Most lakes in Poland are shallow  and vulnerable to degradation mostly due to lake morphology and  landscape  structure.  Other  factors,   like  discharged sewage, internal  loading  and human activities in  the  watershed  are  also important. During  studies  on Brdowskie  Lake, water samples  were  taken twice a  year (spring  and  summer  season)  from  the surface layer   and analyses of soluble reactive   phosphates,  total  phosphorus,  nitrates, conductivity,  five-day biochemical  oxygen  demand,  chlorophyll  a,  dry  mass  of seston  and Secchi depth were undertaken. Brdowskie Lake  is  situated  in  Kujawskie  Lakeland and its catchment basin covers about 155.3  km2. The littoral vegetation is dominated by reed bed with  a  minor presence of other  taxa.   The  lake  is very susceptible to degradation (morphology,  agricultural lands, housing)  and  has several potential  sources  of pollution, e.g. Notec River, a  nameless stream and summer houses.  In  general,  water  quality  of the lake was  better  during the spring season,  especially  in terms  of chlorophyll  a and dry  mass  of seston  concentrations. In 1997-2006,  the  level  of conductivity   was  very changeable with the maximum values observed in 1999-2001. Significant correlations  between some parameters in certain seasons of the year  were found, e.g. between chlorophyll a and Secchi depth  during  spring and total phosphorus and phosphates in summer. The impact of water  level  fluctuations on water quality  parameters, like total  phosphorus  and  chlorophyll a  was  observed. According to the  results, the quality water in  Brdowskie  Lake   is improving. After a biological wastewater treatment plant in the catchment had opened, the rate of salts flowing into the lake with sewage was reduced.