Contrasting seasonal and interannual environmental drivers in bacterial communities within a large shallow lake: evidence from a seven year survey

Bacterial populations experience rapid turnover in both time and space; however, the composition and diversity of their communities in plankton and the dynamics of community changes have not been well investigated over longer time scales. We explored the dynamics of bacterial community composition (BCC) monthly over a 7 yr period from 2004 to 2010, within a large, shallow, eutrophic lake. Seasonal and year-to-year changes in BCC were assessed using a PCR-based 16S rDNA fingerprinting technique based on denaturing gradient gel electrophoresis (DGGE). The bacterial operational taxonomic units differed between ice-free and ice-covered periods, and many recurred in specific seasons in consecutive years. The most significant vari- ables related to within-year variation were temperature and the concentration of nutrients. Year- to-year variability was larger than within-year variability. Therefore, interannual variability tended to mask seasonal changes in BCC. The interannual differences in BCC were strongly related to abiotic conditions such as water level fluctuations that generally lead to water column mixing and sediment resuspension, and the qualitative composition of dissolved humic material. All highly related biotic variables that describe the total plankton community (in addition to bac- teria, including phyto-, protozoo- and metazooplankton) were found to be modulated primarily by changes in the abiotic environment.