Scope for growth and fitness of Daphnia magna in salinity-stressed conditions

1. Scope for growth (SfG) is an integrative physiological indicator reflecting the net energy gain of an organism. It correlates well with growth and reproduction. An experiment was set up to test whether SfG also reflects the capacity to cope with environmental stress. 2. Five groups of Daphnia magna were fed different algal rations to induce different SfG. On day 21, SfG was measured. Thereafter, each ration group was subdivided into a control and a saltwater treatment in which salinity was raised to 0.7%. 3. In the first part of the experiment, mortality was dependent on ration only for the first 6 days. Afterwards, mortality rates did not differ between the rations. SfG increased hyperbolically with increasing ration. Growth and reproduction correlated with SfG. In the second part of the experiment, mortality rates were higher in the salt-stressed groups, but did not differ between rations in either group. Reproduction correlated with SfG in the freshwater group but not in the saltwater group. 4. The results indicate that differences in SfG do not imply different capacities to cope with future stress.