Barombi Mbo

Barombi Mbo is a near-perfect circle of deep water in the rainforest of southwest Cameroon — a volcanic crater lake barely a mile and a half across, around 1.6 square miles (about 3 mi²) of surface, yet plunging to roughly 360 feet (361 ft). It is wholly Cameroonian, with no second country to share it, and it is small enough to walk around in an afternoon. What makes it one of the most famous bodies of water in evolutionary biology is what lives in it: eleven cichlid fishes found nowhere else on Earth, all of them descended from a single ancestor that colonized the crater on the order of a million years ago, and all of them diverged in place, in the same small basin, without ever being separated by a barrier. Barombi Mbo is the textbook case of sympatric speciation — a whole flock of species born in a pond. The lake is also permanently stratified, its deep water locked away below an oxygen-starved boundary, and every one of its eleven endemic cichlids is now listed as Critically Endangered.

Setting & origin

Barombi Mbo sits at about 990 feet (988 ft) above sea level near the town of Kumba in Cameroon's Southwest Region, roughly 40 miles (about 37 mi) north-northeast of the active volcano Mount Cameroon. It is a maar — an explosion crater blasted out by a violent steam-driven eruption — and it belongs to the Cameroon Volcanic Line, the chain of volcanic centers that runs from the Atlantic island of Bioko inland across Cameroon. Radiocarbon and sediment work on long cores from the crater floor (Giresse, Maley and colleagues) places the lake's age at roughly one million years, making it the largest and one of the oldest crater lakes in West and Central Africa. The basin is small and steep-walled: surveys put the surface at about 415 hectares (3 mi², ~1.6 sq mi), the maximum depth near 360 feet (361 ft), and the mean depth around 225 feet (about 226 ft), the latter a sign of a basin that drops away sharply from the shore to a broad, flat floor.

Unlike the great Rift lakes, Barombi Mbo is a one-country lake. It lies entirely within Cameroon and is fed by a handful of small inflowing streams, with a single modest outlet that keeps the water level remarkably stable. The crater is ringed by tropical rainforest and a tuff rim, and the whole site has carried protected status since a 1940 forest-reserve order; in 2006 Cameroon designated it a Ramsar wetland of international importance. The catchment receives heavy equatorial rainfall — on the order of 120 to 160 inches (3,000–4,0.5 in) a year — delivered by the southwest monsoon between roughly March and November, with a short dry season from December to February.

Temperature, oxygen & mixing

The physics of Barombi Mbo is the key to everything that lives in it. It is a strongly stratified, effectively meromictic lake: warm, light surface water floats over cold, dense deep water year-round, and the two almost never fully mix. In the detailed limnological survey of 39 Cameroon lakes by Kling (1988), Barombi Mbo had the highest seasonal water-column stability of any lake measured (about 5,784 J m⁻²), a direct consequence of its depth, clarity and sheltered crater walls. A nine-month thermal record showed the early-morning surface temperature sliding gently from about 84 °F (84 °F) in February down to about 80 °F (80 °F) in September, tracking the annual cycle of air temperature and sunlight; an April reading reached 85 °F (85 °F), and the surface can swing by as much as 1.8 °F (34 °F) over a single day. Below the warm surface layer the water cools to roughly the mid-70s °F (about 75 °F) in the deep, a surface-to-bottom contrast of only a few degrees — small in absolute terms, but in the tropics that is enough, because warm water is just barely lighter than cold, to keep the deep water sealed off.

The oxygen consequence is severe and is the central ecological fact of the lake. Oxygen is confined to the warm upper layer; below a sharp oxycline the deep water is anoxic. The classic study by Green, Corbet and Betney (1973) found the lake strongly stratified in April 1972 with a metalimnion around 60 feet (59 ft) and no measurable oxygen below about 62 feet (62 ft); Kling later measured oxygen reaching down to roughly 130 feet (131 ft) in February 1985, the boundary shifting with season and year. Either way, the great bulk of the lake's water column — everything below roughly 20 to 40 meters — is a dark, oxygen-free monimolimnion in which fish cannot live. The habitable lake is the thin, sunlit, oxygenated shell over the top. One endemic cichlid, the deepwater Konia dikume, is adapted to forage at the very edge of that anoxic zone: it carries an exceptionally high blood-haemoglobin concentration of about 16.55 g per 100 ml, far above the roughly 8–10 g of its relatives, letting it work in water too oxygen-poor for the others.

Water chemistry & clarity

Chemically, Barombi Mbo is a soft, dilute, near-neutral freshwater lake — very different from the alkaline Rift lakes. Field measurements put surface pH in the neutral-to-slightly-basic range, generally about 7.0 to 8.1 (Tabot et al. 2016), and conductivity is low, characteristic of fresh water: Kling measured a surface value near 49 µS/cm rising to about 80 µS/cm in the deep, anoxic water, where dissolved ions accumulate from decomposition. Salinity is negligible, around 0.02–0.03 psu. The modest bicarbonate, calcium and magnesium content the lake does carry is supplied largely by the inflowing streams draining the carbonate-bearing soils of the surrounding catchment rather than by the volcanic crater itself.

The lake's other signature is its clarity. Barombi Mbo is one of the most transparent lakes in Cameroon: Kling recorded a Secchi depth of 36 ft, with the 1-percent-light euphotic zone reaching nearly 98 ft. That deep light penetration is itself part of why the lake stratifies so strongly — clear water lets the sun warm a thick surface layer, which stiffens the density gradient and resists mixing. The flip side is that the deep, light-starved water below the oxycline accumulates dissolved gases and ions over time. That chemistry is not just academic here: Barombi Mbo sits in the same volcanic province as Lakes Nyos and Monoun, the two Cameroonian crater lakes that killed nearly 1,800 people in the 1980s when carbon dioxide stored in their deep water burst to the surface in sudden 'limnic eruptions.' Local fishermen have reported episodes at Barombi Mbo — in 2007 and 2009 — of cloudy water and dead fish, including deepwater Konia dikume, consistent with gas release or an upwelling of anoxic bottom water, though these accounts remain unconfirmed.

