Microbial decomposition of organic matter in the bottom sediments of small lakes of the urban landscape (Lithuania)

Bacterial abundance and the rates of sulfate reduction (SR) and total organic matter decomposition (D-total) were studied in the bottom sediments of nine lakes in the vicinity of Vilnius (Lithuania) during the ice-free seasons of 2006-2009. During the spring mixing of the water, aerobic processes of organic matter decomposition prevailed in the bottom sediments of most lakes, while anaerobic processes predominated (up to 80-90% D-total) in summer and early autumn. SR rates in the bottom sediments made up 0.16-2.6 and 0.09-2.0 mg S2-/(dm(3) day) for the medium-depth and shallow lakes, respectively. The highest numbers of sulfate-reducing bacteria (up to 10(6) cells/cm(3)) and SR rates were observed in summer. SR rate in mediumdepth lakes increased with development of anaerobic conditions at the bottom and elevated sulfate concentrations (up to 96.0 mg/dm(3)). In shallow lakes, where O-2 concentration at the bottom was at least 6.7 mg/L, SR rates increased with temperature and inflow of fresh organic matter, especially during cyanobacterial blooms. The average SR rates in the bottom sediments of the lakes of urbanized areas were 4 times higher than in the shallow lakes of protected areas. Accumulation of organic matter and its intensive decomposition during summer may enhance the processes of secondary eutrophication of these small and shallow lakes.