A front in the Chukchi Sea drives dying phytoplankton to the seafloor four times faster

Four times half a meter a day comes out near two meters a day ^[23]. Across most of the ocean, the trip from the sunlit surface layer to the seafloor takes weeks ^[3]. At the front that trip is compressed, and the compression happens at a boundary: cold water carrying plankton out from under the ice drifts south, meets warmer opposing currents in open water, and the plankton go down with the colder mass ^[18].

"That cold, salty water mass takes a dive, and the warmer, fresher water mass rides up on top," said James Lauer, an Earth system science doctoral student and lead author of the paper on how currents affect the blooms ^[19][20].

The density number needs more care than the sinking rate. Under-ice blooms were up to 10 times more concentrated than blooms sampled a month later in open water, after seasonal melting had begun to open the ice ^[7]. Those two sets of samples differ in place, in date, and by a month of bloom development. "Up to 10 times" is a ceiling; the four-fold difference in sinking is reported as an average across locations ^[5][7].

Sunlight filtering through cracks in the ice was enough to power photosynthesis, the 2023 measurements suggest ^[8]. "We're observing phytoplankton in huge populations underneath substantial sea ice," said Claudette Proctor, lead author of one of the two papers ^[9]. "They are growing in an environment that we previously thought was inhospitable," she said ^[10].

The blooms drew down nutrients until they ran short, nitrate in particular, then withered and began to sink ^[16]. Catching that took traps in the right places and a long enough stay. Few missions have sampled phytoplankton under the ice, and fewer have followed them across almost an entire growing season ^[12]. "We got to see the bloom come up, peak and then start to come down," said Arrigo, senior author of both papers ^[13]. "You almost never get a chance to do that" ^[14].

Earlier surveys had turned up patches of Arctic seafloor with unexpectedly large populations of clams and brittle stars, along with the walruses and whales that feed on them ^[21]. "The discovery that this front is rapidly enhancing rates of sinking helps to explain where the food sources to support that benthic biomass might be coming from," said Lauer ^[22].

A revised Arctic carbon budget would take more than a sinking rate, a density ratio and a nutrient drawdown. It would need the front's extent, whether it forms the same way in other summers, and how much of the shelf it reaches; the samples come from one sea in the summer of 2023 ^[2]. Arrigo's group first found blooms under Arctic ice on a 2011 mission ^[11], and the season-long follow-up came 12 years later ^[24].