Species identity matters: Functional responses to warming in congeneric turfs differ from those of a canopy algae but are species-specific

The decline of kelps and algal canopies and the expansion of algal turfs because of global change is a recurrent pattern reported in temperate coasts worldwide. This pattern has also been identified in recent years in the northwestern Iberian Peninsula where Fucus serratus, a cold-water species that used to form extensive canopies in the intertidal, is reducing its abundance while the warm-water turf-forming Vertebrata hypnoides and V. reptabunda are becoming more abundant in the same tidal level. As ecophysiological performance in seaweeds is strongly influenced by temperature, understanding their response to increasing thermal gradients may assist us to predict their vulnerability to future warming scenarios. In this study, we compared the effects of warming on the photosynthetic and respiratory performance of F. serratus and two species of Vertebrata using a highly resolved temperature gradient (from 7 to 31 degrees C) under controlled laboratory conditions. Only productivity at the optimum temperature showed a consistent difference between the canopy F. serratus and the two turfs, being significantly higher in the latter. Photosynthesis responsiveness to temperature was significantly higher in one of the Vertebrata than in F. serratus while net photosynthesis was significantly higher in V. hypnoides than in V. reptabunda. Altogether, our results show that photosynthesis in algal turfs responded in a species-specific manner to warming, indicating that species composition should be taken into consideration in future ecophysiological studies.