The most notable effect of decreasing temperature, as winter progresses and the ice solidifies, is the reduction of pore space within the ice, and the concurrent increase in the salinity of the brine. Since the organisms have the same temperature as the ice, their survival depends on their ability to prevent the growth of ice crystals in their bodies. Many organisms accumulate large deposits of organic molecules, and fatlike material as a strategy to survive the harsh unproductive winter. Studying the capability of organisms to survive such extreme environments is an active research field of astrobiology , which is a branch of biology concerned with the effects of extraterrestrial environments on living organisms. Wintertime sea ice is used as an analogue for possible extraterrestrial habitats on the frozen surfaces of such solar bodies such as Europa, Ganymede, Mars and Titan.
Ice algae are an important component of the Arctic marine food web.
Sea ice constitutes a thermal barrier against the cold winter atmosphere with the result that the interface between the ice and the seawater remains at the temperature of seawater. During spring, when light begins to be available for photosynthesis, and throughout the summer, a large biomass in the form of unicellular photosynthetic ice algae develops within the lowermost sections of the ice. The interface between the ice and the seawater is therefore critical to the polar marine ecosystem. Larger warm-blooded animals such as birds, seals, whales and polar bears use the ice for migration routes, hunting grounds, rookeries and protection for raising their young. Because sea ice is a diverse and constantly changing habitat, appearing in such varied forms as nilas pancake ice, several meter thick ridged first year and multi-year ice, and pack ice, animals have had to become very good navigators, using homing cues that are as yet not well understood.