By Dan Haifley, Our Ocean Backyard
You’ve probably heard of the North Atlantic Current, an extension of the Gulf Stream which moves warm water from the tropics northward. Many believe the current contributes to Europe’s moderate climate. Its waters cool when mixing with colder, more dense layers as the current travels.
Similarly, waters off western North America arrive via different routes.
Francisco Chavez is a senior scientist at the Monterey Bay Aquarium Research Institute who has been studying the movement of large masses of ocean water. He says that dense, cold, high-oxygen water, from the mixing of the Oyashio Current and Kuroshio Current, sinks near the Kamchatka Peninsula northeast of Japan. That is where the “North Pacific Intermediate Water” system begins its journey to North America.
As it flows along the ocean’s intermediate depths, from 600 feet and 3,000 feet below the surface, it loses oxygen and gains carbon dioxide and nitrate before it upwells off the coast from Oregon and Baja California. Chavez said he believes the age of the water, meaning the time it took to travel, is a relatively young 20 to 50 years old.
“You can think of it as a train,” he told me. “The slower it goes, the more passengers it can pick up. In this case the passengers are the organic compounds that sink from the surface then decay at depth, consuming oxygen and increasing carbon dioxide and nutrients.” Those nutrients help to feed marine life once the “train” arrives off the west coast.
The process by which organisms in surface waters grow and reproduce by photosynthesis, and then die with the resulting organic matter sinking to deeper waters, happens continuously. Chavez said he believes that because the North Pacific Intermediate Water train is going slower it is picking up more passengers — the decaying organic compounds — which is why oxygen is declining in the California Current system. It’s a similar result as when waters becomes more productive, resulting in more sinking particles.
The train helps prevent bottom depths from losing oxygen as it travels across the Pacific, by bringing high-oxygen waters from the surface to those depths. But when the train slows down, there is less oxygen brought to depth and the ocean slowly becomes lower in oxygen. The cycle continues, and will be affected by the projected El Niño which would bring more oxygen to mixing waters near the surface.
Chavez said scientific models suggest climate change could increase the productivity of coastal waters. “There is a debate regarding whether there will be less oxygen and more nutrients in the waters that upwell at the end of the North Pacific Intermediate Water system in 50 to 100 years, as a result of climate change,” he said.
Monterey Bay National Marine Sanctuary’s Research Coordinator Andrew DeVogelaere said that if this happens, then organisms that tolerate low oxygen may benefit by feeding on prey that are lethargic because of lack of oxygen. “Humboldt squid will thrive as they eat oxygen needy, slower moving midwater fishes that we described from recent Shimada cruise,” he said. “It could be good for organisms that live continually near the surface as nutrients upwell, but bad for the organisms that normally migrate to depth and back every day as they have to enter the oxygen minimum zone.”
Dan Haifley is executive director of O’Neill Sea Odyssey. He can be reached at email@example.com.