There is a need to collect and store fast-growing resources of information describing phenomena and processes occurring in the nature, and therefore, databases are increasingly being used in environmental sciences. The possibility of formulating arbitrarily complicated queries enables in the precise determination of the scope and method of analysis using a large amount of historical and actual datasets. This study used a database containing the morphometric parameters of more than 1,000 lakes and their catchments, and basic data describing the quality of water of more than 560 lakes. In the first step of the analysis, the percentage share of agricultural land in the total area of each region was determined. The next stage was to divide the entire set of lakes into equinumerous class breaks based on the values of water quality parameters. Then, the lakes located in individual regions were counted within the groups of low, medium, and high values of studied parameters. Numbers of lakes have been converted to percentages and used to calculate the values of the regional water quality indicator. It was concluded that the values of the indicator are dependent on the type of land use, and in particular, on the percentage of agricultural land that exists within the studied regions.