MICROBIAL DECOMPOSITION OF FRESH-WATER MACROPHYTES IN THE LITTORAL-ZONE OF LAKES

The decomposition of several lake macrophytes was investigated under field conditions. Data on weight and phosphorus loss, numbers of microbial decomposers and their activity were obtained. Experiments were conducted in the littoral of two lakes with different levels of macrophyte development. Weight loss during 40-60 days of decomposition for fast-decomposing plants was 60-95% and after 365-day of incubation, Potamogeton perfoliatus L. lost nearly 100% of its initial weight. Slow-decomposing plants lost 20-50% of their initial weight after 40-60 days of incubation, and Phragmites australis (Cav.) Trin. ex Steud. lost 84% of its initial weight after 365 days. Total phosphorus content in plants did not decrease at the first stages of decomposition. The number of microbial decomposers utilizing both labile and resistant substrates increased 2-6 times during the first 5-25 days period. During this period the community was morphologically diverse and biochemically active (high level of microbial respiration). It coincided with the highest weight loss. After that period, the number of microorganisms utilizing labile substrates, as well as the rate of decomposition decreased. The part of macrophyte organic matter entering the biological cycle in two lakes made up 3.5% and 26% of phytoplankton primary production. Bacterial production on decomposing macrophytes was calculated at 4% and 51% of bacterioplankton production, respectively, in both lakes.