The marginal ice zone
The marginal ice zone is the transition between the open ocean and sea ice. Depending on factors like wind direction and ocean currents, it may consist of anything from isolated, small and large ice floes drifting over a large area to a compact edge of small ice floes pressed together in front of solid pack ice. The marginal ice zone is very dynamic due to the influence of the weather and rapid changes. Changes in its extent may take place over hours or days. Ecological vulnerability is also greater in the marginal ice zone because of high production in spring and summer, and the high density of vulnerable environmental elements in some parts of the year.
Marginal ice zones in polar regions are dynamic in time and space. This makes it difficult to give precise delimitations that are meaningful over time.
One of the most acceptable definitions of the marginal ice zone (MIZ) is "that part of the ice cover which is close enough to the open ocean boundary to be affected by its presence". This definition is open to interpretation, but is generally understood as saying that the MIZ is that part of the ice cover which is close enough to the open ocean to be significant influenced by its physical proximity (e.g. waves). The MIZ is thus largely a physical quantity; it is the outer part of the pack ice belt, with enhanced ice drift, deformation, divergence and primary production.
The MIZ is usually limited to an area some tens of kilometres wide, but information on how far into the pack ice the MIZ extends is sparse.
The marginal ice zone from the open ocean inwards through the pack ice to dense pack ice, fast ice and multiyear ice will be more vulnerable to pressures for large parts of the year. This is because of high production and biodiversity under the water as far as the light penetrates, and the high density of seabirds, marine mammals and polar bears.
The marginal ice zone is biologically important because the stable upper layer of the water is mixed here by a combination of ice melting and wind. This leads to a comparatively brief, but intense production of phytoplankton (primary production) in the water masses near the ice edge and in the marginal ice zone itself (see the illustration). Zooplankton, fish, marine mammals and seabirds exploit this and gather at the ice edge.
A large part of the biological production sinks through the water column and forms a basis for rich communities of benthic organisms in ice-influenced areas. Ice algae, too, which are tied to the actual ice, will contribute to the total production in the area. The bloom of ice algae starts earlier than the phytoplankton bloom in the water masses, thus prolonging the productive season in the marginal ice zone.
The marginal ice zone and ice-edge habitats in general are also essential for the total annual energy budget of many arctic, endemic species like ivory gulls, ringed seals, polar bears, narhvals, beluga whales and bowhead whales. Several of these are Red Listed nationally and internationally. These habitats are also important for many migratory species, like kittiwakes and black guillemots. Disturbances to the habitats may therefore have consequences for the population levels of many species.
Sea ice as a habitat has a unique status and value. The sea ice is the platform on which some species of seals, like harp seals and hooded seals, reproduce and have their pups, and it is also a habitat where arctic whales find protection. The great concentrations of many species, often in very limited areas in the marginal ice zone, mean that these species are very vulnerable when they are there. For instance, in late summer, 80–90% of the global population of ivory gulls are in the marginal ice zone in the Barents Sea.
The ice margin is often depicted as a snapshot from a given month in a given year. Because of the ecological significance of the marginal ice zone and the vulnerability of the species that dwell there, it is expedient to depict the dynamic ice edge over a long period of time.
The Norwegian Polar Institute has therefore constructed maps showing the monthly minimum and maximum extent of the ice, and the sea ice frequency. Daily data on ice concentration obtained from the National Snow and Ice Data Center (NSIDC) were recalculated to the sea ice frequency for 1985-2014. Using a 30-year period of data as the basis, the sea ice frequency can be used to some extent as a measure of the probability of ice, not least in the short term.
More information on the marginal ice zone as a vulnerable area:
Download separate maps by selecting a map in the gallery below, or:
The sea ice frequency in the Norwegian Arctic, monthly for 1985-2014.
- Peter Wadhams 1986. The Seasonal Ice Zone. The Geophysics of Sea Ice, NATO ASI Series, 825–991, 1986. DOI:10.1007/978-1-4899-5352-0_15
- Randall R. Reeves, Peter J. Ewins, Selina Agbayani, Mads Peter Heide-Jørgensen, Kit M. Kovacs, Christian Lydersen, Robert Suydamf, Wendy Elliott, Gert Polet, Yvette van Dijk, Rosanne Blijleven 2014. Distribution of endemic cetaceans in relation to hydrocarbon development and commercial shipping in a warming Arctic. Marine Policy 44, 375-389. 2014 DOI:10.101016/j.marpol.2013.10.005