The pink lake was formerly attached to the Atlantic Ocean. Over time, the installation of the drought has favored its separation and its removal from this ocean by an accumulation of sand dunes. The latter have since been colonized by palm trees and filaos. In 45 kilometers of Dakar, towards the North, one of the wonders of Senegal: a lake with a very particular color, a product of the "fishing" equally original and superb birds. The Lac Rose also called Lake Retba was made famous throughout the world by the Paris-Dakar Rally of which it constitutes the ultimate stage. This lake has particular characteristics: by an unexplained phenomenon it has become, now 40 years, pink. And yes ! According to the hours its color turns from pink to purple exactly as on the postcards. But this color is not only an optical effect: by taking the water in your hand, the water is always pink.
So why ? This color is due to the presence at the bottom of the lake of a microscopic algae which oxidizes the iron of the salt water. The color is more impressive when the sun is at the zenith and during the dry season. A beautiful show not to be missed! The "Dead Sea" of Senegal.
Second characteristic of the lake: the water is hyper salted at a rate of 380 grams per liter (as the Dead Sea) is 10 times more than the ocean. Also, on the south shore of the lake, the product of "fishing" is not fish but salt. Workers, the body coated with shea butter, sink into the waters of the lake to pierce the crust of the bottom of the lake and extract the salt. The latter is then transported to the shores on board flat-bottomed boats which the women will unload to deposit the salt in heaps. Everyone piles up his own pile which he marks of his initials or of a distinctive sign. The salt is of excellent quality and the bag of 25 kilograms is sold for 22 Euros to intermediates of Dakar.
Images: The pink lake, a natural curiosity of Senegal
LAC ROSE OR LAKE Retba
Lake Retba, also known as Rose Lake, is part of the sample of coastal lakes that line up along the Senegal's North Coast, also known as the Grande Côte or the Niayes Zone, from Gandiolais to the Cape Peninsula -Green, on a linear extension of about 253 km. It is bounded to the north by the Atlantic Ocean, to the south by a line passing through Kounoun, Ndiékhirate and Diacksaw, to the west by a Niaga, Médina Thioub, Ngalap line and to the east by Kaniak, Gorom 1 and Mbeute Slope of Lac Rose extends over an area of approximately 155 km2. The contours thus defined correspond approximately to those of the administrative division limited to the Rural Community of Sangalcam, in the department of Rufisque; The rural community grouping 28 villages with a workforce of just under 50000 inhabitants.
In the coastal system of the Grande Côte, Rose Lake differs from other lake formations (Mbaouane, Tanma, Mekhe, Etc) in certain peculiarities, both natural and socio-economic, marked by a pink coloring of its waters, Extraction and marketing of salt, an imposing arboricultural cover as well as problems linked to the intensification of livestock production (adaptation of local livestock). As can be seen, Rose Lake and its watershed concentrate many resources and opportunities but are also particularly vulnerable to nuisances and degradation resulting from human activity (heavy pressure on resources, uncontrolled and uncontrolled settlement, dismantling of Terraces and clumps, clearing, etc.) and whose effects are all the more prejudicial as the economic potential and weight of the Rural Community of Sangalcam are great. In this respect, the intensification of the uses of the resources, the lake and the village terroirs has generated many interactions, externalities and conflicts, the management of which must be based on a set of informative and conceptual tools, Useful for a policy of sustainable development of activities and the application of regulatory bases adapted to the regulation of the local and global problems posed by the integrated development of the rural community.
The morpho-climatic legacy of Rose Lake and its watershed
The present Rose Lake corresponds to a closed lagoon transformed into an over-saline evaporation basin. If one refers to the old formations, the physical environment of the lake fits into the Senegal-Mauritanian sedimentary basin of Meso-Cenozoic age. Among these formations, those of the Paleogene - in particular the lower Eocene clayey and marly, impermeable substrate of the watershed - interest us.
The conditions for setting up the flush formations of the lake and its catchment basin relate to quaternary sedimentation at present. These formations have been the subject of numerous works among which those of L. Hébrard (1973), P. Elouard et al. (1975), P. Michel (1969), P. Elouard et al. (1977), JM Garnier (1978), P. Carbonel and J. Pinson (1979), MM Sall (1982), AT Diaw (1997).
