These seasonally frozen surface waters in polar oceans are one of the most dynamic ecosystems on earth. The sea-ice itself provides habitat for ice-dependent species, such as the microalgal and microbial communities that form the basis of communities in waters below, while plankton, fish and marine birds and mammals feed on and around the ice. Sea-ice plays a crucial role in both pelagic marine ecosystems and biogeochemical processes like ocean-atmosphere gas exchange.