Seasonal and Spatial Variations in the Number of Ammonifying and Denitrifying Bacteria in Communities of Plankton and Epilithon from the Littoral Zone of Lake Baikal

We present the results of a study on the number of indicator bacterial groups of the nitrogen cycle in plankton and epilithon of the littoral zone of Lake Baikal in the spring and autumn of 2017–2019. The number of ammonifying bacteria (AB) varied from 25 to 6.0 × 10 5 cells/mL in lake waters and from 3.5 × 10 3 to 9.5 × 10 6 cells/cm 2 in epilithic biofilms. Stations exposed to anthropogenic impact showed a high number of AB as evidence of the active mineralization of nitrogen-containing organic compounds. The maximum number of AB in the water column and the biofilms from stony substrates was detected in June 2017 in the water area of the town of Severobaikalsk. The average number of AB in the water column over the 3-year period was 5.0 ± 2.8 × 10 2 cells/mL in June and 6.0 ± 1.3 × 10 2 cells/mL in September. In epilithic biofilms, it was an order of magnitude lower in June than in September: 3.4 ± 1.2 × 10 4 cells/cm 2 and 2.6 ± 1.4 × 10 5 cells/cm 2 , respectively. The number of denitrifying bacteria (DB) in water and epilithic biofilms varied from 0 to 60 cells/mL and 6 to 2.5 × 10 3 cells/cm 2 , respectively. Throughout the research period, the highest number of DB in the epilithon was detected in June at the station near Cape Elokhin. The average number of DB over the 3-year period was 13 ± 4 cells/mL in June and 7 ± 6 cells/mL in September in the water column, 7.9 ± 2.4 × 10 2 cells/cm 2 in June and 4.8 ± 0.5 × 10 2 cells/cm 2 in September in epilithic biofilms. The number of planktonic AB positively correlates with the concentration of ammonium ions; the number of DB directly depends on the nitrate concentration and inversely depends on nitrites at some stations.