Arctic Currents Rapidly Send Carbon-Rich Phytoplankton to the Seafloor, Stanford Study Finds

In most of the world’s ocean, the journey of a dead phytoplankton cell from the sunlit surface to the deep seafloor is a slow one, taking weeks of gradual sinking through the water column. In the Chukchi Sea, a shallow shelf sea off the northwestern coast of Alaska, researchers have now documented something strikingly different. There, a collision of opposing ocean currents hurls carbon-carrying phytoplankton downward at roughly four times the typical rate, delivering the organic material to the seabed with remarkable speed. The finding, published in two papers in the Journal of Geophysical Research: Oceans, offers a rare end-to-end view of the seasonal life cycle of Arctic phytoplankton and helps explain some of the most productive benthic ecosystems in the polar north.

The research, led from the Stanford laboratory of biological oceanographer Kevin Arrigo, the Donald and Donald M. Steel Professor in the Stanford Doerr School of Sustainability, draws on an intensive field campaign conducted in the summer of 2023 aboard the research vessel Sikuliaq. The team set out to answer two linked questions: how dense are the phytoplankton blooms that form beneath Arctic sea ice, and what becomes of those blooms when they die? While previous missions have sampled phytoplankton at scattered points in time and space, few have documented the organisms across almost their entire season of growth, from first greening of the water to the final collapse of the bloom.

The story of under-ice phytoplankton blooms begins with a discovery made in 2011, when Arrigo and collaborators reported in Science that phytoplankton were blooming beneath Arctic ice. The observation surprised many researchers, because thick ice had long been assumed to block too much light for photosynthesis. But over the past few decades, Arctic air and sea surface temperatures have climbed, and the ice cover has thinned and cracked, allowing more sunlight to filter through. In the Chukchi Sea, the 2023 measurements suggest that light entering through cracks and thinner patches of ice was sufficient to power substantial photosynthesis in the darker waters below.

The scale of what the team found beneath the ice was remarkable. The researchers observed one of the densest phytoplankton blooms ever recorded, even in areas where the ice was up to two meters thick. These under-ice blooms were up to ten times more concentrated than blooms sampled a month later in open water, after seasonal melting had begun to recede the ice edge. The data show that the blooms eventually withered and began to sink once the phytoplankton had exhausted most of the available nutrients, especially nitrate. As Claudette Proctor, an Earth system science PhD student and lead author of one of the two papers, put it, the organisms are growing in an environment that scientists previously thought was inhospitable.

To follow the bloom through its rise, peak, and decline, the team used a combination of straightforward but demanding methods. They collected seawater samples to measure nutrients and two standard indicators of phytoplankton abundance, carbon and chlorophyll. They also deployed floating sediment traps, moored in the water column both in open water and in holes cut through the ice, to catch phytoplankton as they sank. By tracking how much phytoplankton biomass, by weight, accumulated in the traps over several days, the researchers could estimate both the quantity and the speed of the sinking flux beneath the ice and in open water. They then compared these results with chlorophyll and carbon measured in sediment cores hauled up from the seafloor, connecting the sinking particles to the organic remains accumulating below.

Across much of the Chukchi Sea, the scientists found that phytoplankton sank after their blooms peaked at a rate of about half a meter per day, consistent with expectations for the open ocean. But at particular locations, the sinking accelerated dramatically. Swift waters there pushed the plankton down about four times faster on average, driven by a confluence of ocean currents known as a front. The mechanism is a straightforward consequence of water mass physics: when cold, salty water drifting south just beneath the ice meets warmer, fresher water flowing in the opposite direction in the open water, the denser mass plunges downward, carrying its phytoplankton passengers with it.

James Lauer, an Earth system science PhD student and lead author of the second paper, which focused on how currents affect the blooms, described the encounter in vivid terms: the cold, salty water mass takes a dive, and the warmer, fresher water mass rides up on top. The result is a kind of express elevator to the seafloor for carbon-laden phytoplankton caught at the boundary. Because fronts created by winds, river outflows, or currents are found throughout the oceans, the discovery has implications well beyond the Arctic, suggesting that relatively small-scale physical features may play an outsized role in transporting carbon to the deep sea.

The rapid sinking helps solve an ecological puzzle that has lingered for years. In previous research, scientists had observed areas of the Arctic seafloor supporting unexpectedly high populations of clams and brittle stars, along with the walruses and whales that feed on them, and had struggled to identify the food source sustaining such biomass. The new work provides a plausible answer. As Lauer noted, the discovery that the front is rapidly enhancing rates of sinking helps explain where the food that supports that benthic abundance might be coming from. In effect, the collision of currents is a delivery system that channels surface productivity directly to the animals living in the sediment below.

What happens closer to the surface is less certain, and the timing of the blooms may matter as much as their intensity. Under-ice blooms are a major food source for zooplankton, the tiny drifting animals that in turn feed species such as bowhead whales. But blooms that peak too early in the summer may already be gone by the time seasonal predators arrive, reshuffling who eats what in Arctic food webs. As Proctor suggested, a warmer Arctic could favor bottom-feeding organisms, which benefit from the enhanced rain of sinking food, while pelagic organisms in the water column might face leaner conditions. Such shifts could ripple upward through food webs topped by orcas, whales, and sharks.

The findings also speak to the Arctic’s role in the global carbon cycle. Phytoplankton pull carbon dioxide from the atmosphere as they grow, and the question of what happens to that carbon, whether it is eaten, recycled, or buried in sediments, is central to understanding how the region will respond to climate change. Arrigo said the results point toward an overall increase in the carbon absorbed by phytoplankton and eventually stowed away in sediments, though many unknowns remain. One concern is that warmer freshwater from melting ice could form a buoyant layer at the sea surface, blocking nutrient-rich waters below from mixing upward and thereby limiting future phytoplankton growth. With the Arctic warming four times faster than the global average, the balance between these competing effects will shape how much carbon the fastest-warming region on Earth can lock away.

Subject of Research: The seasonal growth, sinking, and carbon export of under-ice phytoplankton blooms in the Arctic's Chukchi Sea

Article Title: Currents in an Arctic sea accelerate the sinking of carbon-carrying phytoplankton

Article References: Currents in an Arctic sea accelerate the sinking of carbon-carrying phytoplankton. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: phytoplankton, Chukchi Sea, Arctic, ocean currents, carbon cycle, sea ice, fronts, sediment traps, benthic ecosystems, nitrate, climate change, photosynthesis