The event appears similar to one in 2002, when an area of ocean water with low oxygen content formed along the nearshore Oregon coast between Newport and Florence, causing a massive die-off of fish and invertebrate marine species.

Researchers initially thought the 2002 dead zone was a one time occurrance - odd combination of climate, winds and water patterns that led to a low oxygen event that had not been seen in the region's recent history.

But continued research has shown that the same thing almost occurred last year and is now happening in full force again this year.

Dissolved oxygen levels are a great deal lower than those seen in the past 40 years, researchers said, and the event may be part of a fundamental change to ocean conditions in the northern Pacific Ocean.

Dr. Jane Lubchenco, professor of marine biology and zoology at Oregon State University, also chairs the International Council for Science. (Photo courtesy OSU)

In the 2002 event, water at depths of 30 to 50 meters, within a mile or two of the shoreline, had dissolved oxygen levels in the range of 0.5 to 1 milliliters per liter - whereas a normal reading would be about four times that high.

Any dissolved oxygen level below 1.4 milliliters per liter is considered hypoxic, capable of killing a wide range of fish, crabs and other marine species that suffocate in the low oxygen environment.

Researchers said one recent measurement taken at a 30-meter depth station just 1.2 miles off the coast found dissolved oxygen at 0.8 milliliters per liter.

Further offshore and to the south, they found oxygen levels in deeper areas of the continental shelf to be as low as anything seen in 2002

In the current event, researchers said, the "dead zone" of low oxygen water appears to be "sloshing back and forth" between deeper water and, more recently, into shallower, nearshore water.

Oregon coast near Lincoln City looking south towards dead zone (Photo by Bruce Hillard courtesy NOAA)

Last week, hundreds of dead Dungeness crabs and molts were found in nearby tidepools.

The researchers said these hypoxic events are intimately connected to upwelling, which is the movement of cold, nutrient rich water to the surface near the ocean shore.

Upwelling is normally valuable as it brings up nutrients critical for much marine life and key to productive fisheries.

But in the hypoxic events, the upwelled water is coming from the sub-Arctic, and is even colder, more nutrient rich and lower in oxygen than usual.

Summer winds bring this water closer to shore and the high nutrient waters support even more growth than usual of microscopic marine plants.

These plants ultimately sink and decay, leading to consumption of even more of the remaining oxygen in the water

“Hypoxic conditions such as this have been documented in other nearshore upwelling ocean regions of the world," said Jack Barth, a professor of oceanography at Oregon State, "but never on the central Oregon coast."

Scientific data to document the changing ocean conditions is limited, but anecdotal evidence from regional fisherman and other coastal residents suggests that events such as the one that occurred in 2002 have no recent precedent, the researchers said.

"While there is no obvious connection between the hypoxic events and the El Nino/La Nina cycle, the influence of the longer term Pacific Decadal Oscillation can not be ruled out at this point," said Barth.

The possibility that other climatic forces such as global warming could be causing the change in ocean circulation, which sets the initial conditions for the dead zone, is possible but not certain, the researchers said.