Arctic sea ice covers significant portions of the northern hemisphere ocean, forming and persisting at temperatures below the freezing point of seawater. That freezing point is generally around -1.9°C when the salinity is 33 parts per thousand, however, the freezing point changes with the concentration of salt in the seawater. As ice crystals grow in the water during the autumn season, small ice platelets begin to accumulate at the ocean surface, inter-link, and form a porous structure of ice crystals filled with liquid, which is referred to as brine.

The flourishing life within the briny habitat of sea ice. The ice specific ecosystem includes bacteria, viruses, unicellular algae, diatom chains, worms and crustaceans. Click on the image to see a magnified view of the brine channels containing these organisms.

The flourishing life within the briny habitat of sea ice is intricately linked to physical processes. Temperature controls every physical and chemical aspect of ice, including the availability of light. 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. Sea ice serves as habitat for an ice-specific food web (sympagic foodweb) [1] that includes bacteria, viruses, unicellular algae, which often form chains and filaments, and invertebrates sufficiently small to traverse the brine network. The brine network is comprised of passages in the ice, with diameters ranging from micrometers to several centimeters when the temperature remains above -5°C.

During the colder winter months, however, strong gradients of temperature persist throughout the ice, spanning from -2°C at the bottom of the ice in contact with seawater to -35°C at its wind-chilled surface. Here, connectivity of pore space is at a minimum, brine salinities reach 250 parts per thousand [2] and salts begin to precipitate as opaque minerals. 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.

Single-celled (unicellular) algae, which develop in the lowermost sections of sea ice, often forming chains and filaments. 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. They occasionally form long filaments that can extend several meters into the water column (Melosira arctica). Ice algae are a very important part of the marine food web, and contribute on average with 57% to the total Arctic marine primary production [3]. The interface between the ice and the seawater is therefore critical to the polar marine ecosystem. Organisms which eat zooplankton, called zooplankton grazers (such as Gammerus wilkitzkii), seek food in the ice and also protection from their predators. Arctic cod (Breogadus saida), which are an important food source for many marine mammals and birds (Arctic marine food web), use the same habitat as nursery grounds.

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.

At the peak of algae production in spring, the solid ice cover transforms into pack ice with individual floes that transport organisms, sediment (see ice core photograph) and, unfortunately, man-made pollutants over thousands of kilometers before they melt and discharge their contents into the water. It is the aim of scientists to understand how sea ice, its physics, extent and drift patterns affect polar marine populations and the food web in polar waters (the cryopelagic food web) that has formed the basis of sustenance for generations of native Inuit Eskimo peoples.

References

[1] Horner R. A., S. F. Ackley, G. S. Dieckmann, B. Gulliksen, T. Hoshiai, L. Legendre, I. A. Melnikov, W. S. Reeburgh, M. Spindler and C. W. Sullivan (1992). Ecology of sea ice biota. 1. Habitat, terminology, and methodology. Polar Biol. 12, 417-427. Return to article here.

[2] Cox. G. F. N. and W. F. Weeks (1983). Equation for determining the gas and brine volumes in sea-ice samples, J Glaciol. 29, 306-316. Return to article here.

[3] Gosselin M., M. Levasseur, P. A. Wheeler, R. A. Horner, B. C. Booth (1997). New
measurements of phytoplankton and ice algal production in the Arctic Ocean.
Deep-Sea Res. II, 44, 1623-1644. Return to article here.