Several recent experimental studies have examined the effects of macroalgal diversity on community functioning using synthetic assemblages. However, their predictive relevance for natural systems remains uncertain. Unlike terrestrial habitats, where observational studies in natural systems are profuse, studies on the relationship between diversity and ecosystem functioning in natural marine ecosystems are scarce. In the present study, we explored how different components of biodiversity influence the performance of macroalgal assemblages in natural communities (intertidal boulders). Specifically, we examined the relationships between biomass, species richness, spatial aggregation and evenness and the productivity of macroalgal assemblages. We found the expected positive relationship for biomass and species richness. Additionally, we found significant statistical relationships between both spatial aggregation and evenness and some of the productivity-related variables analyzed: assemblages with a higher degree of spatial aggregation reduced their light capture and photosynthetic efficiency, while increasing evenness increased maximum net primary productivity. Although these patterns should be further tested using experimental approaches, observational studies may provide valuable insights by revealing patterns usually overlooked by experimental approaches.