Freshwater salinity varies in natural systems and plays a role in species distribution. Sea level rise and intrusion of saline waters further contribute to freshwater salinization. While mayflies are generalized to be sensitive to increasing salinity, we still know remarkably little about the physiological processes that determine the performance of species in a changing world. Here, we explored life-history outcomes and physiological plasticity in a population of Callibaetis floridanus, a coastal pond that routinely experiences saltwater intrusion. We reared nymphs from egg hatch to adulthood across a gradient of increasing salinities (113, 5,020, 9,921µS/cm). Radiotracer flux studies (22Na, 35SO4, and 45Ca) were conducted in nymphs reared at each condition, revealing a positive correlation between ionic concentration and uptake rates. However, the influence of rearing history on ionic influx rates was apparent when nymphs were transferred from their respective rearing water to the other experimental conditions. For example, nymphs reared in the low salinity treatment (113 µS/cm) had 10.8-fold higher Na uptake rates than nymphs reared at 9,921µS/cm and transferred to 113 µS/cm. Additionally, nymphs acclimated to the higher salinity water exhibited reduced uptake in ion-rich water relative to those reared in more dilute conditions (e.g., in 9,921 µS/cm water, 113 and 5,020µS/cm acclimated nymphs had 1.5- and 1.1-fold higher Na uptake rates than 9,921µS/cm acclimated nymphs, respectively). We found no significant changes in survival (80±4.4%, mean±s.e.m.) or nymphal development time (24±0.3 days, mean±s.e.m.) across these treatments but did observe a significant decrease in subimago female mass associated with the most dilute condition (p=0.0046). This reduction in female biomass was associated with higher oxygen consumption rates in nymphs relative to the other rearing conditions. Collectively, this data suggests that saline-adapted C. floridanus may be more energetically challenged in dilute conditions, which differs from previous observations in other mayfly species.