Making Sense of the Great Barrier Reef's Mysterious Green Donuts

Abstract

On the outer shelf of Australia’s Great Barrier Reef (GBR), under 20–50 meters of water, lies a broad expanse of giant green “donuts.” These seafloor circles, each several hundred meters across, aren’t the result of a rogue offshore baking experiment. Rather, they consist of the remnants of countless generations of green calcareous algae from the Halimeda genus. The green color comes from the current generation of Halimeda living atop these bioherms, as scientists call this type of mounded deposit with a sunken center.The actively accumulating Halimeda bioherms on the northern GBR shelf cover more than 6,000 square kilometers and are the most extensive of such deposits in the world [Whiteway et al., 2013; McNeil et al., 2016]. These globally significant bioherms have complex morphologies that are not yet explained, and compared with the adjacent coral reef systems, little is known about the fundamental processes that control their distribution anddevelopment. Much also remains unknown about the biogeochemical cycling associated with the bioherms, their role as key habitats for benthic (bottom-dwelling) species between the coral reefs and the Australian coast, and how they may be affected by climate change. In August and September 2022, we were part of a multidisciplinary team of scientists that set out on a research voyage on Australia’s R/V Investigator to better understand these enigmatic structures by mapping and sampling them in breathtaking detail. The mission of Project HALO (Halimeda Bioherm Origins, Function and Fate) was to illuminate how the bioherms formed over the past 12,000 or so years (i.e., the Holocene) and their importance in biogeochemical nutrient cycling and as modern habitats amid one of Earth’s most critical but vulnerable biodiversity hot spots