Patterns of use of algal turfs and canopies by reef fishes in Southeastern Brazil

Canopies formed by macroalgae of the genus Sargassum play essential ecological roles in shallow rocky reef ecosystems, contributing to habitat heterogeneity by providing food, shelter, and nursery grounds. However, these macroalgal forests have declined in recent decades, often replaced by low-lying turf-forming algae. We compared reef fish habitat use in Sargassum canopies and turf-dominated plots using remote underwater video recordings at two sites in the subtropical southwestern Atlantic. Fish species richness and Shannon–Wiener diversity index of habitat use did not differ between habitats, although turf plots showed higher total residence time of fish at one site, likely due to prey types supplied at that habitat. However, fish assemblages visiting turfs and canopies were markedly different, with an overall predominance of invertivores in turfs and a larger fraction of larger and less common species in Sargassum canopies. As a result, patterns of habitat use in terms of fish assemblage structure were more variable and unpredictable in the canopy habitat. These findings highlight the complementary roles of structurally distinct benthic habitats, with Sargassum canopies contributing disproportionately to ecological complexity and variability in reef fish assemblages in subtropical rocky reef habitats.