SEASONAL EVOLUTION OF MICROBIAL COMMUNITY STRUCTURE IN A DRINKING-WATER RESERVOIR

The development of bacterial communities in drinking water supply netw orks may give rise to bacterial concentrations exceeding drinking wate r standards and also lead to the establishment of a food chain which a llows the growth of macroorganisms incompatible with water quality req uirements. Under such circumstances, drinking water reservoirs with re latively long residence times undoubtedly represent a very weak link i n efforts to maintain water quality within a distribution network. Nev ertheless, very few studies have examined the microbial communities an d their trophic relationships in drinking water reservoirs. The goal o f this study was to identify and quantify the microbial communities gr owing in a drinking water reservoir, and to follow their seasonal dyna mics. Following cleaning and filling of the reservoir, 15 series of sa mples were collected between November 1991 and July 1992. Microorganis ms of the inflowing water as well as those growing in the reservoir we re analyzed. Bacteria were fixed in formaldehyde, stained with 4.6 dia mino 2 phenylidole (DAPI) and counted by epifluorescence microscopy (P orter and Feig, 1980). Autotrophic and heterotrophic flagellated proto zoa were fixed with glutaraldehyde, stained with primuline and counted by epifluorescence microscopy. Ciliates and amoebae were fixed with m ercuric chloride (HgCl2), microalgae with Lugol's solution, and all th ree types of organisms were counted with an inverted microscope. Rotif ers and crustacea were fixed with formaldehyde and also counted with a n inverted microscope. Chlorophyll a was extracted with 90% acetone an d analyzed by HPLC according to Mantoura and Llewelyn (1983). The biom ass 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 mi crobes were dominated by bacteria. The latter made up 84.5 and 91% of the total biomass of microbial heterotrophs in the reservoir and inflo wing water, respectively (Fig. 9).