Bazaruto Archipelago, Mozambique: Researchers from the Bazaruto Center for Scientific Studies (BCSS) Ocean Observatory have documented ancient forests that grew on the Mozambican coast thousands of years ago and have been found beneath the sea, where they now serve as hotspots teeming with marine life similar in biodiversity to coral reefs. The discovery raises the question of how many more of these hidden peat reefs exist around the world's coastlines and how to evaluate them as a carbon store and develop policies to protect them and their biodiversity.
The paper in Scientific Reports published today by the BCSS Ocean Observatory, working in collaboration with researchers from UK, US and Canadian universities and research institutions, has documented submerged "peat reefs" in the shallow waters of the Bazaruto Archipelago (Western Indian Ocean). It is believed to be the first ecological study of peat reefs anywhere in the tropics, and globally from a biodiversity angle.
The research follows two papers published in Frontiers in Marine Science that described the capabilities of the BCSS Ocean Observatory, which enabled this research. Established in Mozambique's Bazaruto Archipelago in 2017, BCSS is Africa's first permanent Ocean Observatory and 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 km² monitoring area in the greater Bazaruto seascape – from mangroves, seagrass meadows, peat and coral reefs to lagoons, seamounts and the open ocean.
Thousands of years in the making
The reefs are made of peat: compressed, carbon-rich remains of coastal swamps and forests. As sea levels rose over the past few thousand years, and continue to rise, the sea flooded these landscapes. Waves later stripped away the sand above them, leaving firm peat platforms exposed on the seabed. The substrate was then colonized by marine life, which biodiversity is comparable to neighbouring coral reefs.
Radiocarbon dating (14C) shows the peat formed between roughly 300 and 5,000 years ago. Chemical analysis confirms it mostly came from coastal land plants and trees. Some peat samples contain up to 40% organic carbon by dry weight. However, more work needs to be done to determine if these systems can function as a long-term legacy carbon store, and what is their role in the blue-carbon storage
A potential blue-carbon store
This carbon has been stored and sequestered underwater for thousands of years and on land (blue-carbon), peat swamps can hold 1,000–2,000 tonnes of carbon per hectare below ground, but no one yet knows how much the reefs hold, or whether they are storing or releasing it today. The paper itself says the peat reefs' present-day role in carbon cycling remains unresolved and requires dedicated measurements of peat thickness, bulk density, areal carbon stocks, burial or erosion rates, and greenhouse-gas fluxes.
That makes the carbon question a direct scientific consequence of the discovery. Scientists have spent decades working out what warming and disturbance mean for ancient carbon locked in tundra, peatlands and mangroves. Tropical peat reefs are at a much earlier stage of understanding, but they now raise a similar question beneath the sea: what is stored there, how stable is it, and what happens if these newly recognised ecosystems are lost, eroded or disturbed?
The authors also 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. From there, we can explore whether we can classify peat reefs as "blue carbon" habitats like mangroves or seagrass.
A reef with no coral
The team surveyed three peat reefs and a nearby true coral reef, comparing seabed cover, invertebrates, small fish and larger animals such as sharks, rays and turtles.
The peat reefs host no living coral at all but are still teeming with life. Instead, seagrass and seaweeds grow directly on the peat. The crevices and ledges shelter spiny lobsters, oysters, barnacles and juvenile fish, including young snappers and grunts usually linked to coral reefs, likely providing refuge from predators.
Despite looking nothing like coral reefs, the peat reefs share much of their fish wildlife and biodiversity, while the invertebrate animals fixed to the seabed are completely different. Dozens small fish species as well as larger species like sharks, rays and turtles were recorded on both reef types. Each peat reef held 26 to 36 small fish species, with the diversity mix of species comparable to the coral reef. Herbivorous megafauna such as green turtles and dugongs, both found in the area, could also graze the seagrass patches.
The peat reefs also live a harder life. Sitting just 2–4 metres deep and regularly exposed by the tides, they swing between 24.5 and 28.6°C within a day and receive far more sunlight than the deeper coral reef. Storms and cyclones regularly bury, uncover and break them apart. Many of them actually disappear due to erosion and sand movements.
"What was once a forest floor is now a nursery for reef fish. We were surprised by how much life these ancient peat deposits support, and how many species move between them and the coral reefs nearby. This coastline has very little natural hard seabed, mostly shifting sand. When rising seas expose these old peat layers, they create rare, solid habitat where marine life can settle, hide and feed." said Dr Mario Lebrato, lead author and co-founder of the Bazaruto Center for Scientific Studies (BCSS) Ocean Observatory.
Legacy of coastal carbon
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, these overlooked habitats could offer backup refuges for some species.
These findings indicate that peat reefs should be included in coastal habitat maps, conservation plans and assessments of legacy coastal carbon. This study is an important starting point for considerations on how to incorporate these habitats within wider blue economy policies.
Dr Mario Lebrato continued, "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. Their contribution to biodiversity, fisheries habitat, and potential legacy-carbon preservation should be recognised in coastal management and conservation strategies, particularly in regions where coral reef health is declining. The ability to demonstrate the vitality of these habitats as a result of our continuous observation excites me for the prospect of implementing conservation policy that protects similar regions around the world."
