IMIS

Six items (producers or organic matter pools) were identified as potential food sources for primary consumers. Sympagic algae, that mostly occurred as several cm long filaments, were sampled by SCUBA divers by scraping the lower surface of fast ice. The dominant macroalgae, the large Phaeophyceae Himantothallus grandifolius and the Rhodophyceae Phyllophora antarctica, were hand-collected by SCUBA divers. Benthic biofilm was collected by scraping rocks in situ. It was scarce in 2013-2014 but extremely abundant in 2014-2015 (thick layer of several cm covering rocks but also macroalgae and sponges, supporting information S2). Seawater for suspended particulate organic matter (SPOM) was collected through the diving holes, at 10 m depth using a Niskin bottle. 195 Seawater was then pre-sieved to remove items larger than 5 mm, and filtered on pre-combusted (4h at 400 °C) glass fiber filters (Whatman GF/F, sieve size 0.7 μm). For each SPOM sample, 20 liters of seawater were filtered. Finally, samples of the abundant deposits of guano surrounding the extensive colonies of Adélie penguins (Pygoscelis adeliae) were hand collected on land, in vicinity of the diving holes.
Study Extent: Sampling took place in the surroundings of Dumont-d'Urville station (French Polar Institute Paul-Emile Victor - IPEV), on Petrels Island (Adélie Land, East Antarctica) during the austral summers of 2013-2014 (sampling dates: 22 - 26/01/2014) and 2014-2015 (sampling dates: 17/12/2014 - 12/01/2015). During these two austral summers, extreme sea ice conditions occurred, as fast ice did not undergo seasonal breakup for two successive years (ice thickness fluctuating between 40 and over 200 cm during summer, cf. supporting information S1). Two sampling sites were chosen. Site 1 ("Anse du Lion"; 140.003° E, 66.661° S) was visited during both sampling campaigns, while site 2 ("Cap des Eléphants"; 139.997° E, 66.667° S) was sampled only in 2014-2015. In both sampling campaigns, both sites were covered by a thick layer of fast ice (from around 100 to over 200 cm, supporting information S1), and holes were drilled to allow access to the sea.