Spatiotemporal patterns of benthic communities on the shallow reefs of Southwestern Atlantic oceanic islands.

Reef ecosystems are among the most diverse marine systems, with benthic communities underpinning trophic structure and habitat complexity. However, long-term dynamics of benthic assemblages in oceanic reefs remain poorly resolved, particularly across isolated tropical systems under different protection regimes. Here, we assessed spatial and temporal variation in benthic community composition across four Brazilian oceanic islands: St. Peter and St. Paul Archipelago (SP), Rocas Atoll (RA), Fernando de Noronha Archipelago (FN), and Trindade Island (TR). Using annual photoquadrat surveys since 2013, we quantified benthic cover and integrated these data with satellite-derived and modelled oceanographic variables to characterize environmental context. Morphotype richness varied markedly among islands (23-62 taxa). Across all systems, benthic assemblages were dominated by epilithic algal matrix (EAM; 15.6-35.0%) and macroalgae (15.4-23.0%), with lower contributions from coralline algae (CCA + ACA; 10.2-21.8%). Despite these similarities, community composition diverged substantially among islands. Trindade exhibited the most distinct configuration, characterized by higher macroalgal and abiotic cover. FN and RA showed similar benthic profiles, both characterized by high EAM, macroalgal, and coralline algal cover, although Rocas also had a greater contribution of abiotic substrate. This similarity is consistent with their geographic proximity and shared environmental conditions, whereas SP was uniquely structured by suspension feeders. Temporal trajectories were island specific. Between 2013 and 2019, macroalgae increased significantly in SP. In Noronha, scleractinian corals increased and EAM declined significantly, alongside episodic cyanobacterial blooms. At Trindade, directional but non-significant trends were observed, including decreasing EAM and increasing CCA. In RA, no statistically significant temporal trends were detected, although EAM tended to increase and macroalgae tended to decline. These contrasting dynamics reveal asynchronous ecological responses among isolated oceanic reefs and highlight the importance of long-term monitoring to distinguish natural variability from directional change under ongoing environmental forcing.