Lac Rose: the alarming disappearance of one of the strangest lakes in the world

Lake Retba, better known as Pink Lake (Pink Lake) , is located approximately 35 kilometers from the city of Dakar, Senegal. It lies in a depression with a coastline 6.5 meters below sea level.

The lake is isolated from the sea by approximately 1 km of sand dunes . Its fresh water comes from the seasonal water table in the dunes, higher than the lake. The sea therefore provides most of the lake’s water and all its salt.

The waters of the lake They are virtually devoid of life, except for some microscopic algae and bacteria.

Lac Rose is one of the main tourist destinations in the Dakar region, mainly because of the pink color of its waters. But it is losing its appeal for several reasons. Besides tourism, artisanal salt extraction is one of the dominant activities around the lake. Between 1,500 and 3,000 people participate in this activity, with annual production estimated in 2010 at 140,000 tonnes, which are sold throughout the country and in certain neighboring countries.

I have studied this lake since 1995, initially for me doctoral thesis . Since then, hand followed the lake closely through numerous research projects produced by my team and myself. I was also the principal investigator of “The Pink Lake Studies”, a component of a mining sector support program (Application program in the mini sector). The recommendations that I made at the end of this project that I led from 2008 to 2010 were not implemented.

In this article I explain why the waters of this lake are pink and the current challenges it faces, which have led to its decline.

The pink color is due to proliferation of halophilic green algae (which live in a salty environment) , Dunaliella saline, which contains red pigments. The alga is associated with halophilic bacteria of the genus Halobacteria. The resistance of this microscopic algae to salt comes from its high concentration of carotenoid pigments, which protect it from light, and its high glycerol content.

Actually, Dunaliella saline It contains at least four antioxidant pigments (beta-carotene, astaxanthin, lutein and zeaxanthin), rich in vitamins and trace elements. When salinity is high, algae with red pigments thrive, and when salinity is low, they give way to other algae rich in green pigments.

Today, the accumulation of salt poses various challenges for the lake and affects both its southern and northern shores.

On the south coast, the Drainage gutter created in August 2022 for the evacuation of waste and rainwater from the surrounding suburbs allowed:

• The introduction of solid particles which contribute to the filling of the lake
• Dilution of water, with low salinity which prevents the collection of salt and is incompatible with the development of green algae Dunaliella saline . (The high salinity of the water conditions the development of this algae.)

The north shore is the most sensitive part of the lake for several reasons:

• Motorized traffic poses a threat to the stability of the dunes anchored by a group of old casuarina trees. The destruction of these trees would cause the depression to be invaded by quicksand.
• water supply to the lakes by the dunes: these aquifers (fresh water and salt water) being very shallow, their quality is susceptible to rapid urbanization in this northern part.

Ponds are scattered throughout this area, located in the northern part of the old canal that once connected the lake to the sea. Now the dunes fill the northern part of the canal. The canal, like the ponds, is not the work of man but of natural origin. The northern part of the ponds was later filled with sand from the dunes, while the southern part remained a pond.

Since 2004, we have been monitoring the evolution of specific physicochemical and biological factors in these ponds.

We have just received the results of chemical analyzes of samples taken at the end of May 2023 in these basins. They reveal extremely high levels of nitrates (chemical compounds) in May 2023: 12,491.71 to 15,394.75 mg/l compared to 5.82 to 12.05 mg/l in December 2022. He nitrate limit set by the World Health Organization (WHO) is 50 mg/l. so that the water is considered drinkable.

This sudden increase in nitrate levels would be associated with the establishment of hotels near the lake and ponds, as well as the rapid and uncontrolled urbanization of this northern part of the lake, which lacks a sewage system. These polluted waters flow into the lake and eventually reach the salt, making it unfit for consumption.

It is therefore necessary to periodically check the quality of the salt when salt extraction operations resume.

The supply of salt to the lake through the dunes is a permanent phenomenon and, according to the results of our studies, in 2008-2009 it amounted to around 360,000 tonnes per year. This salt deposit exceeded the estimated exploitation rate at the time by around 100,000 tonnes per year.

Contrary to popular beliefs, Salt mining has never harmed the survival of the lake, but rather prevents it from filling up. The shell beds that surround the lake and the sands of the dunes are also fraudulently exploited as construction materials. Imagine what would happen if the amount of salt extracted over the last ten years was dumped into the lake: it would turn into a towering mound of salt. If the salt is so contaminated that it is unfit for consumption, its extraction will be prohibited and the salt deposit will fill the depression.

In this way, the sand from the elevation channel and the accumulation of salt unfit for consumption will fill the depression within a few years and the Pink Lake will disappear completely.

To end the threats to the lake and preserve it, the authorities should:

• Prohibit the extraction of shell pits, which act as paving stones and protect soils from erosion.
• Ban the exploitation of sand from the dunes, which acts as a barrier between the sea and the lake.
• Control agricultural and market gardening activities, avoiding their establishment on the immediate slopes of water points, the construction of tourist facilities and the exploitation of salt to avoid exceeding the maximum limit.
• Rehabilitate the filao belt which stabilizes the dunes
• Prohibit urbanization and tourist development on the dunes that separate the lake from the sea.
• Implement integrated management benefiting both the wildlife for which the lake serves as a “resting place” and the local population, as well as environmental education and awareness.

* El Hadji Sow Teacher-researcher, Cheikh Anta Diop University of Dakar