ABSTRACT: Positive interactions become more important as physical stress increases. Rocky intertidal habitats display marked desiccation and heat stress gradients, increasing from low- to high-tidal levels. The presence of some macroalgae has been shownto facilitate several intertidal organisms by ameliorating stressful conditions. However, few studies have reported positive interactions among 2 or more macroalgal species, and none has addressed how seaweed canopies could modify the physiologicalperformance of other associated algae along intertidal gradients. Here we report on spatial association patterns of 2 macroalgae (the kelp Macrocystis pyrifera and the green alga Ulva lactuca) occurring along a rocky intertidal verticalgradient in southern Chile. We conducted an evaporative water loss experiment and compared temperature and photosynthetic active radiation beneath the canopy of M. pyrifera and on exposed substrates. We compared maximum quantum-yield(F_v/F_m ratio = (F_m–F₀)/F_m, where F₀ and F_m are the minimum and maximum chlorophyll fluorescence yields, respectively) andphotochemical efficiency in Photosystem II (Φ_PSII) of U. lactuca beneath and away from kelp canopies along the intertidal gradient. Positive association patterns between M. pyrifera and U. lactuca were detected athigh-tidal areas, while neutral patterns were evident at low-tidal heights. Evaporation, temperature, and PAR were lower beneath kelp canopies. The photosynthetic performance of U. lactuca was lower in individuals on exposed substrates compared tothose associated with M. pyrifera, with this effect being more pronounced in the upper intertidal zone. Our results support the prediction that the importance of positive interactions increases with stress. We suggest that M. pyrifera couldbe acting as a ‘nurse species’ for U. lactuca, possibly extending the vertical distribution of this green alga to high intertidal zones.