Different responses of coexisting Chara species to foreseeable Mediterranean temperature and salinity increases

An increase in temperature of approximately 4 °C is expected to occur in the Mediterranean by the end of the century. Concomitant to this warming, a foreseeable rise in salinity will affect aquatic species. We addressed the effects of warming and salinity, and their interaction on three coexisting characean species (Chara aspera, C. hispida and C. vulgaris) from a Spanish Mediterranean interdunal pond (spring water temperature 20-23 °C, when charophytes re-grew; salinity 1.3-1.8 PSU). A laboratory experiment was designed with two levels of water temperature treatment (23 and 27 °C), plus two levels of salinity treatment (0.4 and 4.0 PSU). The variables considered were total length, number of ramifications and nodes, dry weight, growth rate and stoichiometric composition. Three clear response patterns arose: (i) C. aspera growth was favoured by the 4 °C rise in temperature and the 10-fold increase in salinity when considered separately, but no synergy occurred when these factors interacted; (ii) C. hispida growth did not change with the temperature, but was negatively affected by the salinity increase, and (iii) C. vulgaris growth was enhanced by the temperature, a fact that compensated for the damaging effect of increased salinity. Variations of N and P concentrations with treatments were similar in C. aspera and C. vulgaris. We demonstrate how charophyte response to the interaction of increased temperature and salinity is species-specific and different under isolated stressors. Therefore, this study highlights how simultaneous drivers of global change might affect the composition of charophyte meadows in Mediterranean shallow wetlands.