In this respect, Rose Lake extends between the coastal dunes - the object of reforestation reinforced since 1988 - and the dulled inland dunes of the Ogolian, set up during a regressive episode -100 to -120 m Under the current level, with the installation of an arid climate and an intensification of the trade winds. This arid episode of low sea level corresponds to a maximum of the Würm glaciation, towards 20000 years BP. These Ogolian cords thus established have undergone during the Chadian a pedological evolution leading to a rubefaction by ferruginization and a stabilization of the sedimentary material by a sufficiently covering vegetation.
Indeed, the phase of the Chadian (11000-6800 years BP) is marked by a humid climate - with a small dry phase around 7000ans BP at the base of the reorganization of ogolian dunes as the erg of Sangalcam - and a rise of the But with a still low level of -10 to -50 m. During this period, the rivers descending from Bargny's impermeable plateau form first, according to L. Hébrard (1973), lakes of dune dams and eventually reach the sea. The latter, ascending, invades the valleys and Inter-dunes; The mouth of the Sangalcam River is backfilled and forms a delta.
Moreover, it is during the Chadian period that the nyayes made of a flora and a fauna of Guinea of which there remain some relics.
During the transgression of the Nouakchottien (6800 to 4500 years BP), especially at its maximum at 5500 years BP, there remains a sandy terrace with shell levels made of Arca, Dosinia, Tympanotonus, Semifusus. This marine terrace forms a plain, partially flooded, which surrounds the lake. On the northeastern, southeastern and southern shores of the lake, there are also large shellfish strings that are the subject of intense exploitation in the form of quarries (north of the lake) or dismantled as in the south.
The post-nouakchotian series, from 4500 years BP to the present, are more specifically marked by the Dakarian wet phase (2800-2000 years BP) framed by two dry periods dated from 4500 to 2800 years BP (Tafolien) and 2000 To 1100 years BP. In this respect, a north-westerly swell established at the time of the withdrawal from the sea, probably from 4000 years, and due to its oblique incidence on the coast, produces a coastal drift directed towards the south; This presides over the accumulation of sandy shorelines (partially fixed yellow dunes).
The remodeling of the cords leads to a gradual closure of the lake at the end of this phase. The bright white dunes set up at the Subactual at the Current, continue to fatten by sandy inlets from the high beach; These recent cords, particularly in the southwestern part of the lake, rest locally on a clayey level (montmorillonite) which creates a zone of springs and basins above the lake's coast (JM Garnier, 1978). The final closure of Lac Rose took place around the 14th-15th century (JM Garnier, 1978) or at the end of the 17th century (P. Carbonel and J. Pinson, 1979). The bottom of the lake consists of a layer of mud resting on the Nouakchottian formations, sands and shells, locally covered with a crust of salt formed of gypsum and halite.
Evolution of the Pink Lake and its watershed
The closure of the original lagoon effective, the lake functions as an evaporation basin with a concentration of its waters and reduction of its surface. The regression of the shores of the lake constitutes - with the development of the live wind forms, the pressure of land, the lowering of the ground water, the modification of the vegetation cover - some of the major forms of evolution of the landscape of the region.
These changes, which lead to a degradation and accelerated transformation of the previous landscapes, are the result of the environmental conditions, the development and the exploitation of the region. In this regard, the triggering of the dry phase of the 1970 decade has contributed to amplify the process of degradation, largely triggered by the widespread agricultural development of the Niayes. To this must be added the effects of pumping by the Water Development Service into local aquifers poorly replenished by successive rainfall deficits; The ensuing lowering of the aquifers is the result of a situation that combines the agricultural use of space and the excessive harvesting of the resource, underlying the hydromorphic nature of the landscapes of the Niayes.
