Successional changes in bacterial community assemblages following anoxia in the hypolimnion of a eutrophic lake

Dynamics of bacterial assemblages following anoxia in the hypolimnion of a eutrophic lake (Lake Aydat) were characterized. The sampling started in spring before complete anoxia and was continued weekly until complete mixing of the water column occurred in autumn. Bacterial community patterns at 3 sampled depths (10, 12 and 14 m) were investigated using temporal temperature gradient gel electrophoresis (TTGE) and terminal restriction fragment length polymorphism (T-RFLP) analyses. Results revealed changes in the structure of the bacterial communities as conditions changed from oxic to anoxic. Once anoxia had been reached, anaerobic bacterial communities continued to change and exhibited gradual successional patterns at the 3 depths. During the anoxic period, bacterial communities at 10 and 12 m were split into 2 groups corresponding to the steps 'until' and 'after' maximum stratification, which occurred on 19 August 2004. While the succession of the bacterial community in the hypolimnion was dynamic and exhibited gradual patterns at the 3 depths, a 'sensitivity gradient' to O-2 depletion was suggested: communities at 10 m appeared to be more affected by the shift in O-2 concentrations than those in deeper water layers.