Low-oxygen waters off Washington, Oregon coasts risk becoming large ‘dead zones’

Low-oxygen waters off Washington, Oregon coasts risk becoming large 'dead zones'

Oceanic measurements collected during a scientific cruise on NOAA Ship Ronald H. Brown last week confirmed that a large area of poorly oxygenated water is growing off the coast of Washington and Oregon. 

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 blooms of plankton that feed small animals like krill, which are food for many marine creatures. When these blooms die off, they sink to the bottom, where their decomposition consumes oxygen, leaving less for organisms such as crabs and bottom-dwelling fish.

Earliest onset in 35 years

“Low dissolved oxygen levels have become the norm ion the Pacific Northwest coast, but this event started much earlier than we’ve seen in our records,” said Oregon State University Professor Francis Chan, director of the NOAA cooperative institute CIMERS. “This is the earliest start to the upwelling season in 35 years.” Typically, hypoxic conditions don’t appear until  late June or early July, he said.  

This map shows the location of transect lines that are being sampled during the 2021 West Coast Ocean Acidification Cruise. Measurements taken in coastal Washington and Oregon waters confirm the presence of a large area of bottom water that has low dissolved oxygen levels. Credit: CICOES

Returning to port from the NOAA-sponsored West Coast Ocean Acidification Cruise, Richard Feely, an oceanographer with NOAA’s Pacific Marine Environmental Laboratory in Seattle, said 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 crabs and other bottom-dwelling fish perish.

West Coast scientific cruise confirms extent

The West Coast Ocean Acidification Cruise left port June 13 for its 45-day mission sampling along several transects from British Columbia to California. Supported by the NOAA Ocean Acidification Program, this recurring scientific cruise surveys ocean conditions for a host of environmental parameters to better understand the factors that influence ocean acidification and hypoxia, which are related. Scientists obtain measurements from a suite of sensors and floats, as well as  collect plankton and other sea life in net trawls. This year, scientists are also conducting the first systematic regional survey of methane gas emitted by thousands of seeps along the west coast.

A surprise in a plankton net 

One discovery on this cruise has Feely and fellow scientists anxious to get back into the laboratory. In U.S. waters, a plankton net retrieved vertically from depths of 100 meters surfaced with a large amount of a greenish black substance in its finely woven fabric. Feely suspects the net was towed through a thick layer of decaying plankton in the water column, the kind of thing responsible for creating hypoxic conditions. 

“We added a little alcohol, and 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 said. Samples are on their way to Seattle for examination under a microscope. 

As the West Coast Ocean Acidification Cruise moves south along the California coast, ongoing measurements will be taken biweekly along the Newport, Oregon transect, and by fishermen deploying dissolved oxygen sensors on commercial crab pots.  

Senior Survey Technician Bryce Dewees and graduate student, Meghan Shea help to guide the bongo nets into the water as the sun rises during the 2021 NOAA West Coast Ocean Acidification Cruise on June 27. The nets are used to collects samples of small zooplankton, crabs and fish for various chemical and biological measurements back in the laboratory. Credit: Meghan Shea

NOAA Research News, 21 July 2021. Full article.

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