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On the basis of the data obtained during field observations in the summer low water period of 2001, the patterns of chlorophyll distribution and its relation to hydrological and hydrochemical factors in two eutrophic reservoirs of the Middle Volga are studied. The hydrological structure of the Gorky Reservoir, where the Volga water mass dominates, is homogeneous, while in the Cheboksary Reservoir along with the eutrophic Volga waters, the mesotrophic Oka water masses can be distinguished keeping their abiotic and biotic features over a long distance. Phytoplankton in the two contiguous reservoirs with different flow regimens and anthropogenic loads responds differently to the external influence. An autotrophic community in the Gorky Reservoir is more stable and depends little on abiotic factors which account for ~63% of the explained chlorophyll variation. In the Cheboksary Reservoir under maximal for the Volga cascade flow velocity and anthropogenic load, the development and distribution of phytoplankton are almost completely ( R 2 = 0.93) controlled by these factors. The trophic state of the reservoirs has not changed as compared to the beginning of the 1990s.

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