ICE FLOE: LOCATION, CHARACTERISTICS AND ORGANISMS - ENVIRONMENT - 2024

The ice pack or sea ice is the set of floating ice sheets that are formed by freezing sea water in the polar oceanic regions of the Earth. The terrestrial polar oceans are covered by sea ice seasonally (only during winter), or permanently throughout the year. They are the coldest environments on the planet.

The cycles of temperature and solar irradiation in the polar oceans present a high variability. The temperature can vary between -40 and -60 ° C and the cycles of solar irradiation oscillate between 24 hours of daylight in summer and total darkness in winter.

Figure 1. Trace on ice pack. Source: LBM1948, from Wikimedia Commons

Sea ice or ice pack covers 7% of the planet's surface and approximately 12% of the total land oceans. Most of them are located in the polar caps: the Arctic polar helmet of the Arctic Ocean to the north, and the Antarctic polar helmet, to the south.

Sea ice undergoes an annual cycle of reduction and reconstruction of its surface area, a natural process on which its life and ecosystem structure depend.

The thickness of the Earth's polar ice sheets is also highly variable; it varies between one meter (in times of melting) and 5 meters (in times of stability). In some places, sheets of sea ice up to 20 meters thick can form.

Due to the combined action of winds, fluctuations in sea currents, and variations in air and sea temperatures, sea ice is highly dynamic systems.

Location and characteristics

Antarctic ice floe

The Antarctic ice pack is located at the south pole, around the continent of Antarctica.

Annually, during the month of December, its ice melts or melts, due to the increase in summer temperature in the southern hemisphere of the Earth. Its extension is 2.6 million km ².

In winter, with the drop in temperatures, it forms again and reaches an area equal to that of the continent, of 18.8 million km ².

Arctic ice floe

In the Arctic ice pack, only the parts closest to the continental areas melt annually. In the northern winter it reaches an area of ​​15 million km ² and in the summer of only 6.5 million km ².

Figure 2. Boat crossing the ice pack. Source: LBM1948, from Wikimedia Commons

Physics of sea ice

Floating of sea ice masses

Ice is less dense than water and floats on the surface of the ocean.

When water passes from a liquid to a solid state, the crystalline structure that is formed has empty free spaces and the mass / volume ratio (density) is lower than that of water in a liquid state.

Channels and internal pores

When pure water solidifies to ice, it forms a brittle solid whose only inclusions are gas bubbles. In contrast, when marine waters freeze, the resulting ice is a semi-solid matrix, with channels and pores filled with the saline solution of seawater.

Salinity

Dissolved substances, including salts and gases, do not enter the crystalline structure, but settle in the pores or circulate through the channels.

The morphology of these pores and channels, the total volume of ice occupied by them and the salinity of the marine solution contained, depend on the temperature and age of ice formation.

There is a drainage of the marine solution due to the force of gravity, which results in the gradual reduction of the total salinity of the sea ice.

This loss of salinity increases in the summer, when the surface layer of the floating ice mass melts and percolates; This destroys the structure of pores and channels and the marine solution they contain flows out.

Temperature

The temperature on the upper surface of a floating sea ice mass (which is around -10 ° C), is determined by the air temperature (which can reach -40 ° C) and by the insulating capacity of the snow cover.

In contrast, the temperature of the underside of a floating ice mass is equal to the freezing point of the seawater on which it rests (-1.8 ° C).

This results in gradients of temperature, salinity - and therefore of dissolved solutes and gases - and of volume of pores and channels, in the sea ice mass.

In this way, during the autumn-winter period the sea ice is colder and has higher salinity.

Organisms that inhabit sea ice

The ice floes are regions of high productivity, as evidenced by the large number of mammals and birds that hunt and feed in these regions. It is known that many of these species migrate over enormous distances to feed in these areas of sea ice.

Polar bears and walruses abound on the Arctic ice pack, and penguins and albatrosses on the Antarctic ice pack. There is presence of seals and whales in both areas of sea ice.

In sea ice there is a considerable seasonal development of phytoplankton, microalgae that carry out photosynthesis and the primary producers of the trophic chain.

This production is what sustains zooplankton, fish and deep-sea organisms, on which, in turn, the aforementioned mammals and birds feed.

The diversity of organisms in the sea ice is less than that of the tropical and temperate zones, but in the ice floes there are also an enormous number of species.

Figure 3. A polar bear jumping off Spitsbergen Island, Svalbard, Norway. Source:

Forms of life in the spaces within the sea ice

The key parameter for the existence of life inside the sea ice is the existence of sufficient space within the ice matrix, space that also allows movement, the uptake of nutrients and the exchange of gases and other substances.

The pores and channels within the matrix of sea ice function as habitats for various organisms. For example, bacteria, various species of algae diatoms, protozoans, peatlands, flagellates, and copepods can live in the channels and pores.

Only rotifers and peatlands have been shown to be capable of traversing channels and migrating across sea ice horizons.

The rest of the organisms, such as bacteria, flagellates, diatoms and small protozoa, live in pores smaller than 200 μm, using them as a refuge where they benefit from the low predation pressure.

Bacteria, archaebacteria, cyanobacteria and microalgae in sea ice

The predominant species in the ice pack are psychrophilic microorganisms, that is, Extremophiles that tolerate very low temperatures.

Heterotrophic bacteria constitute the predominant group within the prokaryotic organisms that inhabit sea ice, which are psychrophilic and halotolerant, that is, they live in conditions of high salinity, as free-living species and also associated with surfaces.

Archaea have also been reported on both ice sheets, Arctic and Antarctic.

Several species of cyanobacteria inhabit Arctic sea ice but have not been found in the Antarctic.

Diatom algae are the most studied group of eukaryotes in sea ice, but there are also dinoflagellates, ciliates, foraminifera and chlorophytes, among others.

Climate change is particularly affecting the polar ice floes and many of their species are threatened with extinction due to this cause.

References

1. Arrigo, KR and Thomas, DN (2004). Large scale importance of sea ice biology in the Southern Ocean. Antarctic Science. 16: 471-486.
2. Brierley, AS and Thomas, DN (2002). Ecology of Southern Ocean pack ice. Advances in Marine Biology. 43: 171-276.
3. Cavicchioli, R. (2006). Cold adapted Archaea. Nature Reviews Microbiology. 4: 331-343.
4. Collins, RE, Carpenter, SD and Deming, JW (2008). Spatial heterogeneity and temporal dynamics of particles, bacteria, and pEPS in Arctic winter sea ice. Journal of Marine Systems. 74: 902-917.
5. Tilling, RL; Shepherd, A.; Wingham, DJ (2015). Increased Arctic sea ice volume after anomalously low melting in 2013. Nature Geoscience. 8 (8): 643-646. doi: 10.1038 / NGEO2489.