Latitudinal cline for hypoxia but not low pH in Tigriopus californicus

Intertidal organisms must tolerate daily fluctuations in environmental parameters, and repeated exposure to co-occurring conditions may result in tolerance to multiple stressors correlating.  The intertidal copepod Tigriopus californicus experiences diurnal variation in dissolved oxygen levels and pH as the opposing processes of photosynthesis and cellular respiration lead to coordinated highs during the day and lows at night.  While environmental parameters with overlapping spatial gradients frequently result in correlated traits, less attention has been given to exploring temporally correlated stressors.  We investigated whether hypoxia tolerance correlates with low pH tolerance by separately testing the hypoxia and low pH stress tolerance separately of 6 genetically differentiated populations of T. californicus.  We independently checked for similarities in tolerance for each of the two stressors by latitude, sex, size, and time since collection as predictors.  We found that although hypoxia tolerance correlated with latitude, low pH tolerance did not, and no predictor was significant for both stressors.  We concluded that temporally coordinated exposure to low pH and low oxygen did not result in populations developing equivalent tolerance for both. Although climate change alters several environmental variables simultaneously, organisms’ abilities to tolerate these changes may not be similarly coupled. 

Methods
Observations were collected in the field or in the laboratory by the authors.