Evolution saisonniere de la structure des communautes microbiennes dans un reservoir d'eau potable

Abstract

The development of bacterial communities in drinking water supply networks may give rise to bacterial concentrations exceeding drinking water standards and also lead to the establishment of a food chain which allows the growth of macroorganisms incompatible with water quality requirements. Under such circumstances, drinking water reservoirs with relatively long residence times undoubtedly represent a very weak link in efforts to maintain water quality within a distribution network. Nevertheless, very few studies have examined the microbial communities and their trophic relationships in drinking water reservoirs. The goal of this study was to identify and quantify the microbial communities growing in a drinking water reservoir, and to follow their seasonal dynamics. Following cleaning and filling of the reservoir, 15 series of samples were collected between November 1991 and July 1992. Microorganisms of the inflowing water as well as those growing in the reservoir were analyzed. Bacteria were fixed in formaldehyde, stained with 4.6 diamino 2 phenylidole (DAPI) and counted by epifluorescence microscopy (Porter and Feig, 1980). Autotrophic and heterotrophic flagellated protozoa were fixed with glutaraldehyde, stained with primuline and counted by epifluorescence microscopy. Ciliates and amoebae were fixed with mercuric chloride (HgCl 2), microalgae with Lugol's solution, and all three types of organisms were counted with an inverted microscope. Rotifers and crustacea were fixed with formaldehyde and also counted with an inverted microscope. Chlorophyll a was extracted with 90% acetone and analyzed by HPLC according to Mantoura and Llewelyn (1983). The biomass of autotrophic microorganisms (essentially microalgae belonging to the class of Diatomophyceae) was very low and consisted of senescent cells in both, the inflowing water and the reservoir. Heterotrophic microbes were dominated by bacteria. The latter made up 84.5 and 91% of the total biomass of microbial heterotrophs in the reservoir and inflowing water, respectively (Fig. 9). Their concentration was greater in the reservoir ( M = 0.29 · 10 6bact · ml −1) than in the inflowing water ( M = 0.15 · 10 6bact · ml −1). The respective differences in biomass were even more pronounced because the inflowing bacteria were considerably smaller (mean biovolume V = 0.24 μm 3) than the reservoir bacteria (mean biovolume V = 0.53 μm 3) (Table 2). Nevertheless, the respective seasonal variations of the bacterial biomasses were parallel and appeared to relate to the temperature changes between November and May. After, the increase of total and free chlorine concentrations and/or the decline of biodegradable organic carbon (Table 3) led to a dramatic drop of the bacterial biomass. Flagellates numerically dominated the protozoa (98 and 94%, respectively, of total protozoan abundance in the inflowing water and reservoir), followed by ciliates (1.7 and 3%) and by naked amoebae (0.3 and 3%). Ciliates, however, had the greatest biomass (Fig. 10). The biomass of all groups of protozoa was always greater in the reservoir than in the inflowing water (Table 3). The seasonal development of the flagellated protozoa followed that of the bacteria (Figs 2 and 3). Conversely, ciliates and amoebae declined during spring with the growth of rotifers and crustacea (Figs 4, 5 and 8). Predation by the latter two appears to be an important factor limiting the development of protozoa. In conclusion, the drinking water reservoir contains a functional ecosystem with well established and structured microbial communities. The reservoir even appears to further the development of micro-organisms, because, excepting autotrophs, their abundance and biomasses are consistently higher in the reservoir than in the inflowing water (Table 2). The population dynamics of bacteria and flagellated protozoa largely depend on the seasonal changes of temperature. However, during the warm water season, drastic measures of water management (e.g. the increase of free chlorine concentration and the decrease of biodegradable organic carbon) dramatically reduce the biomasses of bacteria and flagellated protozoa in the water.