Exploding blooms of Arctic phytoplankton | Polarjournal
A phytoplankton bloom colored the water of the Barents Sea milky-blue in July 2016. Photograph: Jeff Schmaltz and Joshua Stevens, LANCE/EOSDIS Rapid Response, NASA

Big changes appear to be underway in the Arctic Ocean — over the past 20 years, phytoplankton productivity has increased by an enormous 57 percent, Stanford University scientists have found. And a higher concentration of phytoplankton in the Arctic Ocean means, among other things, that larger amounts of carbon dioxide can be absorbed from the atmosphere. Good news for the climate?

According to the new study, the small unicellular algae at the base of the food web have multiplied explosively in extensive plankton blooms. While in the past the loss of sea ice was the main driver of changes in carbon dioxide uptake by phytoplankton, it is now the phytoplankton itself that influences CO2uptake.
Kevin Arrigo, co-author and professor at Stanford University’s School of Earth, Energy & Environmental Sciences, calls the observations a “significant regime shift” for the Arctic, which is warming much faster than any other region on Earth.

In particular, the researchers analyzed the development of net primary production (NPP), which shows how quickly algae can convert inorganic carbon dioxide into organic energy, i.e. sugar. “The rates are really important in terms of how much food there is for the rest of the ecosystem,” Arrigo explains. “It’s also important because this is one of the main ways that CO2 is pulled out of the atmosphere and into the ocean.”

The more light and nutrients the phytoplankton – tiny unicellular algae – gets, the more carbon dioxide is extracted from the atmosphere. Photo: Julia Hager

A thickening soup
The 57 percent increase in NPP in the Arctic between 1998 and 2018 surprised Arrigo and his colleagues. No other ocean basin has ever seen such a jump in productivity. Initially, phytoplankton benefited from the decline of sea ice and the resulting larger open water areas and longer growth periods. But production continued to increase even as less ice melted around 2009. Now the algae grow more concentrated and form a kind of viscous algae soup.

Kate Lewis, lead author of the study, who is a PhD student in the Department of Earth System Science, says: “In a certain volume of water, more phytoplankton were able to grow each year. This is the first time this has been reported in the Arctic Ocean.”

New nutrient sources
Phytoplankton cannot grow without light and nutrients. However, their availability in the water column depends on a number of factors. The Stanford research team therefore compiled an extensive new collection of measurements of the color of the Arctic Ocean and developed new algorithms to estimate the phytoplankton concentration. According to this, the increase in production may not be as limited by nutrient availability as originally assumed. “It’s still early days, but it looks like now there is a shift to greater nutrient supply,” Arrigo said.
This is probably due to a new influx of nutrients from other oceans. The researchers initially thought it possible that production could be increased by recycling the same nutrients. But the study showed something else: “Phytoplankton absorbs more carbon year after year as new nutrients enter this ocean. It was unexpected and it has a big environmental impact,” Arrigo explained.

Left: Nutrient inflows (green arrows) and outflows (purple arrows) in the Arctic Ocean. Right: Rate of change in chlorophyll concentration in the Arctic Ocean between 1998 and 2018 in milligrams per cubic meter per year. Graphics: Kate Lewis, Data Source: NASA

New algorithms “decode” the Arctic
In general, the chlorophyll concentration can be measured relatively easily with satellite sensors or directly on board research vessels. In the Arctic, however, satellites often provide false results because of the interplay of light, color, and life in the Arctic, which is why the new algorithms became necessary.

“When you use global satellite remote sensing algorithms in the Arctic Ocean, you end up with serious errors in your estimates.”

Kate Lewis

One of the biggest difficulties is tea-colored river water, which transports dissolved organic material, but satellites mistakenly recognize it as chlorophyll.
Lewis painstakingly created data sets of ocean color and NPP measurements, and then used the compiled database to develop algorithms that tailored to the unique conditions of the Arctic. Both the database and the algorithms are now available for public use.

With this study, scientists are helping to better understand how climate change will affect future Arctic Ocean productivity, food supplies, and carbon uptake capacity. “There’s going to be winners and losers,” says Arrigo. “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.”
In addition, due to the increased growth of the phytoplankton, the synchronization with the rest of the food web may be lost, as the ice melts earlier in the year. The increased algae growth cannot be seen as a mitigating factor in global climate change, as the Arctic is far too small to absorb much of the world’s greenhouse gas emissions. “It’s taking in a lot more carbon than it used to take in, but it’s not something we’re going to be able to rely on to help us out of our climate problem,” Arrigo said.

Julia Hager, PolarJournal

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