Energy limitation or sensitive predators? Trophic and non-trophic impacts of wastewater pollution on stream food webs

Impacts of environmental stressors on food webs are often difficult to predict because trophic levels can respond in divergent ways, and biotic interactions may dampen or amplify responses. Here we studied food-web level impacts of urban wastewater pollution, a widespread source of degradation that can alter stream food webs via top-down and bottom-up processes. We selected ten pairs of sampling locations (i.e., twenty sites total) in ten small-to-medium tributaries of the Ebro River (NE Iberian Peninsula; see \"Related article\" for locations). These streams were located in low-mountain habitats (365 m – 950 m a.s.l.) under the influence of Mediterranean climate and flow regimes (mean daily discharge 0.05 ± 0.09 m3·s-1). Hydrology in the region is characterized by dry summer periods with reduced flows, and rainfall episodes in spring and late fall. We surveyed the sites in early spring (April 15 - 23, 2016) to avoid spring floods; the last scouring flood preceding our sampling took place...