Global Climate Change May Alter The Dynamics Of Ocean "Dead Zones"

By Dawn Wenzel

Oxygen-starved dead zones in the ocean are emerging as a common and widespread phenomenon worldwide. With global numbers in 2009 estimated at 400 and growing, most new dead zones are thought to be a result of algal blooms seeded by fertilizer and sewage runoff.

Furthermore, coastal systems that experience periodic levels of low oxygen, or "hypoxia," as natural seasonal events are also seeing changes in the extent and severity of hypoxic episodes. With forecasts that the earth's temperature will increase, many researchers are left wondering how climate change in combination with increased human populations may affect the chemistry of the world's oceans.

As outbreaks of ocean hypoxia are becoming more frequent and intense, the ability to predict their occurrence and effects on local ecosystems has become a major focus of ocean research.

Two new studies are providing hints for the future of dead zone dynamics. Looking back in time, researchers from Oregon State University (OSU) have found since the last ice age that dissolved oxygen in coastal sediment decreased at the same time the earth warmed, suggesting climate change may have played a role in hypoxia during earth's history. Additionally, new research characterizing the unique metabolism of a microbe found to populate hypoxic waters is enriching our vision for the future of dead zone ecosystems.

Researchers were able to reconstruct seafloor oxygen levels over the past 30,000 years by measuring the amounts of certain metals in ocean sediment collected off the coast of Chile. Dissolved oxygen levels had been higher during the last ice age and started decreasing about 17,000 years ago, when Antarctica started to warm.

The study is the result of a collaboration between professors Zanna Chase, Jim McManus, and Alan Mix and graduate student Jesse Muratli from the college of oceanic and atmospheric sciences at OSU and is published in the December 13th issue of Nature Geoscience.

The team studied the oxygenation effects of an important body of water known as the Antarctic Intermediate Water. Formed in a band around the Southern Ocean, just south of the tip of South America, this body of water carries dissolved oxygen into the Northern hemisphere, and has a huge influence on the composition and climate of the world's oceans. This water is "by far volumetrically the most important,” explains Chase, one of the authors of the study and assistant professor of chemical oceanography at OSU.

As part of the Ocean Drilling Program, an international collaboration to study the ocean floor, the team received ocean core samples from which they were able to splice together a continuous sediment record spanning tens of thousands of years. Researchers measured amounts of trace metals rhenium and manganese. At low ocean-oxygen concentrations, manganese is able to dissolve and is not present in the sediment. In contrast, the metal rhenium shows the opposite behavior. By comparing the amounts of the two metals, researchers pieced together ocean-oxygen amounts spanning the last 30,000 years.

OSU researchers hypothesized that the cause for the systematic decrease in oxygen could be due to an increase in growth of algae that would then be eaten by other organisms. There was no sign of an increase in organic carbon in the sediment record which would be signature for increased algal growth. They concluded that the low oxygen levels were more likely due to a change in the delivery or circulation of oxygen by the Antarctic Intermediate Water.

Similarly, could a decrease in oxygen delivery triggered by the earth's warming be what's causing the formation of dead zones off the coast of Oregon?

According to Chase, the comparison is "kind of a leap.” She explains that it is very hard to predict how ocean ventilation and circulation is going to change in response to global warming, although over the last ten years there have been hints that the levels of dissolved oxygen in the Antarctic Intermediate Water have decreased.

Francis Chan, a marine ecologist at OSU agrees that there are several reasons why this study should not be interpreted as what's happening with hypoxia off the Oregon coast. "There is incredible value in paleooceanographic studies,” explains Chan, "Nature has been doing studies for a very long time.” He asserts that this study gives us a glimpse of what might happen, and not necessarily what's happening now. Chase hopes that this study might encourage more research into modeling how ocean circulation might change as a result of warming.

