Bacterioplankton of the River Sections of the Kilia Danube Delta

Microbiological research (part of complex hydrobiological investigations of different types of water bodies in the lower Danube over the years 2018-2023) showed that the bacterioplankton community of the river sections of the Kilia Danube Delta is quite abundant (total numbers 5.5-32.2 mln. cells/cm<sup>3</sup>, mean 13.3±6.6 mln. cells/cm<sup>3</sup>), the numbers of bacteria with an intact cytoplasmic membrane (5.9±5.1 mln. cells/cm<sup>3</sup>), eutrophic bacteria (241.0±222.5 th. cells/cm<sup>3</sup>) and oligotrophic bacteria (150.5±184.7 th. cells/cm<sup>3</sup>) also were high. The peculiar feature of the bacterioplankton community of the studied areas consists in significant numbers of dead bacteria (cells with damaged cytoplasmic membrane, 1.85-24.1 mln. cells/cm<sup>3</sup>, mean 7.4±4.6 mln. cells/cm<sup>3</sup>), that is 56.2% of total bacterioplankton numbers. In some areas, the dead cells' portion exceeded 80-90%. It was suggested that this phenomenon reflects the extreme environmental conditions of the considered region, which are characteristic of the ecotone deltaic ecosystems of the «river - sea» type and is probably related to the multi-vector influence of the sea and local pollution from the adjacent territory. It was established that the ratio of the of dead cells number and one live bacterial cell was the highest (>5) in areas subjected to the potential anthropogenic influence by the municipal and industrial effluents (the Vylkove town). This ratio is proposed to be used for ecological indication. The number of eutrophic and oligotrophic bacteria differed significantly in different seasons. ANOVA proved the lowest abundance of eutrophic bacteria (<i>r</i>-strategists) in winter and predominance of oligotrophic bacteria (<i>K</i>-strategists) in autumn (Wilks Lambda = 0.00479, F (32, 22) = 8.7032, <i>p</i><0.001). The conditionally-pathogenic bacteria of different groups were present in most samples and sites. According to the <i>Escherichia coli</i> and <i>Enterococcus faecalis</i> numbers (regulated by Directive 2006/7/EC), which exceeded the regulatory norms by tens and sometimes hundreds times, most of the studied sections of the lower Danube were characterized by the unsatisfactory quality of the water environment. The total bacterioplankton numbers (in the autumn/winter period) in the close to the sea river sections of the Kilia Danube Delta was significantly higher (Wilks Lambda = 0.01288, F (32, 12) = 2.9289, <i>p</i> = 0.02569). The factor analysis (the principal components method) showed confident relationship between the abundance of some groups of the conditionally-pathogenic bacteria in water (<i>Enterococcus, Escherichia, Pseudomonas</i>) and dead bacteria, which indicates significant impact of local conditions (marine water and pollution) on the sanitary state of the water ecosystems of the region.