This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Tiny plantlike organisms called phytoplankton that form the base of marine food webs topped by orcas, whales and sharks usually get eaten or slowly sink to the seafloor, carrying carbon as they go. Across most of the ocean, the descent from sunlit surface layers takes weeks.
But in the Arctic's Chukchi Sea, scientists have found that a convergence of currents pushes phytoplankton from under the ice to the seafloor at four times the usual speed. Two new papers led by researchers in the lab of Stanford biological oceanographer Kevin Arrigo detail this unusual descent and reveal that blooms of phytoplankton, which use energy from the sun to grow, can be 10 times denser in the darker waters below thick sea ice than in open water. Measurements gathered in the Chukchi Sea in the summer of 2023 suggest sunlight filtering through cracks in the ice was sufficient to power photosynthesis.
"We're observing phytoplankton in huge populations underneath substantial sea ice," said Earth system science Ph.D. student Claudette Proctor, lead author of one of the papers published Sept. 23 in the Journal of Geophysical Research: Oceans. "They are growing in an environment that we previously thought was inhospitable." In a 2011 mission, Arrigo and collaborators discovered phytoplankton were blooming under Arctic ice, which has thinned and cracked over the past few decades as air and sea surface temperatures have climbed, allowing more light through. Arrigo and his team returned to the Chukchi Sea in 2023 to study the density of phytoplankton blooms and what happens to them when they die.
While other research missions have sampled phytoplankton, few efforts have sampled the tiny organisms under the ice, let alone documented them across almost their entire growing season. "We got to see the bloom come up, peak and then start to come down," said Arrigo, senior author of the two papers and the Donald and Donald M. Steel Professor in the Stanford Doerr School of Sustainability.
"You almost never get a chance to do that." To study the blooms, the team took seawater samples to measure nutrients and two indicators of phytoplankton density: carbon and chlorophyll. They also placed floating sediment traps in seawater, both in open water and holes within the ice, to catch sinking phytoplankton. Beneath the ice, the researchers observed one of the densest phytoplankton blooms ever recorded, even in areas where the ice was up to 2 meters (6.6 feet) thick.
These under-ice blooms were up to 10 times more concentrated than those sampled a month later in open water, after the ice had begun to recede because of seasonal melting. The data show blooms withered and began to sink as they used up most of the nutrients, especially nitrate. Over several days, the researchers estimated how much phytoplankton by weight sank beneath the ice and in open water, and how quickly.
Extract — continue reading at the source.
