New Hypoxic 'Dead Zone' Found Off Oregon Coast
The event appears similar to one in 2002, when an area of ocean water with low oxygen content formed in the nearshore Oregon coast between Newport and Florence, causing a massive die-off of fish and invertebrate marine species. The fact that it’s happening again is triggering concern among marine scientists.
In 2002, the dead zone appeared to be a one-time anomaly, an odd combination of climate, winds and upwelling patterns that led to a hypoxic event – a situation in which the oxygen level was so low it could not support most marine life – which 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. This is a disturbing trend with an unknown cause that scientists now say may reflect a major change in ocean circulation patterns, with serious impacts on marine biology.
“When you see the same thing happening with this regularity, it suggests that something is fundamentally different,” said Jane Lubchenco, the Valley Professor of Marine Biology at Oregon State University. “This is a significant departure from normal conditions and you have to wonder what’s going on. This ocean system has changed, and we’re paying attention.”
The issue is sufficiently important that OSU scientists from the Partnership for Interdisciplinary Studies of Coastal Oceans and the College of Oceanic and Atmospheric Sciences have joined forces in intensive research with experts from the Oregon Department of Fish and Wildlife, National Oceanic and Atmospheric Administration, and the University of Washington.
Much of the work has been done with the R/V Elakha, a marine research vessel operated by OSU.
“We monitored this situation last year, when another mass of low oxygen water formed but was pushed off the continental shelf by shifting winds and caused little damage,” said Francis Chan, an OSU research associate with PISCO. “Then two months ago, we saw the early signs of another hypoxic event, and we believe we’re now in the process of another dead zone event.”
In the 2002 event, water at depths of 30-50 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 literally suffocate.
“The figures in 2002 were just off the charts compared to the historical norm, and already this year we have had some readings in that same range,” Chan said. “One recent measurement taken at a 30 meter depth station just 1.2 miles off Newport found dissolved oxygen at 0.8 milliliters per liter. Further offshore and to the south, we’ve found oxygen levels in deeper areas of the shelf to be as low as anything we saw in 2002.”
In the current event, Chan 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. What impact this will have on marine life is unknown at this point. Some fish and crab kills have already been documented on beach and intertidal areas, but other dead animals may also be washed out to the deep sea. Last week, hundreds of dead Dungeness crabs and molts were found in tidepools south of Yachats. Area residents have also reported dead fish on some beaches.
“Studies are underway with a remotely-operated submersible vehicle to take video and measurements of the ocean floor environment and better document the current effects of this hypoxic event,” said Hal Weeks of the ODFW Marine Resources Program.
These hypoxic events are intimately connected to upwelling, the researchers say, which is the movement of cold, nutrient rich water to the surface near the ocean shore. Normally, upwelling is valuable - the nutrients it brings up are 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. Upwelling-favorable summer winds bring this water closer to shore. And in this situation, the high nutrient waters support even more growth than usual of microscopic marine plants, which 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 OSU, “but never on the central Oregon coast.”
“This system is normally healthy and productive,” Lubchenco said. “But a change in ocean circulation appears to be shifting the system closer to a tipping point where the right conditions can kick it over the edge and into an hypoxia state. This coastal ecosystem off Oregon seems to be changing in a way we have never seen.”
Scientific data to document the changing ocean conditions only goes back a few decades at best. But anecdotal evidence from regional fisherman and other coastal residents also 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 Niño/La Niña cycle, the influence of the longer term Pacific Decadal Oscillation can’t 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. A key focus of continued research will be not only to determine the extent of these hypoxic events and the marine mortality associated with them, but to identify the underlying cause of the events, the researchers said.