Oceanic measurements taken during a cruise aboard the NOAA vessel Ronald H. Brown have confirmed that a large area of poorly oxygenated water is growing off the coast of Washington and Oregon, with the potential to become a dead zone lethal to certain species.
Research results reported online in late July by the Austria-based Ocean Acidification International Coordination Centre (OA-ICC) note that oxygen-depleted bottom waters occur seasonally along the continental shelf of Washington and Oregon when strong winds blowing along the coast in spring and summer trigger upwellings that bring deep, cold, nutrient-rich water to the surface.
These waters fuel plankton blooms that feed small critters like krill, which themselves are food for other marine creatures. When these blooms die off, they sink to the bottom, where their decomposition consumes oxygen, leaving less oxygen for organisms such as crab and bottom-dwelling fish, the report said.
According to Oregon State University Professor Francis Chan, director of the NOAA Cooperative Institute or Marine Ecosystem and Resources Studies (CIMERS), low dissolved oxygen levels have become the norm on the Pacific Northwest coast.
“But this event started much earlier than we’ve seen in our records,” Chan said. “This is the earliest start to the upwelling season in 35 years. Typically, hypoxic conditions don’t appear until late June or early July.”
Richard Feeley, an oceanographer with NOAA’s Pacific Marine Environmental Laboratory in Seattle, also participated in the voyage. He said the measurements of dissolved oxygen and ocean acidity are consistent with an event that has the potential to create “dead zones” later this summer. Dead zones occur when dissolved oxygen levels drop so low that crab and other bottom-dwelling fish perish.
The West Coast Ocean Acidification Cruise left port on June 13 on a 45-day mission sampling along several areas from British Columbia to California.
The recurring scientific cruise, supported by the NOAA Ocean Acidification Program, surveys ocean conditions for a host of environmental parameters to better understand factors influencing ocean acidification and hypoxia which are related. Measurements are taken from a suite of sensors and floats. Scientists also collect plankton and other sea life in net trawls.
This year, researchers are also conducting the first systematic regional survey of methane gas emitted by thousands of seeps along the West Coast, according to the report.
As the cruise continued to move south along the California coast, the report states, ongoing measurements were taken along the Newport, Oregon transect and by fishermen deploying dissolved oxygen sensors on commercial crab pots.
One discovery of concern was that while in U.S. waters, a plankton net retrieved from depths of 100 meters surfaced with a large amount of a greenish black substance in its fabric. Feely suspects the net was towed through a thick layer of decaying plankton in the water column, a situation responsible for creating hypoxic conditions.
Feeley said researchers added a little alcohol to the greenish black substance in the net fabric.
“We began to realize that it was a large mass of phytoplankton, either still living or dead, sinking into the deeper water and possibly providing the fuel for the oxygen uptake as it decays,” he explained.