Life Under the Ice: How Sea-Ice Algae Prime the Arctic Food Web

Every polar spring, the Arctic's first meal is cooked in a kitchen most people never imagine: inside the sea ice itself. Months before the open water turns green, a layer of algae fastened to the underside of the floes wakes up in the returning light and starts photosynthesising in near-darkness. These sea-ice (or sympagic) algae can deliver the year's first pulse of food to a food web that has been running on empty since autumn. Here is how that hidden meadow works, who depends on it, and why its fate is bound up with that of every polar coastline — including the kelp forests now spreading along newly ice-free shores.

What lives inside sea ice?

Sea ice is not a solid block. As seawater freezes, salt is expelled from the growing crystal lattice and concentrated into tiny pockets and channels of ultra-salty brine — a plumbing system threaded through the bottom of the floe. Those brine channels are habitat. Into them move bacteria, flagellates, ciliates and, above all, microscopic algae, dominated by cold-loving pennate diatoms such as Nitzschia frigida. The whole assemblage — microbes, grazers and all — is called the sympagic community, from the Greek for "ice-associated".

The densest life sits in the lowest few centimetres of the ice, where brine channels stay connected to the seawater below and nutrients can replenish. Turn a floe over at the right time of year and its underside is stained golden-brown with living pigment. In parts of the central Arctic, the centric diatom Melosira arctica grows in metre-long hanging strands, an upside-down meadow trailing beneath the ice — among the strangest sights in polar science and, when it grows thick, a substantial standing crop of food dangling directly into the water column.

The physics of a frozen habitat

Living inside the year's coldest structure takes serious adaptation:

• Light in miniature. Through snow-covered ice, only a sliver of the sun's energy penetrates. Ice algae pack their cells with light-harvesting pigments tuned to the dim blue-green that filters through, and they can photosynthesise at light levels that would starve most phytoplankton. Our explainer on light underwater covers why blue-green wavelengths are the survivors.
• Brutal brine. Brine channels can run far above seawater's normal salinity and far below its freezing point. Sympagic microbes produce cold-active enzymes and exude extracellular polymeric substances (EPS) — sugary gels that buffer cells against ice crystals and subtly alter the ice's own microstructure.
• Light-shock readiness. An alga adapted to perpetual dusk must survive the abrupt glare of melt season without burning out, switching on photoprotective pigments as conditions swing.

The spring handover: three blooms in sequence

The polar production year has a choreography, and the ice algae go first.

1. The in-ice bloom. When daylight returns but the ice still caps the sea, the sympagic community blooms inside the floe. Because this happens weeks to months before open water appears, it is often the first new carbon entering the food web since autumn.

2. The ice-edge bloom. As the floes retreat, meltwater spreads a fresher, lighter layer over the surface. That stratified lid traps phytoplankton in sunlit water — our primer on ocean stratification explains why layers matter — and a classic ice-edge bloom ignites just seaward of the retreating front, tracking it like a green shadow.

3. The pelagic summer bloom. Finally, in open water, the wider phytoplankton community takes over until nutrients run down.

How much of the whole year's production comes from the ice itself? Estimates vary enormously by region and year — from a few percent in dynamic, quickly clearing seas to half or more of total primary production in some heavily ice-covered waters in some years. The honest answer is "a modest slice on average, but a decisive one": what matters is less the tonnage than the timing. The ice-algal pulse arrives when grazers are coming off winter starvation and beginning reproduction.

Who eats the ice meadow?

The short answer: almost everyone, directly or after a few handoffs.

• Under-ice crustaceans. Amphipods loiter beneath floes and rasp the underside directly; Arctic cod shelter among ridge keels and feed on those grazers in turn.
• Copepods on a schedule. Herbivorous copepods such as Calanus species time egg production and the ascent of their young to coincide with the first algal food — for some populations, the ice-algal pulse specifically, rather than the later open-water bloom.
• The benthic dividend. When the bloom ends, dead and dying cells clump and sink. Over the Arctic's enormous shallow shelves, much of that rain reaches the seafloor, feeding amphipods, brittle stars, sea cucumbers and clams — an unusually tight benthic-pelagic handoff compared with deeper seas. In some shelf ecosystems, benthic animals capture the majority of the ice algae's carbon.
• The waiting crowd at the edge. Seabirds, seals, walrus and whales concentrate along the retreating ice edge, where production, prey and access to air all intersect. Iñupiat and other Indigenous hunters have organised seasonal rounds around the ice edge for millennia — the ecological congregation did not need a satellite to be discovered.

