Sub-lethal and chronic salinity tolerances of three freshwater insects: Cloeon sp. and Centroptilum sp. (Ephemeroptera: Baetidae) and Chironomus sp. (Diptera: Chironomidae).

Increased salinity in rivers and streams is a serious environmental concern, and in Australia there is growing information about the acute tolerances to salinity for freshwater macroinvertebrates, but much less information about chronic and sub-lethal tolerances. The effects of increased salinity on the growth and survival of two mayflies, Cloeon sp. and Centroptilum sp. and one midge Chironomus sp. are reported. In both mayfly species survival was variable. Complete mortality was observed in salinities with electrical conductivity of 10 mS cm(-1) and higher. Salinities causing chronic mortality in mayflies were measured as 21-day LC50, and ranged from 0.90 to 2.7 mS cm(-1). Growth rates were not significantly different between treatments. In Chironomus, salinity affected the mean number emerging as flying adults as well as the time to emergence. An inverted 'U' shape response was observed for percentage emergence, with the greatest numbers emerging at intermediate salinities (0.65-5.0 mS cm(-1)). No emergence occurred at salinities of 20 mS cm(-1) and higher. Time to emergence was delayed by 15-88% with increased salinity, however the size of emerged adults was the same for all treatments. Growth rates were reduced with increased salinity, showing a slow, steady reduction up to 10 mS cm(-1) then a steep decline between 10 and 15 mS cm(-1). The implications of altered growth rates and changes in developmental times are discussed. This study illustrates the variability in responses to increased salinity, and highlights the need to continue studying sub-lethal and chronic exposures in a range of freshwater invertebrates, in order to predict impacts of salinisation on freshwater biodiversity.