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Oil Spill vs. Hypoxic Zone

There’s an elephant in the room as summer arrives on the Gulf Coast: hypoxia season.

This year, it’s a different Gulf, one covered in the largest oil slick in our country’s history.  No one is quite sure what the interaction between the oil and hypoxia will be.  Best guess is that both stresses will mean the end for most organisms living in the area and that hypoxia will exacerbate problems associated with the spill and hinder recovery by limiting oxygen availability for detoxifying bacteria.  However, step back for a minute and speculate on other possibilities: could the oil spill actually be helpful if it prevents or slows the eutrophication process? Could the damages associated with the oil spill be less than those associated with a large hypoxic zone?

Around this time of year, nutrients collect and flow down the Mississippi basin from the breadbasket of the US fertilizing farms for the season’s new crops.  Combine loads of nutrients with warm growing conditions to feed blooming algal growth.  Normally, this process of eutrophication would translate into higher bacterial growth, feeding off the algal bodies as they die.  Increase in decomposition uses up most or all of the available oxygen and leaves the Gulf with the largest hypoxic zone in the world.

2009's low oxygen zones

The hypoxic zone is lethal to sessile creatures such as clams and oysters.  Those that can flee the low oxygen water do.  Shrimp surround the edges of the hypoxic zone, traveling only as far as they need to in order to escape the low oxygen conditions.  Fishermen follow suit, creating an annual ring of shrimpers around the hypoxia boundary.

Gulf shrimp boat

Granted, the Gulf fisheries are now largely closed off save some shrimping grounds in the western Gulf due to the toxic effects of the oil on Gulf species and potential human health effects.  If the shrimp survive the spill, they’ll probably be free of fishing pressure this year.  However, what if the oil spill also suppresses algal growth and therefore hypoxia?  Is it possible that oil spills could be less damaging than business as usual?

There is evidence from the Exxon Valdez spill (Jewett et al 1996) that hypoxia is more damaging than oil spills.  Before the spill, there were 24 identified taxa in Prince William Sound.  This was decreased to 6 in the months following the spill but increased to 32 over the next 2 years.  In 1995, there was a hypoxia event in the same area, decreasing the number of taxa from 32  to 4.  At the time of publication almost a year later, there weren’t many signs of rapid recovery.

The Jewett study is one of only a handful that compared the effects of hypoxia and oil spills.  Others show that if the stresses occur together, the effects of both are exacerbated by the presence of multiple stresses.  So as we move into hypoxia season with a massive fresh oil spill, will one suppress the effects of the other or will they act in concert to hurt the Gulf ecosystem even more? Only time will tell.

~Bluegrass Blue Crab

Reference:

Jewett, SC, TA Dean, DR Laur. (1996). Effects of the Exxon Valdex oil spill on benthic invertebrates in an oxygen-deficient embayment in Prince William Sound, Alaska. American Fisheries Society Symposium, 18