A 'regime shift' is happening in the Arctic Ocean, scientists say

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Scientists at Stanford University have discovered a surprising shift in the Arctic Ocean. Exploding blooms of phytoplankton, the tiny algae at the base of a food web topped by whales and polar bears, have drastically altered the Arctic's ability to transform atmospheric carbon into living matter. Over the past decade, the surge has replaced sea ice loss as the biggest driver of changes in uptake of carbon dioxide by phytoplankton.

 

The research appears July 10 in Science. Senior author Kevin Arrigo, a professor in Stanford's School of Earth, Energy & Environmental Sciences (Stanford Earth), said the growing influence of phytoplankton biomass may represent a "significant regime shift" for the Arctic, a region that is warming faster than anywhere else on Earth.

 

The study centers on net primary production (NPP), a measure of how quickly plants and algae convert sunlight and carbon dioxide into sugars that other creatures can eat. "The rates are really important in terms of how much food there is for the rest of the ecosystem," Arrigo said. "It's also important because this is one of the main ways that CO² is pulled out of the atmosphere and into the ocean."

 

A thickening soup

 

Arrigo and colleagues found that NPP in the Arctic increased 57 percent between 1998 and 2018. That's an unprecedented jump in productivity for an entire ocean basin. More surprising is the discovery that while NPP increases were initially linked to retreating sea ice, productivity continued to climb even after melting slowed down around 2009. "The increase in NPP over the past decade is due almost exclusively to a recent increase in phytoplankton biomass," Arrigo said.

 

Put another way, these microscopic algae were once metabolizing more carbon across the Arctic simply because they were gaining more open water over longer growing seasons, thanks to climate-driven changes in ice cover. Now, they are growing more concentrated, like a thickening algae soup.

 

"In a given volume of water, more phytoplankton were able to grow each year," said lead study author Kate Lewis, who worked on the research as a Ph.D. student in Stanford's Department of Earth System Science. "This is the first time this has been reported in the Arctic Ocean."

 

New food supplies

 

Phytoplankton require light and nutrients to grow. But the availability and intermingling of these ingredients throughout the water column depend on complex factors. As a result, although Arctic researchers have observed phytoplankton blooms going into overdrive in recent decades, they have debated how long the boom might last and how high it may climb.

 

By assembling a massive new collection of ocean color measurements for the Arctic Ocean and building new algorithms to estimate phytoplankton concentrations from them, the Stanford team uncovered evidence that continued increases in production may no longer be as limited by scarce nutrients as once suspected. "It's still early days, but it looks like now there is a shift to greater nutrient supply," said Arrigo, the Donald and Donald M. Steel Professor in Earth Sciences.

 

The researchers hypothesize that a new influx of nutrients is flowing in from other oceans and sweeping up from the Arctic's depths. "We knew the Arctic had increased production in the last few years, but it seemed possible the system was just recycling the same store of nutrients," Lewis said. "Our study shows that's not the case. Phytoplankton are absorbing more carbon year after year as new nutrients come into this ocean. That was unexpected, and it has big ecological impacts."

 

Decoding the Arctic

 

The researchers were able to extract these insights from measures of the green plant pigment chlorophyll taken by satellite sensors and research cruises. But because of the unusual interplay of light, color and life in the Arctic, the work required new algorithms. "The Arctic Ocean is the most difficult place in the world to do satellite remote sensing," Arrigo explained. "Algorithms that work everywhere else in the world—that look at the color of the ocean to judge how much phytoplankton are there—do not work in the Arctic at all."

 

The difficulty stems in part from a huge volume of incoming tea-colored river water, which carries dissolved organic matter that remote sensors mistake for chlorophyll. Additional complexity comes from the unusual ways in which phytoplankton have adapted to the Arctic's extremely low light. "When you use global satellite remote sensing algorithms in the Arctic Ocean, you end up with serious errors in your estimates," said Lewis.

 

Yet these remote-sensing data are essential for understanding long-term trends across an ocean basin in one of the world's most extreme environments, where a single direct measurement of NPP may require 24 hours of round-the-clock work by a team of scientists aboard an icebreaker, Lewis said. She painstakingly curated sets of ocean color and NPP measurements, then used the compiled database to build algorithms tuned to the Arctic's unique conditions. Both the database and the algorithms are now available for public use.

