Peter Alexander Bacon Modys

USGS2025

Discovery of late Holocene-aged Acropora palmata reefs in Dry Tortugas National Park, Florida, USA: The past as a key to the future?

Emblematic of global coral-reef ecosystem decline, the coral ecosystem-engineer Acropora palmata is now rare throughout much of the western Atlantic. Understanding when and where this foundation species occurred during the past can provide information about the environmental limits defining its distribution through space and time. In this paper, the present, historical and newly dated geological records of A. palmata are compared to reveal novel insights into the environmental constraints on its occurrence in Dry Tortugas National Park, a subtropical reef system at the south-western terminus of the Florida reef tract. Although past geological investigation found little evidence of the species in the park, a single, moderately sized A. palmata reef existed throughout historical times (1881 Common Era [CE] to present day; ‘historical population’, termed herein). Over the last 140 years, repeated population declines occurred with little to no recovery, culminating in the extirpation of A. palmata from the area during the 2023–2024 CE global coral bleaching event. Reported here for the first time is a significant record of Late Holocene A. palmata populations that existed from ca 4500 to 375 years before present (‘Late Holocene population,’ termed herein) in three broadly distributed areas of the shallow Dry Tortugas platform. This discovery challenges previous assumptions regarding the species' limited contribution to reef development in the area by providing data that extend the known spatial and stratigraphic extent of Holocene populations in this location. It is posited that, although the Late Holocene climate largely suppressed regional reef development, the new records provide evidence for centennial-scale periods of more favourable and stable climate that allowed for short-term expansions of A. palmata populations in the Dry Tortugas. In conclusion, the species' prospects for future success in this and other subtropical location

Florida↗

USGS2024

Modern coral range expansion off southeast Florida falls short of Late Holocene baseline

As thermal stress and disease outbreaks decimate coral reefs throughout the tropics, there is growing evidence that higher latitude marine environments may provide crucial refuges for many at-risk, temperature-sensitive coral species. However, our understanding of how coral populations expand into new areas and sustain themselves over time is constrained by the limited scope of modern observations. Here, we provide geological insights into coral range expansions by reconstructing the composition of a Late Holocene-aged subfossil coral death assemblage on the southeast Florida reef tract and comparing it to modern reefs throughout the region. Our findings show that the Late Holocene coral assemblages were dominated by now critically endangered Acropora species between ~3500 and 1800 years before present, mirroring classic zonation patterns characteristic of healthy pre-1970s Caribbean reefs. In contrast, the modern reefs off southeast Florida are becoming increasingly dominated by stress-tolerant species like Porites astreoides and Siderastrea siderea despite modest expansions of Acropora cervicornis over the past several decades. Our results suggest that ongoing anthropogenic stressors, not present during the Late Holocene, are likely limiting the ability of modern higher latitude reefs in Florida to function as long-term climate refugia.

Florida↗