FORAMINIFERA AS BIOINDICATORS: DID MAY 2024 SEA-SURFACE TEMPERATURES TRIGGER BLEACHING IN AMPHISTEGINA GIBBOSA IN THE FLORIDA REEF TRACT?

On 3 June 2024, sampling of diatom-bearing foraminifera, Amphistegina gibbosa, at Tennessee Reef (~6 m depth) in the Florida Keys, USA, revealed relatively few juveniles and symbiont loss in >60% of adult specimens. This site has been sampled quarterly from 1993–99 and intermittently since; such results had not been seen since 1993–96 when stratospheric ozone levels were at their lowest. Amphistegina share characteristics with reef-building corals: they are prolific producers of calcium carbonate, they are physiologically dependent upon algal endosymbionts, and both groups have suffered bleaching episodes in recent decades. Coral bleaching was first reported on Florida reefs in the 1970s, with notable occurrences in 1983 and 1987, and increasing since, including a peak bleaching in 2023. Since 1991, bleaching has been recorded in populations of Amphistegina in all subtropical oceans and these populations also exhibited notable occurrences of shell breakage, shell deformities, predation, and microbial infestation. Asexual reproduction is profoundly affected; partly bleached adults typically produce fewer viable offspring than normal appearing adults. Bleaching in both corals and Amphistegina is induced by photo-oxidative stress. A key difference is that mass coral bleaching requires photic stress but correlates most consistently with elevated temperatures; color loss becomes evident when corals lose more than half their algal symbionts. Symbiont-loss in Amphistegina previously has been induced by solar radiation, can visibly occur in days, and they are particularly sensitive to the shorter (300–490 nm) wavelengths (blue to UVB). Such radiation has increased relative to longer visible wavelengths (>490–700 nm) over the past 50 years as a consequence of stratospheric ozone depletion. Bleaching in Amphistegina was discovered in the early 1990s following the eruption of Mt. Pinatubo, which resulted in about a 4% reduction in stratospheric ozone at low latitudes. The lowest concentrations occurred in the mid-1990s, with minimal recovery and interannual variability in the 2000s, and no additional ozone depletion in 2023–24. Did anomalously high sea-surface temperatures in late May 2024 produce sufficient photo-oxidative stress to cause symbiont loss and reduced reproductive success in A. gibbosa populations at Tennessee Reef?