High latitude marine ecosystems experience strong seasonality in incoming light and thus food availability. The ongoing reduction in sea ice thickness and extent will change the underwater light climate significantly in the Arctic. Herbivorous calanoid copepods of the genus Calanus of northern higher latitudes are considered to endure times of unfavorable environmental conditions in a state of arrested development, referred to as diapause. However, to date we have very limited knowledge regarding the overwintering physiology and adaptability of Calanus spp. to changes in external cues like light and food. To study potential impacts of changes in the light and primary production regime on Calanus spp. in the Arctic, we measured enzymatic response and biochemical composition of Calanus glacialis copepodite stage V to presence or absence of food (Thalassiosira spp.) and light in winter (November 2009) and summer (July 2010). In situ proteinase and lipase/esterase activity was 10 times and 19 times higher, respectively, while metabolic activity (citrate synthase, CS) was twice as high for C. glacialis CV in summer compared to winter, suggesting that individuals collected at depth in winter had reduced activity and were in an overwintering state. Food availability amplified digestive activity in both seasons, while light had a minor effect in comparison. In winter, gel electrophoresis revealed a change in enzyme pattern over the three weeks of incubation, indicating that feeding induced enzyme expression. Green guts in approximately 90% of fed individuals at the end of the experiment supported that they were actively feeding. However, there was no significant increase in dry mass or carbon and nitrogen content in feeding individuals during the winter experiment. These results suggest that factors other than food and light must be involved in controlling the transition from diapauses to activity in C. glacialis, which is currently under investigation. Understanding the consequences of shifting environmental parameters on the physiology of this energy rich copepod species is crucial to assess to the survival success of the overwintering stock and therefore the fate of the pelagic food web and especially of higher trophic levels in the following spring.