Oldest Recorded Rhodoliths Discovered in Flower Garden Banks National Marine Sanctuary

April 23, 2024

ROV arm collectiong marine rubble — Rhodoliths at a depth of 338 feet (103 meters) were collected by the remotely operated vehicle *Mohawk* from a tilefish pile at Elvers Bank in September 2022. Photo: NOAA/UNCW

Oldest recorded what? Rhodoliths.They're colorful calcareous nodules of coralline red algae that form around marine rubble (e.g., shells, corals, and rocks), and they are important for structure and three-dimensional habitat in marine environments. Other organisms like corals, clams, scallops, and algae attach to rhodoliths, and they provide refuge for the early growth stages of commercially important fishes.

Sarah Olmstead, a master's thesis student at The University of Alabama has discovered what appear to be the oldest documented rhodoliths to date, and they were found at East Flower Garden Bank and Elvers Bank at mesophotic depths (about 130 to 490 feet) in Flower Garden Banks National Marine Sanctuary, located in the northwestern Gulf of Mexico. The oldest rhodoliths found in this study were from Elvers Bank, and were found to have nucleation points that formed roughly 9,500 to 14,800 years before present. That means some of these ancient rhodoliths date back to the end of the Pleistocene, living through the Holocene deglaciation period and sea-level stabilization.

close up view of a rhodolith — A large, fused rhodolith collected from Elvers Bank at 338 feet (103 meters). Photo: Sarah Olmstead/University of Alabama

In addition to the old age of these rhodoliths, the study also shows that they grew slowly and took so long to get to their current sizes, that any major ecological damage to the area would make it impossible for them to recover within human timescales. Olmstead's findings drive home the importance of protecting such habitats from possible human harm. While these rhodoliths are found within protected parts of the national marine sanctuary, there are still many rhodolith beds that exist outside of current sanctuary boundaries.