Seasonal dynamics and spatial distribution of structural indicators of the bacterioplankton community of the Sevastopol Bay (the Black Sea)
Bacterioplankton community determines formation of a significant part of the secondary production and mineralization of organic matter in aquatic ecosystems, as well as responds quickly to any changes in the environment. Data on the state of the microbial community are required for understanding the processes of substance and energy flow transfer in aquatic ecosystems; this is especially important for coastal waters, where significant negative transformations have occurred in recent decades. The aim of this study was to investigate and analyze changes in structural indicators of the bacterioplankton community in different areas of the Sevastopol Bay (the Black Sea) during 1992-2005. Bacterial abundance was determined by direct microscopy, using adsorption (erythrosine) or fluorescent (acridine orange) stains; biomass was calculated using a conversion factor (2·10⁻¹⁴ g C·cell⁻¹) or by direct cell measurements. Cell morphotypes were determined by scanning electron microscopy. The total abundance of microorganisms varied 0.2·10⁶ to 10·10⁶ cells·mL⁻¹; biomass - 2 to 201 mg C·m⁻³. In the morphological structure of bacterioplankton community, cocci (0.36-0.86 μm in diameter) with a volume of 0.02-0.27 μm³ and rod-shaped cells (0.6-1.2 μm length; 0.2-0.4 μm width) with a volume of 0.50-0.65 μm³ prevailed. Maximum values of the bacterioplankton abundance, biomass, and cell size in the Sevastopol Bay were registered in summer and autumn (June to October), while minimum values were recorded in winter and spring. The observed values of bacterioplankton quantitative indicators were comparable with the values for various coastal water areas of the World Ocean, inter alia the Black Sea. The dynamics of bacterioplankton structural indicators of the Sevastopol Bay during the annual cycle was determined by abiotic and biotic environmental factors. High correlation (86 %, p < 0.01) between the hydrological, hydrochemical, and biological variables confirms the non-random nature of the relationship between them. The discriminant analysis revealed significant differences in the structure of bacterioplankton communities for the bay areas with different intensity of water exchange, degree of general pollution, and distance from the open sea. Significantly smaller bacterial cell volume in 2004 [(0.16 ± 0.05) μm³] compared with that of 2005 [(0.20 ± 0.03) μm³] (paired t-test, p < 0.05) was probably related to intense microorganisms' grazing by phagotrophic protozoa. The obtained data on the structure of the bacterioplankton community can be used for forecasting the state of the Sevastopol Bay ecosystem, as well as for developing and verifying mathematical models of coastal ecosystems functioning.