It is now two and a half years since the Deepwater Horizon oil well blowout in the northeastern Gulf of Mexico. Both the people and the ecosystem of the Gulf were changed by this massive spill; how well are they recovering?
Petroleum is a natural part of the Gulf of Mexico ecosystem. Perhaps that is why the spill was assimilated by the system so rapidly. There is a lot of oil extraction activity in the Gulf of Mexico, with an estimated 3200 active platforms and 26,000 miles of underwater pipelines. While small spills are common, it is a testament to the offshore oil drilling industry that more disasters have not occurred.
Oil and gas seeps are normal occurrences in the Gulf of Mexico, releasing oil that can be seen as slicks on the water’s surface by satellite. Natural petroleum tars were used by native Americans to waterproof their pottery, and the Spanish used it to caulk their boats. Oil slicks were noted in Spanish sailing ship logs from the 16th and 17th centuries.
But the magnitude of this spill exceeded the gradual natural releases, resulting in a disturbance to the system. Some parts of the system are still reeling from that hit.
The complexity of oil
Crude oil is a complex mixture of thousands of molecules. The bulk of the oil has little to no toxicity. The most toxic parts are the small aromatic solvents and the complex sheets of cyclic structures called polycyclic aromatic hydrocarbons. These mixtures are used by people in coal tar shampoos.
The solvents tend to evaporate or be degraded by bacteria quickly. The polycyclic aromatic hydrocarbons tend to stick around a bit longer: they’re more difficult for microbes to break down.
As well as the oil, humans added other chemicals to the Gulf ecosystem after the spill. A major toxicity concern emerged from the use of Corexit, an oil dispersant. The active molecule, Dioctyl sodium sulfosuccinate, is available in laxatives for human internal use. It acts by holding onto both water and oil, creating suspensions of microscopic oil droplets in water, breaking up bulk oil and dispersing it into the water.
This increases the exposure of aquatic organisms to oil; they could otherwise swim beneath it without as much worry.
By increasing the exposure or bioavailability of oil, it increases its toxic effects, often lethally. It also increases the surface area of the oil-water interface, where the microbes live that consume the oil for energy, so it also increases the biodegradation rate.
Dispersants are valuable tools for dealing with oil spills, but there are trade-offs. If the concern is coral reefs, seagrass beds, oyster reefs, or other submerged habitats, it is best to not use dispersants and let the oil remain on the surface and be transported elsewhere.
If however the concern is beaches and intertidal habitat such as marshes and mangroves, it is better to use dispersants and keep the spill offshore as much as possible. This is what was done with the Deepwater Horizon well failure.
The amount of dispersant used was unprecedented, as was using dispersant at the wellhead, one mile below the surface. Mid-water plumes of oil developed as the oil rose from the bottom, and there is some debate as to whether using dispersant at the wellhead was effective.
What effects are known?
We (University of Western Florida) started monitoring the shoreline and continental shelf of the Florida Panhandle for oil in sediments and water during the spill, and continue to do so with two of the eight research consortia funded after the spill, DEEP-C and C-IMAGE.
Although visible oil was not found away from the coast once the well was capped, levels of polycyclic aromatic hydrocarbons were elevated during and after the spill, declining to background by a year later.
We chose Coquina surf zone clams as a coastal indicator. Molluscs lack the enzymes to process polycyclic aromatic hydrocarbons (PAHs), and they concentrate these molecules 100-fold or more, making them sensitive indicators.
The levels of PAHs in these clams also declined to stable, low levels by one year later, suggesting that the oil molecules had cycled through the system along the Florida coastline.
However, even with the chemicals gone, some of the effects of the spill were apparent. Fish, like other vertebrates, are able to detoxify PAHs through their liver and bile secretion. But some had delayed wound healing and distended gall bladders. The young of red snapper from 2010 were missing from the reefs in 2011, presumably lost as larvae in the plankton in the year of the spill.
Small reef fishes were also missing. Microscopic benthic forams disappeared in deep water, and did not immediately recover in some areas. Deep water corals were damaged or killed.
Oil, and its toxic effects, still lingers in the Louisiana marshes where it has gotten into the mud and marsh peat. Determining the lasting impacts and ripple effects is the present challenge.
How do we quantify the damage?
So how do we assess the damage? Determining the value of a healthy environment is fraught with difficulty.
We estimate harvest value and even tourism value for healthy ecosystems, but this approach rarely captures the full value of the unspoiled environment. Beyond the direct economic value, aesthetic and even spiritual values are beyond measure, yet very real to many.
How does one put a value on environmental damage when it means so many different things to different people?
Public awareness and passive acceptance play a role. If deep water corals that took hundreds of years to grow are killed, does the average person who never knew they even existed care? If environmental damage occurs as a series of gradual insults with grossly imperceptible effects, or even a series of sudden insults like the Deepwater Horizon spill, where and how do we as a society intervene to stop the process before the resilience of the system is exceeded?
I suspect that we are all a bit poorer for the loss of any of the wonders of our biosphere, and our world becomes smaller and more of a cage with every little bit we lose.