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Positive interactions become more important as physical stress increases. Rocky inter- tidal habitats display marked desiccation and heat stress gradients, increasing from low- to high-tidal levels. The presence of some macroalgae has been shown to 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 phys- iological performance 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 vertical gradient 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 (Fv/Fm ratio = (Fm-F0)/Fm, where F0 and Fm are the minimum and maximum chlorophyll fluorescence yields, respectively) and photochemical efficiency in Photosystem II (ΦPSII) of U. lactuca beneath and away from kelp canopies along the intertidal gradient. Positive association patterns between M. pyrifera and U. lactuca were detected at high-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 to those associ- ated 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 could be acting as a 'nurse species' for U. lactuca, possibly extending the vertical distribution of this green alga to high intertidal zones.

Positive associations between macroalgal species in a rocky intertidal zone and their effects on the physiological performance of Ulva lactuca