Polar marine ecosystems run on ice. Sea ice structures nearly every biological process in the Arctic and Southern Oceans, from the algae that grow inside it to the mammals that haul out on it to the currents and nutrients it helps regulate. These ecosystems support staggering biomass despite months of near-total darkness each year, and the ways organisms have adapted to this environment are among the most remarkable in biology. But the ice is changing fast, and with it, the timing, distribution, and composition of life at both poles.
Ice Algae and the Hidden Base of the Food Web
Before sunlight triggers the open-water phytoplankton blooms that most people associate with polar productivity, microscopic algae are already growing inside and on the underside of sea ice. These “sympagic” (ice-associated) algae photosynthesize through the translucent ice as soon as spring light returns, sometimes weeks before the ice breaks up. A modeling study coupling an ice ecosystem model with a global sea ice model estimated annual Arctic sea ice primary production at roughly 15 million tonnes of carbon, within a range of about 9 to 73 million tonnes estimated from field observations.
1Journal of Geophysical Research: Oceans. Large‐scale modeling of primary production and ice algal biomass within arctic sea ice in 1992That wide range reflects how difficult it is to measure something happening inside ice spread across millions of square kilometers. But even conservative estimates show that ice algae are a critical early-season food source for zooplankton, especially copepods and krill larvae that depend on those calories after a long, dark winter. When the ice melts, the algae sink or are released into the water column, seeding the phytoplankton blooms that follow.
Phytoplankton Blooms and Their Shifting Seasons
Once sea ice retreats in spring, the combination of light, meltwater-driven stratification, and nutrients triggers massive phytoplankton blooms. These blooms are the main engines of polar productivity, fixing carbon, feeding zooplankton, and ultimately supporting everything from fish to whales. But their timing is tightly linked to when and how ice disappears, and that relationship is changing.
West of the Antarctic Peninsula, phytoplankton blooms have been starting and peaking later over the period from 1997 to 2022 in ice-associated waters and over the continental shelf. The most likely explanation is that stronger spring winds are mixing the water column more deeply in a region that is simultaneously losing sea ice, delaying the stratification that phytoplankton need to stay in the sunlit layer long enough to bloom.
2Marine Ecology Progress Series. Changing phytoplankton phenology in the marginal ice zone west of the Antarctic PeninsulaLater blooms carry downstream consequences for carbon uptake by the surface ocean and for animals higher in the food web that time their breeding and feeding to the bloom’s peak.
In the Pacific Arctic, the picture plays out differently but with a similar theme. Earlier ice retreat has shifted the type of bloom that dominates. When ice retreats late, under-ice blooms are more common; when it retreats early, blooms in the marginal ice zone take over. Between 2003 and 2019, the proportion of under-ice blooms declined while marginal ice zone blooms increased, tracking the trend toward earlier ice breakup.
3PLoS ONE. Variability in spring phytoplankton blooms associated with ice retreat timing in the Pacific Arctic from 2003–2019The organisms that graze on these different bloom types are not interchangeable, so a shift in bloom type ripples through the food web.
How Nutrients Reach the Surface
Phytoplankton cannot bloom without nutrients, and polar oceans get theirs through a combination of deep-water upwelling, ice-edge mixing, and currents that carry nutrient-rich water from lower latitudes. The Antarctic Circumpolar Current, the strongest ocean current on Earth, plays an outsized role. Its southern boundary marks the limit of upwelling Circumpolar Deep Water, a water mass rich in nutrients and low in oxygen that originates at depth. Where this water reaches the surface or intrudes onto continental shelves, it fuels primary production.
4Progress in Oceanography. Oceanographic factors determining the distribution of nutrients and primary production in the subpolar Southern OceanOn the West Antarctic Peninsula shelf, intrusions of Upper Circumpolar Deep Water provide a reservoir of warm, nutrient-rich water below about 150 meters. When this water upwells, it brings silicate and other nutrients that diatoms need to thrive. Where upwelling is topographically forced, diatom growth spikes, and large krill are typically found nearby, feeding on the resulting bloom.
5Journal of Marine Research. The Linkage Between Upper Circumpolar Deep Water (UCDW) and Phytoplankton Assemblages on the West Antarctic Peninsula Continental ShelfThis connection between deep-water upwelling, phytoplankton type, and krill distribution illustrates how tightly coupled polar marine food webs are to physical oceanography.
