Hovsgol is the largest deep lake in Central Asia located in the south-west of the Baikal Rift zone. Until now there has been only one study on the autotrophic picoplankton in Lake Hovsgol. The present work summarizes results of picoplankton studies obtained with light, epifluorescence and scanning electron microscopy. Microscopic analysis showed the presence of picoplankton cyanobacteria and algae in Lake Hovsgol. Autotrophic picoplankton of the lake is characterized by morphotype diversity of Synechococcus species, Synechocystis-like aggregates, and picocyanobacteria cells of small sizes. Picocyanobacteria are dominated by Synechococcus species with single, rarely double, coccoid cells. There is also a picoplankton form of the diatom Cyclotella ocellata reaches 4–13 percent of the total species abundance. Three picocyanobacterial cultures belonging to the genus Synechococcus and exhibiting distinct morphological and ultrastructural characteristics were isolated from the plankton samples.

Chlorophyll a content in the pelagic zone of Lake Hovsgol in August was 0.182–0.374 μ g l− 1. In February, the abundance of picoplankton cyanobacteria in the surface layer, which make up the basic autotrophic picoplankton mass, was 2.78 × 105 cells ml−1, in March 1.36 × 105 cells ml−1, and in July 6.06 × 104 cells ml−1. In August, picocyanobacteria abundance fell to 3.42 × 104 cells ml−1 in the 0 to 10 m layer and 7.52 × 104 cells ml−1 in the 0 to 50 m layer. Quantitative variations in autotrophic picoplankton development throughout the lake are negligible. The greatest abundance of picocyanobacteria is recorded in the hypolimnion, 25–50 m deep.

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