Vertical distribution of bacteria and intensity of microbiological processes in two stratified gypsum Karst Lakes in Lithuania

Physical-chemical parameters and the vertical distribution of bacteria and organic matter production-destruction processes were studied during midsummer stratification in two karst lakes (Kirkilai and Ramunelis) located in northern Lithuania. The lakes were characterized by high sulfate concentrations (369-1248 mg.L-1). The O-2/H2S intersection zone formed at 2-3 m depth. In Lake Kirkilai, the highest bacterial densities (up to 8.7 x 10(6) cell.mL(-1)) occurred at the O-2/H2S intersection zone, whereas in Lake Ramunelis the highest densities were observed in the anoxic hypolimnion (up to 11 x 10(6) cell.mL(-1)). Pigment analysis revealed that green sulfur bacteria dominated in the microaerobic-anaerobic water layers in both lakes. The most intensive development of sulfate-reducing bacteria was observed in the anaerobic layer. Photosynthetic production of organic matter was highest in the upper layer. Rates of sulfate reduction reached 0.23 mg S2-.dm(3).d(-1) in the microaerobic-anaerobic water layer and 1.97 mg S2-.dm(3).d(-1) in sediments. Karst lakes are very sensitive to organic pollution, because under such impact in the presence of high sulfate amounts, sulfate reduction may become very intensive and, consequently, the increase in hydrogen sulfide and development of sulfur cycle bacteria may reduce the variety of other hydrobionts.