SPATIAL PATTERN OF MICROBIAL NUMBERS IN THE FREE WATER OF THE SREBARNA LAKE Svetlana Naumova Central Laboratory of General Ecology, BASc The Srebarna Lake is a hypereutrophic freshwater basin situated on the right bank of the Danube between river-km 393 and 391. It covers about 2,5 km² with an area of about 1 km² that is free of macrophytes which is surrounded by reedbeds with smaller and more or less isolated pools within. The water body is polymictic, and the depth of the free water is quite variable depending on the water influx, but usually it is about 2--3 m. The Srebarna Lake as a unique wetland ecosystem was designated as a Monument of the World Cultural and Natural Heritage, a UNESCO Biosphere Reserve, a Ramsar Site, and an Important Bird Area. It was declared a Mana-ged Reserve after a Management Plan was elaborated and approved for im-plementation. The Srebarna Lake was subjected to ecological monitoring from 2001, and certain microbiological analyses were being carried out within the general investigations of the organic matter production and destruction. The objective of this work was to outline the spatial pattern of the microbiological characteris-tics in order to define the sampling strategy and to test the representativeness of the principal monitoring site in the center of the free of mac-rophytes area. Parameters Assessed  Total bacterial number (by direct microscopic count)  Total reproduction rate (after the in situ doubling time of the total number)  Organotrophs numbers (total and ammonifyers by MPN viable count) 1 2 3 Sampling The horizontal distribution of the parameters values was surveyed in integral samples from the whole water column in three points of a South--North transect of the free water: 1 -- at Varban Bozu, 2 -- in the center, 3 -- at the entrance of the Dragayka channel. The change of bacterial numbers with depth was examined in the center of the free water (p. 2) at the surface, at the lower edge of the transparent water column (Secchi depth), and at the bottom. The study was carried out in July and October 2004. The numbers of the two groups of organotrophs we-re confined in the limits of an order in July, as well as in October. Their changes did not reveal any gradient along the transect, naturally ensu-ing from the organic matter and oxygen content, and the water temperature. Conclusion The spatial uniformity of the microbiological pattern proved the adequacy of the site in the center of the free water to the objectives of the ecological monitoring of the Srebarna Lake. No substantial differences were disclosed by the microbiological indicators among the three points along the transect. The total bac-terial count and the reproduction rate were conservative, represen-ting the transitory state of the wa- ter body in the two observations. July The microbiologi-cal parameters cha-racterized the water column in the center of the free of macro-phytes area as fairly homogeneous. The total bacterial count and the reproduction rate changed with the depth insignificantly, which is normal for a shallow lake with active water circulation. The slight vertical dynamics of the total number of carbophilic organotrophs could be related to the development of phytoplankton. The numbers of ammonifyers, being the most variable of the parameters, supervened chiefly the oxygen concentration. Aknowledgements: The Ecological Monitoring of the Srebarna Biosphere Reserve is supported by the National Envionment Protection Fund of MEW, Gant No. 3397