Ecosystem functioning of canopy- and turf- forming algae: contrasting supply of invertebrate prey to pelagic consumers

Macroalgal canopies are declining worldwide and are being replaced by low-lying algal turfs which frequently dominate reefscapes. Their loss may impact reef ecosystems in different ways, including the collapse of small canopy-dwelling invertebrates, and thus the decline of juvenile reef fish that prey on them. To evaluate this potential loss, we assessed (1) the differences between the mobile invertebrate assemblages associated with turf-forming (filamentous and articulated coralline turf) and canopy-forming (Sargassum spp. and Dichotomaria marginata) algae, and (2) the mechanisms underlying those contrasts by examining the invertebrate community assembly of filamentous turf and Sargassum spp. over the main canopy season. Abundance, biomass and diversity almost always differed between canopies and turfs (although not in a consistent way across sampling sites), while differences within canopy and turf algal types were nearly absent. The structure of invertebrate assemblages differed more consistently between canopies and turfs, with certain hard-bodied and soft-bodied invertebrates characterizing canopies and turfs, respectively. This divergence increased as the canopy season advanced. While no temporal changes occurred in turf invertebrate assemblages, clear temporal dynamics occurred in the invertebrate fauna associated with Sargassum. Brittle stars and amphipods were most abundant as early colonizers, followed by hard-shelled gastropods, bivalves and ostracods. By the end of the season, these groups became dominant and decreased diversity in the canopy habitat. As hard-shelled prey are preferred items for the main invertivore fish species in the area, results suggest that canopies may play an important role in the provisioning of trophic resources to pelagic consumers.