JOIN USAn oxygen alarm
There has been a significant drop in oxygen in the waters of the Gulf of Mexico following the Deepwater Horizon spill, according to a number of independent researchers who have gathered data there. But the second US government summary report on the movement and breakdown of subsurface oil around the spill, issued recently, says the depletion is not serious and may be down to flawed data.
The government report from the Joint Analysis Group, which involves a number of agencies including the US National Oceanic and Atmospheric Administration (NOAA), describes oil in the Gulf degrading at a good rate and becoming dilute in waters leading away from the wellhead. The report also indicates that although some oxygen drops have been recorded, oxygen levels in the water column are not low enough to be of concern, and raises the possibility that flaws in instrumentation are responsible for the oxygen depletion that some scientists have reported.
Oxygen is measured using sensors that include a semi-permeable membrane. “There’s some indication that perhaps we’re getting false positives for oxygen decline because the membrane is being fouled,” says Steve Murawski, Director of Scientific Programs and Chief Science Advisor for NOAA Fisheries and head of the joint agency team behind the report.
But scientists who gathered data independently on some of the same cruises on which NOAA is basing its report say they have used a variety of techniques to verify that the oxygen depletion is real.
Oily feast
When oil began pouring into the Gulf of Mexico more than three months ago, it created an abundant food source for those microbes that feast on natural gas and oil. As they degrade these materials, the microbes consume oxygen. Accustomed to much leaner conditions, the oil-loving microbes rapidly multiplied, and some researchers fear that these microbial blooms could create oxygen-poor conditions in parts of the Gulf. So far, scientists agree that oxygen depletion near the broken wellhead and in the surrounding oil plumes is not as serious as the levels found in the seasonal ‘dead zone’ that occurs in shallow waters near the mouth of the Mississippi River and is caused by excess nutrients entering the water from agricultural runoff.
“I don’t think we’ll have a situation of complete anoxia (absence of oxygen) in the deep water,” says Andreas Teske, a microbiologist at the University of North Carolina in Chapel Hill. “The deep sea is quite well aerated, with a lot of oxygen-rich water around.” But the oxygen depletion resulting from the leak is still worrying, experts say. Samantha Joye, a biogeochemist at the University of Georgia in Athens, has found oxygen drops of 30-50% in certain areas, especially in waters some eight kilometres and further out from the wellhead, at depths of around 1,000-1,300 metres.
David Valentine, a geomicrobiologist at the University of California in Santa Barbara, and his team found oxygen losses of 30-35 percent at similar depths as Joye’s team. “It’s a very clear chemical signal,” says Valentine.
Minimal changes
In contrast, the government report indicates there are very minimal changes in oxygen levels at around 1,000-1,400 meter depths at various distances from the wellhead, although it says the anomalies cannot be reliably interpreted at this time.
Data on oxygen levels are gathered using sensors that have a membrane semi-permeable to oxygen. As the government researchers suspect oil in the water could be fouling the membrane and skewing the readings, they recommend in the recent report that scientists use a backup chemical method called Winkler titrations, which determine the amount of dissolved oxygen in a water sample very precisely. According to Murawski, some scientists shared information with them indicating that when titrations were used, they did not observe the oxygen drops. “We have some indications from researchers that (the drawdown) is not real,” says Murawski. “It’s somewhat ambiguous at this point.”
But both Joye and Valentine say that they have used independent methods to validate their data. Valentine’s team used both oxygen-sensor data and Winkler titrations in parallel. “Our data strongly suggest the oxygen anomalies are real,” says Valentine. He also says that a colleague from his team shared their results with NOAA. “I have not seen any, nor have researchers released any data to indicate that,” says Murawski. “I’d be interested in it. Nobody has shared releasable information with me.” Joye says the government’s claim that the oxygen data are false positives is a “red herring”. Joye has also criticised NOAA’s sampling techniques, saying that they are sampling too close to the source, where the oil is fresh and oil-loving microbes have not yet had time to colonise or consume oxygen, and that they are not following the plume of oil.
“They are making these conclusions that there is not an oxygen problem based on samples that are collected right beside the wellhead,” says Joye. “Of course there’s not going to be an oxygen problem because it’s a fresh plume, the bugs haven’t had time to grow in and metabolise anything.” Murawski disagrees, saying that their conclusions take into account measurements at sites near and far from the wellhead and throughout the plume. Scientists agree that the jury is still out on what an oxygen drop could mean for the deep waters of the Gulf of Mexico. It can take years or even decades for deep waters to mix enough for oxygen levels to re-equilibrate.
“What I’m most concerned about is the community effect, what happens when you create this giant bloom of certain kinds of bacteria favoured by the availability of methane and other gases,” says Ian MacDonald, oceanographer at Florida State University in Tallahassee. “These are ecological effects that we’ve never observed naturally.”