The sublethal salinity tolerance of selected freshwater macroinvertebrate species

Increasing salinity levels pose a threat to freshwater ecosystems in many regions of the world. This study investigated the sublethal effects of salinity on four macroinvertebrate species commonly occurring in Australian freshwater ecosystems. These were; the worm <i>Lumbriculus variegatus </i>(Annelida: Oligochaeta), the snail <i>Physa acuta</i> (Gastropoda: Physidae), the shrimp <i>Paratya australiensis (</i>Decapoda: Atyidae) and the midge <i>Chironomus tepperi </i>(Diptera: Chironomidae). The effects of salinity on growth, development and reproduction of these species were investigated through a series of laboratory tests. The tolerances of early life stages of these species were also investigated. The effects of salinity on <i>C. tepperi </i>were assessed across a full life-cycle of exposure. The observed sublethal salinity tolerances of the investigated species were compared to their acute salinity tolerances. These relationships were investigated to assess if there is a correlation between the sublethal and acute salinity tolerance of freshwater macroinvertebrate species. <br><br>With the exception of <i>L. variegatus </i>each of the investigated species displayed life stages with lower salinity tolerances than older aquatic life stages of the species. This was not investigated for <i>L. variegatus</i>. Sublethal effects of salinity were observed for all of the investigated species at salinities significantly lower than those affecting their short term survival. In many cases these effects had obvious implications for the survival ability of the species. Examples include zero reproduction and/or growth at salinities displaying minimal impacts on short term survival.<br><br>The pattern of effect with increasing salinity varied between different end points. Many of the investigated effects displayed non-threshold responses with increased salinity. This necessitated consideration of the magnitude of effect. Many end points, particularly growth, displayed optimal levels at slightly elevated salinities.<br><br>Common trends in the relative salinity tolerance of different life stages and sublethal end points were identified. Sublethal responses were generally more salt sensitive than the survival of eggs and juveniles. Growth and reproduction of freshwater macroinvertebrates were found to be highly sensitive to increased salinity. The cumulative effects of salinity on survival, growth, development and reproduction across a full life-cycle of exposure was found to greatly reduce recruitment potential. A 50% reduction in potential recruitment of second generation <i>C. tepperi </i>was observed at a salinity more than 80% lower than the 96-hr LC₅₀ (the concentration lethal to 50% of test organisms) for the species. <br><br>The sublethal salinity tolerance of the investigated species was closely related to their acute salinity tolerance. The ratio between the EC50 (the concentration producing a response in 50% of test organisms) for the most sensitive sublethal end point, recruitment of <i>C. tepperi</i>, and the 96-hour LC₅₀ for the species was 0.17. Mean ratios between EC50 values for growth and reproduction end points and LC50 values observed for this and other studies were around 0.45. While further research is required, the proportional relationship between the sublethal and acute tolerance of macroinvertebrate species observed in this study allows the sublethal effects of salinity to be taken into account in aquatic ecosystem management where only acute tolerance data are available.<br>