May 31, 2010

Decades later, how has the ecology of coastal Saudi Arabia recovered from the largest oil spill in history?

As the Deepwater Horizon spill progresses, I've been tracking down the science that has been done as a result of other large spills, particularly the monitoring of ecosystem damage and recovery. It's a mixed bag, apples and oranges in some cases, largely dependent on the communities affected, the extent of the spill, the cleanup effort and the environmental/species composition of the affected area.

I went straight to the biggest first, the Gulf War oil spill, which started in January of 1991 and ended up leaking 11 million barrels of oil (one barrel = 42 gallons) into the Persian Gulf, which eventually washed up on to the shorelines of the area, invading the beaches, salt marshes and mangrove forests. In 2001 and then again in 2008, Dr. Hans-Jörg Barth of the University of Regensburg reported on the ecological effects of the spill, which are apparent to this day.

A couple of things to keep in mind: First, every spill is different. Deepwater Horizon is much more widespread than the Gulf War spill, which means the coasts probably won't see as much inundation, but being more widespread could have other dire effects as well. Second, there was no focused cleanup effort in the Persian Gulf. Lastly, it's important to remember that this is an incredibly complicated analytical process. The term "oil" that we use actually describes a conglomerate of many different hydrocarbons and other chemicals which can have different effects in different areas or at varying concentrations. Combine that with the progression of other environmental problems, such as climate change, ocean acidification, lingering pollutants from previous incidents and potential contaminations from dispersants and the like and it becomes very difficult to make accurate predictions. In any case, as this situation changes, so will the outcome.

Two to four years after the spill in the Persian Gulf, natural regeneration began on the beaches and in the mangrove forests. The tides continued to batter the oil on the beaches and between the roots of the trees, breaking it loose, constantly aerating the sediments it clung to, bringing oxygen to the soils. In the mangrove forests, tidal channels brought fresh, oxygenated seawater and benthic invertebrates like crabs, which turned and moved the sediments (termed bioturbation), allowing oxygen to penetrate the substrate. After 10 years, 80 percent of the beaches had returned to a relatively normal state. The predictions from scientists spelled doom for the mangroves, but despite losing 30 percent of the affected Avicennia marina initially, seedlings began to sprout a mere two years after the spill. Those statistics certainly sound hopeful.

A decade after the spill,however, when scientists returned to the area for a quantitative analysis, they still found a million cubic meters of oil sediments remaining. According to Dr. Jacqueline Michel

...the oil got trapped into a very large bay and it was never allowed to keep moving and so it just piled in.  I remember sitting on the shore and looking out and not being able to see clean water, I could just see oil as far as I could see from the shoreline.

In 2001 and then again in 2008, the intertidal regions, particularly the salt marshes, had still not recovered. The oil crept in to the muddy home of halophylic grasses and filled the burrows of crabs. Cyanobacterial mats were allowed to cover the marshes, undisturbed, robbing the water of oxygen beneath. After 10 years only about 20 percent of these areas had recovered; 25 percent were completely dead. Oil remained in the sediments in concentrations similar to the initial estimates. The tar crust and the darker, oil-stained sediments have a lower albedo, absorbing more light and increasing the temperature in the immediate microclimate, in some cases beyond the tolerance of the marsh plants necessary to the ecosystem.

Salt marshes are at a disadvantage. The ecosystem relies on bioturbation, mainly from crabs, to keep the sediments oxygenated and to keep the cyanobacterial mats at bay. But the oil wiped out brachyurans in most of these areas; even in 2006 they had only just begun to recolonize in a few areas. Oxygen is the main fuel that keeps both abiotic and organismic hydrocarbon-metabolizing processes going, and in the anaerobic soup under tar crusts and cyanobacterial mats, those processes are largely halted, so the oil remains and the marsh is slow to regenerate.

So it comes down to oxygen and physical energy, the two main factors that explain the slow recovery of the salt marshes relative to the beaches or mangrove forests. The waves crashing on the beach and the numerous tidal channels through the mangroves helped to break up the oil and allowed organisms like crabs to oxygenate the sediments via bioturbation. In the marshes, as the oil lingered, it killed its residents, became sun-baked and allowed the cyanobacteria to take over, creating a largely hypoxic environment. The tidal channels in the marsh are relatively few, and the process of bioremediation slowed. But as Barth suggests, the existing channels will continue to bring in fresh water and healthy animals which can recolonize via the soft muds from the channel, burrowing up and through the hard, crusted-over surfaces of the marsh. In this situation, it's easy to see that if artificial channels could be created, better access would be given to these keystone animals and the process could be accelerated.

It's too early to make any useful comparisons between our current situation in the Gulf of Mexico and historic spills, but it's important to recognize the research that has been done in the past, and just how lasting of an effect these environmental disturbances can have on different types of ecosystems. Some will recover relatively quickly. Others will need very special attention paid to the functions of organisms that help hold the system together and scientists will need to find ways to assist those functions.

What is not so clear right now is how we're going to handle the resulting toxicity in our fisheries and how that will affect the economy of coastal communities (and the country) for the long term. Ecosystems usually have a particular carrying capacity for toxins, and can continue to function even under certain levels of chemical pressures. I don't think the seafood industry will be able to replicate the same tolerance.

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