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This study deals with the use of a multivariate analysis method in order to understand the functioning of the Bizerte lagoon ecosystem. A set of hydrobiological parameters was collected during 10 monthly campaigns in 2004. A variant of the EOF (Empirical Orthogonal Function) technique is used to examine hydrobiological variability modes in this lagoon. The permanent features characterising the lagoon are obtained by applying the EOF technique to data after removing the annual cycle. Two major modes were obtained. The first one shows a contrast between the northern sector of the lagoon, influenced by the Mediterranean Sea, and the southern one influenced by continental waters. This mode is mostly prominent for temperature and salinity. The second mode shows a contrast between shallow waters along the coastal zones and the deep central area of the lagoon. This contrast is more pronounced towards the southwestern edge of the lagoon, a region of important freshwater input from the Tinja channel, which drifts towards the downstream area of Menzel Bourguiba. This second mode may also correspond to northward water circulation under southerly wind blowing frequently in summer. The spatial and temporal variability in the lagoon is highlighted by applying the EOF to the raw data without removing the annual cycle. A weight series is obtained for each variable by extracting the corresponding data and then projecting them on the corresponding eigenvector. Using this technique, only one mode was found dominant with more than 52% of the total variance. It was also found that the north-south contrast of temperature, salinity and phosphorous and the coast-centre contrast of Chlorophyll a are enhanced in summer. Other variables such as ammonia and nitrate also show significant annual cycles but with local patterns.

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La lagune de Ghar El Melh, située au Nord de la Tunisie (dans le Golfe de Tunis), est caractérisée par une superficie de 28,5 km2, une faible profondeur et une faible communication avec la mer. Actuellement, sous l'effet des actions anthropiques (les rejets terrestres), elle connaît une dégradation progressive de la qualité de ses eaux et sédiments dont les conséquences sont la chute de la richesse biologique et la réduction des ressources halieutiques de la lagune. Dans ce travail, nous présentons les résultats de l'analyse des caractéristiques physico-chimiques et biologiques des eaux de la lagune. Ces analyses montent bien l'hyper-eutrophisation du milieu caractérisé par de fortes concentrations des eaux en azote total et en phosphore total. Les résultats de simulation du fonctionnement hydrodynamique, que nous avons réalisée à l'aide d'un modèle à 2 dimensions intégré sur la hauteur, montrent une stagnation des eaux dans environ 80% de surface de la lagune qui a fortement amplifié la dégradation de la qualité de l'écosystème. Parmi plusieurs scénarios d'aménagements simulés, nous proposons la création d'une nouvelle communication avec la mer dont la simulation hydrodynamique montre une nette augmentation des échanges d'eau mer-lagune. Nous présentons ensuite un modèle écologique homogène de la lagune de Ghar El Melh qui a été mis au point et calibré en s'appuyant sur les mesures de terrain. Les simulations, avec ce modèle, montrent que l'arrêt total des rejets terrestres et la création d'un nouveau grau réduisent considérablement l'eutrophisation du milieu ce qui conduiraient certainement à une nette amélioration de la production halieutique dans la lagune.

Cette étude a pour but la caractérisation hydrobiologique d’une lagune méditerranéenne (lagune de Bizerte, nord de la Tunisie) sur la base de dix campagnes de mesures réalisées à une fréquence mensuelle durant l’année 2004, de janvier à décembre. Deux parmi ces dix campagnes ont fait l’objet d’un maillage fin avec 118 stations en mai et 40 stations en septembre. Le reste n’a couvert que 10 stations seulement. Pendant ces campagnes, des paramètres physicochimiques ont été mesurés (Température, Salinité, Oxygène dissous). Des mesures de paramètres chimiques et biologiques ont été effectuées au laboratoire (Ammonium, Nitrates, Phosphates, Azote total, Phosphore total, Chlorophylle a et Phéopigment).Une analyse multivariée des données collectées a permis de distinguer deux principaux modes de fonctionnement de la lagune. Le premier mode, expliquant 26,3 % de la variance totale, montre un contraste nord-sud et reflète l’effet relatif de la Méditerranée et du bassin versant de la lagune, alors que le second mode, expliquant 18,2 % de la variance totale, montre un contraste est-ouest qui traduit l’effet du lac Ichkeul. Les paramètres qui contribuent significativement au premier mode sont la température, la salinité, la transparence absolue et relative. Ceux qui contribuent au second mode sont la transparence absolue et relative ainsi que la chlorophylle a et le phéopigment. Cependant, les paramètres dont les poids moyens sont non significatifs ont montré des oscillations spatiales importantes en fonction des saisons qui sont en liaison avec les rejets terrestres.The purpose of this study was the hydrobiological characterization of a Mediterranean lagoon (Bizerte Lagoon, North Tunisia) based on ten sampling campaigns carried out during the year 2004, from January to December, on a roughly monthly frequency. Two of these ten campaigns were subject to a high spatial coverage with 118 stations in May and 40 stations in September; the eight remaining campaigns covered only ten stations. During these campaigns, physicochemical parameters were measured (temperature, salinity, dissolved oxygen) and laboratory analyses were performed for chemical and biological parameters (ammonium, nitrate, phosphate, total nitrogen, total phosphorus, chlorophyll-a and pheopigments). A multivariate analysis distinguished two principal functioning hydrobiologic modes. The first mode, explaining 26.3% of the total variance, shows a North-South contrast reflecting the relative effect of the Mediterranean Sea and the catchment area. The second mode, explaining 18.2% of the total variance, shows an East-West contrast that reflects the effect of Lake Ichkeul. The parameters that contribute significantly to the first mode are temperature, salinity, absolute and relative transparency, whereas those that contribute to the second mode are absolute and relative transparency as well as chlorophyll-a and pheopigments. However, parameters for which the average weights were not significant showed important spatial oscillations according to seasons, in relatio...

Nokoué Lake is a complex ecosystem, the understanding of which requires control of physical processes that have occurred. For this, the Surface Water Modeling System (SMS) hydrodynamic model was calibrated and validated on the water depth data. The results of these simulations show a good match between the simulated and observed data for bottom roughness and turbulent exchange coefficients, of 0.02 m −1/3 •s and 20 m 2 /s respectively. Once the ability of the model to simulate the hydrodynamics of the lake is testified, the model is used to simulate water surface elevation, exchanged flows and velocities. The simulation shows that the tidal amplitude is maximum at the inlet of the channel and decreases gradually from the inlet towards the lagoon's main body. The propagation of the tidal wave is characterized by the dephasing and the flattening of the amplitude tide, which increases as we move away from the channel. This dephasing is characterized by a high and low tides delay of about 1 or 4 h and also depends on the tide amplitude and location. The velocities inside the lake are very low and do not exceed 0.03 m/s. The highest are obtained at the entrance of the channel. In a flood period, in contrast with the low-water period, incoming flows are higher than outflows, reinforced by the amplitude of the tide. An average renewal time of the lake has been estimated and corresponds during a flood period to 30 days for an average amplitude tide and 26.3 days on a high amplitude tide. In a low water period it is 40.2 days for an average amplitude tide and 30 days for a high amplitude tide. From the results obtained, several measures must be taken into account for the rational management of the lake water resources. These include a dam construction at the lake upstream, to control the river flows, and the dredging of the channel to facilitate exchanges with the sea.

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