2 February 2015

Wind pattern behind California’s drought also struck at ocean food chain

Posted by nbompey

By Nanci Bompey

Unusual weather that contributed to the California drought also led to an unprecedented drop in small plant-like organisms in the northeastern Pacific Ocean that form the base of the ocean food chain, potentially affecting fish, birds and marine mammals, according to new research.

Winter winds normally drive nutrient-rich water that boosts the growth of phytoplankton — single-cell algae that form the base of the ocean food web — from the cool North Pacific Ocean southward towards the warm sunlit waters of the tropics. In the winter, this transition zone between cool subarctic and warm tropical waters off North America’s west coast is typically rich in food that attracts fish, turtles, seals and other marine animals.

But satellite data from last winter show that the transition zone had noticeably less phytoplankton than scientists had ever seen. The region of high phytoplankton abundance had moved to colder waters 300 kilometers (186 miles) north, according to the new study recently published in Geophysical Research Letters, a journal of the American Geophysical Union.

“That is just very obvious in the data,” said Frank Whitney, a retired chemical oceanographer at Fisheries and Oceans Canada and author of the new study that examined satellite measurements of chlorophyll, a pigment used by phytoplankton to carry out photosynthesis, to determine how much of the algae were present. “There’s less chlorophyll than we’ve ever seen in the transition zone from the satellite data that started in 1997,” he said.

Plots of chlorophyll anomalies for January through May 2014 over the subtropical and subarctic North Pacific Ocean. Chlorophyll anomalies are a comparison of measured chlorophyll concentrations compared to average concentrations. A new study finds that chlorophyll in this region dropped to its lowest level last winter since satellites started taking measurements in 1997.  Credit: Frank Whitney

Plots of chlorophyll anomalies for January through May 2014 over the subtropical and subarctic North Pacific Ocean. Chlorophyll anomalies are a comparison of measured chlorophyll concentrations compared to average concentrations. A new study finds that chlorophyll in this region dropped to its lowest level last winter since satellites started taking measurements in 1997.
Credit: Frank Whitney

Last winter, the nutrient-transporting winds were blocked by an unusually strong high pressure ridge that scientists say contributed to the California drought. The high pressure ridge also kept heat in the ocean, leading to sea surface temperatures in the northeastern Pacific Ocean that were up to 3.5 degrees Celsius (6.3 degrees Fahrenheit) above normal and more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) warmer than any year on record. The unusual winds and warm water inhibited the transport of nutrient-rich water to the south, according to a new study.

As a result, there was up to a 35 percent drop in chlorophyll last winter in the northeast Pacific Ocean that persisted into the spring, according to the new study.

A decline in phytoplankton in this zone and its expansion northward to cooler waters can impact the dozens of fish and marine species that feed on them, including highly-prized tuna, and also predators further up the food chain, like seabirds, according to Whitney.

“You can see this is [an area] that an awful lot of animals use to feed in winter,” he said. “After seeing this large reduction in chlorophyll, then I have to speculate that there’s going to be all sorts of impact on all the animals that usually use this area to feed.”

Scientists have been studying the 2013-2014 winter’s unprecedented warm waters in the eastern North Pacific Ocean and the unusual wind pattern that contributed to it, but the new study is the first to look at the impact this anomaly had on chlorophyll levels in the region, according to Nick Bond, a research scientist at the Joint Institution for the Study of the Atmosphere and Ocean at the University of Washington in Seattle, who was not involved in the new study. He said the unusual weather undoubtedly had an impact on food availability as outlined in the new paper.

“In that region, the sea surface temperature anomalies were so striking – as high as we’ve ever seen them,” Bond said. “It was an extreme event, at least in that pocket of the northeast Pacific.”

“The magnitude of what happened last winter and what is happening this winter is enough that there’s got to be consequences,” he added.

Whitney first heard about the warm water temperatures in the eastern North Pacific Ocean at a conference last year. He initially looked at data that showed the unusual wind patterns in the region and concluded that the odd pattern had to disrupt the normal transport of nutrients from the north Pacific south to where many animals feed. Whitney then used satellite data to examine both the sea surface temperatures and the sea surface chlorophyll levels in the eastern North Pacific Ocean.

It could take time before scientists are able to see the changes that the lower chlorophyll levels had on species further up the food chain, like fish and birds, Whitney said. He said it can take scientists months before they get back information from tagging studies used to look at off-shore species.

“When these data are being looked at in comparison to other years, I think we are going to see some striking results from this last winter,” he said.

Bond added that scientists do know that there are certain species that feed in the zone in the northeastern Pacific Ocean where chlorophyll levels dropped, but it is difficult to know how the abnormal weather patterns and low phytoplankton levels affected any single species until there is data available. Changes in water temperature that affect phytoplankton levels have been known to affect species higher up the food chain, he noted.

Understanding how the warming-induced phytoplankton loss affected the food chain could help scientists understand how marine ecosystems will respond to climate change, Bond said. Previous research has found that the phytoplankton-rich zone in the northeastern Pacific Ocean could shift northward over the next century as the planet warms.

“We know that the biology responds to the physics but the key point that is made is that last winter may produce kind of an example of what is liable to be more frequent in future decades just having to do with climate change,” Bond said.

– Nanci Bompey, AGU public information specialist/writer