Dead zones are hypoxic (low-oxygen) areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. These occur near inhabited coastlines, where aquatic life is most concentrated. Every summer for the past nine years, water with lethally low concentrations of oxygen has appeared off the Oregon coast. The cause is not clear and it does not fit the pattern of several other dead zones associated with man made run off issues. Some other causes have been recently implicated in a research study by Oregon State University.
Dead zones are hypoxic (low-oxygen) areas in the world's oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. These occur near inhabited coastlines, where aquatic life is most concentrated. Every summer for the past nine years, water with lethally low concentrations of oxygen has appeared off the Oregon coast. The cause is not clear and it does not fit the pattern of several other dead zones associated with man made run off issues. Some other causes have been recently implicated in a research study by Oregon State University.!ADVERTISEMENT
Aquatic and marine dead zones can be caused by an increase in chemical nutrients (particularly nitrogen and phosphorus) in the water, known as eutrophication. Additionally, natural oceanographic phenomena can cause deoxygenation of parts of the water column. For example, enclosed bodies of water such as fjords or the Black Sea have shallow sills at their entrances causing water to be stagnant there for a long time.
Dead zones are exactly that. Life does not flourish in such places because there is not enough oxygen to sustain it.
In July 2002, scientists with the Oregon Department of Fish and Wildlife found unusual numbers of bottom-feeding sculpin lying lifeless on the ocean floor, which would normally be teeming with life. Crabs were also dying, and they washed up onto some beaches in large numbers.
Officials at the government agency asked Francis Chan, a biogeochemist at Oregon State University in Corvallis, for help in discovering the cause of the disturbance as quickly as possible. Chan was about to set off on a scheduled research cruise along the Oregon coast, so he grabbed all the extra equipment he could think of, including a brand-new oxygen sensor.
Ocean surface waters normally contain 5â€“8 milliliters of oxygen per liter of water, a number that declines rapidly with depth. But on his first day out, Chan found that at a depth of 150 feet the inner coastal waters off Oregon were hypoxic â€” oxygen levels there were lower than 1.43 milliliters per liter, so low that fish cannot survive.
Similar low oxygen levels were found further offshore, the researchers knew that something unprecedented was happening.
The changes in Oregon may be related to a broader pattern around the globe, in which subsurface patches of permanent hypoxia seem to be growing in size and losing yet more oxygen, for unknown reasons. And whether or not global warming is responsible for the changes to date, ocean models forecast that in the coming decades increasing water temperatures and changes in circulation will drive oxygen concentrations down even further.
"What we have been experiencing is a perfect storm â€” where weather, climate and currents can come together to crash an ecosystem," says Chan.
Storms had repeatedly delayed Chan's cruises during the spring, and he wondered whether the weather was also delaying the oxygen's vanishing act by keeping the winds in a favorable, generally southerly, pattern. Usually in the spring, occasional periods of northerly wind blow surface waters offshore, allowing cool waters, rich in nutrients but poor in oxygen, to upwell from deeper, offshore layers. That upwelling is what makes Oregon's fisheries so productive.
Upwelling can turn deadly for creatures near the sea floor if the winds are unrelenting. Normally, the winds that promote upwelling slacken at various times during spring and summer, enough to mix the waters on the continental shelf, refreshing them with oxygen. But some recent years have brought strong and steady spring winds that prevent oxygen from reaching subsurface waters. At the same time, the constant upwelling spurs blooms of phytoplankton, which die and then decompose, using up oxygen in the near shore waters.
Further out to sea, beyond the continental shelf, water at a depth of roughly 3000 feet is permanently oxygen deprived, at roughly 0.5 milliliters per liter. Called an oxygen minimum zone (OMZ), the layer is a normal feature in many parts of the ocean; it is too far down to mix with the well oxygenated surface waters.
The waters above an OMZ are expected to have slightly lowered amounts of dissolved oxygen. Researchers have discovered that the water above the OMZ off the Oregon coast is steadily losing oxygen. The researchers pulled together 30 years of offshore recordings and, in an unpublished preliminary analysis, found that concentrations of the gas have dropped by 0.5 milliliters per liter and are now about 2 milliliters per liter â€” dangerously close to hypoxic conditions. These are the very waters that well up onto the continental shelf in the spring and summer. Chan estimates that this oxygen decline above the OMZ has pushed the chances of seeing hypoxia in the nearshore waters from 10% to roughly 60% each year.
Around the globe, many OMZs are also losing oxygen and expanding horizontally and vertically. OMZs currently cover about 30 million square kilometres, or 8% of the ocean area. These regions have mostly attracted little attention from researchers, but the best known OMZs sit off the coasts of Namibia, Chile and Peru â€” also areas of strong upwelling.
Chan and other researchers are trying to determine whether the warming climate is to blame for expanding OMZs and the recent upwelling along the Oregon coast. A connection is possible, in theory. Climate models suggest that ocean oxygen concentrations will decline in the future, mainly because increasing temperatures of the surface waters impede mixing with deeper layers, and the warming also reduces the solubility of oxygen in the water. On top of that, heating of the polar regions may slow down the giant currents that carry oxygen from cold regions to warmer ones. Various model runs forecast that the global oxygen content of oceans will decline by 1â€“7% over the next century.
The exact cause of the Oregon dead zone has not yet been established but the trend is troubling.
For further information: http://www.nature.com/news/2010/100811/full/466812a.html