Influence of inlet bacteria on bacterioplankton assemblage composition in lakes of different hydraulic retention time

The influence of inlet bacteria on the assemblage composition of bacterioplankton was investigated in two Swedish forest lakes of different hydraulic retention time. Assemblage composition of the bacteria in lakes and running waters was determined by denaturing gradient gel electrophoresis (DGGE) and sequencing of polymerase chain reaction (PCR)-amplified 16S rDNA. The amount of bacterial cells imported via the inlets in relation to bacterioplankton cells produced in the epilimnion of the lakes was also determined. In the lake with short retention time (theoretical hydraulic retention time of 0.3 yr) the lake bacterioplankton assemblage largely resembled the riverine assemblages, although the extent of similarity varied among inlets, depending on water flow. In the lake with long retention time (theoretical hydraulic retention time of 10 yr) the bacterioplankton assemblage in the lake had low similarity to the inlet assemblages. The degree of similarity between inlets and lakes was well correlated to the amount of imported cells. Thus, our data suggest that import of inlet bacteria could have a large effect on the composition of lake bacterioplankton assemblages and that hydrological factors determined the magnitude of this effect. Since short hydraulic retention times are very common in lakes in the boreal zone, input of allochthonous bacteria can be one major factor influencing bacterioplankton assemblage composition in boreal lakes.