The Amphistegina bleaching index as a tool for evaluating reef-ecosystem health: new data from the Maldives & Australia provide updated insights

Abstract

Abstract The Amphistegina bleaching index (ABI) provides a simple, low-cost method for assessing photo-oxidative stress in reef-dwelling foraminifera. The ABI, developed for Florida reefs, has been applied to assess the extent of such stress in several regions globally, including the Maldives, and now in Australia. Here, we assess the application of the ABI for capturing variability in photo-inhibitory stress across contrasting reef settings and temporal scales. We applied the ABI at four sites in the central Maldivian archipelago, adding a new assessment point to previous surveys conducted in 2015 and 2018, and at two sites at Lizard Island on the northern Great Barrier Reef, comparing austral winter (2022) and summer (2024) conditions. In the Maldives, comparisons with previously published data from the same sites collected in 2015 and 2018 indicate very low levels of bleaching in 2025, with the geometric mean of bleached specimens at 2%, compared to 7% in 2015 and 10% in 2018. Mean abundances of Amphistegina remained broadly consistent, although slight reductions may reflect local environmental differences or residual effects of recent stress events. At Lizard Island, Australia, seasonal variation was pronounced, with austral summer 2024 samples showing elevated bleaching relative to winter 2022. ABI scores reflected these differences in bleaching and population structure across sampling intervals. Combined with environmental measurements, the ABI provides a repeatable approach for assessing photo-oxidative stress and detecting differences between baseline conditions and periods of elevated bleaching risk.