Dynamique Spatio-temporelle du Bactérioplancton d'une aire marine Méditerranéenne perturbée par l'effluent urbain de l'agglomération marseillaise

Abstract

The sewage originated by the city of Marseilles (more than 1 M inhabitants) is entering the sea water at surface level in the calanque of Cortiou (6–8 km east of the city). In the 6 km 2 basin concerned with the polluted flux, bacterial communities demonstrate some behavorial analogies with other plankton. In spite of organic and mineral enrichments, both bacterial and phytoplanktonic densities are similar to those of the oligotrophic surrounding sea water. As for zooplanktonic species, which are able to resist in polluted environments, bacteria show capabilities to grow in various cultural conditions. No effect of the annual temperature cycle is noted on bacterial numbers. But, considering the bacterial community dynamics, the mechanisms leading to these similarities are quite different for bacteria. Heterotrophic bacteria mineralize the exogenous organic matter far from the ‘point source’, as demonstrated by the good correlation between N-NH 4 and viable counts ( r=0·98). The fluctuations of the numbers of viable heterotrophs and total coliforms (incubated at 37°C) are similar ( r=0·98 for n=60) showinga low selective effect of the temperature and the capability for the same bacteria to grow both on selective or not selective medium. The biomass production resulting is shown by high values of the frequency of dividing cells and is due to heterotrophic bacteria. Bacterial production is more important in the more polluted part of the basin (N-NE), but, the bacterial density is regulated by factors varying in relation to the distance to the source of sewage. Near the mouth of the outlet sedimentation, dispersion, zooplanktonic grazing of particles with attached bacteria and bacterial predators grazing on free-living bacteria are the major causes of bacterial disappearance. In the west and south areas, where pollutants and organic material are diluted, the amount of substrate necessary for energy maintenance also plays a role. On the other hand, in these areas, no attempt was made to detect viable but not active cells in direct counts.