Currents bringing vital nutrients to shallow, coastal ocean waters off North America's west coast have weakened and become more variable over the last 50 years a study has found.
That's resulted in short-term drops in reproduction and growth in marine life in the region, including seabirds, fish and marine mammals, research led by the University of Texas at Austin indicates.
Along the coastlines in the region, winter winds pull nutrient-rich water from deep in the ocean up to sunlit surface layers in a process known as coastal upwelling, creating vast blooms of phytoplankton that underpin the entire food chain supporting marine life.
Using tree-ring studies from California coastal forests to reconstruct 600 years of climate, the researchers found the state has seen more winters featuring weak coastal upwelling since 1950 than occurred in the last six centuries.
Of the 10 weakest upwelling years in the last 600, four occurred after 1950, they reported in the journal Science.
Because the weather patterns that generate strong upwelling at sea can block storms from moving onshore, which can cause drought and stunting the growth of trees, the researchers discovered an inverse relationship between growth of tree rings and the health of the ocean ecosystem.
"The winters we see robust growth in the trees, we see poor growth in the marine ecosystem," says Texas marine science Professor Bryan Black.
The researchers turned to tree-ring patterns because direct data on variations in coastal upwelling go back less than 100 years, whereas tree-rings can reveal climate data going back many centuries.
Whether climate change can be linked to the more recent variations in coastal upwelling is unclear, Black says, although evidence suggests a possible link to a climate pattern known as the El Niño-Southern Oscillation, which has been extremely volatile over the past century.
"This is consistent with what we expect from climate change, but at this point, we can't attribute it to that," he says. "This is something we need to continue watching to see how climate variability plays out in the coming years."
Even the 600-year record provided by the tree-ring study may not give the entire picture, the researchers acknowledge, saying recent variations may be part of a larger natural cycle scientists can't look far enough into the past to discern.
The study does highlight the extreme variability of coastal upwelling off California, with unpredictable impacts on human activities as well, Black says.
"You have to keep that in mind if you're managing a fishery -- for example, you can't plan for every year being moderate or reliable," he says. "There are a lot of ups and downs."
