Lake Retba, higher referred to as Lac Rose (the Pink Lake), is situated round 35km from the town of Dakar, Senegal. It sits in a melancholy with a coastline 6.5 metres under sea degree.
The lake is remoted from the ocean by means of about 1km of sand dunes. Its contemporary water comes from the seasonal water desk within the dunes, that are upper than the lake. Thus the ocean supplies many of the lake’s water and all of its salt.
The lake’s waters are just about devoid of lifestyles, except a couple of microscopic algae and micro organism.
The Purple Lake is among the major vacationer locations within the Dakar area, basically on account of the crimson color of its waters. Nevertheless it’s dropping its attraction for a lot of causes. Excluding tourism, artisanal salt mining has been some of the dominant actions across the lake. This job comes to between 1,500 and three,000 other folks, with an estimated annual manufacturing of 140,000 tonnes in 2010, bought all through the rustic and in some neighbouring nations.
I’ve studied this lake since 1995, first of all for my PhD thesis. Since then, I’ve been carefully tracking the lake via a lot of research projects performed by means of my staff and myself. I used to be additionally the lead researcher on “The Purple Lake Research”, an element of a mining sector strengthen programme (Programme d’appui au secteur minier). The suggestions I made on the finish of this undertaking, which I led between 2008 and 2010, weren’t carried out.
On this article, I provide an explanation for why the waters of this lake are crimson and the present demanding situations it faces, that have driven it into decline.
Why does the water flip crimson?
The crimson shade is because of the proliferation of halophilic inexperienced algae (dwelling in a salty surroundings), Dunaliella salina, which include purple pigments. The alga is related to halophilic micro organism of the genus Halobacterium. This microscopic alga’s resistance to salt comes from its top focus of carotenoid pigments, which offer protection to it from mild, and its top glycerol content material.
In truth, Dunaliella salina incorporates no less than 4 antioxidant pigments (beta-carotene, astaxanthin, lutein and zeaxanthin), that are wealthy in nutrients and hint components. When salinity is top, algae with purple pigments thrive, and when salinity is low, they provide approach to different algae wealthy in inexperienced pigments.
Nowadays, salt accumulation poses several challenges for the lake, affecting each its southern and northerly shores.
At the southern shore, the drainage channel created in August 2022 for the disposal of wastewater and rainwater from the encircling suburbs has resulted in:
the creation of cast debris contributing to the lake’s filling in
the dilution of the water, with a low salinity that forestalls salt harvest and is incompatible with the advance of the golf green alga Dunaliella salina. (The top salinity of the water prerequisites the advance of this alga.)
The northern shore is essentially the most delicate a part of the lake for a number of causes:
motorised site visitors poses a danger to the stableness of the dunes anchored by means of a stand of aging casuarina bushes. The destruction of those bushes would result in the encroachment of quicksand into the melancholy.
the lake’s water provide during the dunes: for the reason that those aquifers (contemporary and salt water) are very shallow, their high quality is prone to the speedy urbanisation on this northern phase.
Ponds are scattered throughout this house, located at the northern a part of the previous channel that after attached the lake to the ocean. Now, the dunes fill the northern a part of the channel. The channel, just like the ponds, isn’t man-made however of herbal foundation. The northern a part of the ponds used to be therefore stuffed in by means of sand from the dunes, whilst the southern phase stays as ponds.
Since 2004, we have now been tracking the adjustments in physico-chemical and particular organic components alongside those ponds.
Danger of extinction
We’ve got simply gained the result of chemical analyses of samples amassed on the finish of Might 2023 alongside those ponds. They divulge extraordinarily top ranges of nitrates – chemical substances – in Might 2023: 12,491.71 to fifteen,394.75 mg/l when put next with 5.82 to twelve.05 mg/l in December 2022. The nitrate limit set by means of the Global Well being Group (WHO) is 50 mg/l for water to be thought to be potable.
This surprising building up in nitrate ranges is assumed to be related to the status quo of resorts within the instant neighborhood of the lake and ponds, and to the speedy, out of control urbanisation of this northern a part of the lake, which has no sewage machine. Those polluted waters go with the flow into the lake, ultimately achieving the salt, rendering it mistaken for intake.
The standard of the salt subsequently must be checked ceaselessly when salt mining operations resume.
The availability of salt to the lake during the dunes is an everlasting phenomenon and amounted to round 360,000 tonnes in keeping with yr in line with the result of our studies in 2008-2009. This salt deposit used to be upper than the exploitation price estimated at round 100,000 tonnes in keeping with yr on the time.
Opposite to common trust, salt extraction hasn’t ever been damaging to the lake’s survival, however slightly prevents it from filling up. There could also be fraudulent exploitation of the shell middens that encompass the lake and of the dune sands as development fabrics. Consider what would occur if the quantity of salt extracted within the remaining ten years had been poured again into the lake – it could turn out to be right into a towering salt mound. If the salt is so polluted as to be undeserving for intake, extraction might be prohibited and the salt deposit will fill the melancholy.
On this manner, the sand from the relaxation canal and the buildup of salt rendered undeserving for intake will fill within the melancholy in few years, and the Purple Lake will disappear altogether.
Suggestions to keep the lake
To place an finish to the threats dealing with the lake and keep it, the government must:
restrict the mining of shell middens, which act as paving stones and offer protection to the soil from erosion
restrict the exploitation of dune sand, which acts as a barrier between the ocean and the lake
regulate farming and gardening actions by means of heading off their status quo at the instant slopes of water issues, the development of vacationer amenities and the exploitation of salt to keep away from exceeding the utmost prohibit
rehabilitate the belt of casuarina bushes that stabilise the dunes
ban urbanisation and vacationer amenities at the dunes setting apart the lake from the ocean
enforce built-in control that advantages each flora and fauna for which the lake serves as a “resting position” and the native inhabitants, in addition to environmental schooling and awareness-raising.