The study objective was to analyse the number of tourists present in the shore zone and bathing areas of lakes with regard to their tourist carrying capacity and the amount of biogenic substances potentially entering the ecosystem from the beach and bathing areas. The procedures from project between the EU and Poland, in the module “Development of the sanitary supervision of water quality” were used in three categories: physiological substances – sweat and urine; water-soluble and insoluble organic compounds; and biogenic elements – nitrogen and phosphorus. The research was conducted in two model mesotrophic lakes, Piaseczno and Zagłębocze, located in the Łęczna- Włodawa Lakeland (eastern Poland). The data were analysed in reference to biological trophic status indices defining the limnological status of lakes in the summer of 2014 and 2016. Analyses of gross primary production of phytoplankton using the light and dark bottles method and the analysis of chlorophyll a concentration were applied using the laboratory spectrophotometric method. The relatively small number of tourists recorded in the shore zone of both lakes did not exceed their tourist carrying capacity, and their potential contribution of biogenic substances to the lake ecosystems was small. Biological trophic indices for both lakes indicated that they had been continually late- mesotrophic for decades. The amount of biogenic substances directly linked to beach tourism usually has a minor effect on the limnological status of mesotrophic lakes. Due to the specific character of lake ecosystems, however, even small amounts of these substances can contribute to the destabilisation of the biocenotic system.