Rocky reefs of St Helena and the tropical Atlantic: how the lack of coral and an isolated oceanic location drive unique inshore marine ecology

Abstract

This paper presents the first quantitative assessment of the rocky reef ecology of St Helena, a remote island in the central tropical Atlantic. Monitoring data were used to characterise different habitat types found around St Helena. These findings were compared with 9 other locations in the tropical Atlantic, in different biogeographic, oceanic and reef settings, along with the environmental variables known to limit coral reef formation. St Helena’s rocky and boulder reefs had ~50% cover dominated by turf and other filamentous algae, with lower levels of sessile invertebrates (15%) and macroalgae (4%). Both coral and rocky reef comparison locations also showed a dominance of turf and filamentous algae, with higher levels of macroalgae and sessile invertebrates in areas with higher nutrient concentrations (e.g. south-east continental Brazil). Coral growth in St Helena appeared to be limited by cool average sea temperatures of 22°C, which is near, but not below, accepted lower thresholds for reef formation. The main trophic groups of fish found on rocky reefs in St Helena were comparable to other Atlantic rocky and coral reefs, with a dominance of planktivores, mobile invertivores and roving herbivores, with the major difference in trophic structure being driven by more planktivores in oceanic vs. continental versus locations. St Helena’s narrow rocky coastal strip varied little in terms of reef geomorphology, resulting in high homogeneity around the island. However, endemic fish were numerous, demonstrating that isolation has produced a unique tropical Atlantic marine assemblage.