Répartition et dynamique de différentes populations bactériennes autochtones et allochtones dans les eaux de l'étang de Thau

The purpose of this study is to investigate the behavior of two bacterial communities in the Thau brackish lagoon (Mediterranean coast of France) : the bacterial community from continental origin (allochtonous community) and the bacterial community endemic to the lagoon (autochtonous community). In a first step, the ability of two standard agar media (bioMerieux, nutrient agar at low salinity, 8; Difco, marine agar at high salinity, 34) to distinguish the two communities is tested. Then, abundances sampling of these two bacterial variables for several scales of observation permit to describe their spatial and temporal evolutions. The different encountered behaviors allow us (i) to improve the definition of an optimal sampling design for such bacterial variables and (ii) to examine several hypotheses about the relationships between autochtonous and allochtonous bacteria and environmental variables. Finally, in order to look for the composition and the structure of these two communities, a morpho-macroscopic method is proposed and compared with the biochemical method commonly used API-20B (API-SYSTEM). On one hand this method appears to be a simple, fast and very economic way to estimate structure indices, even if the concordance with the biochemical method is not absolute (Kappa coefficient). On the other hand it seems more usefull to study the spatio-temporal behavior of some "typical" bacterial populations at the ecosystem scale.