New pan-Arctic study finds complex dynamics between ice, algae and nutrients

Ice algae are tiny. But a new pan-Arctic study shows they can leave a big chemical fingerprint inside sea ice. They shift nutrient levels and change nutrient balance in ways that matter for the whole marine food web.

The work helps explain why some Arctic regions support strong spring growth, while others slide into nutrient shortage much earlier as the Arctic warms.

How do ice algae and nutrients move together? 

Sea ice is not just frozen water. It is a porous habitat, full of small brine channels, where microscopic algae can bloom in spring. These ice algae can supply a large share of annual primary (algal) production in ice-covered seas. They also provide an early seasonal pulse of food for zooplankton, fish, seabirds, and marine mammals.

In the new study, a large international team asked a simple question at Arctic scale: How are algae and nutrients inside the ice related to each other across different seas, shelves, and basins? The researchers focused on nutrients that algae need to grow, including different forms of nitrogen, phosphate, and silicate. They used chlorophyll a, a pigment found in algae, as a practical measure of algal biomass.

One clear signal stood out. In many places, nutrient concentrations in the bottom layer of sea ice rose and fell together with algal biomass during the bloom. This suggests that ice algae are not just taking up nutrients from seawater. They are also storing nutrients inside their cells and releasing them when ice samples melt during processing.

 “There appears to be a strong biological influence on what we measure in sea ice,” explains co-author Philipp Assmy of the iC3 Polar Research Hub.

Philipp Assmy hunting sea ice diatoms off Svalbard. Credit: Jenny Ross

Regional dynamics differ strongly

The study revealed strong regional contrasts. The highest bottom-ice nutrient and chlorophyll concentrations were found in Resolute Passage in the Canadian Arctic Archipelago. This region experiences strong tidal mixing at the ice–ocean boundary, which continually resupplies nutrients to the algae growing at the base of the ice.

At the other extreme, the central Arctic Ocean basins showed very low algal biomass and low nutrient concentrations in bottom ice. In these deep and strongly layered waters, it is much harder for nutrient-rich water from below to reach the bottom of the sea ice.

Large-scale ocean circulation also matters. Waters entering the Arctic from the Pacific tend to carry high levels of silicate, while Atlantic-origin waters are relatively richer in nitrogen. These differences in water masses shape nutrient patterns not only in the ocean, but also inside the sea ice itself.

Some coastal regions are especially prone to nutrient shortage. Cambridge Bay stood out as a nutrient-limited system. Here, freshwater input from rivers and melting ice strengthens stratification and cuts off nutrient supply from below. In these conditions, the usual positive link between algae and nutrients weakens or even turns negative as the bloom runs out of fuel.

How the team built a pan-Arctic picture

To move beyond local case studies, the team led by Fowzia Ahmed from the University of Manitoba compiled data from 30 published and unpublished datasets collected between 1983 and 2023. These data cover a wide range of Arctic regions and include measurements from both landfast ice and drifting pack ice.

Philipp photographing sea ice diatoms off Svalbard. Credit: Jenny Ross

The analysis focused on sea-ice cores, especially the biologically active bottom layer, together with nutrient concentrations measured in the seawater just beneath the ice. The team compared regions and seasons and tested how strongly nutrients and algal biomass tracked each other during spring and early summer.

A key methodological point emerged. When sea-ice samples are melted, algal cells can experience osmotic shock. This can release nutrients that were stored inside the cells, meaning that measured nutrient concentrations reflect both dissolved nutrients in the ice and nutrients that were previously locked inside living algae. Understanding this effect is essential for interpreting sea-ice chemistry correctly.

Why this matters for a warming Arctic

The Arctic is warming rapidly. Sea ice is becoming thinner, snow cover is changing, and the upper ocean is becoming more strongly layered in many regions. These changes can improve light conditions for ice algae, but they also make it more likely that algae will face nutrient limitation earlier in the season.

This new synthesis shows that ice algae are not passive passengers in this system. They actively shape nutrient concentrations and nutrient ratios inside sea ice. These changes can influence which organisms grow next, how efficiently carbon moves through the food web, and how Arctic ecosystems respond to ongoing climate change.

Philipp summarises the wider significance:
“We show that ice algae have a pan-Arctic, but regionally specific, influence on nutrient concentrations in bottom sea ice. This biological signal needs to be considered when we think about future Arctic productivity.”

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Philipp is looking for a candidate interested in applying for an MSCA Postdoctoral Fellowship in 2026 to research harmful algae and their toxins in Svalbard fjords. Details on this opportunity will be posted on the iC3 news page in the coming weeks.

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Learn more about the paper and its authors

The study can be accessed in Elementa: Science of the Anthropocene. The lead author is Fowzia Ahmed from the University of Manitoba.

Co-author Philipp Assmy is an iC3 researcher based at the Norwegian Polar Institute in Tromsø. His research focuses on how changes at the ice–ocean boundary affect plankton and marine ecosystems in both the Arctic and the Antarctic. To find out more about Philipp’s work, read this interview with him.