Life exists on and within the floating sea ice cover of polar oceans. In the Arctic, for example, sea ice provides habitat for a variety of organisms, members of an ice-specific food web that include bacteria, viruses, unicellular algae, and small invertebrates. These organisms are adapted to tolerate dramatic changes in light intensity, temperature, and salinity.
As noted in Chapter 3 (pages 66-67) of your DataStreme Ocean textbook, dissolved salts depress the freezing point of seawater. For example, at a salinity of 33 psu, the freezing point is about -1.9 °C. Additionally, salt is excluded as sea ice forms. In autumn as sunshine becomes less intense and daylight shortens, the air temperature drops and surface waters cool. Eventually, ice begins forming on the ocean surface as a porous structure of interlocking ice crystals filled with a salty liquid known as brine. Brine, initially representing about 10% to 30% of the ice volume, occupies tiny channels and pores between and within the ice crystals. As winter progresses, sea ice cover solidifies, pore space decreases, and the salinity of the brine increases. As long as the temperature remains above -5 °C, the ice remains riveted by tiny passages having diameters ranging from a few micrometers to several centimeters. But when the temperature drops to lower values, the connectivity of the brine pores becomes minimal and the salinity of the brine can reach values approaching 250 psu.
Ice organisms have the same temperature as the surrounding ice so that their survival within ice channels and pores hinges on adaptations that prevent the growth of ice crystals in their bodies. An adaptation is defined as a genetically controlled trait or characteristic that enhances an organism's chances for survival and reproduction. For example, some ice organisms accumulate fat-like materials in their bodies to prevent freezing.
Sea ice insulates the underlying seawater from the atmosphere so that a considerable temperature difference develops between the ice surface (where mid-winter temperatures might plunge to -35 °C or lower) and the ice/seawater interface where the temperature is the same as the seawater (perhaps -2 °C). For this reason, most ice organisms are concentrated within the lowermost centimeters of the ice. Beginning in spring with the return of sunlight for photosynthesis, and continuing through summer, populations of unicellular ice algae explode. They live in the lowermost portion of the sea ice and often form chains and filaments that extend many meters into the water. Several hundred species of unicellular algae living in the ice are the main primary producers in the Arctic. In fact, ice algae on average account for 4% to 26% of total marine primary production in seasonally ice-covered Arctic waters and up to 50% or more in perennially ice-covered waters. Dissolved organic matter from the wastes of ice algae is food for ice bacteria. Algae are eaten by crustaceans, rotifers (microscopic invertebrates), and turbellarians (flatworms). Tiny crustaceans (amphipods) live on the underside of the ice, feed on algae and seek shelter from predators in the brine channels. Juvenile stages of zooplankton also feed on the ice bottom community. With rising temperatures in spring, the solid ice cover breaks up into pack ice and individual floes that can transport organisms thousands of kilometers before melting and releasing their contents into the ocean.
Large warm-blooded animals also live on the sea ice. Birds (e.g., penguins in the Antarctic), seals, whales, and polar bears utilize sea ice for migration routes, hunting grounds, and rookeries. Seasonal changes in ice cover and thickness makes for a dynamic environment and requires that animals have excellent navigation skills.
Polar bears have a special adaptation that enables them to survive the extreme cold of the Arctic atmosphere. Scientists became aware of the effectiveness of this adaptation when they came up with a plan to census polar bears. Because white polar bears blend in with the snow-covered ice, scientists decided to locate and count the bears using thermal (infrared) imagery taken from an aircraft. They assumed that polar bears would show up as hot spots on the images. However, scientists discovered that although they could often visually observe the polar bears, the bears were not appearing on thermal imagery. Polar bear fur is so efficient as an insulating blanket that essentially no body heat was getting through the fur so that no heat signals reached the infrared sensor on the aircraft. Polar bear fur is made up of hollow tubes that function as very efficient insulators.
You can monitor the variations of sea ice in the Arctic Ocean, the Bering Sea, and the Southern Ocean around Antarctica through the Global Ice Extent link on the DataStreme Ocean website. The Marine Modeling and Analysis Branch of the Environmental Modeling Center, a part of the National Environmental Prediction Centers (NCEP), routinely analyzes sea ice cover based upon data received from passive microwave sensors onboard polar orbiting satellites.
Return to DataStreme Ocean website
Prepared by H.J. Niebauer, Ph.D. and Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu
© Copyright, 2005, The American Meteorological Society.