Description: Effects of prolonged darkness on Arctic phytoplankton composition were investigated with lab experimentsand a pigment time series in Kongsfjorden, Spitsbergen (78�550N). Chlorophylla(Chla), pigment composition,particulate organic carbon, cell numbers, and photosynthetic characteristics were studied in Arctic diatoms(Thalassiosira antarctica,Thalassiosira nordenskioeldii) andflagellates (Rhodomonassp.,Micromonassp.) during8 weeks of darkness and subsequent recovery in irradiance. Loss of photosynthetic functionality after 2 weeks ofdarkness was reversible in all species when returned to irradiance. Diatoms were more resistant to prolonged dark-ness (〉 2 weeks) compared to theflagellates, with lower decline rates of Chlaand maximum quantum yield ofPSII.T. nordenskioeldiishowed rapid growth during recovery throughout 8 weeks of dark incubation, whereasrecovery offlagellates diminished within 4 weeks. Ratios of taxonomic marker pigments relative to Chlaof allspecies showed limited variation during 8 weeks of dark incubation. The experimentally observed enhanced darksurvival of diatoms was in agreement with pigment observations during four polar nights (2013–2017) inKongsfjorden, which showed increased relative diatom abundance during declining biomass (down to0.02 mg Chlam−3). Therefore, a period of prolonged darkness gives Arctic diatoms a head start during the earlystages of the spring bloom. The taxon-specific survival traits can influence the geographical distribution of dia-toms andflagellates within the polar oceans and their phenology. Furthermore, the persistence of Chlaof non-viable phytoplankton during darkness might influence biomass estimates during the polar night.