Pigment characteristics of phytoplankton in the Belokholunitskoye and Omutninskoye reservoirs

In order to assess the ecological and trophic state of the two largest reservoirs in the Kirov region (Belokholunitskoye and Omutninskoye) experiencing different anthropogenic loads, the study of the species composition of phytoplankton and the determination of its pigment characteristics was carried out. It is noted that the phytoplankton of the studied reservoirs is mainly represented by green algae and diatoms, but cyanobacteria occupy a dominant position during the flowering period. According to the value of Pantle and Bucc saprobity index in the Sladecek modification, both reservoirs correspond to the beta-mesosaprobic zone, water quality class III - moderately polluted waters. During the study conducted in the summer of 2023, for the first time the content of chlorophylls (a, b, c), pheophytin and carotenoids was determined for the studied reservoirs. In the Belokholunitskoye reservoir, the total content of chlorophylls was 6.4-26.2 μg/l, carotenoids - 2.5-10.8 μg/l; in the Omutninskoye reservoir 16.4-18.8 and 4.9-5.2 μg/l, respectively. The proportion of the main phytoplankton pigment - chlorophyll a - from the total content of chlorophylls (a, b, c) in the Belokholunitskoye reservoir was 70-76 %, in the Omutninskoye reservoir - 56-60 %. According to the average concentration of chlorophyll a (9.8-10.5 μg/l), the studied reservoirs occupy a borderline position between mesotrophic and moderately eutrophic reservoirs. For the pheophytin content, negative values were obtained in all cases; this indicates the summer dominant role of active forms of photosynthetic pigments in production processes. The ratio of carotenoids to the content of chlorophyll a averages 0.50 for all studied areas; this demonstrates favorable conditions for the development of phytoplankton and corresponds to “chlorophyll-type” waters characteristic of eutrophic reservoirs. The calculated values of the pigment indices I430/664 (Margalef index) and I480/664 are 2.5-2.8 and 1.3-1.6 rel. units, respectively, show a high diversity of planktonic algocenosis, the predominance of viable active cells in it and sufficient provision of algae with mineral nitrogen nutrition. Accordingly, they indicate a high diversity of planktonic algocenosis, the predominance of viable active cells in it, and a sufficient supply of algae with mineral nitrogen nutrition. No significant differences in the composition of phytoplankton and its pigment characteristics were found between the studied water bodies.