Abstract
<jats:p>Abstract Artificial light at night (ALAN) is transforming natural darkness in freshwater ecosystems, yet its sublethal impacts on aquatic communities remain poorly understood. We addressed this gap by combining a six-week ALAN exposure experiment measuring physiological responses with overnight behavioural tracking to assess ALAN’s effects on two common dragonfly larvae (Aeshna and Sympetrum spp.). Sympetrum larvae reared under nightly cool-white ALAN had ~50% lower immune enzyme (phenoloxidase) activity and significantly reduced energy reserves (~20% less lipids; ~35% less protein) compared to dark controls, while survival and growth rate were unchanged. These sublethal changes indicate chronic physiological stress, consistent with disrupted circadian regulation of metabolism and immunity. In short-term trials, automated video tracking showed Aeshna larvae were significantly more active under both warm- and cool-white ALAN (peaking late at night) relative to darkness, whereas Sympetrum larvae remained as inactive as in darkness, with no behavioural response. Thus, ALAN triggers distinct sublethal effects via different pathways: dramatic behavioural shifts in the active predator Aeshna but cryptic physiological impairment in Sympetrum. Together, these divergent responses shaped by life-history strategies and ecological roles suggest that ALAN may disrupt predator–prey dynamics, weaken immune defences, and alter energy flow in freshwater ecosystems.</jats:p>