2. Relation between sea ice and the pelagic environment

The sea ice is a powerful natural barrier between the atmosphere and ocean. As such, it keeps the biological, biochemical and physical processes at work above and below the ice separated. For example, the sea ice reduces the amount of light that reaches the ocean. It prevents the exchange of gases and heat between the air and water, and it influences the amount of freshwater input in the ocean (Arrigo, 2014). In addition, the sea-ice barrier has a critical influence on surface layering and mixing in the ocean.

The Arctic sea-ice landscape is rapidly changing. Increasing transparency can spark premature seasonal primary production. This early growth can be accompanied by an increase in biomass from ice algae and phytoplankton, which boosts the emission of dimethyl sulphide (DMS) and the binding of carbon dioxide (CO2). Secondary production in the shelf regions can also increase, although the loss of sea ice exacerbates the loss of sea-ice fauna, endemic fish species and megafauna. The loss of sea ice can be  connected to the release of more methane into the atmosphere, but warmer ice may release fewer halogens, which means fewer ozone depletion events. As such, the net changes to the carbon cycle remain highly uncertain (Lannuzel et al., 2020).

The sea-ice cover changes throughout the year. The ice extent begins growing in autumn, spreads to cover vast expanses of the polar region in winter, and shrinks again when the spring melting begins. This dynamic fundamentally shapes the biogeochemical processes and material cycles in the Arctic, in particular the growth of ice algae, the basis of the Arctic food webs. Ice algae are especially an important food source for zooplankton, as they contain high concentrations of various unsaturated and other essential fats. In turn, the higher trophic levels profit from plentiful zooplankton (Arrigo, 2014). The reproduction of the ice algae begins in early spring, when there is little light. As a result of this intensive blooming, a large portion of the light is absorbed by the ice algae (Arrigo et al., 2014).

The light attenuation in snow, produced by scattering and absorption, is roughly one order of magnitude higher than that of the sea ice below it, which in turn is roughly an order of magnitude higher than that of the seawater. Consequently, snow-covered ice allows only very little light to penetrate more than 1 m below the surface of the snow, providing very little support for microbial biomass production. Even under snow-free conditions, the growth of photoautotrophic organisms living in the ice can be limited by the available light, especially in early spring, when the ice brine is still rich in nutrients. The light attenuation in the ice is further intensified by particle absorption, especially via sediments in coastal regions and microalgae containing pigment that grow on the underside of the ice (Arrigo, 2014).
 
Over the past several years public, political and economic interest in the Arctic has grown considerably. This is particularly due to the fact that, over the last few decades, the amount of sea ice in the summer has declined significantly. Climate models predict that this sea-ice retreat will intensify in the years to come, and that the Arctic will have predominantly ice-free (i.e., will have a sea-ice extent of less than 1 million km²) summers by the middle of this century. Experts believe this sea-ice retreat will produce dramatic changes in Arctic habitats. In addition, this development has sparked new commercial and geopolitical interest. After all, as the ice cover dwindles, the Arctic will become more accessible for shipping, raw material exploration and fishing, not to mention tourism.

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Arrigo, K.R. (2014): Sea ice ecosystems, Annual Review of Marine Science, vol. 6, pp. 439-467.
Lannuzel, D., L. Tedesco, & M. van Leeuwe et al. (2020): The future of Arctic sea-ice biogeochemistry and ice-associated ecosystems. Nat. Clim. Chang. 10, pp. 983–992.
https://doi.org/10.1038/s41558-020-00940-4