Antarctic Krill and the Carbon They Move
Antarctic krill are arguably the most consequential single species in the Southern Ocean. Their combined biomass is enormous, and they sit at the nexus between primary production and almost every large predator in Antarctic waters, from penguins to baleen whales to seals. But their importance extends beyond the food web. Krill faecal pellets sink rapidly, and because of the species’ massive population, these pellets transport roughly 20 million tonnes of carbon to the deep ocean each productive season. That is comparable to the carbon stored by major coastal “blue carbon” habitats like mangrove forests and seagrass beds.
6PubMed Central. Antarctic krill sequester similar amounts of carbon to key coastal blue carbon habitatsThe pellets only need to sink to an average depth of about 381 meters to achieve long-term sequestration, and some of that carbon eventually reaches as far as the North Pacific. In economic terms, the carbon service krill provide has been valued at billions of dollars per productive season depending on carbon pricing. The krill fishery, which has been expanding, therefore has implications not just for the food web but for the ocean’s role in regulating atmospheric carbon dioxide.
6PubMed Central. Antarctic krill sequester similar amounts of carbon to key coastal blue carbon habitatsThe Whale Pump
Large whales contribute to polar productivity in a way that sounds counterintuitive: by pooping. Sperm whales in the Southern Ocean feed on squid and fish at depth, then return to the surface to breathe and defecate. Their liquid, iron-rich faeces fertilize the photic zone, the sunlit upper layer where phytoplankton grow. Iron is the limiting nutrient across much of the Southern Ocean, so even small additions can stimulate significant new production.
An estimated 12,000 sperm whales in the Southern Ocean defecate about 50 tonnes of iron into the photic zone each year. Using ratios from natural ocean fertilization events, researchers estimated that this iron drives the export of roughly 400,000 tonnes of carbon per year to the deep ocean, while the whales themselves respire only about 200,000 tonnes. The net effect is that Southern Ocean sperm whales act as a carbon sink, removing more carbon from the atmosphere than they add.
7PubMed Central. Iron defecation by sperm whales stimulates carbon export in the Southern OceanThis “whale pump” mechanism applies to other large marine mammals too, and it means that historical whaling, which dramatically reduced whale populations, likely also reduced the ocean’s capacity to cycle nutrients and sequester carbon.
Life on the Polar Seafloor
Polar benthic communities depend almost entirely on what rains down from above. In most polar seas, the seafloor is food-limited because zooplankton and microbial loops in the water column consume much of the organic carbon before it reaches the bottom.
8Journal of Marine Systems. The influence of oceanographic processes on pelagic-benthic coupling in polar regions: A benthic perspectiveThe exceptions are relatively shallow continental shelves where production is so intense that a substantial fraction sinks through, such as the Bering and Chukchi Seas in the Arctic, which support unusually rich bottom-dwelling communities of clams, worms, and crustaceans.
Along the West Antarctic Peninsula, an interesting workaround sustains the benthos through the dark winter. Organic detritus accumulates in sediments during the productive season, forming a “food bank” that persists for months. This stored material supports sediment respiration, deposit feeding, and even reproduction and recruitment of benthic organisms well after surface production has ceased.
9Oceanography. Pelagic-Benthic Coupling, Food Banks, and Climate Change on the West Antarctic Peninsula ShelfIn shallow Antarctic waters, another process shapes the seafloor: anchor ice. Ice crystals can nucleate on and around benthic organisms, and when enough ice forms, it can lift organisms off the bottom entirely. Some species are more susceptible than others. Sponges like Homaxinella balfourensis show increased ice crystal formation and faster spread compared to sea urchins and sea stars, which helps explain why sponge abundance tends to be lower in areas where anchor ice is common.
10PubMed. Anchor ice and benthic disturbance in shallow Antarctic waters: interspecific variation in initiation and propagation of ice crystalsAnchor ice also forms readily on sediments and can mobilize bed material, playing an ecological role as both a disturbance agent and, paradoxically, a habitat and food source for some organisms.
