During spring 2008 and 2013, the concentrations of nitrogen substances (NO₃^-, NO₂^- and NH₄⁺) were investigated against the background of changes in other selected physical and chemical water quality indices in the surface waters of Lake Warnowo located in Wolin National Park on Volin Island. It was found that the investigated waters were additionally contaminated by nitrogen compounds. This is illustrated by increased values of NO₃^- , NO₂^- and NH₄⁺ concentrations of the investigated waters in 2013 to max. values 0,28 (N-NO₃/dm³), 0,103 (N-NO₂/dm³) and 0,34 (N-NH₄/dm³), causing water quality deterioration in the lake by one class of quality compared to 2008 (where the max. values amounted to 0,39 (N-NO3/dm3), 0,016 (N-NO2/dm3) and 0,13 (N-NH4/dm3), which led to accelerated water eutrophication in the lake in 2008-2013. This paper performs one of the chemometric techniques – principal component analysis (PCA) – used to classify water quality indices.