Zooplankton are crucial for food webs and biogeochemical cycles. However, warming associated with climatechange may alter their seasonal timing and reproductive strategies. This study investigated how long-termwarming impacted zooplankton (mainly copepods) phenology and overwintering strategies by comparing a Bal-tic Sea bay, heated by warm water discharge for more than 50 yr, with an unaffected control bay. Field observa-tions showed that copepod and phytoplankton population growth began earlier in the heated bay than in thecontrol bay, suggesting that copepod abundance was driven by both temperature and food availability in theheated bay and by a stronger temperature dependence in the control bay. Resting eggs are normally producedas a life-history strategy to survive unfavorable environmental conditions. Our laboratory incubation experi-ment showed fewer dormant resting eggs hatched from the heated bay sediment compared with the controlbay, supporting an evolutionary change in overwintering strategy. In conclusion, the results seemed to suggestthat copepods adjusted their life-history in elevated temperatures by relying less on the strategy of usingsediment-stored dormant eggs and instead started their spring development earlier, when phytoplankton foodwas available. Hence, this study suggests that climate change can shift copepod overwintering strategies, leadingto potential cascading effects in the food web and affecting overall biodiversity and productivity.