Climate warming disrupts zooplankton phenology and overwintering strategies

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