Interactive effects of grazing and environmental stress on macroalgal biomass in subtropical rocky shores: Modulation of bottom-up inputs by wave action

• Slight changes in water motion result in substantial increase of algal biomass.
• Nutrient supply may be an important factor controlling algal blooms.
• Herbivore control is restricted to very sheltered areas where algal growth was slower.
• Under increased wave action algal bloom is much faster and swamp herbivore effects.
• The role of desiccation stress was found to be much less important than expected.

In contrast to what is observed in most temperate regions, perennial macroalgae are rare at the mid intertidal level of tropical and subtropical shores, and energy transfer through benthic herbivores largely relies on the consumption of periphyton and ephemeral algae. In this study, we evaluated the interactive effects of environmental stress and mesoherbivore grazing in the regulation of ephemeral macroalgal standing stock along subtropical shores moderately exposed and sheltered from waves in southeastern Brazil. Our results show that grazers can prevent ephemeral algal blooms at the most sheltered shores, and that amelioration of environmental stress, through provision of shade, has no consistent effect on overall biomass or temporal persistence of the algal blooms in these shores. At nearby shores exposed to waves, grazers had no measurable effect on algal biomass and shading rock areas from direct solar radiation can have positive effects on some years, but not on others, probably associated to variation in the species comprising the assemblage. Because nitrate concentration in nearshore waters is remarkably low, we suggest that increased water motion may enhance nutrient flux to the midshore and thus algal blooming. At more exposed sites, algae develop faster and reach a canopy size no longer controlled by grazers. Higher biomass of herbivores at exposed rocky shores is thus best explained as a bottom-up effect of increased plant productivity, without a coupled top-down effect on algae. Thus, besides the well documented effect of waves on temperature and desiccation stresses, wave modulation of nutrient supply may be a very important factor controlling abundance of midshore intertidal macroalgae, and deserves more attention in typically nutrient-depleted tropical and subtropical shores.