Nutrient enrichment scarcely affects ecosystem impacts of a non‐native herbivore in a spring‐fed river

Summary Non‐native invasive species ( NNIS ) are a global issue whose introduction into novel ecosystems can fundamentally alter nutrient recycling and storage. It is therefore important to understand factors that affect the abundance and distribution of NNIS and their effects on ecosystems. We investigated how nutrient enrichment and herbivorous armoured catfish separately and in combination affect ecosystem processes in a subtropical spring‐fed river (San Marcos, River, TX, U.S.A.). A replicated stream channel experiment in which nutrient enrichment was cross‐classified with the presence and the absence of armoured catfish was conducted to evaluate how nutrient enrichment may affect the ecosystem and nutrient cycling effects of catfish. The presence of catfish reduced periphyton biomass and altered detrital decomposition rates and, contrary to predictions, decreased periphyton N:P. In addition, the presence of catfish increased the severity of periphyton P‐limitation. We found little evidence that adding nutrients altered the effects of armoured catfish on ecosystem dynamics. Armoured catfish likely play an important role in the nutrient dynamics of the San Marcos River, but nutrient enrichment has little influence on determining the magnitude of ecosystem and nutrient cycling effects of this invasive and stoichiometrically unique consumer.