Underice

Underice is the zone beneath an ice cover on a body of water, including sea ice on oceans, as well as lake and river ice. It encompasses the water column directly under the ice, the ice–water interface, and, in some contexts, the near-surface sediments near the ice edge. The term describes a distinct physical and ecological environment that differs from open-water conditions.

Physical conditions under ice are characterized by reduced light, temperatures close to the freezing point, and complex brine dynamics in sea-ice environments. Brine rejection, channelized flows, and stratification can create pockets of relatively fresh or saline water and influence vertical mixing, heat exchange, and nutrient distribution. These processes help shape the chemical and physical structure of the underice water column.

Biologically, underice supports specialized communities. In polar oceans, algae that grow on the underside of the ice contribute to primary production when light is available, while a diverse assemblage of microorganisms, crustaceans, and fish such as polar cod can depend on the underice habitat for feeding and shelter. Freshwater under-ice ecosystems also host microbial life and surprisingly active biological activity during winter.

Exploration and study of underice regions rely on ice-strengthened vessels, submarines, remotely operated and autonomous underwater vehicles, and remote sensing approaches. Observations under ice are crucial for understanding seasonal biogeochemical cycles, habitat dynamics, and the impacts of climate change on polar and subpolar aquatic systems.