Habitats

For so small a lake, Barombi Mbo offers a surprising range of inshore habitat, and because life is confined to the oxygenated upper layer, that shallow margin is where everything happens. The shoreline grades from rocky shelves and submerged stones near the crater walls, through coarse debris and leaf litter washed in from the surrounding forest, to sandy and silty bottoms; the open basin drops steeply to the broad, flat, lifeless deep floor. The endemic cichlids partition this space: some patrol open water near the surface, some forage over sand and detritus, some stay close to rock and submerged wood, and at least one — Konia dikume — ranges deeper than the rest, down toward the edge of the anoxic zone. The lake is also home to its own endemic invertebrates, including a freshwater sponge (Corvospongilla) and a shrimp (Caridina), the sponge being food for one specialist cichlid.

The forested crater and the small village on its rim shape the habitat as much as the geology does. Streams entering the lake carry leaf litter and organic debris (and, increasingly, farm runoff and village waste), and dissolved tannins from rainforest leaf fall give the inshore water its tea-stained tint — which is why aquarists who recreate a Barombi Mbo biotope reach for sand, leaf litter, driftwood and dim, slightly acidic water rather than the bright rockscapes of a Malawi tank.

The cichlids

Barombi Mbo's fame rests on eleven cichlid species, every one of them endemic — found in this single crater and nowhere else on the planet. They fall into five genera, four of which exist only here: Sarotherodon (four species: S. linnellii, S. lohbergeri, S. caroli and S. steinbachi), Stomatepia (three species: S. mariae, S. pindu and the famously rare S. mongo), Konia (two species: K. eisentrauti and the deepwater K. dikume), Pungu (one species, P. maclareni), and Myaka (one species, M. myaka). Trewavas, Green and Corbet described and revised most of this flock in their 1972 monograph on the lake's fishes.

What lifted these eleven fish from a regional curiosity to a landmark of evolutionary biology was the genetic work of Schliewen, Tautz and Pääbo, published in Nature in 1994. Their mitochondrial-DNA analysis showed that the entire flock is monophyletic — a single branch of the tree of life, all descended from one ancestral colonist closely related to the riverine Sarotherodon galilaeus of the surrounding region. Because Barombi Mbo is tiny, deep and ecologically uniform, with no plausible way to split a fish population into isolated sub-basins, the only reasonable explanation is that all eleven species arose in place, in the same water, without geographic separation. That is sympatric speciation, and Barombi Mbo became its textbook example — the clearest natural case that new species can bud off from one another while living side by side. Later work (Schliewen & Klee 2004) added a twist: the sponge-eating Pungu maclareni appears to be a hybrid species, the product of two of the lake's own lineages crossing — sympatric speciation by hybridization within the flock itself.

The diet specializations are extraordinary for so confined a radiation. Pungu maclareni is a spongivore: a quantitative study of stomach contents (Martin lab, 2021) found its gut roughly 20 percent freshwater sponge, a feeding strategy used by only about 0.04 percent of all fish species. Stomatepia mariae is a fish predator, Stomatepia pindu takes insect larvae, Myaka myaka is a slender open-water plankton feeder, and Konia dikume works the deep, oxygen-poor water for the phantom-midge larvae (Chaoborus) that migrate up from the depths. Curiously, when researchers measured dietary overlap directly, they found less neat niche partitioning among the littoral species than the textbook story implies — many of them eat broadly similar things — which has kept Barombi Mbo at the center of debate over exactly how disruptive selection drove the split.

People & pressures

A village sits on the rim of the crater, and the lake is the heart of local life: a fishery worked by the Barombi villagers, a transport route, a Ramsar-protected nature site, and the main drinking-water source for the rapidly growing town of Kumba and roughly half a million people in its environs. That intimacy is also the threat. Because Barombi Mbo is a single small, closed basin, it is acutely vulnerable to what happens on its shores. Deforestation and farming — cocoa plots reach close to the water — strip the slopes and feed sediment and nutrients into the lake; village runoff and sewage have driven measurable eutrophication, with macroalgae blooms near the inflows and faecal-bacterial contamination (including Salmonella) documented in the streams and shallows (Tabot et al. 2016). Overlaid on this are the same volcanic hazards as the wider Cameroon Volcanic Line: the slow buildup of dissolved gas in the deep water and the small but real risk of a limnic eruption like those at nearby Nyos and Monoun.

For the cichlids, the consequences are stark. Every one of the eleven endemic species is now listed by the IUCN as Critically Endangered, and the whole flock is the focus of CARES conservation-priority listing and European zoo and aquarium breeding programmes (EAZA) precisely because the lake that made them could also unmake them. Stomatepia mongo, the rarest of all, has been repeatedly feared extinct since the 1980s; recent fieldwork confirms it survives, but only just, and only through careful surveying (Musilová et al. 2014). With no second lake and no other range, the eleven fishes of Barombi Mbo have nowhere to retreat to — which is exactly what makes this small crater both an evolutionary marvel and a conservation emergency.

Sources

Last reviewed 2026-06-06.

How to cite

Aquarist Atlas (2026). Barombi Mbo. Aquarist Atlas. https://www.aquaristatlas.com/water/barombi-mbo/

Setting & origin

Temperature, oxygen & mixing

Water chemistry & clarity

Habitats

The cichlids

People & pressures

Sources

Cichlids recorded here

Konia

Myaka

Pungu

Sarotherodon

Stomatepia