11Progress in Physical Geography: Earth and Environment. Anchor ice in polar oceansPolynyas as Oases in the Ice
Polynyas are persistent areas of open water surrounded by sea ice, maintained by winds or warm water upwelling. They function as ecological hotspots because they allow light and gas exchange at the ocean surface when the surrounding sea is still frozen. The North Water Polynya between Greenland and Ellesmere Island is one of the most productive regions in the entire Arctic, supporting high primary productivity, rich biodiversity, and serving as a key habitat and migration corridor for Arctic species. Its productivity exceeds that of nearby Western Baffin Bay and Western Greenland waters, and it has sustained the world’s northernmost Inuit communities for millennia.
12Arctic Science. Ecosystem structure and function of the North Water PolynyaPolynyas attract concentrations of seabirds, marine mammals, and fish because they offer early access to food when the rest of the ocean surface is still sealed by ice. For migrating species, they serve as stepping stones through otherwise impassable frozen seascapes.
Gelatinous Predators Under the Ice
Jellyfish and comb jellies are not the first animals most people associate with polar seas, but they play a surprisingly important role. In the western Arctic, the comb jelly Mertensia ovum selectively feeds on the largest copepods and amphipods, consuming an estimated two percent of the Calanus copepod population per day. Another species, Beroe, consumes smaller copepods and pteropods at a rate of about four percent per day.
13Deep Sea Research Part II: Topical Studies in Oceanography. Distribution, abundance, and predation effects of epipelagic ctenophores and jellyfish in the western Arctic OceanThose numbers matter because Arctic Calanus copepods have life cycles of two or more years and are a critical food source for bowhead whales and Arctic cod. A gelatinous predator steadily removing even a small percentage of the copepod standing stock each day can significantly affect how much energy is available to vertebrates higher in the food web. As Arctic waters warm and ice retreats earlier, conditions that favor gelatinous zooplankton blooms could shift this balance further.
Built for the Cold
Surviving in water that hovers near or below the freezing point of body fluids requires biological innovation. Many polar fish produce antifreeze proteins that adsorb to small ice crystals in their blood and tissues, halting ice growth and producing a lowering of the freezing point beyond what the fish’s blood salts alone could achieve.
14PubMed. Antifreeze proteins of teleost fishesThese proteins also inhibit the recrystallization of ice and can interact with cell membranes, offering protection at multiple levels.
15PubMed Central. Marine Antifreeze Proteins: Structure, Function, and Application to Cryopreservation as a Potential CryoprotectantThe evolutionary story behind these adaptations is richer than a simple tale of one trait unlocking polar life. In the Southern Ocean, the dominant fish group, the notothenioids, evolved antifreeze glycoproteins between about 42 and 22 million years ago, a window that includes a major global cooling event around 35 million years ago. But the main burst of species diversification did not follow immediately. Instead, the most species-rich lineages radiated at least 10 million years later, during a second cooling event in the Late Miocene. The real ecological opportunity came not from freeze avoidance alone but from the combination of antifreeze capability and the new habitats opened up by increased glacial and ice-sheet activity.
16PubMed Central. Ancient climate change, antifreeze, and the evolutionary diversification of Antarctic fishesMore broadly, a wide variety of Antarctic benthic and pelagic organisms radiated after a major cooling phase around 14 million years ago, linked to shifts in ocean circulation that promoted diatom-based ecosystems.
17Journal of Biogeography. Key stages in the evolution of the Antarctic marine faunaThe Cenozoic cooling that transformed both poles did not just eliminate species; it created conditions for new ones to fill niches that had never existed before.
18PubMed Central. Evolutionary dynamics at high latitudes: speciation and extinction in polar marine faunasMarine Mammals and the Ice They Need
For many Arctic marine mammals, sea ice is not just a surface to rest on. It structures their entire ecology. An index developed to rank Arctic marine mammal sensitivity to climate-driven habitat change identified three types of vulnerability: narrow distribution combined with specialized feeding, seasonal dependence on ice, and reliance on sea ice as a platform for reaching prey or avoiding predators.
19PubMed. Quantifying the sensitivity of Arctic marine mammals to climate-induced habitat changeSpecies like ringed seals, which build birthing lairs in snow on top of sea ice, or walruses, which haul out on ice floes to rest between foraging dives, are losing habitat in the most literal sense as ice thins and retreats. Narwhals, with their narrow range and deep-diving habits tied to specific ice-edge habitats, are among the most sensitive. The pace of ice loss matters because evolutionary adaptations to ice take millennia; behavioral flexibility can only compensate so far when the habitat itself vanishes within a few decades.
