| Summary: | Data pertaining to the characteristics of an arctic fiord in winter were collected at the Polish Arctic Station situated in Hornsund at 77 degrees N, 15 degrees E on Svalbard. Winter in the fiord was defined in terms of climate (November-May), hydrology (January-March) and biology (November-March). The characteristic phenomena of winter in the fiord include a winter drop in the yearly biomass maximum to 0.1% for phytoplankton and 10% for zooplankton; a slowing of the growth rate among pelagic dominants such as Pseudocalanus elongatus and Calanus finmarchicus, as well as among the hyperbenthic dominants Onisimus littoralis and Mysis oculata; and heterotrophy or maintenance of metabolism among living phytoplankton cells found in the middle of the polar night in densities of 10-50 cells/L. Since the life cycles of invertebrates are highly seasonal, no winter breeders were observed and 90% of the examined species were breeding according to a K strategy. Migration takes place among all seabirds in the area, but about 1% of the eiders, fulmars and kittiwakes overwinter, feeding in the open water of polynyas and crevices in the fast ice.Key words: Arctic, winter ecology, fiord ecosystem, Svalbard, Hornsund fiord Mots clés: Arctique, écologie de l’hiver, écosystème de fjord, Svalbard, fjord Hornsund
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