BALWOIS 2008 – Ohrid, Republic of Macedonia – 27, 31 May 2008 1/9 Bacterioplankton Dynamics and the Influence of Environmental Factors on it in the Srebarna Lake Hristina Kalcheva 1 , Michaela Beshkova 1 , Luchezar Pehlivanov 2 and Roumen Kalchev 1 1 Institute of Zoology, Bulgarian Academy of Sciences, 1 Tsar Osvoboditel Blvd., 1000 Sofia, Bulgaria 2 Central Laboratory of General Ecology, Bulgarian Academy of Sciences, 2 Yurii Gagarin Str., 1113 Sofia, Bulgaria E-mail: hristinakalcheva@yahoo.com Abstract The Srebarna Lake is located on the Bulgarian bank (2 kilometers to the south) of the Danube River, 16 km west of the town of Silistra (close to Bulgarian-Romanian boundary) and is the most prominent reserve in Bulgaria. It has been a Protected Site since 1942, a Reserve of the Biosphere since 1977 and was included in the List of the World Heritage Sites in 1983. Srebarna (Silver) is a freshwater hypereutrophic shallow lake. The flora and fauna in the lake and its surroundings have been monitored and the data have been published for many years; however, bacterioplankton and its trophic relationships with environmental factors have been poorly investigated. This study explained the dynamics of bacterioplankton development in spring, summer and autumn during the years 2005- 2006, the influence of physical and chemical factors on this development and the importance of bacterioplankton size structure in the predator-prey relations between the microbial communities and the first two trophic levels of the classical grazer food chain in the pelagial - phytoplankton and zooplankton, by means of statistical analyses. Keywords: bacterioplankton, trophic relationships, microbial food web. Introduction At present Srebarna is a hypereutrophic lake in the initial phase of the classic type of a limnetic succession: hypereutrophic lake – marsh – wet meadow. The first indirect reports on water quality of the Srebarna Lake assumed that the lake trophic state in the beginning of the 20 th century had been mesotrophic to eutrophic. The lake was determined as a Protected Site in 1942 mainly due to its significance as a habitat of the numerous rare waterfowl. In 1949 the lake was isolated from the Danube River by a dike, which reflected negatively on hydrologic conditions and decreased the species composition and diversity in the lake. The connection with the Danube was somewhat restored in 1978 but the river waters did not enter the lake every year. The lack of sufficient connection with the Danube for a long time diminished the lake maximum depth, led to accumulation of bottom sediments and changed the trophic state from eutrophic in 60’s to hypereutrophic. The Srebarna Lake became Reserve of the Biosphere in 1977, was included in the UNESCO's List of the World Heritage Sites in 1983 and as an Important Bird Area (IBA) in 1990. In 1994 a canal connecting the Danube with the lake was built. It improved considerably the ecological conditions in the lake and increases in the number and diversity of species typical for the region have been reported after the substantial ecological monitoring activities by research groups from the Central Laboratory of General Ecology (CLGE), Bulgarian Academy of Sciences and other scientists. The history, changes before and after the connection (the canal) with the Danube River and checklist of the species composition in the Biosphere Reserve Srebarna were particularly presented by Michev et al. (1998). According to Vassilev et al. (in press), in the period before 1994 the lake gross primary production was higher (970 g C m -2 a -1 , high hypertrophy) than in the period after (360-430 g C m -2 a -1 , low hypertrophy). Furthermore, during the years 2005-2006 due to extremely high levels of the Danube River much stronger dilution effects in the Srebarna Lake were observed, leading to substantial improvement of many lake qualitative and quantitative indices (Kalchev et al., 2007). Aquatic bacteria are the most abundant community in the pelagial and for a long time they have been considered only as decomposers of the dead organic matter. Their significance in trophic interactions has been underestimated. After many years’ investigations (Azam et al. 1983, Porter et al. 1988) the “microbial food web” (named “microbial loop”) conception about the trophic relations and the role of the microbial communities in the processes of energy transfer and nutrient recycling has been suggested. The trophic role of microbial communities (microbial loop) weakens when the lake trophic state increases from oligotrophic to eutrophic (Porter et al., 1988, Straškrabová et al., 1999). Both nutrient