Autotrophic picoplankton of Lake Baikal: composition, abundance and structure

Abstract

Investigations of Baikalian phytoplankton in 1960s showed that some very small blue-green algae with a cell diameter of 1.5 μm develop in great abundance in Lake Baikal in summer. They were described as a new endemic species – Synechocystis limnetica Popovsk. S. limnetica was found as mass dominant species of the autotrophic picoplankton. Knowledge on species composition of Baikalian picoplankton is limited to this species. Autotrophic picoplankton (APP) communities were studied monthly from 1997 to 2001 at the monitoring station in southern Lake Baikal. Microscopic examinations of picoplanktonic samples based on the fluorescence of pigments, cell morphology and ultrastructure revealed that Lake Baikal АРР is dominated by chroococcoid phycoerythrin-rich cyanobacteria of genus Synechococcus. Cyanobacteria of this genus were represented by various morphotypes throughout the study: coccoid and ellipsoid Synechococcus spp. 0.85 × 1.1 μm by size, constituting the main mass of autotrophic picoplankton, their proportion varying during the year from 74 to 97%; Synechococcus-species with short and long rod-shaped form of cells, reach maximum (8–18%) in the autumnal period. Endemic species of Lake Baikal – S. limnetica contribute 1–4% in APP communities. Baikalian APP was found to include free aggregated cells of Synechocystis- and Synechococcus-type, to 5% in autumn, as well as the colonial species Aphanothece clathrata f. brevis, the contribution of which to total autotrophic picoplankton fraction changes significantly in seasonal and annual aspects. Picoplanktonic green algae are not especially abundant in Lake Baikal, and, on average did not exceed 8% of the total APP. The sizes of picoalgae are 1.25 × 1.75 μm, both single cells of green algae; aggregated forms are noted in the picoplankton. Maximal abundance of autotrophic picoplankton was registered in a near-surface layer in late July (7.36 × 105 cells ml−1), and the minimum number (2.03 × 104 cells ml−1) in May, during the period of spring isothermal mixing.