Multiple stressors across ecosystem boundaries: Do light pollution and invasive species change the quality of aquatic prey for terrestrial predators?

Abstract

Background: Freshwater ecosystems provide high-quality and essential fatty acids (FAs) to terrestrial food webs via aquatic emergent insects. Anthropogenic stressors, such as artificial light at night (ALAN) and invasive species, may disrupt these aquatic-terrestrial linkages by altering insect emergence and its nutritional quality for terrestrial predators, with cascading effects on terrestrial food webs. The interaction of these individual stressors may have non-additive impacts in the aquatic-terrestrial linkage, for example, by the direct effect of ALAN on the foraging activity of nocturnal invasive species (e.g., signal crayfish). We assessed the impact of ALAN, the invasive signal crayfish, and their interaction on the fluxes of FA via emergent insects, as well as their effect on the FA content in riparian spiders. We conducted a full factorial experiment between spring and summer (six weeks), employing a mesocosm facility composed of 16 artificial streams and adjacent riparian zones. Results: The emergent biomass and FA fluxes were highest during the first week of the experiment in spring, and decreased over time. The presence of signal crayfish further reduced FA fluxes, while the FA content in spiders did not significantly differ among treatments. However, there was a tendency for an increased polyunsaturated FA content in spiders under ALAN, suggesting physiological consequences of the exposure to nocturnal illumination. Further, ALAN reduced signal crayfish activity, but this did not translate into a reduction of its effects. Conclusions: This research demonstrates the effects of global stressors on complex systems such as aquatic-terrestrial meta-ecosystems and calls for a deeper understanding of the interactive effects of multiple stressors in real-world conditions.