Our results indicate that stoichiometrically unique invaders can exert strong impacts on nutrient dynamics and have the ability to alter the functional role of fishes in aquatic ecosystems, especially when they attain high population densities. Relative to the contribution from native fishes, loricariid invasion converted the upper Chacamax River to a system where fishes formed an important pool of stored nutrients, and where fish remineralization had the potential to meet most of the ammonium and P demand. As we predicted, the high population density and unique body stoichiometry of loricariids influenced their effect on nutrient dynamics; they produced a net sink of P relative to their remineralization rates. These results illustrate the utility of ecological stoichiometry as a predictive framework for understanding the potential effects of non-native fishes on nutrient dynamics in freshwater ecosystems. Moreover, this study highlights the importance of estimating both elemental storage and remineralization rates of invaders to elucidate net ecosystem effects of invaders on biogeochemical processes.