April 30, 2008
A review of ocean data records indicates that the low-oxygen events off the Pacific Northwest coast since 2002 are unprecedented and may be linked to the stronger, persistent winds expected to occur with global warming.
Researchers from Oregon State University and the National Oceanic and Atmospheric Administration outline a “potential for rapid reorganization” in basic marine ecosystems and the climatic forces that drive them, and suggest that recent low-oxygen (or “hypoxic”) events may become common. Findings from the team were published in Science.
“Levels of oxygen in the summertime have suddenly become much lower than levels in the previous 50 years,” said Jane Lubchenco, professor of marine biology at OSU. “And 2006 broke all records, with parts of the shallow shelf actually becoming anoxic, meaning that they lacked oxygen altogether. We’ve never seen that before.”
“We’re now seeing low-oxygen levels that are much more widespread and far more intense than what has been recorded in the past,” said William Peterson, an oceanographer at the NOAA Fisheries Service’s science center in Newport, Ore.
“The fish have simply moved out of these areas and are probably doing fine elsewhere,” said Peterson. “But animals that can’t move to better waters, like Dungeness crabs, sea urchins and starfish, will perish.”
The rapid shift of ocean conditions has occurred in what is considered one of the world’s more productive marine areas — the California Current Large Marine Ecosystem. The California Current, according to NOAA, is a slow-moving mass of cold water that travels south along the coast from British Columbia to Baja California. The increasing areas of water off Washington and Oregon with little or no oxygen are possibly resulting in the deaths of marine animals that cannot leave the low-oxygen areas.
“The low oxygen levels we’ve measured in the last six years are abnormally low for our system,” said Lubchenco. “We haven’t seen conditions like this in many, many decades, and now with varying intensity we’ve seen them in each of the last six summers.”
In these events, water oxygen levels have repeatedly reached hypoxic levels, below which most marine animals suffocate or are severely stressed if they cannot escape the area. If oxygen levels drop to zero, most animals die.
During the summer of 2006, anoxia, a complete lack of oxygen in the water, was recorded off the central Oregon coast for the first time.
According to NOAA, the 2006 event covered at least 3,000 square kilometers, lasted four months, and occupied up to 80 percent of the water column in shallow shelf areas. Fish died or fled these areas; thousands of crabs died; and marine seafloor life that could not move faced almost total mortality. Recovery has been slow.
The team conducted a submersible-based survey in the summer of 2006 and discovered no fish living along the rocky reefs that are normally healthy habitat for varieties of commercially important rockfishes. Similar surveys completed from 2000 through 2004 registered abundant fish populations. In shallow areas in particular, the team found an almost complete absence of bottom-dwelling organisms and a rise in bacteria that flourish under conditions of little or no oxygen.
It’s less certain why this is happening, but the events are consistent with global climate change, according to OSU.
“There have always been unusual weather events, such as hurricanes, droughts, and changes in wind patterns,” said Jack Barth, OSU professor of physical oceanography. “So it’s difficult to prove that any one event is caused by global warming. Having said that, we expect global warming to generally cause stronger and more persistent winds. These winds contribute to the hypoxic events by increasing plankton production and holding low-oxygen water on the continental shelf for longer periods.”
“At this point, I’d be surprised if this trend towards hypoxic events didn’t continue,” said Barth.
Francis Chan, a marine ecologist with OSU, conducted a survey of all known records of oxygen levels on the Oregon continental shelf over the last 60 years, with measurements taken by research cruises and ocean-going vessels from more than 3,000 stations.
“The data make it pretty clear that the recent conditions are unprecedented during any period that has been measured,” Chan said. “We’re now seeing very low-oxygen water, lasting for long periods, and closer to shore than at any time in more than 50 years.”
That period included several El Nino and La Nina events and shifts in the Pacific Decadal Oscillation, all potential factors in near-term climate trends. However, none of these appeared to have any correlation to the hypoxic events.
Hypoxic conditions in ocean waters — often called “dead zones” — are usually associated with serious nitrate loads or other nutrient pollution, such as in the Gulf of Mexico or Chesapeake Bay. Pollution-caused hypoxic zones are found with much less frequency in regions where significant upwelling occurs — a process that is usually beneficial to productive marine food webs.
“Coastal upwelling ecosystems occupy only about 1 percent of the ocean surface area, but they produce about 20 percent of global fishery production,” said Lubchenco. “These areas have historically been highly productive. The appearance or increase in severity of hypoxia in these ecosystems would be cause for concern.”
Sources: Oregon State University, National Oceanic and Atmospheric Administration.