Flood events, worse condition of surface waters, and pollution of sea waters are associated with the anthropogenic changes in the environment. Increasing pollution and the amount of rainwater – raise the pollution load as a result of increasing the built-up area. The aim of the study was to investigate the pollution load coming from a catchment in Gdansk to the Baltic Sea by stream. The study presents the results of water quality tests in the Oliwa Stream. It is one of the largest rainwater receivers in Gdansk with a catchment area of almost 29 km². Water samples were collected at six points (three in the natural catchment and three in urbanized catchment). The pollutant concentrations were obtained from the tests On the basis of the results, the loads of biogenic compounds introduced into the Gulf of Gdansk were calculated. The largest load of nitrates (V) and total phosphorus is discharged in the summer and amounts to 4 tons NO₃^- and almost 2 tons of total phosphorus per year, respectively. Orthophosphates are discharged mainly in autumn, and their annual load is almost 4 tons. The majority of biogenic substances comes from the urbanized catchment; therefore, it is necessary to locate and limit the sources of pollution in this area.