Phytoplankton Surge in Arctic Waters

Phytoplankton Surge in Arctic Waters

Each summer, parts of the Arctic Ocean and peripheral seas lose their ice cover and bathe in ample sunlight. In this window of time, some of these open-water areas come to life with phytoplankton blooms so large and vivid they can be seen from space.

The summer of 2020 has been one of those summers. Phytoplankton—the floating plant-like organisms that, like plants on land, need sunlight and nutrients to thrive—have exploded in numbers, or “bloomed,” across ice-free northern latitudes. The image above, acquired on July 26 with the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra and Aqua satellites, shows a bloom of phytoplankton in the Barents Sea, north of Scandinavia and Russia.

In the Barents Sea, blooms that show up in spring and early summer are typically composed of diatoms—a microscopic form of algae with silica shells and ample chlorophyll, which makes them appear green in satellite images. By late July and into autumn, waters become warmer and more stratified. This change promotes blooms of coccolithopohres, a type of phytoplankton that turns the water milky white-green in satellite imagery due to their calcium carbonate shells. Sometimes, as is likely the case here, both types of phytoplankton show up to at once.

While blooms around the Arctic are somewhat common, new research has shown that phytoplankton biomass is continuing to increase in the Arctic Ocean. In a recent paper published in Science, researchers from Stanford University found that the rate of growth of phytoplankton biomass across the Arctic Ocean increased by 57 percent between 1998 and 2018.

The success of phytoplankton in this increasingly ice-free environment is apparent in the map above. It shows changes in the amount of chlorophyll a—the pigment used by phytoplankton to harness sunlight for making food—during the past two decades across the Arctic Ocean. The map was produced using a regional algorithm that incorporates satellite data from NASA’s SeaWiFS and MODIS instruments. Notice that most of the increase is in the Chukchi and Barents seas.

“We were certainly surprised to see that phytoplankton biomass has begun to increase in the Arctic,” said Stanford biological oceanographer Kevin Arrigo, whose doctoral student Kate Lewis led the study. “Many people have been saying that with increased melt, nutrient inventories would decline, but that is not what we saw.”

Between 1998 and 2008, the increase in phytoplankton biomass was likely the result of increasingly vast expanses of ice-free waters and a longer growing season. Since 2009, the increases have been due to phytoplankton growing in ever-higher concentrations. The implication is that nutrients might be increasing in some parts of the Arctic Ocean.

Not all areas saw increases; the waters east of Greenland stand out. Arrigo speculated that the decline there could be related to higher nutrient consumption “upstream” in waters that have become more productive. “But we would have to go there and look to know for sure,” he said.

It remains unclear how phytoplankton biomass will change in the future as Arctic sea ice cover continues to shrink. “The Arctic Ocean is changing in an alarming rate and soon may lose a lot of its uniquely polar characteristics,” Arrigo said. “There will always be winter ice, but for the rest of the year, it’s starting to look a lot like our temperate oceans.”

NASA Earth Observatory images by Joshua Stevens, using MODIS data from NASA EOSDIS/LANCE and GIBS/Worldview and data courtesy of Lewis, K. M., van Dijken, G. L., & Arrigo, K. R. (2020). Story by Kathryn Hansen.

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