Lake Retba, located in Senegal and commonly known as Lac Rose, draws international attention for its vivid pink hue. This body of water transforms from shades of rose to deep scarlet depending on the time of day and season. The lake’s coloration is not a trick of the light or mineral deposits, but the direct result of a specialized biological process. This phenomenon is caused by a convergence of extreme environmental chemistry and a unique microorganism adapted to survive in a harsh aquatic environment.
The Unique Conditions of Lake Retba
The foundation for Lake Retba’s pink color is its extremely high salt concentration, a condition known as hypersalinity. The lake is separated from the Atlantic Ocean by a narrow strip of sand dunes, allowing seawater to seep in. The shallowness of the lake and the high rate of evaporation in the arid West African climate cause the water to concentrate salt far beyond typical ocean levels.
Salt saturation can reach as high as 40 percent in some areas, significantly surpassing the 3.5 percent salinity of the open ocean. This level of salinity makes the water dense enough that people can float effortlessly on its surface. The dissolved salt creates an environment too chemically harsh for most aquatic life, allowing only specialized organisms to survive the intense osmotic pressure.
The Microorganism Responsible for the Hue
The striking pink coloration is directly caused by the massive presence of the single-celled microalga, Dunaliella salina. This organism is classified as a halophile, meaning it thrives in environments with extremely high salt concentrations. Although Dunaliella salina is a type of green algae, it only produces its characteristic pigments when stressed by the lake’s severe conditions.
When exposed to high salinity and intense sunlight, the algae produces high concentrations of red/orange carotenoid pigments, primarily beta-carotene. This organic compound, which is also found in carrots, serves a biological function as a photoprotective agent. Beta-carotene shields the algae’s chlorophyll and cellular machinery from the damaging effects of the strong light and high salt.
The sheer density of these beta-carotene-rich cells suspended in the water visually transforms the lake from blue-green to its signature pink or reddish-orange. The color is most intense during the dry season, from November to June, when evaporation is highest. This increases the concentration of both salt and the protective pigment.
The Economic Importance of the Salt
Lake Retba is a major center for artisanal salt harvesting, providing income for thousands of local workers. The traditional industry relies on workers, predominantly men, wading into the buoyant water to manually break up the crystallized salt crust on the lakebed using long sticks. This salt layer can be several feet thick in some places.
Once freed, the salt is shoveled into baskets and ferried back to the shore in canoes. To protect their skin from the corrosive, hyper-concentrated brine, workers apply a thick layer of shea butter (beurre de Karité) before entering the water. The harvested salt is then piled into white mounds on the shore, where women collect and process it for commercial use.
The annual yield from the lake is substantial, with tens of thousands of tons of salt harvested each year. This salt is a valuable commodity, used locally by Senegalese fishermen for preserving fish. The salt is also a significant export, distributed to neighboring West African countries such as Mali and Benin.