Sea Ice as Habitat: Why Frozen Water Is One of Earth's Most Important Ecosystems

To the casual observer, Arctic sea ice appears to be a uniform, lifeless expanse of frozen water. In reality, it is one of Earth's most biologically complex habitats — a three-dimensional matrix of ice crystals, brine channels, and gas pockets that supports a community of organisms from bacteria and algae to amphipods, fish, seals, polar bears, and Arctic foxes. The organisms that inhabit sea ice have evolved over millions of years to exploit this ephemeral, dynamic, and physically extreme environment — and they collectively support the productivity that sustains the entire Arctic marine food web.

The Brine Channel Network

When seawater freezes, salt is largely excluded from the ice crystal lattice — concentrating in liquid brine that remains unfrozen within a network of tiny channels threading through the ice. These brine channels, typically a fraction of a millimetre to a few millimetres in diameter, create a labyrinthine three-dimensional habitat within the ice. The brine within them can be extremely saline — up to five times the salinity of seawater — and extremely cold, but it remains liquid and provides a refuge for microorganisms that have evolved to tolerate these extreme conditions. The brine channel community is dominated by bacteria, viruses, and unicellular algae — but it supports a complex microbial food web that forms the base of the sea ice ecosystem.

Ice-Dependent Mammals

Several Arctic mammal species are fundamentally dependent on sea ice for hunting, resting, breeding, or travelling. Ringed seals use sea ice as a platform for pupping and moulting, and excavate lairs in snow drifts above the ice for their pups. Bearded seals rest on ice floes between dives. Walruses haul out on sea ice in enormous aggregations to rest between foraging bouts. Polar bears use sea ice as a hunting platform to access seals. Arctic foxes follow polar bears onto the sea ice in winter to scavenge seal remains. As sea ice retreats, each of these species is forced to adapt their behaviour — with varying degrees of success.