The underside of sea ice in Polar Regions represents a natural habitat for heterotrophic organisms, such as copepods and amphipods. This under-ice fauna plays a key role in transferring carbon synthesized by sea ice-associated (sympagic) microalgae into associated pelagic and benthic food webs of polar ecosystems. Animals at higher trophic levels are adapted to feed on the under-ice fauna as well as on pelagic zooplankton and nekton. Polar ecosystems thrive significantly on ice algae-produced carbon depending on different periods of the year. Thus, the under-ice fauna and the associated pelagic food web are largely affected by multi-scale climate changes accompanied by the reduction of sea ice coverage and an increasing duration of the melt season. Until now, however, the degree to which polar food webs depend on sea ice-derived carbon is unclear. The overall aim of this thesis is to quantify the transfer of ice algae-produced carbon from the sea ice into the under-ice community and from there into pelagic food webs in Arctic and Antarctic ecosystems, in order to improve our understanding of the potential ecological consequences of a changing sea ice environment for marine food web dynamics. Furthermore, spatial and seasonal differences in the utilization of ice algaeproduced carbon within and between both hemispheres are investigated. The sample collection in the central Arctic Ocean was carried out during the RV ‘Polarstern’ expedition ARK XXVII-3 (PS80, August-September 2012) within the Amundsen and Nansen Basins. In the Southern Ocean, samples were collected during the RV ‘Polarstern’ expeditions ANT XXIX-7 (PS81, August-October 2013) in the northern Weddell Sea and ANT XXIX-9 (PS82, December 2013-March 2014) offshore from the Filchner Ice Shelf. Trophic interactions of important representatives of Arctic and Antarctic food webs are studied using lipid fingerprinting, stable isotope analysis (SIA) of natural abundance bulk carbon and nitrogen (BSIA), and compound-specific SIA (CSIA) of fatty acids (FAs). From the distribution of algae-produced FAs in the consumers (= marker FAs), the origin of carbon produced by diatoms versus dinoflagellates in key Arctic species (Chapter I and II) and key Antarctic species (Chapters III-VI) is investigated. Stable isotope mixing models are used to quantify the relative contribution of bulk carbon and marker fatty acids derived from ice algae versus pelagic phytoplankton to the carbon budget of the organisms. Additionally, the stomach contents of polar cod Boreogadus saida (Chapter II) and Antarctic krill Euphausia superba (Chapter IV) are investigated to provide information on the most recent diet composition and carbon sources compared to the long-term trophic signal derived from FA proportions and stable isotope compositions. In the Arctic food web, a high contribution of ice algal carbon with up to 90% of the carbon budget of species with a known strong sea ice association, such as the amphipods Apherusa glacialis and Onisimus glacialis, is demonstrated. The results also suggest a substantial ice algae-carbon assimilation by rather pelagic species, such as Calanus copepods and the pelagic amphipod Themisto libellula during late Arctic summer, in which sympagic carbon contributed up to 55% of the carbon budget of these species (Chapter I). Furthermore, a high trophic dependency of polar cod on sea ice-associated resources is shown (up to 95% ice algal carbon of body carbon), indicating their high vulnerability in regards to alterations of the sympagic food web (Chapter II). Chapter III addresses differences in the utilization of ice algal carbon by different developmental stages of Antarctic krill (Furcilia larvae, juveniles, adults) during late austral winter. It is shown that young developmental stages thrive significantly on ice algae produced carbon to survive their first winter, receiving up to two thirds of their carbon uptake from ice algae. The high spatial and temporal variability in diet and carbon sources of AC0 krill (larvae, juveniles) across the sampling area in the northern Weddell Sea is discussed in Chapter IV. Besides young E. superba, the amphipod Eusirus latircarpus demonstrates a particularly high trophic dependency on sea ice-related primary production during late austral winter, indicating a proportional contribution of ice algal carbon of up to 67% of their energy budget. Other important energy linkers indicate a switch from a predominantly pelagic lifestyle to a strong dependency on ice algae-produced carbon as the winter season progressed (Chapter V). Among the other abundant euphausiids collected offshore from the Filchner-Ronne Ice Shelf, Euphausia crystallorophias and Thysanoessa macrura show that ice algal carbon can serve as important carbon source during austral summer, accounting for up to 43% of the dietary carbon in these species (Chapter VI). In summary, the applied state-of-the art techniques and statistical models allow for a reliable quantification of the contribution of ice algae-produced carbon to the carbon budget of ecological key species in both Polar Regions. The results imply that functioning and carbon dynamics of food webs in both Polar Regions are likely affected by changes in sea ice coverage and thus ice algal primary production. Due to the close connectivity between the sea ice ecosystem and the pelagic system, these consequences will subsequently impact the entire polar ecosystems, their fish populations and subsequently mammal populations. Moreover, these large amounts of required carbon for the nutrition of polar food webs, currently fulfilled by ice algae, can likely not be substituted by an increased pelagic primary production.