Energy flow across ecosystems, commonly referred to as spatial subsidies, is a fundamental property of nature. In river-terrestrial ecotones, for example, terrestrial consumers prey on emerging aquatic insects with strong aggregative responses. Due to the tight trophic linkage across ecosystems, anthropogenic impacts on a donor ecosystem could influence recipient ecosystem functioning exerted by subsidized consumers. Here, we examined how riverine eutrophication mediates decomposition by terrestrial ground beetles, which may rely on allochthonous inputs. Field surveys were conducted in unproductive gravel bars of the Tokachi River basin. Bayesian path analysis revealed a strong connection between riverine eutrophication and terrestrial decomposition rate: increased secondary production due to eutrophication resulted in increased density of ground beetles and rapid decomposition of dead worms, which was provided artificially on gravel bars. Our results suggest that recipient ecosystem functioning can be modified by the state of an adjacent, but distinct donor ecosystem through spatial subsidies.