Solar ultraviolet radiation (UVR) is an important environmental threat for organisms in aquatic systems, but its temporally variable nature makes the understanding of its effects ambiguous. The aim of our study was to assess potential fitness costs associated with fluctuating UVR in the aquatic zooplankter Daphnia magna. We investigated individual survival, reproduction and behaviour when exposed to different UVR treatments. Individuals exposed to fluctuating UVR, resembling natural variations in cloud cover, had the lowest fitness (measured as the number of offspring produced during their lifespan). By contrast, individuals exposed to the same, but constant UVR dose had similar fitness to control individuals (not exposed to UVR), but they showed a significant reduction in daily movement. The re-occurring threat response to the fluctuating UVR treatment thus had strong fitness costs for D. magna, and we found no evidence for plastic behavioural responses when continually being exposed to UVR, despite the regular, predictable exposure schedule. In a broader context, our results imply that depending on how variable a stressor is in nature, populations may respond with alternative strategies, a framework that could promote rapid population differentiation and local adaptation.

太阳紫外线辐射 (UVR) 是水生系统中生物的重要环境威胁，但其随时间变化的性质使得对其影响的理解模糊不清。我们研究的目的是评估与水生浮游动物Daphnia magna波动 UVR 相关的潜在健康成本. 我们调查了暴露于不同 UVR 处理时的个体生存、繁殖和行为。暴露于波动的 UVR（类似于云层的自然变化）的个体具有最低的适应度（以在其一生中产生的后代数量来衡量）。相比之下，暴露于相同但恒定 UVR 剂量的个体与对照个体（未暴露于 UVR）具有相似的适应性，但他们的日常运动显着减少。因此，对波动的 UVR 治疗再次发生的威胁反应对D. magna造成了巨大的适应成本，并且我们没有发现持续暴露于 UVR 时塑料行为反应的证据，尽管有规律的、可预测的暴露时间表。在更广泛的背景下，我们的结果意味着，根据压力源本质上的可变性，人口可能会以替代策略做出反应，这是一个可以促进快速人口分化和本地适应的框架。