1 Sea -ice inhabitants

At first sight, the sea ice in the Arctic and Antarctic might seem inhospitable and hostile to life. In fact, however, it is a unique habitat that countless organisms call home. The organic community of the sea ice forms a complete food web, in which there are a range of interactions, some of which make certain organisms heavily reliant on one another. The base of the sea-ice food web consists of primary producers, especially microalgae and other microorganisms capable of photosynthesis. This is followed by three to four higher trophic levels, ranging from herbivores to predators – such as seals and polar bears.

As a habitat, sea ice is characterised by extreme environmental conditions, especially in terms of available food, temperature, salinity and light. In the winter, snow accumulates on the ice, reducing the amount of light that penetrates it in the spring. In the summer, broad expanses are ice-free, in the winter everything is covered in ice. The local fauna have adapted their lifecycles to these changing conditions regarding light and dark, available food, and ice cover versus open water. The formation and melting of sea ice greatly affect these animals’ living conditions. While the ice is forming, the seawater below it is highly saline. In contrast, during the melting phase the conditions become more brackish, as freshwater collects below the sea ice. The melting and freezing of the ice in the course of the year produces varying and in some cases extreme temperatures and salinities, which the organisms living here have adapted to. Due to these conditions, the population sizes vary throughout the annual cycle.