831 ISSN 0097-8078, Water Resources, 2017, Vol. 44, No. 6, pp. 831–839. © Pleiades Publishing, Ltd., 2017. Original Russian Text © N.M. Mineeva, L.G. Korneva, V.V. Solov’eva, 2017, published in Vodnye Resursy, 2017, Vol. 44, No. 6, pp. 653–662. Phytoplankton Production Characteristics in the Upper and Lower Pools of Volga Reservoir HPPs N. M. Mineeva*, L. G. Korneva, and V. V. Solov’eva Papanin Institute for Biology of Inland Waters, Russian Academy of Sciences, Borok Settl., Nekouzskii raion, Yaroslavl oblast, 152742 Russia *e-mail: mineeva@ibiw.yaroslavl.ru Received February 24, 2015; in final form, May 25, 2017 Abstract—Data of field observations of 1989–1991 were used to analyze the habitat conditions and the struc- tural–functional characteristics of phytoplankton in near-dam reaches of Volga reservoirs. Averaged data show negative trends, manifesting themselves in a decline in the characteristics of phytoplankton develop- ment and production–destruction processes in the lower pools of HPPs against the background of a decrease in water temperature, water transparency, and dissolved oxygen content. Saprobity indices show water quality in near-dam reaches to correspond to β-mesosaprobic zone and not to deteriorate in the lower pools, despite the decline in organic matter destruction. By the combination of abiotic characteristics and phytoplankton production indices, the upper and lower pools of Volga HPP reservoirs occupy close positions in the shortest network and fall in the clusters formed by stations in appropriate reservoirs. Keywords: phytoplankton, composition, production, environmental factors, Volga reservoirs, upper and lower HPP pools DOI: 10.1134/S0097807817060069 INTRODUCTION The present-day Volga is a system of eight large, relatively shallow, lowland reservoirs, separated by HPP dams [1]. The dams refer to the channel or dam- channel type and are classified as low-head. The con- struction of the dams and the regimes of HPP opera- tions cause adverse ecological transformations, including changes and depletion of the species com- position of aquatic organisms in the lower pools of HPPs, a decline in primary production (PP) and self- purification potential, changes in fish population, and disturbance of fish migration [21, 24, 28]. Data on the effect of HPP dams on river plankton are few and not enough to reliably assess the effect [4, 18, 23, 24, 26, 27]. In the case of Volga phytoplankton, the primary producer of organic matter (OM) in Volga reservoirs, no such data are available now. The focus of this study is the comparative analysis of the habitat conditions and production characteris- tics of the phytoplankton in the upper and lower pools of Volga reservoir HPPs. MATERIALS AND METHODS The study was based on the data on the composi- tion and production of Volga phytoplankton in reaches upstream and downstream of HPP dams, i.e., in the upper (UP) and lower (LP) pools. The materials were collected in multipurpose route expeditions of Papanin Institute for Biology of Inland Waters, Rus- sian Academy of Sciences, in June 1990 and 1991 (Middle Volga), August and October 1990 (Middle and Lower Volga), and August 1989 and 1991 (the entire chain of reservoirs). The data on the pools of the Rybinsk HPP were collected in May and August 1992 [14, 15]. Water sampling, phytoplankton biomass eval- uation, determination of chlorophyll and the PP and destruction of OM were carried out by conventional methods described elsewhere [6, 10–12, 31]. OM destruction in water mass was calculated for the actual depth of stations. The similarity of phytoplankton in the upper and lower pools was assessed by Sjerensen indices [19]. Phytoplankton species diversity was eval- uated by Shannon indices [16], and water quality, by saprobity indices [18], which were evaluated in terms of biomass. All these characteristics were evaluated in 58 water samples. The study used the results of hydro- logical and hydrochemical studies [9, 11]. Statistical calculations were made using standard computer soft- ware; the coefficients of correlation r given in the text are significant at p < 0.05. RESULTS AND DISCUSSION Situated in different natural-climate zones, Volga reservoirs significantly differ in their morphological characteristics and hydrological regime [1, 30]. The WATER QUALITY AND PROTECTION: ENVIRONMENTAL ASPECTS