When glaciers feed sea-ice life: a hidden Arctic bloom

Sea ice is often thought of as a frozen barrier between the atmosphere and the ocean. But, particularly in spring, microscopic life can turn this seemingly hostile environment into a productive ecosystem.

A new study, co-authored by Pedro Duarte from the iC3 Polar Research Hub, has documented exceptionally high sea-ice algal production in a Greenland fjord close to marine-terminating glaciers. The findings suggest that the interaction between glaciers, fjord circulation and turbulence may create local hotspots of productivity beneath the ice.

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A hidden bloom in Arctic sea ice

The researchers studied landfast sea ice in Nuup Kangerlua, West Greenland, during early spring—a period when light is returning but sea ice still covers the fjord.

They found an unexpectedly active community of sea-ice algae, including abundant pennate diatoms such as Nitzschia frigida. Primary production and chlorophyll a accumulation reached levels higher than previously detected inGreenland.

“These observations remind us that sea ice is not simply a physical barrier. It can be a highly productive habitat, even during the early spring,” says Pedro Duarte, iC3 researcher and co-author of the study.

Figure: Comparison of sea ice and under-ice water algal biomass and primary production. (a) Nitzschia frigida colonies in sea ice at 200 × magnification. (b) Mean primary production (blue bars, mg C m⁻² d⁻¹) and Chl a values (green bars, mg Chl a m⁻²) for sea ice and under-ice water. Credit: Søgaard et al. (2026), Nature Scientific Reports.

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The missing ingredient: nutrients

Light is essential for photosynthesis, but for algae growing at the bottom of sea ice, nutrients can be just as important. The researchers found that nitrate and phosphate were still available in the water beneath the ice, while silicic acid was strongly depleted within the ice, suggesting that it was the main limiting nutrient for the algal community.

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So where did the nutrients come from?

The answer may lie in the fjord's circulation. Marine-terminating glaciers can influence water movement and promote the transport of nutrient-rich water towards the surface. In addition, turbulence beneath the ice can enhance the exchange of nutrients across the ice–ocean boundary.

“It's the combination of physics and biology that makes this system so interesting,” Pedro explains. “Water movement can deliver nutrients to the ice–ocean interface, allowing algae to take advantage of the increasing spring light.”

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Glaciers, ice and ecosystems are connected

The study highlights how closely physical processes and biological productivity are linked in Arctic fjords. Rather than functioning as isolated components, glaciers, ocean circulation, sea ice and microorganisms form an interconnected system.

The authors stress that their observations represent a single early-spring event, so more measurements are needed to determine how widespread and persistent this phenomenon is. Nevertheless, the results challenge the idea that landfast sea ice in Greenland fjords is consistently biologically unimportant.

For Pedro, this is precisely why studying the Arctic across disciplines is so important:

“Small-scale processes at the ice–ocean interface can have consequences for the larger Arctic ecosystem. Understanding those connections is essential if we want to predict how Arctic marine ecosystems will respond to a changing climate.”

The study offers another reminder that some of the Arctic's most important biological activity may be happening in places—and at times—that remain surprisingly difficult to see.

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Find out more

The paper “High sea ice algal primary production near marine-terminating glaciers linked to turbulence-driven nutrient fluxes across the ice–ocean interface” is available open access in Nature Scientific Reports.

Lead author Dorte H. Søgaard is a senior researcher at the Greenland Climate Research Centre and the Department of Biology at Aarhus University.

Co-author Pedro Duarte is a researcher at the Norwegian Polar Institute and the iC3 Polar Research Hub in Tromsø, Norway. His work focuses on developing coupled physical-biochemical models to study the interactions between oceanic and sea ice processes.