.

In contrast to the above, the oil spill impacts will be regional or basin-wide. Toxins from the oil spill will likely integrate into the food chain and eventually arrive to the deep in the form of food.  Flux of material from the ocean surface is also likely to transport oil and toxins to the deep ocean. Shading by the oil slick might also inhibit phytoplankton production and reduce carbon flux to the deep sea meaning less food for seafloor organisms. An overall reduction of biodiversity both in terms of species and genetic diversity is expected. Of course with the oil spill, it needs to be recognized that 48 million gallons of oil are leaked naturally in the Gulf of Mexico annually (the current spill is near 34 million gallons). On one hand this may have produced a more resilient fauna. On the other hand this might provide a tipping point that pushes many species and communities past which they can survive. It also should be considered that many organisms will be exposed to oil concentrations much higher than they would naturally.

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