Artificial light at night and invasive signal crayfish alter aquatic‐terrestrial food webs

Abstract Resource flows across aquatic‐terrestrial boundaries are increasingly affected by multiple stressors, such as artificial light at night (ALAN) and aquatic invasive species, which can alter the availability of resources for consumers. However, how these stressors, individually or in combination, affect the diets of aquatic and terrestrial consumers remains poorly understood. We conducted a 6‐week mesocosm experiment in Southwest Germany using 16 artificial streams with adjacent riparian areas, assigned to one of four treatments: control, ALAN, signal crayfish presence and their combination. We used stable isotope ratios of carbon and nitrogen to assess the diet and trophic niche width of the riparian spider Tetragnatha extensa and the invasive signal crayfish Pacifastacus leniusculus . Stable isotope mixing model shows that aquatic sources comprised 56% (95% HDI: 40%–64%) of T. extensa 's diet in the control, 57% (49%–73%) under ALAN, 64% (49%–79%) under crayfish and 58% (35%–79%) under combined ALAN and crayfish. The normalized median niche width of T. extensa increased under ALAN by 14% (95% CI: 6%–24%) and combined ALAN and crayfish treatment by 13% (6%–23%) compared to 6% (3%–10%) in the control, indicating greater dietary diversity, while crayfish‐only was similar at 7% (3%–12%). This pattern reflects shifts in resource availability to T. extensa under both individual and combined ALAN exposure. The signal crayfish showed a similar response, with diet consisting of roughly half of the combined Chironomidae–Gammaridae prey source in the absence of ALAN and increasing to about three quarters under ALAN treatment, with a 6.4% (3.6%–10.1%) wider isotopic niche width under ALAN conditions. Overall, these shifts, driven by ALAN despite the presence of signal crayfish, highlight how multiple stressors can reshape resource flows across linked aquatic‐terrestrial ecosystems, with implications for riparian food‐web dynamics and biodiversity. Read the free Plain Language Summary for this article on the Journal blog.