Lakes are natural ecosystems that are daily exposed to pollution. The main culprits are the chemical elements phosphorus and nitrogen. Thankfully lakes have the ability to be cleaned by itself to a certain point. We call that the self-cleaning ability. Lake water pollution is caused by anthropogenic and natural environmental influences. The quantities of nitrogen and phosphorus are affected by rainfall, type of landscape, land use, level of urbanization, the number of households and the arrangement of drainage and wastewater treatment.In this diploma work, I interpret and compare Lake Račeva in the municipality of Logatec and Lake Divje jezero in Idrija. The aim was to calculate the indicative annual amounts of nitrogen and phosphorus that lakes get from their own catchment areas. The calculations are only indicative, because currently there are no analysis of water quality of lakes. On the web pages of ARSO I first sought information of the ten-year average amount of rainfall on a few of meteorological stations and determine the size and proportions of each land use in relation to the total surface hydrological areas of both lakes. With the help of guidelines from various authors I determined the quantities of two elements that contribute roads, hardened surfaces, household wastewater treatment plants, industry and agricultural land. I acquired quantities for each reservoir separately and compared the final results with each other.Because it is predominantly wooded area the forest also contributes the largest amount of nitrogen and phosphorus. Among the major emitters we can count the road and households which do not have drainage and cleaning waste water. The quantities of pollutants will be significantly reduced when the waste water will be discharged to the treatment plant and when water from busy roads will be retained in reservoirs.