Antarctic pack-ice is a class of sea ice defined by the fact that it is able to drift with ocean currents and the wind. The overall formation and decay of the pack ice is controlled by the seasonal cycle of solar radiation and temperature. It is a highly dynamic habitat structured by numerous physical processes both spatially and temporally. The sea ice is inhabited by complex communities comprised of algae, bacteria, heterotrophic protozoa and metazoans which exist on, under and within the pack ice structure. In addition to this, the pack ice is an incredibly important to several Antarctic seals and penguins for foraging, moulting and breeding.
Sea ice cover in the Antarctic covers an area between 4 million km2 in summer and approximately 19 million km2 in winter. The majority of this consists of annual pack ice (Cavalieri et al., 1999, Arrigo & Thomas, 2004).
Sea ice climatology: Antarctic sea ice concentration climatology from 1979-2000, showing the approximate seasonal maximum and minimum sea ice extent based on passive microwave satellite data. This image is a modified version of this image provided by National Snow and Ice Data Center, University of Colorado, Boulder, CO.
What is Pack ice?
Pack ice is a class of Antarctic sea ice. It is characterised by the fact that it drifts with the winds and ocean currents (Herdman, 1948). There are two types of pack ice classified based on their age these are; first year ice and multiyear ice. First year ice forms over fall and winter and then melts away again in spring to summer each year, while multi-year ice persists for two or more years (Garrison, 1991, Brierley and Thomas, 2002). Though it can persist for multiple years in some regions, up to 90% of the sea ice in the Southern Ocean is composed of first year pack ice which forms and decays annually (Garrison, 1991, Wadhams, 2000, Brierley and Thomas, 2002).
Formation and structuring processes
In Antarctica the overall formation and decay of the pack ice is controlled by the seasonal cycle of solar radiation and temperature (Garrison, 1991, Garrison et al., 2003). Sea ice formation begins when seawater temperatures drop to the freezing point of seawater, approximately -1.9˚C, in autumn. The initial growth of ice is in the form of small crystals, about 3-4mm in diameter, called frazil ice. These frazil ice crystals form at both the surface and at depth and coalesce at the surface in the form of grease ice. This grease ice slowly thickens into either a sheet of nilas, in calm conditions, or pancake ice when there is wind or wave action. Regardless this ice eventually fuses into a solid sheet of ice. Once this solid sheet forms, frazil crystals stop forming and the growth of the ice slows. Subsequent thickening of the ice occurs by ice formation at the ice-water interface. The ice crystals forming at this interface, under these conditions, have a very regular orientation are termed congelation or columnar ice (Garrison, 1991, Haas et al., 2001, Brierley and Thomas, 2002, Worby et al., 2008).
A number of physical processes can act upon the pack ice to modify its structure. The solid pack ice sheet can be broken up into smaller floes by ocean swell, wind and waves. Floes can be forced apart by wind and currents to create patches of open water (Garrison, 1991, Massom et al., 2008). They can also be compacted together by these winds or forced onto or underneath each other in a process known as ice rafting, resulting in significant thickening of the ice pack (Brierley and Thomas, 2002, Massom et al., 2008). At a more local scale floe, depression by snow accumulation and/ or ice rafting can cause surface flooding and the formation of layers of snow-ice or infiltration ice. (Brierley and Thomas, 2002, Worby et al., 2008, Worby et al., 2011). Seasonal warming of the pack ice can result in internal melting and the formation of internal gap layers (Ackley and Sullivan, 1994, Ackley et al., 2008, Meiners et al., 2011). These consist of a partly melted honeycomb-like matrix filled with seawater, and can be quite extensive in the pack ice in the summer months (Fritsen et al., 2001, Haas et al., 2001). Together these processes impart significant spatial and temporal heterogeneity to the pack ice habitat both on the large and small scales (Thomas and Dieckmann, 2002, Kattner et al., 2004, Meiners et al., 2011).
Biota: Communities within the ice
Sea ice colonisation:
The surface, interior and bottom of the sea ice all provide an extensive habitat for sea ice communities (Staley and Reysenbach, 2002). Colonisation of this habitat is largely determined by the physical processes controlling ice formation and growth (Ackley and Sullivan, 1994, Garrison et al., 2003). Surface flooding, due to ice rafting or snow loading, provides the opportunity for the colonisation and development of surface-layer communities (Ackley and Sullivan, 1994, Garrison et al., 2003). During ice formation itself processes such as water circulation, wave action and scavenging by frazil ice incorporate particulate material and organisms into the forming pack ice (Ackley and Sullivan, 1994, Weissenberger and Grossmann, 1998, Staley and Reysenbach, 2002).
