Homogeneous microbial diversity in the upper sediment layers of a shallow lake

The variability and patchiness in composition of bacterial communities were studied in a large eutrophic boreal lake (280 km2) with a mean depth of <3 m. The present study was undertaken to determine the factors that influence species dispersal in the sediment environment of large shallow lakes and to test the hypothesis that species dispersal in the sediment is heterogeneous. Surficial sediment samples were collected according to both transect and ‘log-wise’ sampling designs. For the vertical slicing of sediment samples, steps were chosen according to average deposition rates. Bacterial community composition (BCC) was examined by denaturing gradient gel electrophoresis (DGGE) of PCR amplification products from DNA extracted from the total community. The relationships of the DGGE fingerprint to space, bacterial abundance, production, and dry weight of the sediment were analyzed using multivariate statistical methods. The BCC was found to be uniform over both the 1 to 1000 m (‘log-wise’) and kilometer (>10 km transect) scales. An increase in bacterial diversity from the river bed to mid-lake corresponded to the sediment type and was related to an increase in productivity. Vertically, the bacterial diversity decreased with sediment depth while the bacterial community in the upper 2 to 5 cm of sediment was almost homogeneous. These data suggest that BCC of organic-rich sediment is very homogeneous horizontally, and the upper centimeters of the sediment undergo frequent mixing, leading to similar homogeneity.