The number of extinct or urgently threatened species rapidly accelerates and almost one-third of freshwater biodiversity face extinction. Consequently, the need to identify means to help protect and conserve species is paramount. Here, we explore mechanistic links between eutrophication and species declines. Specifically, we hypothesised that declining species within the plant genus Potamogetonaceae exhibit a low degree of ecophysiological trait plasticity and a suboptimal trait expression under enhanced levels of nutrients rendering these species prone to extinction under eutrophication. Individuals of five species including common species (S. pectinatus, P. perfoliatus and P. crispus) and declining species (P. compressus and P. gramineus) were transplanted into three replicate lakes along a gradient in nutrient availability. After four weeks, ecophysiological traits were measured and the phenotypic plasticity was assessed. We found that declining species were unable to convert increased nutrients availability into enhanced rates of growth. Additionally, we found that the ecophysiological plasticity was lower both regarding nutrient acquisition and the ability to adjust physiologically to maximise growth under the prevailing nutrient regime. We conclude that the mechanisms behind species declines link to inappropriate ecophysiological adjustments under nutrient enrichment that have severe consequences for their competitive capabilities, eventually leading to local extinction.