Contaminants in Polar Waters
Polar ecosystems are remote but not pristine. Mercury reaches high-latitude oceans through atmospheric deposition and ocean currents, and once there it biomagnifies through the food web. In a Svalbard Arctic marine food web, total mercury increased with trophic level, with methylmercury as the main form accumulating up the food chain.
20Science of The Total Environment. Biomagnification of mercury in selected species from an Arctic marine food web in SvalbardA study tracking mercury concentrations across seasons found that the trophic magnification factor, essentially how much mercury increases per step up the food web, was highest in late summer and dropped to about half that value in early winter, though this seasonal pattern was largely driven by one outlier species, the pteropod Clione limacina. Without that species, the magnification factor held relatively steady across seasons at about 2.6.
21Progress in Oceanography. Little seasonal variation of mercury concentrations and biomagnification in an Arctic pelagic food webMicroplastics are another contaminant reaching polar oceans in unexpected ways. Sea ice acts as a trap: as ice forms, it can incorporate floating plastic particles from the surrounding water. Modeling shows that positively buoyant microplastics dominate in Arctic sea ice, while in the Southern Ocean, neutrally buoyant plastics transported via deep-water currents are more prevalent.
22Journal of Geophysical Research: Oceans. Modeling the Accumulation and Transport of Microplastics by Sea IceWhen ice melts, those concentrated plastics are released into the water column, potentially exposing filter-feeding organisms to higher concentrations than they would encounter in the open ocean.
Cold water also makes polar seas particularly vulnerable to ocean acidification, because carbon dioxide is more soluble at lower temperatures. The lowest aragonite saturation values are found in polar and subpolar waters, including the Kara, Laptev, and Beaufort Seas in the Arctic and across the Southern Ocean. Shelled pteropods, tiny swimming snails that are an important food source for fish and seabirds, are already showing severe shell dissolution. Projections suggest that as much as 40 percent of pteropods in the Southern Ocean south of 60°S, half in the North Pacific around 50°N, and 60 percent in the Arctic are undergoing severe shell damage.
23Oceanography. Global Synthesis of the Status and Trends of Ocean Acidification Impacts on Shelled PteropodsBorealization and the Arrival of New Species
As Arctic waters warm, species that were historically confined to subarctic and boreal regions are expanding poleward. Long-term fisheries surveys of the Siberian Arctic have documented North Atlantic species extending their ranges eastward and North Pacific species pushing westward, a process termed “borealization.”
24Progress in Oceanography. Ongoing borealization of Siberian Arctic marine ichthyofauna: Further evidenceAtlantic cod turning up in Greenland fjords where they were previously rare is one well-documented example. For the Arctic species already there, particularly the cold-adapted specialists, newcomers can mean increased competition for prey and habitat, or the introduction of new predators into ecosystems that evolved without them.
Indigenous Communities and Changing Harvests
Polar marine ecosystems are not just study sites for scientists. They are home territories for Indigenous peoples whose cultures, food security, and identities are intimately connected to the ice and the animals it supports. In northwest Alaska, the community of Qikiqtaġruk (Kotzebue) has seen the spring bearded seal hunting season shrink dramatically. Between 2003 and 2019, the timing of regional sea ice breakup shifted roughly three weeks earlier, and hunts now end about 26 days sooner than in the previous decade. Historical records show that May ice coverage now resembles what used to be July conditions in the mid-20th century.
25Environmental Research Letters. Co-production of knowledge reveals loss of Indigenous hunting opportunities in the face of accelerating Arctic climate changeIn Greenland’s Ilulissat Icefjord, local fishers report shifting ice conditions, changes in fjord accessibility, and the appearance of Atlantic cod in numbers not seen before. They are adapting their fishing strategies in response, but the pace of change is testing the limits of that flexibility.
26Human Ecology. Inuit and Local Knowledge on the Marine Ecosystem in Ilulissat Icefjord, GreenlandThe knowledge these communities hold, built over generations of direct observation, often captures ecological changes before scientific monitoring programs detect them. Collaborative research that brings Indigenous and scientific knowledge together is increasingly recognized as essential for understanding how polar ecosystems are actually changing on the ground, and on the ice.