The effect of road salt runoff on mayflies (Ephemerella subvaria): An investigation into the energetic costs that mayflies suffer via freshwater salinization

Salinization from road salt runoff negatively affects mayfly (Ephemeroptera) individuals at concentrations below their hemolymph osmolality. Mayflies in saline water have been shown to have increased sodium turnover, contradicting the conventional model of osmoregulation which holds that they should be able to tolerate increases in salinity below their iso-osmotic point. One hypothesis for why mayflies are affected at salinities below their iso-osmotic point is that there are energetic costs associated with increased sodium turnover. Here I compared the metabolic rate, percent carbon, percent nitrogen, and percent sodium of Ephemerella subvaria from increasing salinities. This study involved both lab and field-based work; results were obtained from Ephemerella subvaria raised in stream chambers for seven days (70-750 μS/cm) and from Ephemerella subvaria collected from urban stream sites in Burlington, Vermont (32.37-331.45 μS/cm). Metabolic rate in mayflies increased along with salinity, which is evidence that osmoregulation in saline environments requires more energy as a result of increased sodium turnover (p = 0.003). Although there was no significant linear relationship between salinities for percent carbon, percent nitrogen decreased along with salinity for mayflies from urban stream sites collected in the field (p = 2e-05). This result suggests that salt may affect how mayflies allocate energy when living in salinized streams. Further, mayfly sodium concentration did not vary with salinity among streams which is consistent with the hypothesis that maintaining sodium homeostasis is energetically costly in saline streams.