Current State of Bacterial Community of Lake Ladoga

The data are presented on a quantitative assessment of the bacterioplankton community of Lake Ladoga and total bacterial numbers dynamic in the period from 2009 to 2019. The trophic state of the lake and water quality in different areas have been characterized by microbiological parameters. Maximum concentrations of microorganisms and highest water temperatures were commonly observed in the shallow coastal area (Volkhov Bay, up to 6.40 × 10⁶ cells ml⁻¹), and minimum—in the deepest part of the lake (ca. 0.45 × 10⁶ cells cm⁻³). The bacterial abundances correlate with water temperatures both within each year of observations and over the whole study period. The trophic state of the lake expressed by the total bacterial numbers during summer seasons closely matches the state during the first decade of the 21st century. In the period of 2009-2019, the trophic state varied from oligo-mesotrophic in the hypolimnion of deep-water areas to mesotrophic eutrophic in the shallow southern coastal area, which corresponds to the trophic state indicated by chlorophyll-a concentrations. Within the previous decade (2000–2008), the waters of Lake Ladoga were “clean” and “especially clean”, while during the 2017–2019 period, water masses all over the lake can be identified as “especially clean”. From 2017 to 2019 the coccoid forms of bacterial cells prevailed over rod-shaped cells comprising 57.6 ± 5.1% (2017), 64.4 ± 4.5% (2018), 61.1 ± 4.6% (2019), which imply the high quality of the waters.