The copepod Calanus glacialis comprises up to 80% of the zooplankton biomass in Arctic shelf seas and plays a key role in Arctic marine ecosystems. It is primarily a grazer, accumulating essential polyunsaturated fatty acids from its algal diet as well as converting low-energy carbohydrates and proteins in algae into high-energy wax ester lipids. It is able to survive long periods without food by descending to depth and lowering its metabolism to a minimum, a state referred to as diapause. Although C. glacialis may be in this physiological state for up to 8 months each year we know very little about the energetic costs required during diapause. We therefore initiated an extensive field campaign in a high-Arctic fjord, sampling the local population monthly from June 2012 to July 2013. Monthly carbon demand was estimated by measuring respiration, image analysis was used to analyse variability in lipid content over the season. The carbon demand during winter differed among C. glacialis CIV, CV, females and males, with CV and adults being active much earlier previously assumed. Lipid reserves in CV and females remain largely untouched throughout autumn but decrease from January on, most likely to fuel moulting and maturation. The C. glacialis population declined steeply from January to May suggesting that individuals may run out of energy stores during winter. Of the verwintering stages, only IV seems to stay in diapause over an extended period, utilizing little of its lipid storage from fall through winter