Exceptional phenomenon of colored waters during spring 2007 in the Lebanese coastal zone of Zouk-Nahr el Kelb

A brownish coloring in the Lebanese coastal water of the Zouk-Nahr el Kelb area was detected in May 2007. The qualitative and quantitative analyses, in comparison with preceding data, showed that this phenomenon was due mainly to the massive and exceptional bloom of both a centric diatom Skeletonema costatum (Greville) Cleve reaching 1,6 t 10 super(7) L super(-1) cells and of an ichthyotoxic raphidophyte Heterosigma akashiwo (Hada) Hada reaching approximately 9 t 10 super(6) L super(-1) cells. The maximum of this proliferation was located in surface water opposite Zouk power plant where the measured rates of chl-a reached 32 mg.m super(-3) indicating a phenomenon of eutrophication. Attempts were made at explaining the mechanism of development of this algal bloom. The hydro-climatic conditions prevailing during a heat wave coupled with nutrient availability are probably the cause to the development of this phenomenon. The fact that the benthic cysts produced by the cells of H. akashiwo may develop each year when the conditions of the environment become favorable thus causing a risk of the widening of this phenomenon.Original Abstract: Une coloration brunatre a ete detectee en mai 2007 dans les eaux cotieres libanaises de la region Zouk-Nahr el Kelb. Les analyses qualitative et quantitative ont montre, en comparaison avec des donnees precedentes, que ce phenomene etait du principalement a l'efflorescence massive et exceptionnelle d'une diatomee centrique Skeletonema costatum (Greville) Cleve atteignant 1,6 t 10 super(7) cellules L super(-1) et d'un raphidophycee ichtyotoxique Heterosigma akashiwo (Hada) Hada atteignant environ 9 t 106 cellules L super(-1). Le maximum de cette proliferation a ete situe en surface en face de la centrale electrique de Zouk ou les taux de chl-a mesures ont atteint 32 mg.m super(-3) indiquant un phenomene d'eutrophisation. Des essais ont ete realises visant a expliquer le mecanisme de developpement de cette efflorescence algale. Les conditions hydro-climatiques regnant durant une vague de chaleur reunies a la disponibilite des nutriments sont probablement la cause de developpement de ce phenomene. Le risque vient du fait que les kystes benthiques produits par les cellules de H. akashiwo peuvent se developper chaque annee quand les conditions du milieu sont favorables et ainsi l'envergure de ce phenomene peut s'elargir.