Calanus glacialis strongly contributes to the energy fluxes in Arctic shelf ecosystems. During its life cycle, this copepod species overwinters at depth with physiological adjustments which are yet not fully understood. We therefore assessed key metabolic activities and extracellular pH (pHe) in the haemolymph, seeking to identify the mechanisms which characterize this so called diapause state. For one year we sampled copepods in a high Arctic fjord and found that their enzymatic activities and pHe followed a clear seasonal trend. Both, activities and pHe were high when the copepods were feeding in surface waters in spring. In winter at depth, metabolic enzyme activities and pHe were low (pHe<6), while lipid catabolism on internal reserves increased. Metabolic activity matched the pHe pattern suggesting that low pH supports metabolic depression. Metabolic enzyme syntheses decreased and copepods descended before food had disappeared. In late winter, metabolic activity increased and the copepods migrated to the surface long before the spring bloom. Our data support that diapause timing is regulated internally, with an early enzyme synthesis that prepares the copepods for activity before spring.