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sur le développement des zones arides et semi-arides

Titre : Ecological and biological monitoring of Yarqon river (central Israel) by using microscopic algae

Auteur : Tavassi, Mordechay

Etablissement de soutenance : Haïfa University

Grade : Doctor of Philosophy (PhD) 2008

Résumé
Deteriorating water quality is a major environmental problem worldwide and has already restricted development country in arid and semiarid environments. In Israel this problem was most dramatically observed in the pollution of the largest river, the Yarqon. Dumping of wastes into the river and exploiting most of the Yarqon water by the national water system caused detrimental changes in the ecological balance of the river. Environmental risk assessment using indicator organisms has become widely recognized, especially in developing countries, in order to provide an objective means for judging the quality needed to maintain particular environmental regulations. A recently developed example of an environmental risk assessment is the Water Framework Directive of the European Union. According to this framework, the biological assessment of European surface waters is an integral part of the whole aquatic assessment concept. However, in Israel this method is not well recognized. We had hoped to find more relevant chemical physical and biological parameters of water quality for the monitoring system in Israel. Algal assemblages respond rapidly (within days) to changes in their environment with concomitant changes in overall abundance, growth rates, and species composition. Algal species have characteristic optimal nutrient and trace element requirements, and specific tolerances for cation concentrations, salinity, and pH. Changes in physical, nutrient concentrations, salinity, temperature, oxygenation and chemical water quality characteristics can thus result in rapidly changing species composition. Algae are a highly diverse group of photosynthetic organisms with unicellular reproductive structures. In most habitats these structures function as primary producers in the food chain, producing organic material from sunlight, carbon dioxide, and water. Photosynthetic organisms (mostly algae and plants) synthesize biomass, which is consumed by animals and biodegraded by bacteria and fungi through respiration. Besides constituting the basic food source for these food chains, the algae also form the oxygen necessary for the metabolism of the consumer organisms. When an excess of organic wastewaters is discharged into natural waters, the ecological balance is disturbed. It has long been recognized that pollution can change the structure and function of the natural algal assemblage, and thus have substantial utility for biological assessments. Algae are most useful as indicators in the context of eutrophication but have also detected organic pollution as well as influencing various environmental changes due to their well-documented tolerance. Based on the environmental variables of water quality during the investigation period, the Yarqon River could be clearly divided into three segments : (a) an upper relatively unperturbed section (7.5 km) with very low concentrations of phosphorus and ammonia (<0.1 mg/l), (b) a central section of about 17.5 km, which begins at the point of confluence with the Qane tributary, where municipal effluents from a waste-water treatment plant (Kefar-Sava–Hod-Hasharon) enter the Yarqon River. In the central section we measured higher levels of phosphorus and ammonia of 9.5 and 33 mg/l, respectively, (c) the lowermost section (4 km), inundated by marine water within the tide range, is saline or brackish increasing up to 10.5 mS/cm. Algae of the Yarqon River have been accidentally studied since 1944 till 2000. Their species list till this study was initiated include 32 species only, most of which are the euglenoids. In the present study 319 species of algae and cyanoprokaryotes identified belonging to eight taxonomical divisions, which, among them, the greens (Chlorophyta), diatoms (Bacillariophyceae), blue-greens (Cyanoprokaryota), and euglenoids (Euglenophyta) prevailed, and other divisions were quite rare. The taxonomical distributions of all algal species diversity throughout the entire investigation period, which were dominated by diatom groups, are very similar to the distribution of seasonal algal species (found in dry or wet seasons only). However, we found a different taxonomic distribution for dominant species of each seasonal community. The green chlorococcoid algae were dominated in the dry season communities. These species survived in middle-polluted water with medium contents of organic matter. The diatom species prevailed in rainy season communities indicating a decrease in organic matter pollution. The taxonomic preference for self-purification in the river was more intensive during the wet season, probably due to rainy-floods contributing fresh water to the river.

Présentation -> http://digitool.haifa.ac.il/R/VPDHY...

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