Benthic communities and foraminifera at Lizard Island (Great Barrier Reef, Australia): what can we learn about ecosystem health status?

The Great Barrier Reef, the world’s largest coral reef system, has been extensively studied at Lizard Island, a classical site for investigating reef ecosystems located 240 km north of Cairns (NE Australia). However, relatively few studies have focused on foraminifera, protists with biomineralized tests. This study, conducted during August and September 2022 assessed coral reef ecosystem health. We simultaneously examined foraminifera and benthic communities (e.g., corals, calcareous coralline algae, bacterial mats), as well as nutrient and seawater physiochemical parameters. We applied the Amphistegina Bleaching Index (ABI) to gauge photo-inhibitory stress during an Autumn/Winter season with average seawater temperatures around 25°C. The ABI revealed stress that was either chronic and mild, or recent and moderate, indicated by Photosynthetic Active Radiation (PAR) at the surface reaching 2268 μmol m-2 s-1, approximately 1000 units higher than the averages measured in the Maldives during the same period in 2018. The Foraminifera in Reef Assessment and Monitoring Index (FoRAM-Index or FI), shows that the Lizard Island reefs are overall healthy and growing. A mean FI value of 7 and a median value of 6.89 at North Point, together with a mean FI value of 7 and a median value of 7.19 at Watson Bay, indicate that these reef ecosystems support calcification and growth. Data collected indicate that reefs at Lizard Island remained healthy without significant temperature-induced bleaching during 2022. We postulate that a thin ozone layer may still be present to explain the chronic mild and/or recent moderate stress. A further sampling campaign in February 2024 (summertime) will provide essential data, allowing for comparison with the same environment under substantially higher seawater temperatures and stressors. We warmly thank Anne Hogget and Lyle Vail of the Australian Museum’s Lizard Island Research Station for facility assistance and Swiss National Science Foundation Grant 200020_201106 for funding this research.