PHYTOPLANKTON ASSEMBLAGES IN BIR M'CHERGA FRESHWATER RESERVOIR (TUNISIA)

Amel Ben Rejeb Jenhani, Afef Fathalli, Mohamed Salah Romdhane
Unité de Recherche Ecosystèmes et Ressources Aquatiques, Institut National Agronomique de Tunisie 43, Avenue Charles-Nicolle, 1082 Tunis Mahrajène, Tunisia

Abstract

Tunisia is a Mediterranean country characterized by aridity on the major part of its territory. To this aridity is added the variability of the Mediterranean climate, with erratic and unpredictable periods of large drought and violent floods, to make of water an often limited resource and distributed unequally in the time and in the space. Facing this shortage of water, Tunisia developed a strategy of surface water resources mobilization. Consequently, the continuous increase of artificial water systems (reservoirs) provided a privileged investigation field for limnological research. Being the first link in the trophic web, phytoplankton plays a basic role in aquatic ecosystems. In fact, the processes, related to it, affect directly or indirectly the environment. Thus, a monitoring of the environmental conditions and phytoplankton assemblages was carried out in Bir M'cherga reservoir, from witch problems of fish mortality was reported in the recent years. In this way, we have identified 50 phytoplankton species spreading across seven classes, namely chlorophyceae, diatoms and cyanobacteria. These latter was the most diversified group since it represented 36% of the species richness, against 34% for chlorophyceae and 14% for diatoms. In quantitatively terms, the successions of the main phytoplankton species showed large seasonal and inter-annual variations, but with a clear dominance of cyanobacteria. Indeed, it is very important to note that 80% of  samples were dominated by filamentous cyanobacteria mainly Limnothrix sp, at more than 50% and up to 99%. Thus, the phytoplankton in Bir M'cherga reservoir did not present the classically succession described in the Tunisian reservoirs, where the Chlorophyceae and diatoms are dominant. It seems that the high trophic level of the studied water body contributed largely to the expansion of these micro-organisms. Cyanobacteria pose a potential risk to the environment and public health since they can produce toxic secondary metabolites including hepatotoxins that have carcinogenic potential, neurotoxins and lipopolysaccaride endotoxins. In fact, in Bir M’cherga dam, eight potentially toxic cyanobacteria species, such as Cylindrospermopsis raciborskii, Microcystis aeruginosa and Planktothrix agardhi, have been identified. It is important to note that the hepatotoxicity in Bir M’cherga freshwater, evaluated by PP2A assay, didn’t exceed the WHO guideline value for drinking water, but it was positively correlated with temperature, salinity and chlorophyll a.

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