We investigated the emergence of Daphnia pulicaria from resting eggs in Río Seco, an alpine lake in which the zooplankton growing period is short. In situ studies were carried out using sediment emergence traps and plankton sampling, and the sediment egg bank was analyzed. A laboratory experiment was performed to explore the role of photoperiod and temperature as hatching cues for D. pulicaria. We hypothesized that the time window for emergence would be short and that hatching cues would be linked to the variable ice-break period. In the field, D. pulicaria showed a highly synchronized emergence in both years. The hatching period was restricted to 3 weeks during and immediately after the thaw. In the laboratory experiment, no hatchings were observed in the dark treatment and neither photoperiod nor temperature affected hatching rates. Therefore, the presence of light, which corresponds with the period of ice-break, was essential for triggering dormancy termination of D. pulicaria. Our results show that the period when the ice-break occurs is a key event for Daphnia population development. The longer ice-free periods predicted by climate change in the alpine lakes could therefore have important effects on the crustacean population dynamics of these systems.