Urban freshwater salinization reshapes <i>Erythemis simplicicollis</i> (Odonata: Libellulidae) life history timing and predator–prey interactions

As urban areas spread and winter road salt use increases, freshwater salinization has become an increasingly common threat facing aquatic organisms. Previous research has shown high salt exposure to have lethal and sublethal effects on aquatic macroinvertebrates. However, few studies have assessed how salt exposure during aquatic life stages may carry-over to affect terrestrial adults in semi-aquatic organisms. We used Erythemis simplicicollis (Say, 1839) dragonflies as a model organism with a biphasic life cycle to assess the impacts of freshwater salinization on aquatic larvae and the potential for carry-over effects. We used an experimental mesocosm approach to investigate the effects of salt pollution on larval size, foraging activity and immune health, and carry-over effects on adult immunity, morphology, and emergence timing. We found that although salt pollution did not impact E. simplicicollis larval size or immune system strength, it decreased larval foraging rates and accelerated adult emergence. However, we did not find carry-over effects of salt pollution on adult morphology or immune strength. At broader scales, the impacts of salt pollution on larval behavior and life history timing could alter interactions within freshwater food webs. In biphasic, semi-aquatic organisms, this may restructure terrestrial food webs, compounding changes in predator-prey interactions we observed in aquatic food webs. We highlight the need for further research on the sublethal effects of salt pollution on diverse aquatic taxa and across life history transitions to better predict the impacts of this common urban pollutant on freshwater ecosystems.