• Species interaction
• facilitation
• competition
• stress gradient hypothesis

Species interactions are integral drivers of community structure and, according to the stress gradient hypothesis, can change from competitive to facilitative with increasing levels of environmental stress. In subtidal marine ecosystems, however, interactions along physical stress gradients have seldom been tested.
In this study, we combined observational and experimental approaches in order to investigate the variation in the interaction between two red macroalgae: the canopy forming species Halopithys incurva and its epiphyte Jania rubens across a depth gradient. Our work was generated by the observation that the percentage cover of the epiphyte J. rubens on the canopy H. incurva tended to decrease along a depth gradient ranging between 1 m and 10 m. Our hypothesis is that the high load of J. rubens could buffer high light stress on its host at shallow depth. As light condition becomes less stressful, the relationship between J. rubens and H. incurva should turn into competition.
First, we performed a correlative study in order to investigate how the photosynthetic efficiency and the amount of epiphyte of the host H. incurva vary along a depth gradient. Furthermore, we carried out two experiments in order to understand the role of the epiphyte J. rubens under different environmental conditions: a superficial experiment (1m) involving the removal of the epiphyte from some host plants of H. incurva; and a deep experiment (10m) where J. rubens was added to some host samples. Effective quantum yield and pigments content were measured in treated H. incurva samples and compared with a control and a procedural control group.
Our results reveal a shift from competition to facilitation under increasing light stress. The presence of epiphyte J. rubens was somewhat effective in reducing light stress on its host, possibly extending the distribution range of H. incurva toward more surface water. These findings provide a support for the stress gradient hypothesis within subtidal seaweed communities and highlight the importance of biotic interactions in driving species distribution and the maintenance of subtidal marine habitats under adverse environmental conditions.