Dead Dungeness crabs that washed up along the Oregon coast after succumbing to low-oxygen conditions during 2004. Photo by Elizabeth Gates. Click on the image for an expanded view.
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Dead Zones
Dead zones (low oxygen waters where living creatures are scarce) in coastal areas of the ocean are a growing concern worldwide. Not only can they disrupt the local ecosystem, the events can be economically devastating, leading to massive die-offs or migration of commercially important fish and invertebrates. Recovery can take anywhere from days to months, and perhaps even years.
Low oxygen in the water (called hypoxia) generally occurs after a large phytoplankton bloom is followed by a strong increase in bacteria. The bacteria feed on the plankton after it dies and sinks; since bacteria require oxygen, they decrease the amount of oxygen in the water. The initial cause of the large phytoplankton bloom could be pollution from rivers (like fertilizer runoff which provides food for the phytoplankton), changes in ocean circulation patterns, or just natural events.
Two recurring hypoxic zones are present off of the west coast of the United States. The first occurs yearly in the Gulf of Mexico; it is the largest dead zone recorded in the western hemisphere, first noticed in the early 1970s. The second was first documented in 2002 off the Oregon coast, and has been seen almost every year since. These two zones differ in many ways.
In the Gulf of Mexico, hypoxia appears to result from a combination of terrain and pollution. A shelf in the Gulf of Mexico helps to contain the productive water. River pollution provides several nutrients essential to phytoplankton in large quantities. In the summer another required nutrient (silica) becomes available, leading to rapid phytoplankton growth. At the same time the circulation changes, causing the water to stratify (when water stays in layers and doesn't mix vertically). This further feeds the hypoxic conditions in the Gulf of Mexico.
The dead zones seen off the Oregon Coast are of great interest because hypoxic events are rare in open ocean systems. The main driver for these zones seems to be recent changes in circulation patterns in the ocean and air. The coastal waters off Oregon have an "oxygen minimum zone" at 700-900 m depth; these waters don't usually come close to shore. However, in 2002 these waters did move close to shore, making the shallow waters hypoxic before the phytoplankton had even bloomed. Furthermore, warm weather, causing water stratification, helped prolong the low oxygen event. The reason for the shallowing of the oxygen minimum zone is still in debate, but appears to be a result of a change in climate (a combination of winds and temperature).
Because of their large economic and biological impact, understanding the causes of dead zones is critical. Research is ongoing worldwide; for information about research on this topic at Oregon State University, see: http://www.piscoweb.org/outreach/topics/hypoxia. More information can also be found online in numerous newspaper and science articles. A nice summary can also be found at: http://www.cnie.org/NLE/CRSreports/06Oct/98-869.pdf.
- feature story by Maria Kavanaugh |
