Résumés
Résumé
La lagune de Khnifiss, située au sud du Maroc, est une réserve biologique d’intérêt mondial pour l’avifaune. Des études pluridisciplinaires portant sur la physico-chimie des eaux, la biologie, la sédimentologie et les courants ont été réalisées par l’INRH dans la lagune en septembre 1998 et février 2001. Les résultats du courant obtenus montrent que la circulation des eaux à l’intérieur de la lagune est gérée par un courant alternatif et bidirectionnel : les courants du jusant sont plus importants que ceux du flot et varient également en fonction du rythme de la marée vive-eau/morte-eau. Ce mode de circulation des eaux attribue à la lagune une richesse en éléments azotés et phosphatés, surtout en période de vive-eau. La biomasse chlorophyllienne, assez importante dans la lagune, est causée par la richesse du milieu en éléments nutritifs et le phénomène d’upwelling qui se manifeste en été et au printemps dans la zone sud du Maroc. Ces études ont montré que la lagune de Khnifiss est un milieu propice à la conchyliculture. Néanmoins, l’installation de tout projet aquacole dans le site devra tenir compte de sa capacité trophique et de son équilibre écologique.
Mots-clés:
- Conchyliculture,
- courant de marée,
- lagune de khnifiss,
- site RAMSAR,
- sud du Maroc,
- remontée
Abstract
The Moroccan coastal marine area has several zones that are targets for exploitation but must simultaneously be protected. Lagoons are considered to be among the most productive natural systems in the world, but they are also vulnerable systems due to natural and human constraints. Lagoons can also be a target of economic expansion. This is why an understanding about the state of these ecosystems is important in order to proceed with rational use of the resource. The Khnifiss lagoon, situated on the South Atlantic coast of Morocco, is a potential site for aquaculture and tourism if planned with an ecological awareness. The large surface area and high biological production for the avifauna qualify it to be among the four protected sites by the RAMSAR convention for humid zone conservation (lagoons of Khnifiss and Moulay Bousselham, lakes of Sidi Boughaba and Afennourir). The objective of this work was to complete previous studies and to estimate the nutritive richness of the Khnifiss lagoon. To understand the spatio-temporal variability in the water currents, and the physico-chemical and biological and characteristics of this lagoon, two surveys were organized in September 1998 and February 2001.
The Khnifiss lagoon (20 km long and 65 km2 surface area) is situated between Tantan and Tarfaya (28°02'54'' N, 12°13'66'' W). It opens up into the Atlantic ocean by a narrow inlet called ''Foum Agouitir'', about 100 m wide. The lagoon continues upstream beyond a salt marsh, called "Sebkha Tazra". Physical, chemical and biological variables were monitored: temperature and salinity; dissolved oxygen, nitrates and phosphates; chlorophyll a. The current (direction and intensity) and the sediment were also studied. Samples were collected four times from the surface and bottom waters during spring tide (ST), neap tide (NT), low tide (LT) and high tide (HT) during two seasons (September 1998 and February 2001). Seven hydrological stations were sampled, representing the lagoon system and one reference station in a neighbouring zone (Hréf), which represents the marine zone of the lagoon. Temperature and salinity were measured with a multi-sensor probe. The chemical analyses were carried out according to methods suggested by AMINOT & CHAUSSEPIED (1983). Thirty (30) sediment samples were also collected. Currents were measured at two fixed points by two current-meters.
Based on the hydrological characteristics of the lagoon, especially temperature, salinity, water circulation and the nature of sediment, three zones were identified :
1. Zone I (downstream from the lagoon) was dominated by the ocean’s influence and included the Hréf, H1 and H2 stations. Maximum depths varied from 2.5 to 6.6 m, depending on the tide and location. The concentrations of the different hydrological parameters were controlled by the plug effect of the oceanic environment. The most important current in the lagoon was recorded in this zone in the spring tide (ST: 110 cm.s-1). The sediment was coarse and important water - sediment exchange was caused by the strong current recorded in this zone.
2. Zone II (middle of the lagoon) was a transition zone with an intermediate oceanic influence. It was situated between the H3 and H5 stations. The size of the inlet and the important exchanges governed by currents caused by strong tides resulted in concentrations of the various parameters being similar to those in Zone I. Depths varied from 2.64 to 8.7 m. Waters were more saline during the LT and NT periods (36-41 practical salinity unit, or psu). The current decreased relative to that is zone I (56 cm.s-1 in September 1998 and 78 cm.s-1 in February 2001). In this zone the current moved preferentially towards the right strand at high tide and toward the left strand at low tide.
3. Zone III (upstream in the lagoon) covered a different biotope compared to the first two zones. The waters were less deep (2.64 to 5.3 m) and included the stations H6, H7 and the upper part of the lagoon. This zone was characterized by a very low oceanic influence. Water temperature and salinity were higher, 24-26°C and 41-44 psu respectively, during the LT and NT periods.
On the basis of the hydrological zonation, the conjunction of the climate descriptors (moderate wind, rare precipitation, absence of storms, moderate air temperature) of the lagoon and the strong hydrodynamic influence in the lagoon suggest that the Knifiss lagoon is a relatively unconfined ecosystem. The hydrodynamics of this lagoon permit a fast water renewal rate and insure a good environmental quality. In addition, studies carried out on the healthiness of this lagoon demonstrated not only a healthy environment, but also healthy shellfish. This diagnosis seemed to be related to the geographical location and important rates of water exchange. However, the absence of continental freshwater sources leads to an important increase in salinity and temperature at the level of the upstream zone (zone III). Zone III therefore cannot be considered for shellfish breeding.
On the basis of the biological zones: With respect to the bathymetry of the lagoon (depths varied from 2.5 to 8.7 m), shellfish, in contrast to fish, require less water depth. Therefore, it is sufficient to search for a less turbulent zone, which offers sufficient planktonic food for the shellfish growth. In the Khnifiss lagoon, the taxonomy of the phytoplankton has not been yet studied. However, BENNOUNA (1999) demonstrated that the phytoplankton community was dominated by dinoflagellates and diatoms in the Oualidia and Sidi Moussa lagoons. RHARBI (2000) found that picophytoplankton were the principal breeding food source of oysters and clams in Oualidia lagoon. The source of shellfish for breeding is located in the neighbouring upwelling marine waters. Development projects planned for the lagoon must include an awareness programme for fishermen about the role that moderate and responsible shellfish breeding programs can play in the preservation and reconstitution of the marine resource as well as in the improvement of their incomes.
Keywords:
- Shellfish culture,
- tidal current,
- Khnifiss lagoon,
- RAMSAR site,
- south of Morocco,
- upwelling area
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