Abstract
Abstract
The features of summer stratification and vertical distribution of photosynthetic pigments (chlorophylls and bacteriochlorophylls) in five karst lakes in the Middle Volga region (Republics of Tatarstan and Mari-El) are studied. The water of the investigated lakes has low color and sulfate concentration and low-to-medium total dissolved salts content. Anoxic conditions develop in the hypolimnion in all lakes after the establishment of summer thermal stratification. Reduced iron and sulfur compounds are found in the anaerobic zones of most lakes. In these lakes, deep maxima of chlorophyll and bacteriochlorophylls are formed, indicating the presence of biomass maxima of microbial plankton, consisting of both oxygenic and anoxygenic phototrophs. The presented data demonstrate also a wide variety of features of both chemical stratification and vertical distribution of phototrophic organisms in the studied lakes, which are probably associated with differences in the limnological characteristics of lakes, their water composition, and the history of their existence.