The triggers for ocean dead zones are complicated. Some, like off the coasts of Oregon and Washington, are naturally occurring, driven by winds that propel nutrient-rich, oxygen-poor deep water to the surface. The deep water is like "the bottom of a compost bin,” explains Francis Chan. Microscopic plants in surface waters quickly gobble up the fresh nutrients and their populations explode into what's known as an algal bloom. Once the food runs out, the algae die and sink to the ocean floor where their decomposition by bacteria triggers the rapid depletion of dissolved ocean oxygen. In contrast, some dead zones like the one found seasonally in Washington's Hood Canal originate from algal blooms fed on fertilizer run-off, pollution, sewage leaks and other human sources that are rich in nutrients.

Ocean ecosystems are drastically altered by outbreaks of hypoxia. While organisms mal-adapted for low-oxygen environments die or become displaced, researchers are finding that the hypoxic water becomes an attractive niche that a specific group of microbes known as SUP05 can exploit. How this organism's metabolism supports growth in such an extreme environment is the focus of a recent study published in the October 23rd issue of Science led by Steven Hallam, professor of microbiology and immunology at the University of British Columbia in collaboration with US Department of Energy Joint Genome Institute (DOE JGI).

Because the presence of SUP05 has been detected in many oxygen deficient ocean zones, Hallam suggests SUP05 might be utilized as a "sentinel or indicator species” for detecting emerging ocean dead zones where oxygen is being depleted. "Think of this the same way a medical practitioner monitors disease progression or remission at both the molecular and systems levels,” explains Hallam. "A patient manifesting a diagnostic marker for disease may be asymptomatic at the time of sampling. However, detection of the marker could be an indicator of future clinical manifestation, i.e. disease progression. By the same token, the residual signature of the marker could indicate a previous infection that has not yet cleared the system.” Hallam adds that just finding the presence of this organism "might give you a picture of what's going on.”

Microbes respond rapidly to change, so they make great indicators for environmental processes, agrees Robert Morris, assistant professor of biological oceanography at the University of Washington. As microbe populations are dynamic themselves, there is a large push to get better, high resolution data on how microbial populations respond to their environments, which will make future surveillance and response to environmental changes such as increases in hypoxia more tenable. A long term goal would be to "link microbial structure and function with key geochemical processes,” says Morris.

Analysis of SUP05 DNA revealed it obtains chemical energy through transferring electrons from sulfide to nitrate, producing nitrous oxide. Researchers expected that SUP05 might be able to utilize sulfide for its metabolism, as these conditions are often found in poorly ventilated, low oxygen environments. What was most surprising was that SUP05 could also convert carbon dioxide from the atmosphere into useable sugars, which is a relatively rare metabolic feat. The machinery to utilize carbon dioxide is "clearly present in the genome,” explains Susannah Tringe, a scientist at the DOE JGI and coauthor of the study.

As the atmosphere's carbon dioxide levels rise, and oxygen depleted ocean dead zones expand globally, what might be the long term ecological effects of SUP05? "The genetic blueprint for SUP05 suggests a paradox,” answers Hallam. "On the one hand SUP05 has the potential to provide valuable ecosystem services, i.e. sulfide detoxification and carbon fixation. On the other hand it has the potential to produce a potent greenhouse gas, nitrous oxide.”

"This is a super cool bug,” says Hallam, "and we are only just beginning to appreciate its significance.”

As research on dead zones advances, the outlook for how they will respond to global climate change remains murky. "We don't have all the pieces of the puzzle to hone in on an answer that we can have a consensus on,” says Francis Chan. The process of global warming might "change the fundamental levers that we think are important for hypoxia.”

Overall, most people would agree that the oxygen content in oceanic and coastal water is declining, says Nancy Rabalais, professor in the department of oceanography and coastal sciences at Louisiana State University, Baton Rouge. Climate change is one thing, but these activities are "paralleled by human activities on land and on water,” explains Rabalais. With increased population growth, fertilizer use will increase; causing run-off that will continue to fuel algal blooms. Increased temperature could also change ocean currents and shift winds in ways that are hard to predict. It isn't going to be a "smooth trajectory, but a bumpy trajectory,” says Rabalais. What can we do about ocean hypoxia problem? Rabalais suggests one solution is to "do something about excess nutrients now, don't wait.”

Dawn Wenzel is a graduate student at the University of Washington in the department of biochemistry.