Fast ice is a type of sea ice that remains fixed and attached to coastlines, the seafloor in shallow areas, or grounded icebergs and islands. Its immobility, stemming from this physical connection, prevents it from drifting with ocean currents or winds. It forms a stable boundary between land and the open ocean in polar and subpolar regions.
Formation and Defining Features
Fast ice begins to form as water temperatures drop below freezing, growing outward from the coast or around fixed structures like icebergs and islands. This initial growth creates a stable platform. As the winter progresses, this ice sheet thickens and expands, becoming more robustly attached.
Fast ice is “fastened” to the land or seafloor and does not drift. This stability leads to variable thickness, ranging from a few centimeters to several meters, and in some areas, up to 20 meters in Arctic seas or about 10 meters in subarctic seas. Fast ice can be smooth and level or rugged, especially when subjected to pressure. It remains frozen in place throughout the winter, sometimes persisting for multiple years, which is then categorized as multi-year fast ice.
Role in Polar Ecosystems
Fast ice provides a stable and consistent platform that is beneficial for polar wildlife. Animals such as seals, including Weddell seals, and emperor penguins use fast ice for breeding, giving birth, resting, and protection from predators. Polar bears also rely on fast ice as a hunting ground, primarily for seals.
The ice also supports unique under-ice communities, acting as a nursery for various marine organisms. Microscopic algae, particularly pennate diatoms and flagellated protists, grow on the underside of the ice, forming the base of the polar food web. These algae are a food source for zooplankton, which in turn support larger marine life. Fast ice also influences the distribution of freshwater and salinity in the ocean, impacting local currents and water circulation patterns.
Distinguishing Fast Ice from Other Sea Ice
General sea ice is a broad category encompassing any ice formed from frozen seawater. Fast ice differs from other types, such as pack ice, which is free-floating and drifts with ocean currents and winds.
Pack ice is composed of smaller, individual ice floes that can collide, raft, or consolidate. While fast ice is anchored to the coast or seafloor, pack ice is in constant motion, often creating open water areas called leads and polynyas.