Acidified lakes in the Tatra Mountains (Slovakia) can be divided according to their pH as a measure of acidification into three categories (non-acidified lakes with pH higher than 6, acidified lakes with pH between 5 and 6, and strongly acidified lakes with pH lower than 5). A process of oligotrophication was recognised within the lakes in the second category resulting to the lowest total phosphorus, organic carbon and chlorophyll-a concentrations compared to non-acidified and strongly acidified lakes, and by planktonic Crustacea extinction. The suggested mechanisms of oligotrophication are precipitation of phosphorus containing compounds with aluminium in the water column, and further retention of phosphorus in the acidified watershed. The thesis was focused on studying the processes which can influence the phosphorus concentration in the water column. Two long-term and one short-term mesocosm experiments and a series of microcosm experiments were conducted in order to simulate lakes in the three categories of acidification described in the Tatra Mountains and to study phosphorus forms transformations under these conditions. The measured parameters were: pH, concentration of phosphorus, organic carbon and aluminium fractions and mostly also conductivity, alkalinity and concentration of major ions, nitrogen fractions and chlorophyll-a. In mesocosm experiments presence of zooplankton and zoobentos was recorded. The experimental results indicated that precipitation of phosphorus had depended on pH and the initial aluminium and organic matter concentrations. In the samples with initial aluminium concentration higher than 250 mg l-1 the most effective phosphorus removal was found at pH ~5.5. In the samples with initial aluminium concentration equal to 250 mg l-1 and lower the concentration of phosphorus also decreased but no single minimum was found. A simple theoretical model based on a solubility of aluminium hydroxide species was constructed and explained a large portion of variability of the both experimental and field data. An evaluation of data from 1500 Norwegian lakes revealed the lowest total phosphorus concentrations in lakes with pH between 5 and 6 what is in agreement with the "Tatra's phenomenon". This is further evidence supporting the meaning that acidification is accompanied by oligotrophication. Mechanisms leading to oligotrophication were found in the experiments under conditions simulating acidified lakes in the Tatra Mountains. The agreement between experimental and field data suggests that the same mechanism can cause oligotrophication of acidified lakes.