Temporal and spatial variation in the community composition of lake bacterioplankton

Sammanfattning: Bacterioplankton are among the smallest and most abundant organisms in lake ecosystems. Due to the lack of relevant methods, the taxonomic composition of bacterioplankton has been studied less thoroughly than that of larger organisms. The work described in this thesis has focused on analyzing temporal and spatial variations in bacterioplankton community composition, especially in relation to environmentalparameters. The community composition was determined by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified (polymerase chain reaction) 16S rDNA (16S rRNA encoding DNA).Temporal changes within a bacterioplankton community in a mesotrophic brownwater lake were observed over a two-year period. These changes appeared to occur slowly and gradually, without any apparent coupling to season. The inflow of water from the drainage area was one possible factor causing the observed changes. The bacterioplankton community composition at different depths in two stratified lakes was investigated in a combined field survey and field experiment. The results suggest that differences between strata in the amount of dissolved oxygen, the availability of phosphorus, and temperature were important factors contributing to differences in community composition.Differences in community composition were also detected between five lakes differing in trophic status and humic content. The results indicated that the amount of potential grazers (mixotrophic flagellates and microzooplankton) present in the lakes had an impact on bacterioplankton community structure. The abundance of these organisms appeared to be coupled to the trophic status of the systems, indicating that trophic status at least indirectly influenced the composition of the bacterioplankton communities.Differences in bacterioplankton community composition were also found in five lakes of similar trophic status but different humic content. In these lakes grazers again appeared to be important, as did the size of both the lakes and their drainage areas. The coupling to humic content was less evident.In summary, many physical, chemical, and biological factors within the pelagic environments as well as in surrounding ecosystems appear to influence the taxonomic structure of bacterioplankton communities, which was found to vary in time and space.