Bacterioneuston Abundance and Physicochemical Properties of the Surface Microlayer of Lake Baikal

Abstract—The aquatic surface microlayer at the interface between the atmosphere and the hydrosphere occupies 70% of the Earth’s surface, covering all water bodies. The surface microlayer is about 50 μm thick. The special microbial community formed here is referred to as neuston. The total bacterial abundance in the surface microlayer (SML) and underlying waters (UW) of Lake Baikal was studied by epifluorescence microscopy. The physicochemical features of the surface microlayer of Lake Baikal have been established for the first time. SML was sampled throughout all of Lake Baikal in May–June of 2013–2016 and in August of 2013, 2015, and 2016. SML samples were taken from a boat with Garret’s metal mesh screen (26.5 cm in diameter) mostly in calm weather. The average values of total bacterial abundance in SML varied from year to year within the range of (0.93–1.49) × 10⁶ cells/mL in May–June and (1.73–2.24) × 10⁶ cells/mL in August; in the UW (at a depth of about 15–20 cm), these values were 0.79–0.89 × 10⁶ cells/mL in May–June and 1.15–1.4 × 10⁶ cells/mL in August. There was a significant difference and direct relationship between the total abundances of neuston and plankton bacteria of Lake Baikal in the summer period. The differences in chemical composition between the surface microlayer and the subsurface water layer were revealed in all the seasons under study. The surface microlayer was enriched in \({\text{PO}}_{4}^{{3 - }}\), total organic carbon, and suspended particulate matter compared to the underlying water layers. There was a direct relationship between bacterial numbers in the surface microlayer and the concentration of suspended particulate matter.