Following the formation of the sea ice, these organisms are distributed the span of the ice column (Kottmeier and Sullivan, 1988). This habitat presents a harsh and varied physiochemical environment to biological organisms (Sullivan and Palmisano, 1984).Temperatures within the ice range from <-20°C at the surface to -1.8°C at the ice-water interface (Palmisano et al., 1985, Garrison, 1991, Haas et al., 2001, Meiners et al., 2011). The salinity of the sea ice can drop lower than 6ppt as the sea ice melts and rise as high as 178ppt in the brine within the brine channels (Lytle and Ackley, 1996, Haas et al., 2001, Meiners et al., 2011). Ambient light is attenuated by snow cover and the ice itself and is typically less than 1% of the surface irradiance (Palmisano et al., 1985, Garrison, 1991, Fritsen et al., 1994). The development and abundance of the sea ice organisms within the ice habitat is primarily determined by these physiochemical conditions (Ackley and Sullivan 1994).
Sea ice communities:
Sea ice is inhabited by complex communities comprised of algae, bacteria, heterotrophic protozoa and metazoans (Gradinger and Ikävalko, 1998, Lizotte, 2001, Staley and Reysenbach, 2002). In terms of biomass, these communities tend to be dominated by algae (Meiners et al. 2011). The algal assemblage within the ice is composed of many of the same algal groups present in the phytoplankton (Staley and Reysenbach, 2002). Of these groups pennate diatoms tend to dominate, and are considered to be the most important contributors to primary production within the ice (Fritsen et al., 2001, Garrison et al., 2005, Meiners et al., 2011). Bacterial populations are closely tied to algal productivity in the sea ice (Kottmeier et al., 1987). Primary productivity by the algae provides bacteria with the organic substrates required for secondary bacterial production (Grossi et al., 1984, Kottmeier et al., 1987). At the same time, the bacteria recycle this organic material into inorganic nutrients which can be utilised by the algae for further primary production (Sullivan and Palmisano, 1984, Arrigo et al., 1995). Production by these combined algal and bacterial communities provides a rich source of organic material and detritus for grazers (Sullivan and Palmisano, 1984, Garrison et al., 1986, Lizotte, 2001, Staley and Reysenbach, 2002, Kattner et al., 2004).
Within the matrix of the sea ice grazers include copepods, turbellarians, ciliates, foraminifera and heterotrophic flagellates among others (Schnack-Schiel et al., 2001a, Schnack-Schiel et al., 2001b, Garrison et al., 2005). The algae, bacteria and other particulate material being released from the sea ice also attracts a wide variety of protozoan and metazoan and larger grazers to the underside of the ice (Lizotte, 2001, Arrigo and Thomas, 2004). These organisms include amphipods, copepods, krill and other euphausiids and fish (Obrien, 1987, Voronina et al., 2001, Brierley and Thomas, 2002, Krapp et al., 2008). These grazers, especially krill, are in turn a vital food source to higher trophic levels including seals, whales, seabirds and other fish (Ainley et al., 1998, Brierley and Thomas, 2002, Arrigo and Thomas, 2004, Lyver et al., 2011). The productivity of the sea ice communities is especially important to grazers in the winter months, when many other regular food sources are absent (Ackley and Sullivan, 1994, Lizotte, 2001, Brierley and Thomas, 2002)
Biota on the ice
Marine predators such as Antarctic seals and penguins depend almost entirely on the pack-ice for their habitat (Lyver et al., 2011). Among penguins pack ice is a particularly important habitat for the Emperor (Aptenodytes forsteri) and Adelie (Pygoscelis adeliae) penguins. Emperor and Adelie penguins forage within the pack ice during the chick rearing season (Ainley et al., 1998, Rodary et al., 2000, Lyver et al., 2011). For Adelie penguins in particular, the proximity of the pack ice to their nesting colonies significantly affects their foraging success, with closer pack ice proximity resulting in higher foraging success (Ainley et al., 1998). In addition to this, both Adelie and Emperor penguins moult on the pack ice (Kooyman et al., 2000, Ackley et al., 2003). During moult they are unable to enter the water and finding a stable region of pack ice, which will survive the moulting period, is critical to their survival (Kooyman et al., 2000).
Among the Antarctic seals there are four pack ice seal species, these are; the crab-eater (Lobodon carcinophaga), leopard (Hydrurga leptonyx), Ross (Ommatophoca rossii), and Weddell (Leptonychotes weddellii) seals (Ackley et al., 2003, Bester et al., 1995). Pack-ice seals use the pack-ice area for breeding and birthing of pups, especially in the case of crabeater seals, moulting (Ackley et al., 2003, Stewart et al., 2003) and as a resting platform between foraging trips (Ackley et al., 2003, Stewart et al., 2003, Forcada et al., 2012).
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