 

The work helps to illuminate how climate change will shape the Arctic Ocean's future productivity, food supply and capacity to absorb carbon. "There's going to be winners and losers," Arrigo said. "A more productive Arctic means more food for lots of animals. But many animals that have adapted to live in a polar environment are finding life more difficult as the ice retreats."

 

Phytoplankton growth may also peak out of sync with the rest of the food web because ice is melting earlier in the year. Add to that the likelihood of more shipping traffic as Arctic waters open up, and the fact that the Arctic is simply too small to take much of a bite out of the world's greenhouse gas emissions. "It's taking in a lot more carbon than it used to take in," Arrigo said, "but it's not something we're going to be able to rely on to help us out of our climate problem."

 

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A Major Food Chain Shift Appears to Be Happening in The Arctic Right Now

Phytoplankton blooming in the Barents Sea (Jeff Schmaltz & Joshua Stevens, LANCE/EOSDIS Rapid Response, NASA)

 

There's a major change happening in the Arctic. Dark waters are blooming with algae, as sunlight floods spaces long obscured by sheets of ice.

 

Over the past two decades, there's been a 57 percent increase in phytoplankton in the Arctic ocean, an analysis by researchers from Stanford University has revealed.

 

That's outpaced scientist's expectations, and it's changing the way the ocean stores carbon, as well as sucking up resources needed for the rest of the ecosystem. And no one's

 

sure what it means.

 

"The rates are really important in terms of how much food there is for the rest of the ecosystem," says Earth system scientist, Kevin Arrigo.

 

"It's also important because this is one of the main ways that CO2 is pulled out of the atmosphere and into the ocean."

 

At first glance this expansion of the photosynthesising part of the food chain shouldn't be all that surprising. Global warming has caused the Arctic's ice sheets to wither away

 

over the decades, opening up new frontiers for phytoplankton to blossom.

 

But according to the researchers, from around 2009, the rate at which new open water was being exposed dropped off significantly.

 

By all accounts, this should have been followed by a similar decline in greenery. After all, no matter how much the Sun shines, population numbers should taper off as the

 

amount of available nitrogen and other essential elements gets used up.

 

But that's not what happened. This expansion at the base of the food pyramid – described in eco-jargon as a gain in the rate of net primary production (NPP) – just kept going.

 

"The increase in NPP over the past decade is due almost exclusively to a recent increase in phytoplankton biomass," says Arrigo.

 

It's hard to know whether we should be alarmed or appreciative. After all, more green stuff means more food for herbivores, which means more meat for the carnivores.

 

Not to mention more carbon being locked away in organic molecules.

 

But the Arctic Ocean isn't really a big player when it comes to sinking carbon. Especially if vanishing sea ice simply makes way for more marine traffic.

 

And as Arrigo puts it, life in the Arctic is also better adapted to having plenty of ice around.

 

"There's going to be winners and losers," he says.

 

More to the point, the extended surge in NPP observed by the team has been perplexing enough to force them to look at existing explanations and asked what they might have

 

missed.

 

The study's lead author, environmental scientist Kate Lewis, explains it was initially assumed there just wasn't a big store of nutrients to chew through, a question that's been

 

addressed previously by the team's studies.

"We knew the Arctic had increased production in the last few years, but it seemed possible the system was just recycling the same store of nutrients," says Lewis.

 

"Our study shows that's not the case. Phytoplankton are absorbing more carbon year after year as new nutrients come into this ocean. That was unexpected, and it has big

 

ecological impacts."

 

Getting a grip on the influx of nutrients is easier said than done since it depends so much on the complexities of ocean currents spreading mixes of different materials through

 

water columns and following trends that are also at the whim of a changing global climate.

 

Even just getting to this point in mapping the changes in phytoplankton required a huge rethink on how to measure the shades of colour that are traditionally used to analyse

 

NPP.

 

"Algorithms that work everywhere else in the world – that look at the colour of the ocean to judge how much phytoplankton are there – do not work in the Arctic at all," says

 

Arrigo.

 

Armed with improved, Arctic-specific processes, Lewis and her team can now be confident that the changes we're seeing in the planet's far north do point to a sustained

 

blooming of producers fed by nutrients pouring in.

 

Further studies on our planet's vast circulating network of atmospheric and oceanic streams could help us better nail down what to expect of this vast algal bloom and what it

 

means for the Arctic's future.

 

This research was published in Science.

 

 

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