Electronic Journal of Polish Agricultural Universities is the very first Polish scientific journal published exclusively on the Internet, founded on January 1, 1998 by the following agricultural universities and higher schools of agriculture: University of Technology and Agriculture of Bydgoszcz, Agricultural University of Cracow, Agricultural University of Lublin, Agricultural University of Poznan, Higher School of Agriculture and Teacher Training Siedlce, Agricultural University of Szczecin, and Agricultural University of Wroclaw. ELECTRONIC JOURNAL OF POLISH AGRICULTURAL UNIVERSITIES 2003 Volume 6 Issue 2 Series ENVIRONMENTAL DEVELOPMENT Copyright © Wydawnictwo Akademii Rolniczej we Wroclawiu, ISSN 1505-0297 GOTKOWSKA-PŁACHTA A., NIEWOLAK S., KORZENIEWSKA E. 2003. VERTICAL DISTRIBUTION AND SEASONAL CHANGES IN THE NUMBER OF BACTERIOPLANKTON IN THE WATER OF LAKE HAŃCZA, PARTICULARLY IN THE PERIOD OF RESERVOIR SUMMER STRATIFICATION Electronic Journal of Polish Agricultural Universities, Environmental Development, Volume 6, Issue 2. Available Online http://www.ejpau.media.pl VERTICAL DISTRIBUTION AND SEASONAL CHANGES IN THE NUMBER OF BACTERIOPLANKTON IN THE WATER OF LAKE HAŃCZA, PARTICULARLY IN THE PERIOD OF RESERVOIR SUMMER STRATIFICATION Anna Gotkowska-Płachta, Stanisław Niewolak, Ewa Korzeniewska Department of Environmental Microbiology, University of Warmia and Mazury, Olsztyn, Poland ABSTRACT INTRODUCTION MATERIAL AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES ABSTRACT The paper presents the results of the study on seasonal changes in the number of bacterioplankton, its vertical distribution and morphological composition in the waters of the deepest Polish lake, Lake Hańcza (108.5 m deep). The research was performed from 1997 till 2000. Water samples were collected at a research station placed at a location where the lake depth was maximum, at monthly intervals from May to October. Water was sampled along the following depth profile: from 0-1 cm layer, at 0.3 m and at the depth of 1m, 2m, 5m, 10m, deeper down at 10m intervals towards the bottom. Bacteriological analyses were completed with measurements of selected physico-chemical parameters such as temperature, pH, oxygen, nitrogen and phosphorus contents. Obtained results with regards both to the number of bacterioplankton and its morphological structure, namely 68% for cylindrical rods forms, and physico-chemical assays confirmed the purity of Lake Hańcza. The vertical distribution of bacterioplankton was usually of a micro-zonal character, indicating the 'focal' distribution of organic and nutrient matter, with higher population reported at the surface (during summer stratification at 1-5 m depth) or in demersal water that can be attributed to organic matter of auto- or allochtonic origin accumulating there. Key words: Bacterioplankton, lake, physico-chemical parameters.