Running waters are subjected to quick physicochemical changes due to the water fluxes and to to the exchanges with surrounding ecosystems (rural or urbanized area). These fluctuations in physicochemical parameters generate changing local spatial and temporal structures and make necessary a continous adaptation for the living organisms (at the individual or community level).

  Bacterial communities especially adapt themselves very quickly to these changes (short generation time, genetic adaptability) and could be relevant indicators of the ecological status of aquatic ecosystems (Servais et al., 1999). Bacteria are also basic contributors to the evolution of the chemical quality of aquatic systems by controlling the dynamic of biogeochemical cycles and the so called self purification processes. For good understanding of the role of bacteria in these fluctuations it is required that many parameters be taken into account (Bianchi, 1998), for example, accurate measurement of bacterial densities and of their physiological state, metabolic activities and regulation factors. Finally, it is important to know the taxonomic or functional diversity and the relative abundance of the main strains of bacterial assemblages. The consequences should then be addressed in terms of self-purification or assimilation capacity.

  A good example of that is given by the biogeochemical cycle of nitrogen which is of great significance for the ecological quality of freshwater systems (Pauer and Auer, 2000). Particularly the oxidation step of NH4+ and NO2- (harmful or toxic for aquatic life) into NO3- is essential to allow the denitrification to export nitrogen compounds from the aquatic phase to gazeous forms. This major pathway is the nitrification (performed by Gram negatiave bacteria, Nitrobacteraceae family) and is a two sub-step process : ammonium oxidation (from NH4+ to NO2-) and nitrite oxidation (from NO2- to NO3-), which are specifically performed by different bacterial species (McCaig et al., 1999; Cebron, 2001).

  In freshwater environments, the presence of dissolved or particulate nitrogen often results less from an autochthonous production (N2 assimilation) than from anthropic inputs such as agricultural fertilizers lixiviation (non point-source pollution) or wastewater discharges (point-source pollution). These human activities could have a marked effect on the "health" of aquatic ecosystems (Karr, 1999) and, as other microbial-mediated processes, nitrification could be strongly modified by these environmental changes. Wastewater treatment plant (WWTP) discharges in rivers are causes of important and global chemical and biological disruptions of the N cycle (Brion and Billen, 1999, Brion et al., 2000). They have a particular significance for the nitrification as they could supply large quantities of mineral and organic nitrogen compounds, organic carbon, bacterial communities acting as bacterial seeding in rivers, suspended matter, which modify environmental parameters in the water column and the sediment downstream the discharge point.

  Both aspects of the nitrifying communities specificity and of the chemical characteristics of WWTPs make an interesting model for studying the resilience of aquatic communities to a chemical stress.

  What are the characteristics and the changes in the ammonium-oxidizing communities and the nitrite-oxidizing communities, upstream and downstrean a WTP discharge ? Is there any change in the biodiversity of nitrifiers in connection with a chemical stressor ? Could the WWTP discharges act as a seeding for freshwater ecosystems ? The objectives of this paper are to enlighten these questions with some aspects of the dynamics of nitrifying communities in freshwater sediment subjected to an anthropic pressure:

Three study levels have been started on with different experiments : in situ, in microcosms and in batches at the laboratory, using chemical, microbial and molecular tools.
The field approach allowed us to show outwards discrepancies between the activity (markedly influenced by WWTP discharges) and the global densitiy of nitrifiers in river sediment (for both the ammonium-oxidizing communities and the nitrite-oxidizing communities). Data acquired on 6 differing small water courses illustrated the importance of the physico-chemistry of the sediment and the impact of WWTP was found to be site-specific. Besides the use of pure culture of nitrifier strains, the tools or of techniques coming from the molecular biology (such as molecular probes, fluorescent in situ hybridization, immunofluorescence, Féray et al., 1999; Cebron, 2001) are very useful for studying the dynamic of nitrifiers populations, wether at the laboratory level (microcosm) or in situ (Féray, 2000).

It was shown in this way that the WWTP discharges could act as seeding in nitrifiying bacteria for freshwater ecosystems (Brion and Billen, 1999 ; Montuelle et al., this paper) and that they could change the 'natural' biodiversity of the in-place communities (Montuelle et al., 1996 ; Bonnet et al., 1997). However the characterization of in situ changes in biodiversity still remains tricky although the FISH technique is quite promising for ecological studies (Cebron, 2001). A precise approach of the resilience of bacterial communities in polluted freshwater systems and the assessment of the functional consequences of the changes in the bacterial biodiversity in term of self purification capacities is still developing.

References

Bianchi M., 1998, Nouvelles approches d'étude des réseaux microbiens, Annls. Limnol., 34, 4, 465-473.
Bonnet C., Volat B., Degrange V., Bardin R., Montuelle B., 1997, Use of immunofluorescence technique for studying a Nitrobacter population from wastewater treatment plant following discharge in river sediments: first experimental data, Wat. Res., 31, 3, 661-667.
Brion N., Billen G., 1999, Wastewater as a source of nitrifying bacteria in river systems : the case of the river Seine downstream from Paris, Wat. Res., 34, 12, 3213-3221.
Brion N., Billen G., Guezennec L., Ficht A., 2000, Distribution of nitrifying activity in the Seine river (France) from Paris to the estuary, Estuaries, 23, 5, 669-682.
Cebron A., 2001, Approche de la diversité in situ des nitrifiants en sédiment d'eau douce: limites et apports de la technique FISH, Rapport de DEA, U. Claude Bernard Lyon I - Cemagref, 35p.
Féray C., 2000, Nitrification en sédiment d'eau douce : incidence de rejets de stations d'épuration sur la dynamique de communautés nitrifiantes, Thèse Université Cl. Bernard, Lyon I, n° 58-2000, 202p.
Féray C., Volat B., Degrange V., Clays-Josserand A., Montuelle B., 1999, Assessment of three methods for detection and quantification of nitrite-oxydizing bacteria and Nitrobacter in freshwater sediments (MPN-PCR, MPN-Griess, Immunofluorescence), Microb. Ecol., 37, 208-217.
Karr J.R. (1999) Defining and measuring river health, Freshwat. Biol., 41, 221-234.
McCaig A. E., Phillips CJ , Stephen JR, Kowalchuk G., Harvey SM, Herbert RA, Embley TM, Prosser JI. (1999) Nitrogen cycling and community structure of proteobacterial b-subgroup amonia-oxidizing bacteria within polluted marine fish farm sediments. Appl. Environ. Microbiol. 65:213-220.
Montuelle B., Volat B., Torio-Fernandez M.M., Navarro E., 1996, Changes in Nitrobacter serotypes biodiversity in a river: impact of a wastewater treatment plant discharge, Wat. Res., 30, 5, 1057-1064.
Pauer, J. J., Auer MT, (2000) Nitrification in the water column and sediment of a hypereutrophic lake and adjoining river system. Wat. Res. 34:1247-1254.
Servais P., Garnier J., Demarteau N., Brion N., Billen G., 1999, Supply of organic matter and bacteria to aquatic ecosystems through wastewater effluents, Wat. Res., 33, 16, 3521-3531.