In this context, the comparative study of the general hydrography of the Pink Lake watershed shows, within a few decades, the strong alteration of the humidity in the area. Indeed, the hydrographic physiognomy observed in 1941 for example shows a high moisture content of the watershed. This situation results in the existence of small depressions on the coastal fringe of Lac Rose. The 1941 basin incorporates many pools that have disappeared; These pools, according to many testimonies were fishy. They occupied depressions, some of which, before the drought of the 70 years, were planted in rice by the diola colony which alternated this activity with the harvesting of palm wine. With the sinking of the water table, these pools have dried up and, despite an intermittent revival (in a particularly rainy year), their cultivation will necessitate systematic watering from 1976.
The then competent Sangalcam and Wayambam backwaters remained so until 1965 to become completely dry between 1973 and 1975.
The Land Cover of Pink Lake and its watershed
Market gardening / arboriculture around the lake
This component of the vegetation of the zone encountered between the halophyte meadow and the dune at Rose Lake is a woody formation dominated by introduced species. Market gardening is highly developed in this area. Cultivated vegetable speculation varies according to the season. The most abundant are cabbage, salad, tomato, sweet pepper, chilli, parsley, cucumber. The woody species of the original vegetation cover were replaced by new species introduced for the purposes of arboriculture.
Clearing, market gardening, planting of exotic trees and salinization of the land have led to a profound change in the flora and the original vegetation of this flooded part of the lake.
Plantations FILAO (Casuarina equisetifolia)
These massive plantations which border the beach occupy the still active coastal dunes. They are essentially constituted by Casuarina equisetifolia, which is the species which has best withstood the difficult climatic and climatic conditions of this zone. These plantations made it possible to limit the progression of the dunes to the vegetable basins.
The background of the filao strip is occupied by a coastal steppe that has settled on the transitional yellow-orange dunes. The species that grow there are succulent and perennial non-grass herbaceous. These include Cyperus maritimus, Sporobolus spicatus, Ipomoea brasiliensis, Alternanthera maritima, Opuntia tuna, Philoxerus vermicularis. Most of these species are halophytes and psamophytes capable of surviving in difficult edaphic and climatic conditions. The few woody species found there are Maytenus senegalensis, Chrysobalanus orbicularis, Calotropis procera and Casuarina equisetifolia.
Dunes bright / bare soil
This unit, made up of bright dunes, has little vegetation cover. The edaphic conditions of these dunes are not conducive to the development of spontaneous vegetation. The few rare species encountered on these dunes are Sporobolus spicatus, Cyperus maritimus, Leptadenia hastata, Calotropis procera, Dodonea viscosa.
This area is occupied by sparse vegetation as a result of clearing for agricultural purposes. Woody individuals are generally fairly spaced and clearly leave the soil uncovered. Most native woody species are eliminated to give way to new introduced species which have a relatively large overlap in some places. Among the native species that can still be found there are Acacia albida, Adansonia digitata, Borassus aethiopum, Aphania senegalensis, Fagara leprieurii, Ficus ovata, Calotropis procera, Capparis tomentosa, Maytenus senegalensis, Annona glauca, Chrysobalanus orbicularis, Opuntia tuna, Leptadenia hastata, Borreria verticilata. Some of the woody species have become rare or even threatened.
The most abundant introduced species are Mangifera indica, Anacardium occidentale, Cocos nucifera, Annona muricata, Carica papaya, Manihot esculenta, Eucalyptus sp., Azadirachta indica, Leucaena leucocephala, Euphorbia balsamifera, Euphorbia turicalli. These species are either associated with crops or planted with hedgerows or shelterbelts.
The physiognomy of the original vegetation of this area and its floristic composition have undergone a profound modification mainly linked to agricultural activities.
This formation borders the lake of Rose Lake, particularly in its northern part. It is mainly composed of herbaceous halophytic species at ground level, the main ones being: Philoxerus vermicularis, Sporobolus robustus, Sesuvium portulacastrum, Paspalum vaginatum. In some places, higher species such as Phragmites vulgaris, Typha australis, Cyperus sp.
Terraces (grassy steppe)
This morphological unit presents mainly herbaceous vegetation which does not completely cover the soil at the end of the rainy season. The most common species are Schoenefeldia gracilis, Andropogon pinguipes, Cenchrus biflorus, Aristida stipoides, Pennisetum pedicellatum, Leptadenia hastata, Borreria verticillata, Waltheria indica, Centaurea perrottetii, Indigofera secundiflora, Indigofera linearis.
