Seasonal evolution of microbial community structure in a drinking water reservoir

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 (HgCl2), 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).