Résumés
Résumé
La lagune de Smir a été sujette à un aménagement (construction d'un barrage et d'un port de plaisance) qui a contribué à la modification de son système hydrologique notamment, la disparition de certaines espèces de flore et de faune et la réduction de superficies considérables de cet écosystème.
Une caractérisation des eaux de cette lagune a porté sur la mesure pendant le reflux, d'un ensemble de paramètres physico-chimiques. Une méthodologie a été suivie afin d'exprimer de façon cartographique les résultats. De même, des suivis d'une journée ont été réalisés pour les eaux entrantes et sortantes au niveau de deux stations fixes, dans une tentative de comprendre l'action de la marée sur l'hydrologie au sein de la lagune, de cerner les caractéristiques des eaux entrantes dans la lagune et celles sortantes de celle-ci et de suivre l'évolution des différents paramètres physico-chimiques au cours des cycles marégraphiques.
L'évolution spatiale des paramètres hydrologiques au niveau de la lagune, se réalise sous forme de gradients entre l'aval et l'amont. Elle est fonction des saisons, de la marée et des apports de l'Oued Smir et du Chenal principal venant des marais. La lagune subit le flux et le reflux de la marée d'un mètre d'amplitude environ. Combinée avec le déficit en eaux douces, la communication permanente avec la mer a eu pour conséquence l'augmentation générale de la salinité dans le plan d'eau lagunaire et dans les marais. Le fonctionnement hydrologique de la lagune de Smir se trouve actuellement régi par les facteurs hydrodynamiques liés à la marée (réguliers) et aux apports d'eaux véhiculés par le chenal des marais (irréguliers), auxquels peuvent être ajoutés des facteurs physiques (évaporation intense), bathymétriques (faible profondeur) et physiologiques (photosynthèse des macrophytes et des phanérogames dans la lagune et dans les marais limitrophes).
Mots-clés:
- Méditerranée,
- lagune,
- hydrologie,
- activités humaines,
- impacts
Abstract
The Mediterranean Smir lagoon (35°43' N 5°20' W) is located in the extreme west of the Mediterranean basin, northwest of Morocco, 25 km south from the Gibraltar strait. The area of this lagoon is about 3 km2 with a maximum depth of 2.5 m. The bottom is dominated by silty and fine substrates with some sandy zones close to the entrance. The bottom is extensively covered with macrophytes (e.g., Enteromorpha and Ulva) and phanerogams (e.g., Rupia maritima and Zostera noltii). The lagoon receives water from the Smir Wadi but this input has decreased. It also receives fresh water via another channel (the marshes channel) from swamps that lie between the water body and M'diq city. This ecosystem has been subjected to unplanned dam and pleasure port constructions, and these changes led to a modification of the hydrologic system. Currently, this ecosystem communicates with the Kabila harbour (marina) and the sea across a narrow gully and is regularly subjected to tidal movements.
This ecosystem, which functioned before as a lake, has been transformed into a coastal lagoon. This lagoon is changing and trends suggest a progressive evolution towards a neutral type lagoon, where water movement is exclusively influenced by the tidal rhythm. The salinity in the lagoon is comparable to sea salinity. This increase in salinity clearly influences the hydrologic conditions, and has altered the composition, structure and functioning of the biotic compartment. The biota has progressively changed and there has been a progressive colonization of halophyte plants within the marsh and a disappearance of fresh water species from the lagoon.
The physicochemical parameters (salinity, conductivity, dissolved oxygen, pH, water temperature and sediment temperature) of the lake water were characterized. The methodology used allowed the results to be expressed cartographically. Similarly, the measurements for one day were analyzed for incoming and retreating waters at two stationary points, thus increasing the understanding of tidal action on the lagoon-lake hydrology.
The salinity analyses clearly show a mixohalinity and strong spatial and temporal variabilities (9 - 40) within the lagoon. This parameter was influenced by tidal changes, rainfall and sunshine during the summer. The latter contributes to evaporation of lagoon water, which therefore increases salinity. Gradients established between the upstream and downstream regions are a function of tide and season. The salinity distribution is marked by maximum values recorded in the summer and minimum values recorded in the winter. The same evolution scheme evolution is also noticed for conductivity since it is a parameter closely linked to salinity.