Vegetation 1 (Shrub steppe)
This type of vegetation has virtually the same herbaceous species as those found on the terrace. The woody component is dominated by shrubs with a height between 2 and 5 meters. The species most represented are Boscia senegalensis, Euphorbia balsamifera, Maytenus senegalensis, Ziziphus mauritiana, Calotropis procera, Opuntia tuna, Acacia seyal. Among the rare tree species encountered are Acacia albida, Adansonia digitata and Casuarina equisetifolia which is introduced.
This vegetation formation is relatively rich and presents numerous uncommon Sudanian and Guinean species confined to depressed areas.
At the level of the coastal dunes still subjected to a relatively intense geodynamics (northern part of the lake), this formation characterized by pseudo-steppe by Trochain (1940) is considered as a pioneer formation of edaphic origin. Inland (southern part of the lake), it is considered to be an anthropogenic secondary formation. It is most likely derived from a savanna due to the frequency of agricultural activities and livestock pressure.
Vegetation 2 (Shrub / Tree Savannah)
This type of vegetation is characterized by a herbaceous stratum which forms an almost continuous carpet at the end of the rainy season. This formation would have an essentially anthropic origin (crop clearings, plantations).
The dominant species of the herbaceous stratum are Eragrostis tremula, Cenchrus biflorus, Leptadenia hastata, Waltheria indica.
The shrub stratum that covers 5 to 15% of the soil is dominated by Maytenus senegalensis, Piliostigma reticulatum, Annona senegalensis, Guiera senegalensis, Neocarya macrophylla, Balanites aegyptiaca, Ziziphus mauritiana, Commiphora africana.
The tree stratum which covers only 1 to 5% of the soil is sparse and mainly composed of Acacia albida, Elaeis guineensis, Adansonia digitata, Ficus elasticoides, Mangifera indica, Casuarina equisetifolia.
Vegetation 3 (Wet / palm plantation complex, fruit tree plantations)
In some depressions below sea level such as Lake Mbawane, this formation is a complex of several types of vegetation. From the center of the depressions to the foot of the dunes, one finds successively a herbaceous carpet formed of hydrophytic species, the remains of a palm grove to Elaeis guineensis and a thicket testifying the presence in the past of a humid forest.
The main species of herbaceous cover on the carpet are Cyperus sp., Imperata cylindrica, Philoxerus vermicularis, Sporobolus robustus, Sesuvium portulacastrum, Paspalum vaginatum, Leersia hexandra, Cynodon dactylon.
In the palm grove and vestiges of the degraded wet forest we find Elaeis guineensis, Cocos nucifera, Ficus sp., Detarium senegalense, Syzygium guineense, Landolphia heudelotti, Cassia sieberiana, Prosopis africana, Entada africana, Celtis integrifolia, Acacia raddiana, Neocarya macrophylla, Dalbergia ecastaphyllum, Alchornea cordifolia, Fagara xanthoxyloides.
This type of vegetation has been severely affected by drought, despite the shallow depth of the water table. The increasing salinity has caused the death of several hydrophytes and in particular of Elaeis guineensis. The intensification of vegetable and fruit crops has also reduced the extension of relict communities.
Inland, this plant formation is often represented by plantations of mango and cashew nut trees and shreds of shrub savannas with relatively conserved trees and dominated by Acacia albida.
As for the flood zone, the flora and the original vegetation of these areas have been profoundly modified during the last decades. The main factors affecting this change are salinization of land, overexploitation of certain species, clearing, market gardening, planting of fruit trees.
This Botanical Reserve of 15,9 hectares created in 1957 presents a geomorphology consisting of lowlands with semi-peat hydromorphic soils bordered by low dunes. Its vegetation is relatively dense on the slopes of the shallows and sparse on the dunes. Its flora has a dominant Sudanian background with a marked Guinean affinity. It is composed of 192 species of which 131 have a Sudanian affinity and 43 a Guinean affinity. The results of a study published in 1992 show that the flora of this reserve has depleted 57% between 1957 and 1992. The majority of the extinct species are species with Guinean affinity.