Abstract
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.
Highlights
In natural environments, organisms are exposed to various threats, and escaping from them generally implies a cost, in both energy and missed opportunities for feeding and reproduction
The aim of this study was to assess the costs of fluctuating exposure of ultraviolet radiation (UVR) in Daphnia magna, and we hypothesized that fluctuations in UVR would reduce the number of offspring and the survival of individuals owing to the allocation of energy to threat response movements
Individuals exposed to the fluctuating UVR treatment showed the lowest reproductive success, measured as the total amount of neonates produced during the experiment
Summary
Organisms are exposed to various threats, and escaping from them generally implies a cost, in both energy and missed opportunities for feeding and reproduction. Most studies on effects of UVR have focused on effects from constant exposure as the treatment [8,10,11,12,13], despite the intensity of UVR in nature fluctuating strongly over short time scales with the position of the Sun and rapidly occurring variations in cloudiness Several studies include these natural variations in UVR in their experimental design, but they do not explicitly address the costs of the fluctuating threat per se [4,14,15]. Responding to these short-term fluctuations in UVR through avoidance behaviour likely implies a cost, in terms of both energy and missed opportunities for feeding and reproduction [8]. We hypothesized that there may be behavioural plasticity in the individual responses, possibly accounting for part of the considerable variance observed in natural ecosystems
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