The number of urgently threatened species rapidly accelerates and almost one-third of freshwater biodiversity face extinction. Here, we explore mechanistic links between eutrophication and ecophysiological trait plasticity to investigate if plasticity can predict species being predisposed to extinction. Individuals of five species belonging to the widespread and prominent genus Potamogeton were selected according to their historical trend and transplanted into three replicate lakes along a gradient in nutrient availability. After four weeks of growth, it was observed that declining species were unable to convert increased nutrient availability into enhanced rates of growth and that the ecophysiological plasticity was lower regarding nutrient acquisition and the ability to adjust physiologically to maximise growth under the prevailing nutrient regime. We conclude that an important mechanism behind species declines link to inappropriate ecophysiological adjustments under nutrient enrichment that likely have severe consequences for species competitive capabilities under eutrophication, eventually leading to local extinction.