The lagoon and the main channel waters had higher salinity in the summer (S‰=40) due in part to marine waters that pervaded this channel, and the evaporation of inflow currents. Low salinity aureoles were detected in the Smir Wadi mouth indicating that fresh waters dilute marine waters. Marine waters that enter the Wadi channel by the inflow currents blend with fresh rainfall waters or stagnant saline waters or with groundwater inputs.
Two periods were distinguishable based on the surface water temperature; hot summer conditions and cold winter conditions. The continental effect was compensated by the sea buffer effect that played an important role in the regulation of the temperature within the Smir lagoon. During the period of decreasing temperature gradients from the downstream region towards the upstream region (by stretching from sea towards continent), marine waters played a significant role by moderating the freshwater temperatures cooled down from the continent during cold periods (winter and autumn). In contrast, during decreasing temperature gradients from the upstream region towards the downstream region, the oceanic waters cooled the high temperatures of internal lagoon waters resulting from hot seasons (summer and spring). Furthermore, the shallow lagoon depth and the long exposure to sunshine, especially in the summer, resulted in water heating.
In addition, the water temperature was influenced by hot marsh channel waters that occupy the upper region and spread with the low tide up to the lagoon entrance. Waters brought by the marsh channel also were distinguishable by high oxygen values, which demonstrate the role played by the swamp vegetation in the oxygenation of these waters. The low oxygen values recorded at the Smir Wadi mouth reflected the effect of suspended elements (turbidity). The Wadi contribution to the oxygen distribution in the lagoon waters was notable as it brings less oxygenated waters because of their strong turbidity.
Nevertheless, the lagoon waters showed good dissolved oxygen saturation and no under-saturation was observed during the study period. This relates not only to the permanent exchanges with the sea, but also to the rather remarkable abundance of marsh vegetation and to the development of macrophytes and phanerogams within the lagoon. The dissolved oxygen isovalue distribution curves overlapped remarkably with isotherms, even as the temperature increased, because the entire zone that stretches up to the gully was occupied by a dense meadow. This biomass supplies oxygen to the central zone and can contribute to over-saturation. The lagoon inside was less covered with vegetation and showed comparatively low oxygen values. Besides the photosynthesis activity, the shallow depth and the uninterrupted lagoon waters mixed by the tidal currents explain the over-saturation sometimes observed within lagoon, especially in its downstream region.
Variations in pH appeared to be closely linked to dissolved oxygen and water temperature. The three parameters present comparable distribution patterns showing the influence of the marsh waters on the lagoon hydrology. The pH varied between 8.0 and 9.0, demonstrating an elevated alkalinity in some lagoon waters. This alkalinity trend was due to the constant seawater entrance and to the stopping of Smir Wadi fresh water supplies.
The sediment temperature measured at 5 cm deep gave insight into different daily and seasonal variations. The temperature of both air and marsh waters was comparatively well reflected in the sediments; the temperature distribution in the sediments overlaps with the water isotherms, with temperature values varying from 14 to 28°C. The inside part of the lagoon showed temperatures widely different from those recorded close to the entrance.
The spatial evolution of hydrologic parameters in the lagoon resulted from gradients between the downstream and upstream regions. These gradients are a function of seasons, tide, Smir Wadi inputs and also the main channel coming from the swamps. The lagoon is subjected to high and low tides. In addition to freshwater shortages, the permanent communication with the sea resulted in a general increase of salinity in lagoon waters and marshes. The lagoon hydrologic functioning is currently governed by hydrodynamic factors that are related to the tide (regular) and to the fresh water supplied by the marsh channel (irregular). In addition, physical (intense evaporation), bathymetrical (shallow depth) and physiological (photosynthesis by macrophytes and phanerogams in the lagoon and the neighboring marshes) variables also influence the functioning of the lagoon.
Keywords:
- Mediterranean,
- lake,
- hydrology,
- man-made,
- impacts