The oxygen content of Oregon’s near-shore Pacific Ocean waters plummeted to dangerously low levels this summer before a timely storm arrived in mid-September to “flush” the system and ease the threat to many marine creatures. Hypoxia has become a seasonal threat.
“We are now living on a knife edge in terms of hypoxia, and this year we crossed the threshold into danger,” said Francis Chan, an Oregon State University marine ecologist and an expert on ocean chemistry. “It was one of the worst years we have had in a while and it looked like it was going to get really bad before that storm came in.
“This is something that only happened occasionally in the 20th century, but has been taking place on a near-yearly basis for the past 15 years. The leading hypothesis for why this is happening is that the ocean is changing. Warmer water holds less oxygen, for one, but there also may be increased stratification and other factors.”
Chan said he and his colleagues began hearing anecdotal reports about abnormal conditions and animal behavior in July. Researchers from the Oregon Department of Fish and Wildlife collected video of crabs in a research trap dying from lack of oxygen. Marine educators at OSU’s Hatfield Marine Science Center said crabs were leaving the ocean to enter bays and estuaries – some even burying themselves in sandy flats exposed to the air at low tide.
Researchers on survey ships run by the National Oceanic and Atmospheric Administration told Chan that when they sampled the ocean waters off Oregon this summer for juvenile fish, they caught almost nothing.
Jack Barth, director of the Marine Studies Initiative at Oregon State and a principal investigator with the National Science Foundation-funded Ocean Observatories Initiative, retrieved data that showed the level of oxygen in the ocean off Yaquina Head – in 25 meters of water – was down to 0.5 milliliters of oxygen per liter of water. That is classified as “severe” hypoxia, he noted.
“It lasted from mid-August to early September,” Barth said, “which is enough time to do some damage, but not as bad as the event in 2006, which killed thousands of crabs and other marine organisms. Oxygen levels were down to zero that year and it persisted well into September. We were lucky this year – we dodged a bullet.”
When water near the seafloor reaches hypoxic levels – below 1.4 milliliters of oxygen per liter of water – some fish and other creatures have the ability to flee the area and find more oxygenated water. However, some animals don’t have that ability and those that do, when the hypoxia is severe and widespread, may not find a place to go.
“The good thing is that we now have a lot of eyes on the ocean, with more and more people reporting abnormalities,” Chan said. “We can use instrumentation from the Ocean Observatories Initiative, research ships and gliders to determine where the levels of low oxygen are and when they occur, as well as where there may be areas of more oxygenated water. We are learning more each year.”
Near-shore hypoxia, which can lead to the aforementioned marine “dead zones,” first came to researchers’ attention in 2002 when crabbers pulled up pots of dead crabs, Chan said.
“When you look back to data from the 1950s and 1960s, the low oxygen values were just not there,” he noted. “It’s been much more prevalent over the past 15 or so years. It’s like a special season out there in mid- to late-summer that we can’t see, but is very important. On land, we get smoke and fires. In the ocean, it’s dead crabs.”
The Oregon legislature has recognized the threat and established an ocean acidification and hypoxia council, which Barth co-chairs. Chan is meeting with fishermen in October to brief them on the oceanographic data OSU has recorded, and to get their insights and observations from up and down the Oregon coast.
“Every year, things get a little weird and though we are observing more of it, there’s still a lot to learn,” Chan said. “This spring, for example, millions of pyrosomes showed up in the water – they are a luminescent, jellyfish-like sea creature that can grow up to two feet long – and no one is sure why, or if they may have contributed to the hypoxia.
“As the ocean changes, and we experience an annual hypoxia season, we can expect more surprises.”
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