Structural and taxonomic diversity and ecological features of phytoplankton in the Velikaya River delta (Pskov Oblast)

The basis of the existence of aquatic ecosystems is microalgae communities, which produce most part of biological products, play an important role in the sustainability of ecosystems and provide many types of different ecosystem services. The short life cycle and rapid metabolism of microalgae make them ideal targets for monitoring research. The structure and dynamics of producers determines the formation of the entire trophodynamic structure of the aquatic ecosystem and provides the necessary indicators for assessing the aquacultural potential of any water body. The aim of this research was to study the structural and taxonomic diversity of phytoplankton of the Velikaya River delta. The Velikaya River delta (57°86'96''N, 28°11'54"E) presents a unique natural complex where connection and transformation of the river water masses and the water-receiving basin of Lake Peipus take place. We collected samples for phytoplankton study and basic physical and chemical measurements in summer 2016 at five sampling stations representing various ecological areas of the Velikaya River delta (See Fig. 1). The hydrobiological material (five samples) was collected by 0.5 liter plastic samplers from a depth of 0.3-0.5 m. Phytoplankton samples were fixed with a 40% formalin solution until a weak odor and were concentrated by the sediment method. We identified the species of all organisms, as far as possible, using a "Carl Zeiss Axio Lab. A1" microscope with the help of Russian and foreign determinants. The laboratory treatment was performed with a 0.05 cm³ Najotta camera using conventional methods. The biodiversity was studied at alpha and beta levels. To study the parameters of alpha-diversity of communities, we calculated the Shannon's diversity index, the Simpson's dominance index, the Margalef's index and the Pielou's evenness index. Analysis of algae taxonomic composition similarity at different sampling sites (beta-diversity) was conducted using the S0rensen-Chekanovsky Index. For complex presentation of species richness parameters and algae community evenness, we constructed dominance-diversity curves. To compare the microalgae communities of the Velikaya river delta according to species structure, we used cluster analysis. We used the Ward's method as a method of linkage. To identify ecological groups and specify ecological and geographical characteristics of algae, we used determinants and data from a number of monographs. The index of saprobity was calculated according to Pantle-Buck in the modification of Sladechek. We identified one hundred sixty five species taxa of microalgae belonging to 8 phylums: Bacillariophyta (37%), Chlorophyta (33.9%), Cyanophyta/Cyanoprokaryota (9.7%), Chrysophyta (6.1%), Euglenophyta (6.1%), Cryptophyta (3%), Dinophyta (3%), Xanthophyta (1.2%) (See Tables 1 and 2). The Shannon index values indicate the average complexity of microalgae community structure. The Margalef index values describe the Velikaya River delta as an ecosystem of high species richness (See Table 3). Compared to the previous studies, a significant increase in the level of information diversity is observed, indicating an increase in the number of possible substance and energy flows in the ecosystem. The forms of dominance-diversity curves indicate that the structure of microalgae communities corresponds to the concept of Hutchinson's multidimensional niche space (See Fig. 3). Identification of differences in the structure of species communities also points out the existence of ecological differentiation in microalgae complexes of the delta, which is due to the habitat heterogeneity. Ecological and geographical analysis proves that the absolute dominance of cosmopolitan freshwater forms is typical of the algoflora of the Velikaya River delta. In relation to the pH-reaction, the inhabitants of neutral and slightly alkaline water dominate (See Fig. 4). The Pantle-Buck saprobity index is used for water quality assessment, which shows beta-mesosaprobic water quality in the ecosystem. Thus, the water of the Velikaya River delta could be referred to the category of moderately polluted water (class III of water quality) (See Fig. 5; Table 4). The paper contains 5 Figures, 4 Tables and 31 References.

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