Conservation programs for imperiled freshwater taxa often include a propagation and reintroduction component designed to enhance existing population sizes but may also be used to recover lost ecosystem functions resulting from population declines. Freshwater mussels are imperiled organisms that enhance habitat and control aquatic food webs through bottom-up subsidies via their nutrient recycling traits. Improving efforts to recover mussel populations and subsequent mussel-mediated nutrient cycling requires information about how species’ performance and functional traits are influenced by conditions of the reintroduction environment. We addressed how environmental conditions influence the growth and stoichiometric traits of species by conducting a 16-week reciprocal transplant experiment with broods of captivity-raised mussels in two rivers within the Mobile River Basin (MRB; Cahaba and Sipsey) and two rivers within the Tennessee River Basin (TRB; Duck and Paint Rock). The reciprocal transplant occurred during week eight. We expected growth rates to differ among rivers due to underlying environmental conditions, and nutrient excretion rates and stoichiometric ratios would differ among individuals stocked into different rivers because tissue stoichiometry should remain stable regardless of water column and food nutrient stoichiometry as animals have evolved post-assimilatory mechanisms to balance tissue demands with resource supply to maintain homeostasis. MRB mussels grew most in the Cahaba, or when transplanted from the Sipsey to the Cahaba. TRB mussels grew most in the Paint Rock, or when transplanted from the Paint Rock into the Duck. TRB mussel tissue C:N, C:P, N:P, and P excretion were highest in the Duck, but transplanting had no effect. Transplanting mussels between rivers led to opposing excretion N:P ratios between transplanted and non-transplanted MRB mussels. Mussel growth, tissue stoichiometry, and nutrient excretion rates were strongly shaped by river-specific environmental conditions, with clear evidence that environmental context creates lasting physiological legacies that influence mussel performance and ecosystem nutrient cycling following transplantation. The persistence of growth and stoichiometric effects following transplantation suggests that conservation efforts involving mussel translocations should consider source populations and recipient river conditions. Relocating mussels to seemingly favorable habitats may not immediately yield expected growth or ecosystem benefits if individuals are physiologically constrained by prior conditions.