«Black» lakes are widely widespread in the area of humid climate. Their waters contain many humic matters and cut-in iron, therefore have different intensity with a brown – yellow tint. On this background lakes meetings lakes with clear colourless water considerably stand out. Due to mass development on the bottom of these lakes of dark blue-green aigae (cyanobacterias) water in them is of greenish-blue tint («blue» lakes). In the course of life, these microorganisms consume dissolved organic substances and sulfates and produce in the daytime – oxygen, and at night – hydrogen sulfide which reacts with dissolved iron in water. This transition increases the acidity of the pyrite in the lake waters and is also accompanied by acidification of the water. Periodic formation of hydrogen sulfide is the cause of «acidification» of lakes. In addition, it leads to precipitation of dissolved iron in the form hydrotroilit. It’s not the «acid» rains that are only an additional supplier of sulfates in the lake under consideration but the activity of specific groups of microorganisms cyanobacteria mats that considered is the main cause of «acidification» of lakes (low pH). These processes should be taken into account when considering the anthropogenic impact on the state of water objects.
