The current generally flows south, driven by winds roughly parallel to the coast. It runs some 1,600 feet deep within about 600 miles of the shoreline from British Columbia to Baja California.
The key to the ecosystems’ biological productivity is called “upwelling,” a condition in which winds push surface water offshore and it is replaced from below. The movement of deep waters to the surface circulates nitrates, phosphates and trace metals from below to the top water layers where they can be taken up by phytoplankton, which in turn grow and provide energy other organisms.
There are just four comparable upwelling regions on Earth, according to a new Atlas of Coasts & Oceans published by the University of Chicago. Three are off Africa, while the fourth is next to Chile and Peru.
Like all ecosystems, the California Current is subject to numerous forces — from pollution to global ocean circulation. A major challenge for oceanographers is distinguishing between short-term variations such as El Niño and La Niña weather phenomena versus those that span decades and others that play out over hundreds or even thousands of years.
Scientists at Scripps, which is part of the University of California San Diego, have been studying changes in the regional marine environment since 1949 as part of CalCOFI, along with colleagues from federal and state agencies. Four times a year, scientists voyage hundreds of miles along a set course to sample temperature, salinity, oxygen, zooplankton and other components of the southern California Current.
“Without such a record, there is no way to see a long-term change,” Koslow said.
The research has helped to define El Niño weather patterns and the Pacific Decadal Oscillation, a marine climate cycle that shifts every few decades. It also has provided foundational data for numerous other scientific endeavors.
Scripps scientist Peter Bromirski was lead author of a recent paper that said complex interactions of air and water currents have held sea-level rise along the West Coast far below the global average in recent decades. He said signs point to a major shift in ocean dynamics that is expected to reduce the upwelling of cold, nutrient-rich waters off the coast. If that happens, waters stuck near the surface are expected to heat, expand and creep higher onto land more like they have elsewhere.
Such changes would hamper coastal development and ripple through the ecosystem in less obvious ways.
“Alteration of the rate of upwelling will have consequences for plankton production, for growth and survival of fish population, for bottom-dwelling invertebrates, sea urchins, squids, sea birds and marine mammals,” Ohman said.
Just as unsettling, “dead zones” have been detected off the coast of Oregon starting in 2002. Researchers found oxygen levels too low for marine organisms to survive, something they couldn’t easily explain. While scientists explore links to global warming, climate models suggest more low-oxygen zones due to rising water temperatures and changes in mixing patterns.
Then there’s the issue of acidification, which is the result of carbon dioxide from the atmosphere being absorbed by the ocean. As the seas become more acidic, they are less hospitable to sensitive shell-bearing organisms such as sea snails that are an important part of the food web.