Effects of total dissolved solids on growth and mortality predict distributions of stream macroinvertebrates

1. Spatial distributions of many freshwater invertebrates are associated with the concentration of total dissolved solids (TDS) in streams (usually measured as specific electrical conductivity EC). These patterns are presumably caused by taxon-specific effects of TDS on the maintenance of ion and water balance, which is energetically expensive and thus could affect fitness. However, no studies have assessed if TDS-related effects on fitness metrics, such as growth and survival, are predictive of distributions. 2. We used stream-side, flow-through microcosms to experimentally determine how TDS concentration influenced three life-history performance measures (growth, mortality and emergence) of 13 species of aquatic insects. We then modelled the relationship between experimentally derived performance measures and both EC optima and minima estimated from field surveys. 3. In the streamside experiment, we exposed taxa to stream water with naturally low (<25 mu S cm(-1)) and moderate (>125 mu S cm(-1)) EC for 83 days, controlling for differences in habitat, temperature, food availability and pH. We measured performance as the differences in specific growth rates, survival and adult emergence between the two treatments. 4. Treatment effects on performance varied among taxa and with performance metric. Survival differences between treatments varied from 1 to 21 days, and specific growth rates differed by 0.07-1.5 day(-1). Emergence success was less variable (>20% for only two taxa). Three taxa performed better in the low EC treatment, six in the moderate EC treatment, one had a mixed response and three taxa did not differ between treatments. 5. The best model predicting field-derived EC optima included both survival and specific growth rate as predictors (R-2 = 0.68). Estimates of EC minima were not as strongly related to performance measures as were optima estimates. Taxa with greater performance in the moderate EC treatment had the highest EC optima, indicating that low TDS concentrations could restrict the distributions of taxa adapted to higher TDS concentrations. In contrast to taxa with higher EC preferences, taxa that can tolerate low EC environments will likely be increasingly stressed as stream TDS increases in response to climate and land-use change expected this century.