The story of these reefs begins thousands of years ago on dry land. The peat that forms them consists of compressed, carbon-rich remains of coastal swamps and forests that once flourished along the Mozambican shoreline. As sea levels rose over the past few thousand years, a process that continues today, the ocean flooded these landscapes and buried them beneath sand. Waves later stripped away the overlying sediment, leaving firm peat platforms exposed on the seabed. Once exposed, the substrate was rapidly colonized by marine life, and the resulting communities now support biodiversity comparable to that of neighbouring coral reefs.

Radiocarbon dating reveals that the peat formed between roughly 300 and 5,000 years ago, meaning these deposits span a remarkable window of coastal history. Chemical analysis confirms that the material originated mostly from coastal land plants and trees, preserving the signature of a long-vanished terrestrial world. Some peat samples contain up to 40 percent organic carbon by dry weight, a figure that immediately raises questions about the role these formations might play in regional and global carbon cycling.

That carbon question is now a direct scientific consequence of the discovery. On land, peat swamps can hold between 1,000 and 2,000 tonnes of carbon per hectare below ground, and the submerged deposits off Bazaruto have been sequestered underwater for thousands of years. Yet no one yet knows how much carbon the reefs hold, or whether they are storing or releasing it in the present day. The paper itself is explicit on this point: the peat reefs’ current role in carbon cycling remains unresolved and requires dedicated measurements of peat thickness, bulk density, areal carbon stocks, burial and erosion rates, and greenhouse-gas fluxes before any definitive assessment can be made.

Scientists have spent decades working out what warming and disturbance mean for ancient carbon locked away in tundra, peatlands and mangroves. Tropical peat reefs sit at a far earlier stage of understanding, but they now pose a strikingly similar question beneath the sea: what is stored there, how stable is it, and what happens if these newly recognized ecosystems are lost, eroded or disturbed? The authors call for research to establish whether these systems primarily function as long-term legacy carbon stores, whether living communities contribute to new carbon burial, and how these processes interact as the environment changes. Only then, they argue, can scientists explore whether peat reefs should be classified as blue-carbon habitats alongside mangroves and seagrass meadows.

To understand the ecology of these formations, the team surveyed three peat reefs and a nearby true coral reef, comparing seabed cover, invertebrate communities, small fish and larger animals such as sharks, rays and turtles. The contrast is striking. The peat reefs host no living coral at all, yet they teem with life. Instead, seagrass and seaweeds grow directly on the peat surface, while crevices and ledges shelter spiny lobsters, oysters, barnacles and juvenile fish, including young snappers and grunts usually associated with coral reefs, likely providing refuge from predators.

Despite looking nothing like coral reefs, the peat formations share much of their fish wildlife and biodiversity, while the invertebrate animals fixed to the seabed are completely different. Dozens of small fish species as well as larger species like sharks, rays and turtles were recorded on both reef types. Each peat reef held between 26 and 36 small fish species, with a diversity mix comparable to the coral reef. Herbivorous megafauna such as green turtles and dugongs, both found in the area, could also graze the seagrass patches that carpet the ancient surfaces.

Life on these reefs is far from easy. Sitting just 2 to 4 metres deep and regularly exposed by the tides, the peat platforms swing between 24.5 and 28.6 degrees Celsius within a single day and receive far more sunlight than the deeper coral reef nearby. Storms and cyclones regularly bury, uncover and break them apart, and many of the formations actually disappear due to erosion and shifting sand. This harshness makes the survival of such rich communities all the more remarkable, and it hints at the resilience of life when a rare hard substrate appears in an otherwise shifting environment.

Dr Mario Lebrato, lead author and co-founder of the BCSS Ocean Observatory, described the transformation in vivid terms. What was once a forest floor is now a nursery for reef fish, he said, adding that the team was surprised by how much life these ancient peat deposits support and how many species move between them and the coral reefs nearby. He noted that this coastline has very little natural hard seabed, consisting mostly of shifting sand, so when rising seas expose these old peat layers, they create rare, solid habitat where marine life can settle, hide and feed.

The discovery carries implications far beyond Mozambique. Coastal planners and conservationists rarely map or protect peat reefs, and similar habitats may lie undiscovered across Southeast Asia, the Caribbean and West Africa. As coral reefs decline under mounting environmental pressure, these overlooked ecosystems could offer backup refuges for some species. The findings indicate that peat reefs should be included in coastal habitat maps, conservation plans and assessments of legacy coastal carbon, and the study offers an important starting point for incorporating these habitats within wider blue economy policies. Lebrato emphasized that the existence of tropical peat reefs underscores the resilience and adaptive reinvention potential of coastal ecosystems under pressure from sea-level rise and shoreline change, and that their contribution to biodiversity, fisheries habitat and potential legacy-carbon preservation should be recognized in coastal management and conservation strategies, particularly in regions where coral reef health is declining. The research was enabled by the BCSS Ocean Observatory, established in the Bazaruto Archipelago in 2017 as Africa’s first permanent ocean observatory, which has generated more than 22 million measurements across over 30 chemical, environmental and biodiversity variables spanning six interconnected ecosystems and 14 long-term monitoring stations across an approximately 6,850 square kilometre monitoring area in the greater Bazaruto seascape, from mangroves, seagrass meadows, peat and coral reefs to lagoons, seamounts and the open ocean.

Subject of Research: Submerged ancient peat reefs formed from drowned coastal forests in the Bazaruto Archipelago, Mozambique, and their biodiversity and carbon storage potential

Article Title: Drowned 5,000 years old forests reborn as thriving peat reefs and biodiversity hotspots off Mozambique

Article References: Drowned 5,000 years old forests reborn as thriving peat reefs and biodiversity hotspots off Mozambique. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: peat reefs, Bazaruto Archipelago, Mozambique, blue carbon, sea-level rise, marine biodiversity, coral reefs, Scientific Reports, coastal ecosystems, carbon storage, ocean observatory, marine conservation