Impact de la sécheresse sur l'évolution de la qualité des eaux du lac Mansour Eddahbi (Ouarzazate, Maroc) The effect of dryness on water quality of lake Mansour Eddahbi (Ouarzazate, Morocco)

Résumé

L'objectif de ce travail est d'étudier l'impact d'une année hydrologique très sèche (98/99) sur la qualité physico-chimique et l'évolution de l'état trophique du lac Mansour Eddahbi. Ce lac est situé dans la région de Ouarzazate, au sud du Maroc, caractérisée par un climat aride continental, avec des écarts thermiques hiver-été importants, des précipitations très faibles et une forte évaporation. Ceci induit une réduction considérable du volume total du lac et par suite une baisse de 14 m de son niveau.

À la lumière des valeurs de températures enregistrées au niveau de la colonne d'eau, le lac peut être classé dans la catégorie des lacs monomictiques avec une seule période de mélange hivernale. Le pH est légèrement alcalin. La réduction du volume d'eau au niveau du lac a engendré une augmentation de la salinité des eaux. Une corrélation significative est enregistrée entre les deux paramètres (r² =0,60 pour n=13 et p<0,05).

Un déficit marqué en oxygène dissous (7,3 mg d'O₂ /l comme moyenne en surface et des valeurs inférieures à 2 mg d'O₂ /l voire nulles en profondeur durant la stratification), les teneurs en Chl "a" (24 µg/l) permettent de classer le lac dans la catégorie des lacs eutrophes. Selon les teneurs enregistrées en azote (0,2 mg/l) et en phosphores (0,02 mg/l), le lac est hyper-eutrophe. Deux années auparavant, le lac était considéré comme mésotrophe (ONEP/BRL, 1998). Les conditions climatiques sévères qui ont sévi durant l'année hydrologique 98/99 ont contribué à une évolution accélérée de l'état trophique du lac.

Mots-clés:

• Lac,
• Climat aride,
• État trophique,
• Qualité de l'eau,
• Minéralisation,
• Nutriments,
• Azote,
• Phosphore

Abstract

Eutrophication is one of the most important water quality problems in lakes. Due to the effects of excessive external inputs of nutrients (phosphorus and nitrogen), compounded with unfavorable climatic and hydrologic conditions, the lake undergoes major changes that involve both the physical-chemical properties of water and the aquatic communities living there. There have been many studies on lake ecosystems and their evolution (HENRY et al., 1984; AFDALI, 1993; HARRAK, 1991; LOUDIKI et al., 1994; SBIYYAA, 1998, MOUHRI et al., 1999). These studies relate especially to the role of nutrients (nitrogen and phosphorus mainly) in the evolution of lakes. The influence of climatic and hydrologic conditions on the structure and dynamics of these ecosystems has been often underestimated. Indeed, there are few studies on the relationship between climatic conditions and the trophic status of aquatic ecosystems.

The aim of this paper is to study the behavior of the physical-chemical qualities of water in lake Mansour Eddahbi (Ouarzazate, Morocco) and its trophic status under severe climatic conditions, especially during the very dry hydrological year 1998/99. The Ouarzazate region is characterized by an arid continental climate with very hot summers and cold winters. Due to both great evaporation and increasing demand for water for irrigation, the water levels of the reservoir has been reduced by 14 m and its storage capacity has been considerably reduced. The approach used was to assess water quality in a station at the deepest area of the lake during the hydrological year 1998/99. Sampling was undertaken monthly and different parameters were analyzed including temperature, pH, total suspended solids (TSS), salinity, dissolved oxygen (DO), chlorophyll a, phosphorus (TP and PO₄^3-), and nitrogen (TNK, NO₃ - and NH₄ +).

The results of this study demonstrated important seasonal variations of water temperature in the lake. Temperature varied from 9.5°C in the winter to as high as 28.5°C in the summer. Based on this parameter, the reservoir could be classified as a monomictic hot lake with one mixing period occurring in winter. In the entire water column, the pH values were slightly alkaline and oscillated between 7.8 and 8.5 at the surface and 7.4 to 8.2 at the bottom of the lake. This reflects a well-buffered system with moderate benthic decomposition activity. The decrease in water volume by evaporation and intensive use for irrigation resulted in an increase in water salinity. These two parameters presented a significant correlation (r² =0.60 for n=13 and p<0.05). A decrease in dissolved oxygen from 7.3 mg O₂ /l at the surface to 0 mg O₂ /l at the bottom of the lake occurred during summer stratification. However, water of the lake reservoir was well oxygenated during preceding years (concentration of dissolved oxygen oscillated between 2 and 11 mg/l (ONEP/BRL, 1998)). Concentrations of chlorophyll a in the euphotic zone (8 m depth) were very important and varied from 12 µg/L (August 1998) to 49 µg/L (September 1999) with an average of 25 µg/L. Based on the chlorophyll a value, the lake Mansour Eddahbi could be classified as eutrophic.

The lake has become shallower and the thermocline, beginning at 12 m below the surface, has moved closer to the deeper layers where exchange between sediments and the water column is important. These exchanges between the two compartments of the lake contributed to enrichment of the water column in summer, creating an internal nutrient load. During the hot period when the lake was stratified, there was a large loss of nitrogen from the system. During the mixing period, nitrates were present at substantial concentrations in the deeper regions, but in summer nitrates were consumed rapidly in the surface layer, leading to a limitation on phytoplankton growth in this period. This same phenomenon was also noticed by HENRY et al. (1984). Furthermore, these losses of nitrogen induce a strong reduction in the N/P ratio during this period.

Orthophosphate concentrations decrease from the bottom layers to the surface, with almost total impoverishment in the upper strata. This impoverishment reaches 10 m during the summer period and these values are often limiting for the phytoplankton production. The correlation obtained between orthophosphate concentrations and chlorophyll a was significant, especially in summer (r² =0.79 for n=13 and p<0.05).

Nutrient exchange at the water and sediment interface was favoured by the anoxic conditions at the lake bottom that prevailed during the eight months of stratification. The release of these substances can alone, without external contributions, maintain important primary production. This was confirmed by the importance of phytoplankton development compared to the preceding years. The algae production seemed to be enhanced, even if the reported N/P ratio was not optimal, particularly during the period of stratification and had no significant correlation with chlorophyll a content (r² =0.19, n=13 and p<0.05). This important production appeared to be the result of an internal enrichment of the lake in nutrients coming from the sediment. According to nitrogen (0.2 mg/L) and phosphorus values (0.02 mg/L), the lake could be classified as hypereutrophic whereas two years earlier it was considered mesotrophic (ONEP/BRL, 1998). This work shows clearly that the severe climate conditions during the hydrological year 1998/99 contributed to a rapid degradation of the trophic status of this lake. It changed from a mesotrophic to a hypereutrophic state, resulting from an important enrichment in nutrients and the consequent algal production.

Keywords:

• Lake,
• Arid climate,
• Trophic status,
• Water quality,
• Salinity,
• Nutrients,
• Nitrogen,
• Phosphorus