The importance of subsidies as sources of energy to adjoining ecosystems is receiving increasing attention. In many cases the mechanisms controlling these subsidies are naturally derived however we investigate the response of a terrestrial predator to a human-mediated aquatic subsidy. The responses of web-building spider populations using bridges spanning the Susquehanna River in Pennsylvania were investigated in relation to patterns of aquatic insect prey populations. Potential prey and spider densities were measured on seven bridges. Prey and spider densities were also measured along a light-extinction gradient caused by proximity to streetlights. Representative prey and spiders were collected to determine cross-ecosystem trophic linkages using stable isotopes. Based on δ¹³C and δ¹⁵N values, aquatic insects constitute a larger proportion of spider diets on bridges than away from bridges. Both aquatic insects and spiders were proportionally more abundant in the middles of bridges. Furthermore, spiders from the middles of bridges had higher δ¹³C and larger body size relative to spiders at the ends of bridges. We estimated that spider populations on these bridges consumed in the region of 1.6 million adult mayflies over the peak emergence period. Human construction of bridges enhanced the cross-ecosystem subsidy by providing predators access to the aquatic resource.