Temporal dynamics of structural and functional characteristics of the Yenisei river phytoplankton downstream of the Krasnoyarsk hydroelectric power station

The Yenisei River is the biggest waterway in Siberia. In the second half of the 20th century, the Yenisei River was overlapped by dams, and it deeply damaged the natural hydrological, hydrochemical and hydrobiological regimes of the river. As a result of constructing the Krasnoyarsk hydroelectric power station, a significant river component - phytoplankton, which occupies the central place in the formation of the aquatic ecosystem - has changed downstream of the river. In the river, phytoplankton is formed from phytoperiphyton and allochthonous algae, including phytoplankton species of upstream reservoirs. Therefore, knowledge of transformation laws of phytoplankton species composition, which occurs during the regulation of the river flow and formation of reservoirs, may contribute to the development of ideas about changing the aquatic communities. The aim of this research was to explore the formation of phytoplankton under the conditions of water discharge in the Krasnoyarsk HPS and to study chlorophyll a concentration. We collected phytoplankton samples at a distance of 34 km below the Krasnoyarsk HPS (55°59'1.8"N, 92°47'13.4"E) in 2008-2014 and analyzed them according standard hydrobiological methods (See Table). In 2012-2014, we measured fluorescent water characteristics along with the study of algae. We took the nomenclature for algae considering taxonomic transformations according to the system adopted by the International algae database (http://www.algaebase.org). A quantitative processing of phytoplankton samples was conducted daily, species composition and biomass of algae was determined weekly. In size, algae cells were differentiated according to Rainey. The degree of complexity of phytoplankton communities was determined calculating the Shannon species diversity index (by biomass). The dominant algae included species making the main contribution into the total phytoplankton biomass. In 20122014, along with the study of algae, we determined chlorophyll а concentration by fluorimetric method, weekly (See Fig. 3). We collected and processed a total of 2453 qualitative and quantitative phytoplankton samples for the period of study; 89 samples to determine chlorophyll а content. Altogether, we found 99 species, kinds and forms of algae from 6 groups in the river phytoplankton. In the Yenisei River, diatoms, which predominate in plankton in all seasons, are characterized by the maximum number of species. There is a certain sequence in the appearance of dominant algae. From late April to late May, Diatoma vulgare Bory and Hannaea arcus (Ehrb.) Patr. were recorded in quantities, in June -Aulacoseira islandica (O. Mull.) Sim., in July - Cyclotella radiosa (Grun.) Lemm., in August - Fragilaria crotonensis Kitt. (See Fig. 1). Upstream of the Krasnoyarsk reservoir A. islandica had a dominant position in nephotic cold layer in June, in July and August - C. radiosa; F. crotonensis was a subdominant. It is known that deep water discharge through the dam (18-40 м) is specific for the Krasnoyarsk reservoir. This causes the flow of algae over the dam. In general, the dynamics of the total number and the total biomass of phytoplankton had the form of unimodal curve within the year (with a peak in June) (See Fig. 2). Nanophytoplankton (2-20 mkm) and microphytoplankton (20-64 mkm) made the biggest contribution to the total biomass of phytoplankton. We recorded peaks of number and biomass of phytoplankton in June in all years in question. We found that the percentage of chlorophyll content per phytoplankton biomass unit had seasonal variations and increased in winter (See Fig. 5). It is known, that the concentration of chlorophyll a depends on dimensional structure of phytoplankton. In spring and summer, the domination of algae cells with bigger volumes in phytoplankton caused a decrease in chlorophyll а concentration.

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