Climate-change-driven cooling can kill marine megafauna at their distributional limits

The impacts on marine species from secular warming and heatwaves are well demonstrated, however the impacts of extreme cold events are poorly understood. Here, we link the death of organisms from 81 species to an intense cold upwelling event in the Agulhas Current, and show trends of increasing frequency and intensification of upwelling in the Agulhas Current and East Australian Current. Using electronic tagging, we illustrate potential impacts of upwelling events on the movement behaviour of bull sharks, Carcharhinus leucas including alterations of migratory patterns and maintenance of shallower dive profiles when transiting through upwelling cells. Increasing upwelling could result in "bait-and-switch" situations, where climate change expands subtropical species' distribution, while simultaneously exposing climate-migrants to increased risk of cold-stun/mortality events at poleward distributional limits. This shows the potential impacts of increased cold events, an understudied aspect of climate-change research, and highlights the complexities of climate change effects on marine ecosystems.长期变暖与热浪对海洋物种的影响已得到充分证实，但极端寒冷事件的影响却鲜为人知。本研究将81个物种的生物死亡与厄加勒斯海流（Agulhas Current）中的一次强冷上升流事件相关联，并揭示了厄加勒斯海流与东澳大利亚海流（East Australian Current）中上升流频率增加、强度增强的趋势。本研究通过电子标记技术，阐明了上升流事件对公牛鲨（Carcharhinus leucas）运动行为的潜在影响，包括其洄游模式的改变，以及在穿越上升流单元时维持较浅潜水深度的行为。上升流活动增强可能引发“诱饵转换”困境：气候变化推动亚热带物种的分布范围扩张，但同时却使这些气候迁徙物种在向极分布边界遭遇冷击/死亡事件的风险显著升高。本研究阐明了极端寒冷事件增多的潜在影响——这是气候变化研究中尚未得到充分关注的领域，同时也凸显了气候变化对海洋生态系统影响的复杂性。