Lethal and sublethal effects of the saline stressor sodium chloride on Chironomus xanthus and Girardia tigrina

Salinization in freshwaters is gradually increasing as a result of human activities and climatic changes. Higher salt content causes stress for freshwater organisms. Sodium chloride (NaCl) is among the most frequently occurring salts in freshwater ecosystems. The objective of the present study was to investigate the lethal and sublethal effects of NaCl on freshwater ecosystems, using as test organism the dipteran Chironomus xanthus and the planarian Girardia tigrina. Acute tests showed that C. xanthus was more sensitive (48-h LC₅₀ (median lethal concentration) of 2.97 g NaCl L^-1) than G. tigrina (48-h LC₅₀ of 7.77 g NaCl L^-1). C. xanthus larvae growth rate (larvae length and head capsule width) was significantly reduced under exposure to concentrations as low as 0.19 g L^-1 NaCl and higher. A delay in the emergence time (EmT₅₀) was also demonstrated for the same concentration. Sublethal NaCl effects in G. tigrina included feeding inhibition (LOEC (lowest observed effect concentration) of 0.4 g L^-1), reduced locomotion (LOEC = 0.2 g L^-1), and 24-48-h blastema regeneration (LOEC = 0.2 g L^-1 and 0.1 g L^-1, respectively). The results demonstrated the toxicity of NaCl to C. xanthus and G. tigrina including sublethal effects that can result in negative consequences for populations in natural freshwaters under salinization.