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Ancient coastal forests in Mozambique’s Bazaruto Archipelago have become underwater peat reefs: hard, eroded platforms that now shelter fish, invertebrates, seagrass and seaweeds. They are not coral reefs, but each surveyed formation supported 26–36 small fish species, a mix comparable to nearby coral-reef sites.
The findings, published in Scientific Reports on 12 October 2026 and described by the Bazaruto Center for Scientific Studies (BCSS) the following day, trace peat deposits dated to roughly 300–5,000 years before present (BP). The oldest deposits are linked to mid-Holocene sea-level rise that submerged forest and peat-swamp landscapes. Later erosion and wave action exposed the buried material as solid seabed in otherwise highly mobile, sandy coastal waters.
How did a forest become a reef?
The formations began on land. Tropical coastal forests and peat swamps accumulated partially decomposed plant matter. As sea level rose during the mid-Holocene, low-lying parts of the landscape were drowned and covered by shallow marine sediments. Radiocarbon dates from sampled peat span approximately 300 to 5,000 years BP, showing that the deposits formed and were submerged at different times rather than in one event.
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Modern waves, tides and erosion have stripped away some of that cover. Where peat is exposed, it forms ledges, cracks, hollows and uneven platforms. Those surfaces function like a reef foundation: organisms can attach to them, and small animals can hide in their crevices. The material is terrestrial peat, not limestone produced by reef-building corals.
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- Forest and peat accumulation: vegetation grew on the former coastal plain and built organic-rich peat.
- Sea-level rise: rising water submerged portions of the forest and peat-swamp terrain.
- Burial and preservation: submerged deposits remained beneath or within shallow seabed sediments.
- Erosion and exposure: waves, tides and shifting sand revealed harder peat surfaces.
- Marine colonisation: algae, seagrass, sponges, oysters, barnacles, lobsters and fish occupied the newly available structure.
Are Mozambique’s peat reefs coral reefs?
No living coral was documented on the peat reefs. Their ecological role comes from physical structure rather than coral growth or carbonate construction. In the Bazaruto setting, where much of the natural seabed is shifting sand, an exposed peat platform can provide rare stable attachment and refuge habitat.
| Feature | Peat reef | Nearby coral reef |
|---|---|---|
| Substrate origin | Ancient terrestrial forest and peat deposits | Carbonate framework associated with reef-building corals |
| Living reef-building coral | None documented on the peat formations | Present as part of the coral-reef ecosystem |
| Typical setting in the survey | About 2–4 metres deep and regularly exposed by tides | Coral-reef habitat; exact comparison depth was not stated |
| Physical complexity | Peat ledges, cracks, hollows and uneven eroded surfaces | Coral branches, bommies, crevices and other carbonate structures |
| Fish and megafauna | 26–36 small fish species per surveyed peat reef; sharks, rays and turtles recorded in broader surveys | Fish community overlaps with peat sites |
| Attached invertebrates | Sponges, oysters, barnacles and spiny lobsters; composition differs strongly from coral reefs | Distinct coral-reef invertebrate assemblage |
| Carbon accounting | High organic-carbon content in samples, but system-wide stock and fluxes are unresolved | Not directly comparable to peat carbon measurements |
What animals and plants live on the peat?
Seagrass and seaweeds grow directly on exposed peat. Sponges, oysters and barnacles attach to the surface, while spiny lobsters use cracks and ledges. Juvenile snappers and grunts shelter among the irregular surfaces, where they can avoid predators and find feeding opportunities.
Each surveyed peat reef contained 26–36 small fish species. BCSS reports 41 small-fish and megafauna species shared between peat and coral-reef habitats. Sharks, rays and turtles appeared in the wider surveys around the archipelago. This overlap means the peat can provide nursery and refuge functions, not that it reproduces the full biological composition of a coral reef.
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How does biodiversity compare with a coral reef?
The strongest comparison is functional and community-based. Small-fish diversity on individual peat reefs was comparable to nearby coral-reef sites, and many fish and larger-animal species moved between the habitat types. Attached invertebrate communities, however, differed substantially, reflecting the different substrates and physical conditions.
A peat reef therefore adds habitat to the local seascape rather than replacing coral habitat. Its value is especially clear along a coastline dominated by mobile sand, where even a relatively small stable platform can offer shelter and attachment surfaces.
Do the drowned forests store blue carbon?
The peat contains a substantial amount of carbon-rich terrestrial material. Its carbon-isotope values, around δ13C −27‰, support a predominantly land-plant origin, and samples contained up to 40% organic carbon by dry weight. That percentage describes the tested material; it is not a measurement of the carbon stock of an entire reef or archipelago.
The study does not establish how much carbon the reefs contain overall, how quickly carbon is buried or eroded, whether it remains stored for the long term, or what greenhouse-gas balance the system has. Calling the formations a confirmed blue-carbon sink would go beyond the evidence. BCSS identifies several measurements still needed:
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- peat thickness and the area covered by each formation;
- bulk density needed to convert carbon percentage into areal stock;
- burial, exposure and erosion rates;
- carbon exchange and greenhouse-gas fluxes; and
- the permanence of carbon under storms, tides and changing sediment conditions.
Until those data exist, the most accurate description is carbon-rich ancient terrestrial peat that may have blue-carbon significance, not a quantified climate-mitigation resource.
Why are the reefs productive but vulnerable?
The reefs lie in shallow water, roughly 2–4 metres deep, and are regularly exposed by tides. Daily temperatures in the BCSS observations ranged from approximately 24.5 to 28.6°C. Shallow exposure can increase access to light for seagrass and algae, but it also subjects the peat to strong physical disturbance.
- Storms and cyclones can bury, uncover, fracture or remove sections.
- Wave-driven erosion can enlarge cracks or wear away entire platforms.
- Shifting sand can smother a formation or expose a different section of peat.
- Tidal exposure and heat can repeatedly stress organisms living on the surface.
As a result, an individual formation may change shape, disappear beneath sand or be lost to erosion even while the broader archipelago continues to contain other peat outcrops. The same instability that creates new hard surfaces can also destroy established habitat.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What do the BCSS measurements cover?
BCSS describes its observatory, established in 2017, as Africa’s first permanent multi-ecosystem ocean observatory. Its program spans coral reefs, seagrass, mangroves, peat, lagoons, seamounts and open ocean. The centre reports more than 22 million measurements across more than 30 variables from 14 monitoring stations covering approximately 6,850 km².
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Those figures describe the observatory’s regional monitoring program, not the size of the peat reefs. The peat-reef results are a habitat study within that wider observation effort.
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Could similar underwater peat reefs exist elsewhere?
They could plausibly occur wherever low-lying peat-forming forests were submerged and later exposed by erosion, but the Mozambique study does not establish a global distribution. Finding comparable habitats would require geological and ecological surveys that combine seabed mapping, peat dating, substrate sampling and biological monitoring.
The Bazaruto discovery shows why apparently ordinary shallow seabed can contain a much older coastal record. A formation that looks like a small rocky patch may instead be preserved land-based peat now functioning as marine habitat.
What the discovery changes about coastal habitat?
The study adds a third category to the usual coral-versus-sand view of tropical shallow seas. Ancient terrestrial deposits can become modern nursery habitat without ever becoming coral reefs. Protecting that habitat will require accounting for its shifting boundaries and physical fragility, while carbon claims must wait for measurements of thickness, density, stocks and fluxes.
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