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Diatom assemblages from the sediment traps in Lake Onega : preliminary results 446

Studying the patterns of sedimentation processes is essential for understanding the current state of the lake ecosystems and predicting future environmental changes. A pioneer study of the sediment-trap diatom assemblages in Lake Onega was performed as a part of the research aimed at assessing the spatial and temporal distribution of sedimentary particles in different part of the Lake Onega basin. The sediment-traps diatom assemblages accumulated during a certain period provide an integrated characteristic of the present-day diatom flora of the lake. Comparing their composition with the living diatom communities contributes to understanding the sedimentation processes, and enables assessing the current state of the lake ecosystem.


Introduction
Studying the patterns of sedimentation processes is essential for understanding the current state of the lake ecosystems and predicting future environmental changes. A pioneer study of the sediment-trap diatom assemblages in Lake Onega was performed as a part of the research aimed at assessing the spatial and temporal distribution of sedimentary particles in different part of the Lake Onega basin. The sediment-traps diatom assemblages accumulated during a certain period provide an integrated characteristic of the present-day diatom flora of the lake. Comparing their composition with the living diatom communities contributes to understanding the sedimentation processes, and enables assessing the current state of the lake ecosystem.

Materials and methods
The study sites are located in different parts of Lake Onega (the Petrozavoskaya, Kondopogskaya, Bolshoe Onego, Lizhemskaya, Unitskaya, Maloe Onego, Zaonezhskiy and Povenetskiy bays, and in the southern open-water part of the lake). The water depth at the stations ranges from 12 to 96 m. 12 sediment traps were installed at 1 m from the bottom and exposed for 1 year (2018-2019). The samples treatment for the diatom analysis was performed using the standard procedure (Davydova, 1985).
Previous studies also reported the increased contribution of Diatoma tenuis, Fragilaria crotonensis and Stephanodiscus hantzschii to the phytoplankton assemblages at the areas with highest anthropogenic pressure, such as Petrozavoskaya and Kondopogskaya bays (Chekryzheva, 2012;2015). In the sediment traps, however, these species are rather uncommon, and their proportion rarely exceeds 1%. It could be partly explained by their fragile valves that could already start dissolving in the water column after the cell death. On the other hand, it might also somehow reflect their contribution to the living communities at least during the study period, thus suggesting decreased human impact on the Lake Onega ecosystem compared to the recent past.
Benthic diatoms contribute to 8-39% of the total diatoms reflecting their secondary role in the sedimenttrap diatom assemblages. They are dominated by epiphytes (8-33%) with Tabellaria fenestrata being the most common species, abundant in all study sites (6-13%) except for the Kondopogskaya Bay where it only accounts for <3%. T. fenestrata, the most widespread species in epiphytic diatom communities in Lake Onega, is also common in summer and autumn planktonic assemblages due to the transportation of its long-chain colonies from the shallow-water parts of the lake where it growths on macrophytes (Petrova, 1971). Highest abundances of epiphytes (>20%) were observed in the sediment traps from Lizhemskaya, Unitskaya, Maloe Onego, Zaonezhskiy and Povenetskiy bays, where the largest areas of the littoral zone are occupied by macrophytes (Onezhskoe…, 2010). Surprisingly, the highest abundances of both epiphytic and bottom-living diatoms (33% and 6%, respectively) were found at the deepest station, the Povenetskiy Bay (96 m). Apparently, it results from the complex bottom topography of this remote sheltered bay, where higher water transparency favors growth of the benthic species in the littoral zone, which are later transported to the deepest part of the basin.

Conclusions
The preliminary results of the study of the sediment-trap diatom assemblages in Lake Onega demonstrated that: 1. the composition of the most abundant species generally reflects the composition of the most abundant taxa in the living communities; 2. the specifics of the diatom assemblages composition are thought to result from the local specifics of the study sites, such as anthropogenic pressure, macrophytes stocks, water transparency, bottom topography, etc.; 3. the species indicative of eutrophication are underrepresented in the sediment-trap diatom assemblages, which might reflect both their poor preservation and their minor contribution to the living communities at least during the study period.