As krill feed primarily on algae and phytoplankton, one might not expect them to associate with the underside of sea ice, where the passage of light into the water column is severely limited by the ice, acting against autotrophic organisms. This could not be further from the truth, however, as the numerous ways autotrophic organisms have adapted to grow in and on sea ice and this in turn has allowed some species of krill to utilise sea ice as a significant source of food, with Euphausia superba feeding on algae such as diatoms attached to the sea ice as well as free-floating phytoplankton in the water column (Cripps and Hill, 1998).
Within sea ice, complex networks of brine tubes created during the forming of the ice are inhabited by numerous communities of organisms, including autotrophic organisms such as diatoms, also trapped during the ice’s formation (Brierly and Thomas, 2002) . These organisms must not only be adapted for their cramped conditions, but also to survive in concentrated brine. This can manifest in characteristics such as enhanced cell wall thickness, which would combat increased salinity. While this extreme environment is not inhabited by krill or krill larvae, the amount of nutrients contained in the organisms, as well as algal blooms caused by the release of nutrients, provide a significant source of food for zooplanktonic organisms. This influx of nutrients is also a significant source of food for krill larvae during the summer months, giving them a stable supply of food whilst also allowing them to stay within a sheltered habitat (Meyer et al, 2003). These released autotrophs also provide a dietary supplement, providing consumers with compounds they cannot produce themselves, such as polyunsaturated fatty acids, with the diet of an individual greatly affecting it’s fatty acid makeup (Cripps et al, 1999), thus making their presence all the more significant. This combined with the increased primary production caused by the loss of sea ice during these thawing periods makes clear the fact that even the indirect effects of sea ice’s nature as a habitat for autotrophs have a significant impact on polar ecosystems.
As well as this, a significant amount of algae grows on the underside of the sea ice, providing krill and other grazers with a food source all year round. This is includes a significant amount of juvenile krill that are able to utilize the ice as a constant source of food in conjunction with it’s function as a shelter from predators (Hegseth and Von Quillfeldt, 2002).
It is these provisions of a stable food source and vital nutrients from this ephemeral habitat that sustains krill as a foundation of polar ecosystems.