This is the same architecture that powers a kelp forest food web, drawn on a different substrate: a fixed primary producer blooming on a schedule, a queue of grazers calibrated to the calendar, and a detrital overflow that feeds everything below.

Why kelp people care

Arctic kelp forests and sea ice are closer neighbours than a map suggests. Around Svalbard, Greenland, northern Alaska and the broader North Atlantic and Arctic, the spring ice edge often sits directly over rocky, kelp-suitable bottom. Three connections matter:

1. A shared pulse. Meltwater stratification and ice-edge blooms set the nutrient background against which polar kelp starts its growth season.
2. Ice as wave shield. Fast ice suppresses winter storm waves, protecting coastal kelp from the season's worst tearing — and from ice scour when the floes finally move.
3. A moving frontier. As ice-free seasons lengthen, historical surveys and recent observations agree that kelp is expanding into Arctic coastlines that light and ice scour once closed to it. The forest is literally following the ice's retreat.

A habitat in transition

The Arctic Ocean's old, thick, multi-year ice has been thinning and shrinking for decades, replaced by a younger, more seasonal cover. For the sympagic community the consequences cut both ways, and honest science is still sorting the balance:

• Thinner ice admits light earlier. In the Chukchi Sea in 2011, researchers documented a vast phytoplankton bloom underneath the ice, fed by light passing through melt ponds — a phenomenon once thought impossible. Some production may be migrating from brine channel to water column rather than disappearing.
• But the habitat itself is shrinking. Less multi-year ice means less of the stable, months-old floe underside where the richest algal layers develop, and melt ponds can darken the lower ice when the in-ice bloom needs light most.
• Timing is the fragile variable. If the ice-algal pulse arrives weeks earlier than copepod egg-laying or amphipod breeding expect, carbon produced at the bottom of the food web sinks uneaten — a phenological mismatch whose full costs are still being measured.

Rising ocean heat content underwrites all of these shifts: the ice's loss is the surface expression of a heat budget that has been changing for two generations.

How scientists study the under-ice world

Ice cores bored through floes and melted down in the dark remain the workhorse measurement. Ice-tethered profilers and under-ice ROVs now photograph and sample the floe underside across whole seasons — it was ROV survey work that revealed how extensive the hanging Melosira meadows can be. And because satellites cannot see through ice, researchers increasingly validate models against biological archives: the annual growth bands of long-lived Arctic clams record each year's food supply in shell chemistry, a library of past ice edges.

Key takeaways

• Sea-ice algae live in brine channels and on the floe underside (the sympagic community), dominated by cold-adapted diatoms, sometimes forming metre-long hanging strands.
• They bloom first — weeks before open-water phytoplankton — delivering the food web's first meal after winter; timing, not tonnage, is their superpower.
• Copepods, under-ice amphipods, Arctic cod and shelf seafloor communities are all tuned to the pulse; the retreating ice edge concentrates birds and mammals.
• Polar kelp forests share the ice-edge system: ice shields coasts from storm waves, melt sets the nutrient stage, and forests are expanding as ice retreats.
• Thinner, younger ice is shifting production from the ice into the under-ice water column and shrinking the habitat itself; the net ecological cost is an open and urgent question.

FAQ

Is sea ice really alive inside?

Yes. Brine channels within the ice hold liquid water year-round, hosting bacteria, algae and small grazers — a functioning, if miniature, ecosystem running at temperatures and salinities that would kill most marine life.

How much of Arctic food production comes from ice algae?

Regional estimates span from a few percent of annual primary production to over half in heavily ice-covered waters in some years. Even at the low end, the ice-algal bloom's early timing makes it disproportionately important to grazers ending their winter fast.

What happens to ice algae when the ice melts?

Cells are released into the water column, where some seed the following phytoplankton bloom while much of the material clumps and sinks — fertilising seafloor communities across the shallow Arctic shelves.

Does more light under thinner ice mean more algae?

Sometimes — under-ice phytoplankton blooms through melt ponds are real and were considered impossible not long ago. But thinner ice is also shorter-lived and more ponded, so the long-run trend for the in-ice habitat itself is downward.

What does sea-ice algae have to do with kelp restoration?

On Arctic coasts, the sea-ice system dictates light, nutrients and wave exposure for kelp. Restoration planning on those shores has to model the ice edge and its future as carefully as it models water temperature — part of the habitat is the ice.